Recent interesting articles

The electrical conductivity of solid-state matter is a fundamental physical property and can be precisely derived from the resistance measured via the four-point probe technique excluding contributions from parasitic contact resistances. Over time, this method has become an interdisciplinary characterization tool in materials science, semiconductor industries, geology, physics, etc, and is employed for both fundamental and application-driven research. However, the correct derivation of the conductivity is a demanding task which faces several difficulties, e.g. the homogeneity of the sample or the isotropy of the phases. In addition, these sample-specific characteristics are intimately related to technical constraints such as the probe geometry and size of the sample. In particular, the latter is of importance for nanostructures which can now be probed technically on very small length scales. On the occasion of the 100th anniversary of the four-point probe technique, introduced by Frank Wenner, in this review we revisit and discuss various correction factors which are mandatory for an accurate derivation of the resistivity from the measured resistance. Among others, sample thickness, dimensionality, anisotropy, and the relative size and geometry of the sample with respect to the contact assembly are considered. We are also able to derive the correction factors for 2D anisotropic systems on circular finite areas with variable probe spacings. All these aspects are illustrated by state-of-the-art experiments carried out using a four-tip STM/SEM system. We are aware that this review article can only cover some of the most important topics. Regarding further aspects, e.g. technical realizations, the influence of inhomogeneities or different transport regimes, etc, we refer to other review articles in this field.

http://iopscience.iop.org/article/10.1088/0953-8984/27/22/223201/pdf

November 3.

Boros Csanád Örs

Margination of micro- and nano-particles in blood flow and its effect on drug delivery

Kathrin Müller, Dmitry A. Fedosov & Gerhard Gompper

Drug delivery by micro- and nano-carriers enables controlled transport of pharmaceuticals to targeted sites. Even though carrier fabrication has made much progress recently, the delivery including controlled particle distribution and adhesion within the body remains a great challenge. The adhesion of carriers is strongly affected by their margination properties (migration toward walls) in the microvasculature. To investigate margination characteristics of carriers of different shapes and sizes and to elucidate the relevant physical mechanisms, we employ mesoscopic hydrodynamic simulations of blood flow. Particle margination is studied for a wide range of hematocrit values, vessel sizes, and flow rates, using two- and three-dimensional models. The simulations show that the margination properties of particles improve with increasing carrier size. Spherical particles yield slightly better margination than ellipsoidal carriers; however, ellipsoidal particles exhibit a slower rotational dynamics near a wall favoring their adhesion. In conclusion, micron-sized ellipsoidal particles are favorable for drug delivery in comparison with sub-micron spherical particles.

http://www.nature.com/articles/srep04871

Szeptember 8-15.

Scherübl Zoltán

Guiding of Electrons in a Few Mode Ballistic Graphene Channel

Peter Rickhaus, Ming-Hao Liu, Péter Makk, Romain Maurand, Samuel Hess, Simon Zihlmann, Markus Weiss, Klaus Richter, Christian Schönenberger

http://arxiv.org/abs/1509.02653

Hexagonal boron nitride as an atomically thin oxidation barrier for ferromagnetic nano structures

Simon Zihlmann, Péter Makk, C. A. F. Vaz, Christian Schönenberger

http://arxiv.org/abs/1509.03087

Giga-Hertz quantized charge pumping in bottom gate defined InAs nanowire quantum dots

S. d'Hollosy, M. Jung, A. Baumgartner, V.A. Guzenko, M.H. Madsen, J. Nygård, C. Schönenberger

http://arxiv.org/abs/1509.01574

Coherent coupling of a single spin to microwave cavity photons

J. J. Viennot, M. C. Dartiailh, A. Cottet, T. Kontos

http://arxiv.org/abs/1509.03839

Negative local resistance due to viscous electron backflow in graphene

D. A. Bandurin, I. Torre, R. Krishna Kumar, M. Ben Shalom, A. Tomadin, A. Principi, G. H. Auton, E. Khestanova, K. S. Novoselov, I. V. Grigorieva, L. A. Ponomarenko, A. K. Geim, M. Polini

http://arxiv.org/abs/1509.04165

Point contacts in encapsulated graphene

Clevin Handschin, Balint Fülöp, Péter Makk, Sofya Blanter, Markus Weiss, K. Watanabe, T. Taniguchi, Szabolcs Csonka, Christian Schönenberger

http://arxiv.org/abs/1509.04137

Coherent manipulation of Andreev states in superconducting atomic contacts

C. Janvier (1), L. Tosi (1), L. Bretheau (1), Ç. Ö. Girit (1), M. Stern (1), P. Bertet (1), P. Joyez (1), D. Vion (1), D. Esteve (1), M.F. Goffman (1), H. Pothier (1), C. Urbina (1) ((1) QUANTRONICS)

http://arxiv.org/abs/1509.03961

Realization of Microwave Quantum Circuits Using Hybrid Superconducting-Semiconducting Nanowire Josephson Elements

G. de Lange, B. van Heck, A. Bruno, D. J. van Woerkom, A. Geresdi, S. R. Plissard, E. P. A. M. Bakkers, A. R. Akhmerov, and L. DiCarlo

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.115.127002

Semiconductor-Nanowire-Based Superconducting Qubit

T. W. Larsen, K. D. Petersson, F. Kuemmeth, T. S. Jespersen, P. Krogstrup, J. Nygård, and C. M. Marcus

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.115.127001

Charge Number Dependence of the Dephasing Rates of a Graphene Double Quantum Dot in a Circuit QED Architecture

Guang-Wei Deng, Da Wei, J. R. Johansson, Miao-Lei Zhang, Shu-Xiao Li, Hai-Ou Li, Gang Cao, Ming Xiao, Tao Tu, Guang-Can Guo, Hong-Wen Jiang, Franco Nori, and Guo-Ping Guo

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.115.126804

Spin pairs in a weakly coupled many-electron quantum dot

S. Hellmüller, D. Bischoff, T. Müller, M. Beck, K. Ensslin, and T. Ihn

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.92.115401

Observation of negative refraction of Dirac fermions in graphene

Gil-Ho Lee, Geon-Hyoung Park, Hu-Jong Lee

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3460.html

Parity lifetime of bound states in a proximitized semiconductor nanowire

A. P. Higginbotham, S. M. Albrecht, G. Kiršanskas, W. Chang, F. Kuemmeth, P. Krogstrup, T. S. Jespersen, J. Nygård, K. Flensberg, C. M. Marcus

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3461.html

Reliable Exfoliation of Large-Area High-Quality Flakes of Graphene and Other Two-Dimensional Materials

Yuan Huang, Eli Sutter, Norman N. Shi, Jiabao Zheng, Tianzhong Yang, Dirk Englund, Hong-Jun Gao, and Peter Sutter

http://pubs.acs.org/doi/abs/10.1021/acsnano.5b04258

Aug. 25. - szept. 7.

Coupling Two Distant Double Quantum Dots with a Microwave Resonator

Guang-Wei Deng, Da Wei, Shu-Xiao Li, J. R. Johansson, Wei-Cheng Kong, Hai-Ou Li, Gang Cao, Ming Xiao, Guang-Can Guo, Franco Nori, Hong-Wen Jiang, and Guo-Ping Guo

We fabricated a hybrid device with two distant graphene double quantum dots (DQDs) and a microwave resonator. A nonlinear response is observed in the resonator reflection amplitude when the two DQDs are jointly tuned to the vicinity of the degeneracy points. This observation can be well fitted by the Tavis–Cummings (T–C) model which describes two two-level systems coupling with one photonic field. Furthermore, the correlation between the DC currents in the two DQDs is studied. A nonzero cross-current correlation is observed which has been theoretically predicted to be an important sign of nonlocal coupling between two distant systems. Our results explore T–C physics in electronic transport and also contribute to the study of nonlocal transport and future implementations of remote electronic entanglement.

http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b02400

Reliable Exfoliation of Large-Area High-Quality Flakes of Graphene and Other Two-Dimensional Materials

Yuan Huang, Eli Sutter, Norman N. Shi, Jiabao Zheng, Tianzhong Yang, Dirk Englund, Hong-Jun Gao, and Peter Sutter

Mechanical exfoliation has been a key enabler of the exploration of the properties of two-dimensional materials, such as graphene, by providing routine access to high-quality material. The original exfoliation method, which remained largely unchanged during the past decade, provides relatively small flakes with moderate yield. Here, we report a modified approach for exfoliating thin monolayer and few-layer flakes from layered crystals. Our method introduces two process steps that enhance and homogenize the adhesion force between the outermost sheet in contact with a substrate: Prior to exfoliation, ambient adsorbates are effectively removed from the substrate by oxygen plasma cleaning, and an additional heat treatment maximizes the uniform contact area at the interface between the source crystal and the substrate. For graphene exfoliation, these simple process steps increased the yield and the area of the transferred flakes by more than 50 times compared to the established exfoliation methods. Raman and AFM characterization shows that the graphene flakes are of similar high quality as those obtained in previous reports. Graphene field-effect devices were fabricated and measured with back-gating and solution top-gating, yielding mobilities of ∼4000 and 12 000 cm2/(V s), respectively, and thus demonstrating excellent electrical properties. Experiments with other layered crystals, e.g., a bismuth strontium calcium copper oxide (BSCCO) superconductor, show enhancements in exfoliation yield and flake area similar to those for graphene, suggesting that our modified exfoliation method provides an effective way for producing large area, high-quality flakes of a wide range of 2D materials.

http://pubs.acs.org/doi/abs/10.1021/acsnano.5b04258

Influence of Impurity Spin Dynamics on Quantum Transport in Epitaxial Graphene

Samuel Lara-Avila, Sergey Kubatkin, Oleksiy Kashuba, Joshua A. Folk, Silvia Lüscher, Rositza Yakimova, T. J. B. M. Janssen, Alexander Tzalenchuk, and Vladimir Fal’ko

Experimental evidence from both spin-valve and quantum transport measurements points towards unexpectedly fast spin relaxation in graphene. We report magnetotransport studies of epitaxial graphene on SiC in a vector magnetic field showing that spin relaxation, detected using weak-localization analysis, is suppressed by an in-plane magnetic field B∥, and thereby proving that it is caused at least in part by spinful scatterers. A nonmonotonic dependence of the effective decoherence rate on B∥ reveals the intricate role of the scatterers’ spin dynamics in forming the interference correction to the conductivity, an effect that has gone unnoticed in earlier weak localization studies.

http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.115.106602

Highly thermally conductive and mechanically strong graphene fibers

Guoqing Xin, Tiankai Yao, Hongtao Sun, Spencer Michael Scott, Dali Shao, Gongkai Wang, Jie Lian

Graphene, a single layer of carbon atoms bonded in a hexagonal lattice, is the thinnest, strongest, and stiffest known material and an excellent conductor of heat and electricity. However, these superior properties have yet to be realized for graphene-derived macroscopic structures such as graphene fibers. We report the fabrication of graphene fibers with high thermal and electrical conductivity and enhanced mechanical strength. The inner fiber structure consists of large-sized graphene sheets forming a highly ordered arrangement intercalated with small-sized graphene sheets filling the space and microvoids. The graphene fibers exhibit a submicrometer crystallite domain size through high-temperature treatment, achieving an enhanced thermal conductivity up to 1290 watts per meter per kelvin. The tensile strength of the graphene fiber reaches 1080 megapascals.

https://www.sciencemag.org/content/349/6252/1083.abstract

A quantum circuit rule for interference effects in single-molecule electrical junctions

David Zsolt Manrique, Cancan Huang, Masoud Baghernejad, Xiaotao Zhao, Oday A. Al-Owaedi, Hatef Sadeghi, Veerabhadrarao Kaliginedi, Wenjing Hong, Murat Gulcur, Thomas Wandlowski, Martin R. Bryce, Colin J. Lambert

A quantum circuit rule for combining quantum interference (QI) effects in the conductive properties of oligo(phenyleneethynylene) (OPE)-type molecules possessing three aromatic rings was investigated both experimentally and theoretically. Molecules were of the type X-Y-X, where X represents pyridyl anchors with para (p), meta (m) or ortho (o) connectivities and Y represents a phenyl ring with p and m connectivities. The conductances GXmX (GXpX) of molecules of the form X-m-X (X-p-X), with meta (para) connections in the central ring were predominantly lower (higher), irrespective of the meta, para, or ortho nature of the anchor groups X, demonstrating that conductance is dominated by the nature of QI in the central ring Y. The single-molecule conductances were found to satisfy the quantum circuit rule Gppp/Gpmp = Gmpm/Gmmm. This demonstrates that the contribution to the conductance from the central ring is independent of the para versus meta nature of the anchor groups.

http://xxx.lanl.gov/abs/1509.00990

Spawning rings of exceptional points out of Dirac cones

Bo Zhen, Chia Wei Hsu, Yuichi Igarashi, Ling Lu, Ido Kaminer, Adi Pick, Song-Liang Chua, John D. Joannopoulos & Marin Soljacˇic

The Dirac cone underlies many unique electronic properties of graphene1 and topological insulators, and its band structure— two conical bands touching at a single point—has also been realized for photons in waveguide arrays2 , atoms in optical lattices3 , and through accidental degeneracy4,5. Deformation of the Dirac cone often reveals intriguing properties; an example is the quantum Hall effect, where a constant magnetic field breaks the Dirac cone into isolated Landau levels. A seemingly unrelated phenomenon is the exceptional point6,7, also known as the parity–time symmetry breaking point8–11, where two resonances coincide in both their positions and widths. Exceptional points lead to counter-intuitive phenomena such as loss-induced transparency12, unidirectional transmission or reflection11,13,14, and lasers with reversed pump dependence15 or single-mode operation16,17. Dirac cones and exceptional points are connected: it was theoretically suggested that certain non-Hermitian perturbations can deform a Dirac cone and spawn a ring of exceptional points18–20. Here we experimentally demonstrate such an ‘exceptional ring’ in a photonic crystal slab. Angle-resolved reflection measurements of the photonic crystal slab reveal that the peaks of reflectivity follow the conical band structure of a Dirac cone resulting from accidental degeneracy, whereas the complex eigenvalues of the system are deformed into a two-dimensional flat band enclosed by an exceptional ring. This deformation arises from the dissimilar radiation rates of dipole and quadrupole resonances, which play a role analogous to the loss and gain in parity–time symmetric systems. Our results indicate that the radiation existing in any open system can fundamentally alter its physical properties in ways previously expected only in the presence of material loss and gain.

http://www.nature.com/nature/journal/vaop/ncurrent/pdf/nature14889.pdf

A Short Course on Topological Insulators: Band-structure topology and edge states in one and two dimensions

János K. Asbóth, László Oroszlány, András Pályi

This course-based primer provides newcomers to the field with a concise introduction to some of the core topics in the emerging field of topological band insulators in one and two dimensions. The aim is to provide a basic understanding of edge states, bulk topological invariants, and of the bulk--boundary correspondence with as simple mathematical tools as possible. We use noninteracting lattice models of topological insulators, building gradually on these to arrive from the simplest one-dimensional case (the Su-Schrieffer-Heeger model for polyacetylene) to two-dimensional time-reversal invariant topological insulators (the Bernevig-Hughes-Zhang model for HgTe). In each case the model is introduced first and then its properties are discussed and subsequently generalized. The only prerequisite for the reader is a working knowledge in quantum mechanics, the relevant solid state physics background is provided as part of this self-contained text, which is complemented by end-of-chapter problems.

http://xxx.lanl.gov/abs/1509.02295

Március 24-31.

Fülöp Gergő

Graphene-templated directional growth of an inorganic nanowire

Won Chul Lee, Kwanpyo Kim, Jungwon Park, Jahyun Koo, Hu Young Jeong, Hoonkyung Lee, David A. Weitz, Alex Zettl & Shoji Takeuchi

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2015.36.html

Coaxial lithography

Tuncay Ozel, Gilles R. Bourret & Chad A. Mirkin

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2015.33.html

Coupling between electrons and optical phonons in suspended bilayer graphene

Antti Laitinen, Manohar Kumar, Mika Oksanen, Bernard Plaçais, Pauli Virtanen, and Pertti Hakonen

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.121414

Nonlocal electromagnetic response of graphene nanostructures

Arya Fallahi, Tony Low, Michele Tamagnone, and Julien Perruisseau-Carrier

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.121405

Nonequilibrium spin transport in Zeeman-split superconductors

Tatiana Krishtop, Manuel Houzet, and Julia S. Meyer

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.121407

Finite-frequency noise in a quantum dot with normal and superconducting leads

Stephanie Droste, Janine Splettstoesser, and Michele Governale

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.125401

Március 24.

Boros Csanád Örs

Spin and charge transport in graphene-based spin transport devices with Co/MgO spin injection and spin detection electrodes

'F. Volmer, M. Drögeler, G. Güntherodt, C. Stampfer, B. Beschoten'

In this review we discuss spin and charge transport properties in graphene-based single-layer and few-layer spin-valve devices. We give an overview of challenges and recent advances in the field of device fabrication and discuss two of our fabrication methods in more detail which result in distinctly different device performances. In the first class of devices, Co/MgO electrodes are directly deposited onto graphene which results in rough MgO-to-Co interfaces and favor the formation of conducting pinholes throughout the MgO layer. We show that the contact resistance area product (RcA) is a benchmark for spin transport properties as it scales with the measured spin lifetime in these devices indicating that contact-induced spin dephasing is the bottleneck for spin transport even in devices with large RcA values. In a second class of devices, Co/MgO electrodes are first patterned onto a silicon substrate. Subsequently, a graphene-hBN heterostructure is directly transferred onto these prepatterned electrodes which provides improved interface properties. This is seen by a strong enhancement of both charge and spin transport properties yielding charge carrier mobilities exceeding 20000 cm2/(Vs) and spin lifetimes up to 3.7 ns at room temperature. We discuss several shortcomings in the determination of both quantities which complicates the analysis of both extrinsic and intrinsic spin scattering mechanisms. Furthermore, we show that contacts can be the origin of a second charge neutrality point in gate dependent resistance measurements which is influenced by the quantum capacitance of the underlying graphene layer.

http://arxiv.org/abs/1503.01735v1

Nov. 1-21.

Scherübl Zoltán

Unusual resistance-voltage dependence of nanojunctions during electromigration in ultra-high vacuum

D. Stöffler, M. Marz, B. Kießig, T. Tomanic, R. Schäfer, H. v. Löhneysen, R. Hoffmann-Vogel

The electrical resistance R of metallic nanocontacts subjected to controlled cyclic electromigration in ultra-high vacuum has been investigated in-situ as a function of applied voltage V. For sufficiently small contacts, i.e., large resistance, a decrease of R(V) while increasing V is observed. This effect is tentatively attributed to the presence of contacts separated by thin vacuum barriers in parallel to ohmic nanocontacts. Simple model calculations indicate that both thermal activation or tunneling can lead to this unusual behavior. We describe our data by a tunneling model whose key parameter, i.e., the tunneling distance, changes because of thermal expansion due to Joule heating and/or electrostatic strain arising from the applied voltage. Oxygen exposure during electromigration prevents the formation of negative R(V) slopes, and at the same time enhances the probability of uncontrolled melting, while other gases show little effects. In addition, indication for field emission has been observed in some samples

http://arxiv.org/abs/1411.0105

Pb/InAs nanowire Josephson junction with high critical current and magnetic flux focusing

J. Paajaste, M. Amado, S. Roddaro, F. S. Bergeret, D. Ercolani, L. Sorba, F. Giazotto

We have studied mesoscopic Josephson junctions formed by highly n-doped InAs nanowires and superconducting Ti/Pb source and drain leads. The current-voltage properties of the system are investigated by varying temperature and external out-of-plane magnetic field. Superconductivity in the Pb electrodes persists up to ∼7 K and with magnetic field values up to 0.4 T. Josephson coupling at zero backgate voltage is observed up to 4.5 K and the critical current is measured to be as high as 615 nA. The supercurrent suppression as a function of the magnetic field reveals a diffraction pattern that is explained by a strong magnetic flux focusing provided by the superconducting electrodes forming the junction.

http://xxx.lanl.gov/abs/1411.0990

Room temperature magnetic order on zigzag edges of narrow graphene nanoribbons

Gabor Zsolt Magda, Xiaozhan Jin, Imre Hagymasi, Peter Vancso, Zoltan Osvath, Peter Nemes-Incze, Chanyong Hwang, Laszlo P. Biro, Levente Tapaszto

Magnetic order emerging in otherwise non-magnetic materials as carbon is a paradigmatic example of a novel type of s-p electron magnetism predicted to be of exceptional high-temperature stability. It has been demonstrated that atomic scale structural defects of graphene can host unpaired spins. However, it is still unclear under which conditions long-range magnetic order can emerge from such defect-bound magnetic moments. Here we propose that in contrast to random defect distributions, atomic scale engineering of graphene edges with specific crystallographic orientation, comprising edge atoms only from one sub-lattice of the bipartite graphene lattice, can give rise to a robust magnetic order. We employ a nanofabrication technique based on Scanning Tunneling Microscopy to define graphene nanoribbons with nanometer precision and well-defined crystallographic edge orientations. While armchair ribbons display quantum confinement gap, zigzag ribbons narrower than 7 nm reveal a bandgap of about 0.2 - 0.3 eV, which can be identified as a signature of interaction induced spin ordering along their edges. Moreover, a semiconductor to metal transition is revealed upon increasing the ribbon width, indicating the switching of the magnetic coupling between opposite ribbon edges from antiferromagnetic to ferromagnetic configuration. We found that the magnetic order on graphene edges of controlled zigzag orientation can be stable even at room temperature, raising hope for graphene-based spintronic devices operating under ambient conditions.

http://xxx.lanl.gov/abs/1411.1196

Using polymer electrolyte gates to set-and-freeze threshold voltage and local potential in nanowire-based devices and thermoelectrics

Sofia Fahlvik Svensson, Adam M. Burke, Damon J. Carrad, Martin Leijnse, Heiner Linke, Adam P. Micolich

We use the strongly temperature-dependent ionic mobility in polymer electrolytes to 'freeze in' specific ionic charge environments around a nanowire using a local wrap-gate geometry. This enables us to set both the threshold voltage for a conventional doped substrate gate and the local disorder potential at temperatures below 200 Kelvin, which we characterize in detail by combining conductance and thermovoltage measurements with modeling. Our results demonstrate that local polymer electrolyte gates are compatible with nanowire thermoelectrics, where they offer the advantage of a very low thermal conductivity, and hold great potential towards setting the optimal operating point for solid-state cooling applications.

http://xxx.lanl.gov/abs/1411.2727

Revisiting the measurement of the spin relaxation time in graphene-based devices

H. Idzuchi, A. Fert, Y. Otani

A long spin relaxation time (tausf) is the key for the applications of graphene to spintronics but the experimental values of tausf have been generally much shorter than expected. We show that the usual determination by the Hanle method underestimates tausf if proper account of the spin absorption by contacts is lacking. By revisiting series of experimental results, we find that the corrected tausf are longer and less dispersed, which leads to a more unified picture of tausf derived from experiments. We also discuss how the correction depends on the parameters of the graphene and contacts.

http://xxx.lanl.gov/abs/1411.2949

Detecting nonlocal Cooper pair entanglement by optical Bell inequality violation

Simon E. Nigg, Rakesh P. Tiwari, Stefan Walter, Thomas L. Schmidt

Based on the Bardeen Cooper Schrieffer (BCS) theory of superconductivity, the coherent splitting of Cooper pairs from a superconductor to two spatially separated quantum dots has been predicted to generate nonlocal pairs of entangled electrons. In order to test this hypothesis, we propose a scheme to transfer the spin state of a split Cooper pair onto the polarization state of a pair of optical photons. We show that the produced photon pairs can be used to violate a Bell inequality, unambiguously demonstrating the entanglement of the split Cooper pairs.

http://arxiv.org/abs/1411.3945

Current noise cross correlation mediated by Majorana bound states

Hai-Feng Lu, Hai-Zhou Lu, Shun-Qing Shen

We study the transport properties of a quantum dot-Majorana hybrid system, in which each of paired Majorana bound states is connected to one quantum dot. With the help of non-equilibrium Green's function method, we obtain an exact solution of the Green's functions and calculate the currents through the quantum dots and nonlocal noise cross correlation between the currents. As a function of dot energy levels ϵ1 and ϵ2, we find that for the symmetric level configuration ϵ1=ϵ2, the noise cross correlation is negative in the low lead voltage regime, while it becomes positive with the increase of the lead voltages. Due to the particle-hole symmetry, the cross correlation is always positive in the anti-symmetric case ϵ1=−ϵ2. In contrast, the cross correlation of non-Majorana setups is always positive. For comparison, we also perform the diagonalized master equation calculation to check its applicability. It is found that the diagonalized master equations work well in most regimes of system parameters. Nevertheless, it shows an obvious deviation from the exact solution by the non-equilibrium Green's function method when all eigenenergies of the dot-Majorana hybrid system and simultaneously the energy intervals are comparable to the dot-lead coupling strength.

http://arxiv.org/abs/1411.4260

Detecting bit-flip errors in a logical qubit using stabilizer measurements

D. Ristè, S. Poletto, M.-Z. Huang, A. Bruno, V. Vesterinen, O.-P. Saira, L. DiCarlo

Quantum data is susceptible to decoherence induced by the environment and to errors in the hardware processing it. A future fault-tolerant quantum computer will use quantum error correction (QEC) to actively protect against both. In the smallest QEC codes, the information in one logical qubit is encoded in a two-dimensional subspace of a larger Hilbert space of multiple physical qubits. For each code, a set of non-demolition multi-qubit measurements, termed stabilizers, can discretize and signal physical qubit errors without collapsing the encoded information. Experimental demonstrations of QEC to date, using nuclear magnetic resonance, trapped ions, photons, superconducting qubits, and NV centers in diamond, have circumvented stabilizers at the cost of decoding at the end of a QEC cycle. This decoding leaves the quantum information vulnerable to physical qubit errors until re-encoding, violating a basic requirement for fault tolerance. Using a five-qubit superconducting processor, we realize the two parity measurements comprising the stabilizers of the three-qubit repetition code protecting one logical qubit from physical bit-flip errors. We construct these stabilizers as parallelized indirect measurements using ancillary qubits, and evidence their non-demolition character by generating three-qubit entanglement from superposition states. We demonstrate stabilizer-based quantum error detection (QED) by subjecting a logical qubit to coherent and incoherent bit-flip errors on its constituent physical qubits. While increased physical qubit coherence times and shorter QED blocks are required to actively safeguard quantum information, this demonstration is a critical step toward larger codes based on multiple parity measurements.

http://arxiv.org/abs/1411.5542

In-situ Raman Spectroscopy of the Graphene / Water Interface of a Solution-Gated Field Effect Transistor: Electron-Phonon Coupling and Spectroelectrochemistry

J. Binder, J. M. Urban, R. Stepniewski, W. Strupinski, A. Wysmolek

We present a novel measurement approach which combines aqueous solution-gated field effect transistors based on epitaxial bilayer graphene on 4H-SiC (0001) with simultaneous Raman spectroscopy. Since SiC is transparent in the visible wavelength range, we took advantage of the fact that one can measure graphene through the substrate. By sweeping the gate voltage, we observed Raman signatures related to the resonant electron-phonon coupling. The positions of these features allowed the extraction of the geometrical capacitance of the system and thus facilitated the accurate calculation of the Fermi levels for bilayer graphene. An intentional application of higher gate voltages allowed us to trigger electrochemical reactions, which we followed in-situ with Raman spectroscopy. The reactions showed a partially reversible character, which was monitored by an emergence / disappearance of peaks assigned to C-H and Si-H vibration modes as well as an increase / decrease of the defect-related Raman D band intensity. Our setup provides chemically specific information and electrical control by using a graphene solution-gated field-effect transistor and constitutes a highly interesting platform for future spectroelectrochemical research on electrically induced sorption processes of graphene in the micrometer scale.

http://arxiv.org/abs/1411.4837

Study of dynamical spin injection at ferromagnet-graphene interfaces

S. Singh, A. Ahmadi, C.T. Cherian, E. R. Mucciolo, E. del Barco, B. Özyilmaz

We present a study of dynamical spin injection from a three-dimensional ferromagnet into two-dimensional single-layer graphene. Comparative ferromagnetic resonance (FMR) studies of ferromagnet/graphene strips buried underneath the central line of a coplanar waveguide show that the FMR linewidth broadening is the largest when the graphene layer protrudes laterally away from the ferromagnetic strip, indicating that the spin current is injected into the graphene areas away from the area directly underneath the ferromagnet being excited. Our results confirm that the observed damping is indeed a signature of dynamical spin injection, wherein a pure spin current is pumped into the single-layer graphene from the precessing magnetization of the ferromagnet. The observed spin pumping efficiency is difficult to reconcile with the expected backflow of spins according to the standard spin pumping theory and the characteristics of graphene, and constitutes an enigma for spin pumping in two-dimensional structures.

http://arxiv.org/abs/1411.5339

Inverted singlet-triplet qubit coded on a two-electron double quantum dot

Sebastian Mehl and David P. DiVincenzo

The sz=0 spin configuration of two electrons confined at a double quantum dot (DQD) encodes the singlet-triplet qubit (STQ). We introduce the inverted STQ (ISTQ) that emerges from the setup of two quantum dots (QDs) differing significantly in size and out-of-plane magnetic fields. The strongly confined QD has a two-electron singlet ground state, but the weakly confined QD has a two-electron triplet ground state in the sz=0 subspace. Spin-orbit interactions act nontrivially on the sz=0 subspace and provide universal control of the ISTQ together with electrostatic manipulations of the charge configuration. GaAs and InAs DQDs can be operated as ISTQs under realistic noise conditions.

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.195424

Majorana fermions in Ge/Si hole nanowires

Franziska Maier, Jelena Klinovaja, and Daniel Loss

We consider Ge/Si core/shell nanowires with hole states coupled to an s-wave superconductor in the presence of electric and magnetic fields. We employ a microscopic model that takes into account material-specific details of the band structure such as strong and electrically tunable Rashba-type spin-orbit interaction and g factor anisotropy for the holes. In addition, the proximity-induced superconductivity Hamiltonian is derived starting from a microscopic model. In the topological phase, the nanowires host Majorana fermions with localization lengths that depend strongly on both the magnetic and electric fields. We identify the optimal regime in terms of the directions and magnitudes of the fields in which the Majorana fermions are the most localized at the nanowire ends. In short nanowires, the Majorana fermions hybridize and form a subgap fermion whose energy is split away from zero and oscillates as a function of the applied fields. The period of these oscillations could be used to measure the dependence of the spin-orbit interaction on the applied electric field and the g factor anisotropy.

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.195421

Revealing Topological Superconductivity in Extended Quantum Spin Hall Josephson Junctions

Shu-Ping Lee, Karen Michaeli, Jason Alicea, and Amir Yacoby

Quantum spin Hall–superconductor hybrids are promising sources of topological superconductivity and Majorana modes, particularly given recent progress on HgTe and InAs/GaSb. We propose a new method of revealing topological superconductivity in extended quantum spin Hall Josephson junctions supporting “fractional Josephson currents.” Specifically, we show that as one threads magnetic flux between the superconductors, the critical current traces an interference pattern featuring sharp fingerprints of topological superconductivity—even when noise spoils parity conservation.

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.197001

Axially Tunable Carbon Nanotube Resonators Using Co-integrated Microactuators

Stuart Truax , Shih-Wei Lee , Matthias Muoth , and Christofer Hierold *

Tuning of the mechanical resonance frequency of single-walled carbon nanotubes (SWCNTs) is achieved by application of uniaxial strain by purely mechanical means, utilizing both directly grown and dry-transferred SWCNTs. The induction of a beam-to-string transition is achieved, resulting in an axial tension sensitivity of 9.4 × 1010 Hz/ε in the vibrating string regime. Increases in the resonant Q-factor, removal of residual slack, and resonance frequency changes from 10 to 60 MHz are affected.

http://pubs.acs.org/doi/abs/10.1021/nl501853w

Giant Magnetoconductance Oscillations in Hybrid Superconductor−Semiconductor Core/Shell Nanowire Devices

Ö. Gül †, H. Y. Günel *†‡, H. Lüth †, T. Rieger †, T. Wenz †, F. Haas †, M. Lepsa †, G. Panaitov §, D. Grützmacher †, and Th. Schäpers *†

The magnetotransport of GaAs/InAs core/shell nanowires contacted by two superconducting Nb electrodes is investigated, where the InAs shell forms a tube-like conductive channel around the highly resistive GaAs core. By applying a magnetic field along the nanowire axis, regular magnetoconductance oscillations with an amplitude in the order of e2/h are observed. The oscillation amplitude is found to be larger by 2 orders of magnitude compared to the measurements of a reference sample with normal metal contacts. For the Nb-contacted core/shell nanowire the oscillation period corresponds to half a flux quantum Φ0/2 = h/2e in contrast to the period of Φ0 of the reference sample. The strongly enhanced magnetoconductance oscillations are explained by phase-coherent resonant Andreev reflections at the Nb-core/shell nanowire interface.

http://pubs.acs.org/doi/abs/10.1021/nl502598s

Quantum Noise and Asymmetric Scattering of Electrons and Holes in Graphene

Atikur Rahman , Janice Wynn Guikema , and Nina Marković *

We present measurements of quantum interference noise in double-gated single layer graphene devices at low temperatures. The noise characteristics show a nonmonotonic dependence on carrier density, which is related to the interplay between charge inhomogeneity and different scattering mechanisms. Linearly increasing 1/f noise at low carrier densities coincides with the observation of weak localization, suggesting the importance of short-range disorder in this regime. Using perpendicular and parallel p–n junctions, we find that the observed asymmetry of the noise with respect to the Dirac point can be related to asymmetric scattering of electrons and holes on the disorder potential.

http://pubs.acs.org/doi/abs/10.1021/nl503276s

Multimode Silicon Nanowire Transistors

Sebastian Glassner †, Clemens Zeiner †, Priyanka Periwal ‡§, Thierry Baron ‡§, Emmerich Bertagnolli †, and Alois Lugstein *†

The combined capabilities of both a nonplanar design and nonconventional carrier injection mechanisms are subject to recent scientific investigations to overcome the limitations of silicon metal oxide semiconductor field effect transistors. In this Letter, we present a multimode field effect transistors device using silicon nanowires that feature an axial n-type/intrinsic doping junction. A heterostructural device design is achieved by employing a self-aligned nickel-silicide source contact. The polymorph operation of the dual-gate device enabling the configuration of one p- and two n-type transistor modes is demonstrated. Not only the type but also the carrier injection mode can be altered by appropriate biasing of the two gate terminals or by inverting the drain bias. With a combined band-to-band and Schottky tunneling mechanism, in p-type mode a subthreshold swing as low as 143 mV/dec and an ON/OFF ratio of up to 104 is found. As the device operates in forward bias, a nonconventional tunneling transistor is realized, enabling an effective suppression of ambipolarity. Depending on the drain bias, two different n-type modes are distinguishable. The carrier injection is dominated by thermionic emission in forward bias with a maximum ON/OFF ratio of up to 107 whereas in reverse bias a Schottky tunneling mechanism dominates the carrier transport.

http://pubs.acs.org/doi/abs/10.1021/nl503476t

Realization of the Meminductor

Jiahao Han , Cheng Song *, Shuang Gao , Yuyan Wang , Chao Chen , and Feng Pan

The meminductor was proposed to be a fundamental circuit memdevice parallel with the memristor, linking magnetic flux and current. However, a clear material model or experimental realization of a meminductor has been challenging. Here we demonstrate pinched hysteretic magnetic flux–current signals at room temperature based on the spin Hall magnetoresistance effect in several-nanometer-thick thin films, exhibiting the nonvolatile memorizing property and magnetic energy storage ability of the meminductor. Similar to the parameters of the capacitor, resistor, and inductor, meminductance, LM, is introduced to characterize the capability of the prepared meminductor. Our findings present an indispensable element of memdevices and open an avenue for nanoscale meminductor design and manufacture, which might contribute to low-power electronic circuits, information storage, and artificial intelligence.

http://pubs.acs.org/doi/abs/10.1021/nn502655u

Szept.19-30.

Márton Attila

Előadás: SNS junctions in nanowires with spin-orbit coupling: role of confinement and helicity on the sub-gap spectrum (http://arxiv.org/abs/1410.6074) pptx (pptx)

Photons made to dance together

Physicists have made two beams of light interact at the level of individual photons.

Getting photons to interact is important for all-optical computation and for producing new quantum states of light. Kristin Beck at the Massachusetts Institute of Technology in Cambridge and her colleagues crossed two beams of light inside a cavity filled with trapped and cooled caesium atoms. When photons from each beam tried to pass through the system at the same time, the trapped atoms changed their internal state, allowing only one photon to be transmitted, while the other one was reflected or scattered.

The interaction creates two entangled beams of light, which the authors say could eventually be used to improve the accuracy of measurements, such as of a gyroscope's rotation, that would otherwise be limited by the laws of quantum mechanics.

http://www.nature.com/nature/journal/v513/n7519/full/513463a.html

Quantum bits get their first compression

Without algorithms that compress data to encode information into fewer bits, hard drives would clog up and Internet traffic would slow to a snail's pace. Now, a group of physicists in Canada has shown for the first time that it is possible to compress the kind of data that might be used in the computers of tomorrow — known as quantum bits, or qubits.

http://www.nature.com/news/quantum-bits-get-their-first-compression-1.15961

Transmission Phase in the Kondo Regime Revealed in a Two-Path Interferometer

S. Takada, C. Bäuerle, M. Yamamoto, K. Watanabe, S. Hermelin, T. Meunier, A. Alex, A. Weichselbaum, J. von Delft, A. Ludwig, A. D. Wieck, and S. Tarucha

We report on the direct observation of the transmission phase shift through a Kondo correlated quantum dot by employing a new type of two-path interferometer. We observed a clear π/2-phase shift, which persists up to the Kondo temperature TK. Above this temperature, the phase shifts by more than π/2 at each Coulomb peak, approaching the behavior observed for the standard Coulomb blockade regime. These observations are in remarkable agreement with two-level numerical renormalization group calculations. The unique combination of experimental and theoretical results presented here fully elucidates the phase evolution in the Kondo regime.

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.126601

Entanglement Entropy in Fermi Gases and Anderson’s Orthogonality Catastrophe

A. Ossipov

We study the ground-state entanglement entropy of a finite subsystem of size L of an infinite system of noninteracting fermions scattered by a potential of finite range a. We derive a general relation between the scattering matrix and the overlap matrix and use it to prove that for a one-dimensional symmetric potential the von Neumann entropy, the Rényi entropies, and the full counting statistics are robust against potential scattering, provided that L/a≫1. The results of numerical calculations support the validity of this conclusion for a generic potential.

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.130402

Experimental realization of a Coulomb blockade refrigerator

A. V. Feshchenko, J. V. Koski, J. P. Pekola

We present an experimental realization of a Coulomb blockade refrigerator (CBR) based on a single - electron transistor (SET). In the present structure, the SET island is interrupted by a superconducting inclusion to permit charge transport while preventing heat flow. At certain values of the bias and gate voltages, the current through the SET cools one of the junctions. The measurements follow theoretical model down to about 80 mK, which was the base temperature of the current measurements. The observed cooling increases rapidly with decreasing temperature in agreement with the theory, reaching about 15 mK drop at the base temperature. CBR appears as a promising electronic cooler at temperatures well below 100 mK.

http://xxx.lanl.gov/abs/1409.5637

Klein-tunneling transistor with ballistic graphene

Quentin Wilmart, Salim Berada, David Torrin, V. Hung Nguyen, Gwendal Fève, Jean-Marc Berroir, Philippe Dollfus, Bernard Plaçais

Today the availability of high mobility graphene up to room temperature makes ballistic transport in nanodevices achievable. In particular, p-n-p transistor in the ballistic regime gives access to the Klein tunneling physics and allows the realization of devices exploiting the optics-like behavior of Dirac Fermions (DF) as in the Vesalego lens or the Fabry P\'erot cavity. Here we propose a Klein tunneling transistor based on geometrical optics of DF. We consider the case of a prismatic active region delimited by a triangular gate, where total internal reflection may occur, which leads to the tunable suppression of the transistor transmission. We calculate the transmission and the current by means of scattering theory and the finite bias properties using Non Equilibrium Green's Function(NEGF) simulation.

http://xxx.lanl.gov/abs/1409.6170

Directly accessible entangling gates for capacitively coupled singlet-triplet qubits

Fernando A. Calderon-Vargas, Jason P. Kestner

The recent experimental advances in capacitively coupled singlet-triplet qubits, particularly the demonstration of entanglement, opens the question of what type of entangling gates the system's Hamiltonian can produce directly via a single square pulse. We address this question by considering the system's Hamiltonian from first principles and using the representation of its nonlocal properties in terms of local invariants. In the analysis we include the three different ways in which the system can be biased and their effect on the generation of entangling gates. We find that, in one of the possible biasing modes, the Hamiltonian has an especially simple form, which can directly generate a wide range of different entangling gates including the iSWAP gate. Moreover, using the complete form of the Hamiltonian we find that, for any biasing mode, a CNOT gate can be generated directly.

http://xxx.lanl.gov/abs/1409.6292

Zeeman splitting spin filter in a single quantum dot electron transport with Coulomb blockade effect

Wenxi Lai

Electron spin filter induced by Zeeman splitting in a few-electron quantum dot coupled to two normal electrodes is studied considering Coulomb blockade effect. Based on the Anderson model and Liouville-von Neumann equation, equation of motion of the system is derived and analytical solutions are achieved. Transport windows for perfectly polarized current, partially polarized current and non-polarized current induced by the Zeeman splitting energy and Coulomb blockade potential are exploited. We will give the relations of voltage, magnetic field and temperature for high quality spin filtering.

http://xxx.lanl.gov/abs/1409.6389

Majorana bound states without topological superconductivity

Pablo San-Jose, Jorge Cayao, Elsa Prada, Ramón Aguado

Recent experimental efforts towards the detection of Majorana bound states have focused on creating the conditions for topological superconductivity. Here we demonstrate an alternative route, which achieves fully localised zero-energy Majorana bound states when a topologically trivial superconductor is opened to a normal region. The emergence of Majorana states is a consequence of non-hermitian degeneracies of the resulting open quantum system, while arbitrarily large Majorana lifetimes follow from high junction transparency and helicity of the normal side. At these degeneracies, also known as `exceptional points', both the eigenvalues and the eigenstates coalesce, and acquire Majorana properties (zero-energy, self-conjugation, 4π-periodic braiding...) despite the trivial band topology. Exceptional Majoranas are thus the open-system counterparts of conventional Majorana bound states, to which they are continuously connected, and exhibit all their phenomenology while not requiring topological superconductivity.

http://xxx.lanl.gov/abs/1409.7306

Majorana Fermions in Ge/Si Hole Nanowires

Franziska Maier, Jelena Klinovaja, Daniel Loss

We consider Ge/Si core/shell nanowires with hole states coupled to an s-wave superconductor in the presence of electric and magnetic fields. We employ a microscopic model that takes into account material-specific details of the band structure such as strong and electrically tunable Rashba-type spin-orbit interaction and g factor anisotropy for the holes. In addition, the proximity-induced superconductivity Hamiltonian is derived starting from a microscopic model. In the topological phase, the nanowires host Majorana fermions with localization lengths that depend strongly on both the magnetic and electric fields. We identify the optimal regime in terms of the directions and magnitudes of the fields in which the Majorana fermions are the most localized at the nanowire ends. In short nanowires, the Majorana fermions hybridize and form a subgap fermion whose energy is split away from zero and oscillates as a function of the applied fields. The period of these oscillations could be used to measure the dependence of the spin-orbit interaction on the applied electric field and the g factor anisotropy.

http://xxx.lanl.gov/abs/1409.8645

Szept.13-19.

Tóvári Endre

Crossover from Josephson Effect to Single Interface Andreev Reflection in Asymmetric Superconductor/Nanowire Junctions

'H. Y. Günel, N. Borgwardt, I. E. Batov, H. Hardtdegen, K. Sladek, G. Panaitov, D. Grützmacher, and Th. Schäpers'

We report on the fabrication and characterization of symmetric nanowire-based Josephson junctions, that is, Al- and Nb-based junctions, and asymmetric junctions employing superconducting Al and Nb. In the symmetric junctions, a clear and pronounced Josephson supercurrent is observed. These samples also show clear signatures of subharmonic gap structures. At zero magnetic field, a Josephson coupling is found for the asymmetric Al/InAs-nanowire/Nb junctions as well. By applying a magnetic field above the critical field of Al or by raising the temperature above the critical temperature of Al the junction can be switched to an effective single-interface superconductor/nanowire structure. In this regime, a pronounced zero-bias conductance peak due to reflectionless tunneling has been observed.

http://pubs.acs.org/doi/abs/10.1021/nl501350v

Robust Electron Pairing in the Integer Quantum Hall Effect Regime

'Hyungkook Choi, Itamar Sivan, Amir Rosenblatt, Moty Heiblum, Vladimir Umansky, Diana Mahalu'

Electron pairing is a rare phenomenon appearing only in a few unique physical systems; e.g., superconductors and Kondo-correlated quantum dots. Here, we report on an unexpected, but robust, electron "pairing" in the integer quantum Hall effect (IQHE) regime. The pairing takes place within an interfering edge channel circulating in an electronic Fabry-Perot interferometer at a wide range of bulk filling factors, 2<νB<5. The main observations are: (a) High visibility Aharonov-Bohm conductance oscillations with magnetic flux periodicity Δϕ=φ0/2=h/2e (instead of the ubiquitous h/e), with e the electron charge and h the Planck constant; (b) An interfering quasiparticle charge e∗∼2e - revealed by quantum shot noise measurements; and (c) Full dephasing of the h/2e periodicity by induced dephasing of the adjacent edge channel (while keeping the interfering edge channel intact) : a clear realization of inter-channel entanglement. While this pairing phenomenon clearly results from inter-channel interaction, the exact mechanism that leads to e-e attraction within a single edge channel is not clear.

http://xxx.lanl.gov/abs/1409.4427

Robust 2D Topological Insulators in van der Waals Heterostructures

'Liangzhi Kou, Shu-Chun Wu, Claudia Felser, Thomas Frauenheim, Changfeng Chen, and Binghai Yan'

We predict a family of robust two-dimensional (2D) topological insulators in van der Waals heterostructures comprising graphene and chalcogenides BiTeX (X = Cl, Br, and I). The layered structures of both constituent materials produce a naturally smooth interface that is conducive to proximity-induced topological states. First-principles calculations reveal intrinsic topologically nontrivial bulk energy gaps as large as 70–80 meV, which can be further enhanced up to 120 meV by compression. The strong spin–orbit coupling in BiTeX has a significant influence on the graphene Dirac states, resulting in the topologically nontrivial band structure, which is confirmed by calculated nontrivial Z2 index and an explicit demonstration of metallic edge states. Such heterostructures offer a unique Dirac transport system that combines the 2D Dirac states from graphene and 1D Dirac edge states from the topological insulator, and it offers ideas for innovative device designs.

http://pubs.acs.org/doi/abs/10.1021/nn503789v

Pseudospin-driven spin relaxation mechanism in graphene

'Dinh Van Tuan, Frank Ortmann, David Soriano, Sergio O. Valenzuela & Stephan Roche'

The prospect of transporting spin information over long distances in graphene, possible because of its small intrinsic spin–orbit coupling (SOC) and vanishing hyperfine interaction, has stimulated intense research exploring spintronics applications. However, measured spin relaxation times are orders of magnitude smaller than initially predicted, while the main physical process for spin dephasing and its charge-density and disorder dependences remain unconvincingly described by conventional mechanisms. Here, we unravel a spin relaxation mechanism for non-magnetic samples that follows from an entanglement between spin and pseudospin driven by random SOC, unique to graphene. The mixing between spin and pseudospin-related Berry’s phases results in fast spin dephasing even when approaching the ballistic limit, with increasing relaxation times away from the Dirac point, as observed experimentally. The SOC can be caused by adatoms, ripples or even the substrate, suggesting novel spin manipulation strategies based on the pseudospin degree of freedom.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3083.html

Observation of topological surface state quantum Hall effect in an intrinsic three-dimensional topological insulator

'Yang Xu, Ireneusz Miotkowski, Chang Liu, Jifa Tian, Hyoungdo Nam, Nasser Alidoust, Jiuning Hu, Chih-Kang Shih, M. Zahid Hasan, Yong P. Chen'

A three-dimensional (3D) topological insulator (TI) is a quantum state of matter with a gapped insulating bulk yet a conducting surface hosting topologically-protected gapless surface states. One of the most distinct electronic transport signatures predicted for such topological surface states (TSS) is a well-defined half-integer quantum Hall effect (QHE) in a magnetic field, where the surface Hall conductivities become quantized in units of (1/2)e2/h (e being the electron charge, h the Planck constant) concomitant with vanishing resistance. Here, we observe well-developed QHE arising from TSS in an intrinsic TI of BiSbTeSe2. Our samples exhibit surface dominated conduction even close to room temperature, while the bulk conduction is negligible. At low temperatures and high magnetic fields perpendicular to the top and bottom surfaces, we observe well-developed integer quantized Hall plateaus, where the two parallel surfaces each contributing a half integer e2/h quantized Hall (QH) conductance, accompanied by vanishing longitudinal resistance. When the bottom surface is gated to match the top surface in carrier density, only odd integer QH plateaus are observed, representing a half-integer QHE of two degenerate Dirac gases. This system provides an excellent platform to pursue a plethora of exotic physics and novel device applications predicted for TIs, ranging from magnetic monopoles and Majorana particles to dissipationless electronics and fault-tolerant quantum computers.

http://xxx.lanl.gov/abs/1409.3778

MoS2: a Choice Substrate for Accessing and Tuning the Electronic Properties of Graphene

'Chih-Pin Lu, Guohong Li, K. Watanabe, T. Taniguchi, Eva Y. Andrei'

One of the enduring challenges in graphene research and applications is the extreme sensitivity of its charge carriers to external perturbations, especially those introduced by the substrate. The best available substrates to date, graphite and hBN, still pose limitations: graphite being metallic does not allow gating, while both hBN and graphite having lattice structures closely matched to that of graphene, may cause significant band structure reconstruction. Here we show that the atomically smooth surface of exfoliated MoS2 provides access to the intrinsic electronic structure of graphene without these drawbacks. Using scanning tunneling microscopy and Landau-level spectroscopy in a device configuration which allows tuning the carrier concentration, we find that graphene on MoS2 is ultra-flat producing long mean free paths, while avoiding band structure reconstruction. Importantly, the screening of the MoS2 substrate can be tuned by changing the position of the Fermi energy with relatively low gate voltages. We show that shifting the Fermi energy from the gap to the edge of the conduction band gives rise to enhanced screening and to a substantial increase in the mean-free-path and quasiparticle lifetime. MoS2 substrates thus provide unique opportunities to access the intrinsic electronic properties of graphene and to study in situ the effects of screening on electron-electron interactions and transport.

http://xxx.lanl.gov/abs/1409.5179

Aharonov-Bohm Oscillations in a Quasi-Ballistic 3D Topological Insulator Nanowire

'S. Cho, B. Dellabetta, R. D. Zhong, J. Schneeloch, T. S. Liu, G. Gu, Matthew J. Gilbert, Nadya Mason'

In three-dimensional topological insulators (3D TI) nanowires, transport occurs via gapless surface states where the spin is fixed perpendicular to the momentum[1-6]. Carriers encircling the surface thus acquire a \pi Berry phase, which is predicted to open up a gap in the lowest-energy 1D surface subband. Inserting a magnetic flux ({\Phi}) of h/2e through the nanowire should cancel the Berry phase and restore the gapless 1D mode[7-8]. However, this signature has been missing in transport experiments reported to date[9-11]. Here, we report measurements of mechanically-exfoliated 3D TI nanowires which exhibit Aharonov-Bohm oscillations consistent with topological surface transport. The use of low-doped, quasi-ballistic devices allows us to observe a minimum conductance at {\Phi} = 0 and a maximum conductance reaching e^2/h at {\Phi} = h/2e near the lowest subband (i.e. the Dirac point), as well as the carrier density dependence of the transport.

http://xxx.lanl.gov/abs/1409.5095

Experimental Realization of a Three-Dimensional Dirac Semimetal

'Sergey Borisenko, Quinn Gibson, Danil Evtushinsky, Volodymyr Zabolotnyy, Bernd Büchner, and Robert J. Cava'

We report the direct observation of the three-dimensional (3D) Dirac semimetal phase in cadmium arsenide (Cd3As2) by means of angle-resolved photoemission spectroscopy. We identify two momentum regions where electronic states that strongly disperse in all directions form narrow conelike structures, and thus prove the existence of the long sought 3D Dirac points. This electronic structure naturally explains why Cd3As2 has one of the highest known bulk electron mobilities. This realization of a 3D Dirac semimetal in Cd3As2 not only opens a direct path to a wide spectrum of applications, but also offers a robust platform for engineering topologically nontrivial phases including Weyl semimetals and quantum spin Hall systems.

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.027603

Terahertz Generation by Dynamical Photon Drag Effect in Graphene Excited by Femtosecond Optical Pulses

'J. Maysonnave, S. Huppert, F. Wang, S. Maero, C. Berger, W. de Heer, T. B. Norris, L. A. De Vaulchier, S. Dhillon, J. Tignon, R. Ferreira, and J. Mangeney'

Graphene has been proposed as a particularly attractive material for the achievement of strong optical nonlinearities, in particular generation of terahertz radiation. However, owing to the particular symmetries of the C-lattice, second-order nonlinear effects such as difference-frequency or rectification processes are predicted to vanish in a graphene layer for optical excitations (ℏω ≫ 2EF) involving the two relativistic dispersion bands. Here we experimentally demonstrate that graphene excited by femtosecond optical pulses generate a coherent THz radiation ranging from 0.1 to 4 THz via a second-order nonlinear effect. We fully interpret its characteristics with a model describing the electron and hole states beyond the usual massless relativistic scheme. This second-order nonlinear effect is dynamical photon drag, which relies on the transfer of light momentum to the carriers by the ponderomotive electric and magnetic forces. The model highlights the key roles of next-C-neighbor couplings and of unequal electron and hole lifetimes in the observed second-order response. Finally, our results indicate that dynamical photon drag effect in graphene can provide emission up to 60 THz, opening new routes for the generation of ultrabroadband terahertz pulses.

http://pubs.acs.org/doi/abs/10.1021/nl502684j

Exceptional Charge Transport Properties of Graphene on Germanium

'Francesca Cavallo, Richard Rojas Delgado, Michelle M. Kelly, José R. Sánchez Pérez, Daniel P. Schroeder, Huili Grace Xing, Mark A. Eriksson, and Max G. Lagally'

The excellent charge transport properties of graphene suggest a wide range of application in analog electronics. While most practical devices will require that graphene be bonded to a substrate, such bonding generally degrades these transport properties. In contrast, when graphene is transferred to Ge(001) its conductivity is extremely high and the charge carrier mobility derived from the relevant transport measurements is, under some circumstances, higher than that of freestanding, edge-supported graphene. We measure a mobility of ∼5 × 105 cm2 V–1 s–1 at 20 K, and ∼103 cm2 V–1 s–1 at 300 K. These values are close to the theoretical limit for doped graphene. Carrier densities in the graphene are as high as 1014 cm–2 at 300 K.

http://pubs.acs.org/doi/abs/10.1021/nn503381m

Direct Laser Writing of Graphene Electronics

http://pubs.acs.org/doi/abs/10.1021/nn504946k

Room-temperature coupling between electrical current and nuclear spins in OLEDs

'H. Malissa, M. Kavand, D. P. Waters, K. J. van Schooten, P. L. Burn, Z. V. Vardeny, B. Saam, J. M. Lupton, C. Boehme'

The effects of external magnetic fields on the electrical conductivity of organic semiconductors have been attributed to hyperfine coupling of the spins of the charge carriers and hydrogen nuclei. We studied this coupling directly by implementation of pulsed electrically detected nuclear magnetic resonance spectroscopy in organic light-emitting diodes (OLEDs). The data revealed a fingerprint of the isotope (protium or deuterium) involved in the coherent spin precession observed in spin-echo envelope modulation. Furthermore, resonant control of the electric current by nuclear spin orientation was achieved with radiofrequency pulses in a double-resonance scheme, implying current control on energy scales one-millionth the magnitude of the thermal energy.

http://www.sciencemag.org/content/345/6203/1487

Large, non-saturating magnetoresistance in WTe2

'Mazhar N. Ali, Jun Xiong, Steven Flynn, Jing Tao, Quinn D. Gibson, Leslie M. Schoop, Tian Liang, Neel Haldolaarachchige, Max Hirschberger, N. P. Ong & R. J. Cava'

Magnetoresistance is the change in a material’s electrical resistance in response to an applied magnetic field. Materials with large magnetoresistance have found use as magnetic sensors1, in magnetic memory2, and in hard drives3 at room temperature, and their rarity has motivated many fundamental studies in materials physics at low temperatures4. Here we report the observation of an extremely large positive magnetoresistance at low temperatures in the non-magnetic layered transition-metal dichalcogenide WTe2: 452,700 per cent at 4.5 kelvins in a magnetic field of 14.7 teslas, and 13 million per cent at 0.53 kelvins in a magnetic field of 60 teslas. In contrast with other materials, there is no saturation of the magnetoresistance value even at very high applied fields. Determination of the origin and consequences of this effect, and the fabrication of thin films, nanostructures and devices based on the extremely large positive magnetoresistance of WTe2, will represent a significant new direction in the study of magnetoresistivity.

http://www.nature.com/nature/journal/vaop/ncurrent/full/nature13763.html

Fast non-thermal switching between macroscopic charge-ordered quantum states induced by charge injection

'I. Vaskivskyi, I. A. Mihailovic, S. Brazovskii, J. Gospodaric, T. Mertelj, D. Svetin, P. Sutar, D. Mihailovic'

The functionality of logic and memory elements in current electronics is based on multi-stability, driven either by manipulating local concentrations of electrons in transistors, or by switching between equivalent states of a material with a degener- ate ground state in magnetic or ferroelectric materials. Another possibility is offered by phase transitions with switching between metallic and insulating phases, but classical phase transitions are limited in speed by slow nucleation, proliferation of domains and hysteresis. We can in principle avoid these problems by using quantum states for switching, but microscopic systems suffer from decoherence which prohibits their use in everyday devices. Macroscopic quantum states, such as the superconducting ground state have the advantage that on a fundamental level they do not suffer from decoherence plaguing microscopic systems. Here we demonstrate for the first time ultrafast non-thermal switching between different metastable electronically ordered states by pulsed electrical charge injection. The macroscopic nature of the many-body quantum states(1-4) - which are not part of the equilibrium phase diagram - gives rise to unprecedented stability and remarka- bly sharp switching thresholds. Fast sub-50 ps switching, large associated re- sistance changes, 2-terminal operation and demonstrable high fidelity of bi-stability control suggest new opportunities for the use of macroscopic quantum states in electronics, particularly for an ultrafast non-volatile quantum charge-order resistive random access memory (QCOR-RAM).

http://xxx.lanl.gov/abs/1409.3794

Imaging the two-component nature of Dirac–Landau levels in the topological surface state of Bi2Se3

'Ying-Shuang Fu, M. Kawamura, K. Igarashi, H. Takagi, T. Hanaguri & T. Sasagawa'

Massless Dirac electrons in condensed matter1, 2, 3, 4, 5, 6 are, unlike conventional electrons, described by two-component wavefunctions associated with the spin degrees of freedom in the surface state of topological insulators5, 6. Hence, the ability to observe the two-component wavefunction is useful for exploring novel spin phenomena. Here we show that the two-component nature is manifest in Landau levels, the degeneracy of which is lifted by a Coulomb potential. Using spectroscopic-imaging scanning tunnelling microscopy, we visualize energy and spatial structures of Landau levels in Bi2Se3, a prototypical topological insulator. The observed Landau-level splitting and internal structures of Landau orbits are distinct from those in a conventional electron system7 and are well reproduced by a two-component model Dirac Hamiltonian. Our model further predicts energy-dependent spin-magnetization textures in a potential variation and provides a way for manipulating spins in the topological surface state.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3084.html

Ultrafast non-local control of spontaneous emission

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.190.html

http://www.sciencedirect.com/science/article/pii/S0008622314007465

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.125428

For fun:

Spraying Quantum Dot Conjugates in the Colon of Live Animals Enabled Rapid and Multiplex Cancer Diagnosis Using Endoscopy

http://pubs.acs.org/doi/abs/10.1021/nn5009269

Szept.2-12.

Tóvári Endre

Fabry-Pérot Interference in Gapped Bilayer Graphene with Broken Anti-Klein Tunneling

'Anastasia Varlet, Ming-Hao Liu (劉明豪), Viktor Krueckl, Dominik Bischoff, Pauline Simonet, Kenji Watanabe, Takashi Taniguchi, Klaus Richter, Klaus Ensslin, and Thomas Ihn'

We report the experimental observation of Fabry-Pérot interference in the conductance of a gate-defined cavity in a dual-gated bilayer graphene device. The high quality of the bilayer graphene flake, combined with the device’s electrical robustness provided by the encapsulation between two hexagonal boron nitride layers, allows us to observe ballistic phase-coherent transport through a 1−μm-long cavity. We confirm the origin of the observed interference pattern by comparing to tight-binding calculations accounting for the gate-tunable band gap. The good agreement between experiment and theory, free of tuning parameters, further verifies that a gap opens in our device. The gap is shown to destroy the perfect reflection for electrons traversing the barrier with normal incidence (anti-Klein tunneling). The broken anti-Klein tunneling implies that the Berry phase, which is found to vary with the gate voltages, is always involved in the Fabry-Pérot oscillations regardless of the magnetic field, in sharp contrast with single-layer graphene.

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.116601

Anomalous Sequence of Quantum Hall Liquids Revealing a Tunable Lifshitz Transition in Bilayer Graphene

'Anastasia Varlet, Dominik Bischoff, Pauline Simonet, Kenji Watanabe, Takashi Taniguchi, Thomas Ihn, Klaus Ensslin, Marcin Mucha-Kruczyński, and Vladimir I. Fal’ko'

Bilayer graphene is a unique system where both the Fermi energy and the low-energy electron dispersion can be tuned. This is brought about by an interplay between trigonal warping and the band gap opened by a transverse electric field. Here, we drive the Lifshitz transition in bilayer graphene to experimentally controllable carrier densities by applying a large transverse electric field to a h-BN-encapsulated bilayer graphene structure. We perform magnetotransport measurements and investigate the different degeneracies in the Landau level spectrum. At low magnetic fields, the observation of filling factors −3 and −6 quantum Hall states reflects the existence of three maxima at the top of the valence-band dispersion. At high magnetic fields, all integer quantum Hall states are observed, indicating that deeper in the valence band the constant energy contours are singly connected. The fact that we observe ferromagnetic quantum Hall states at odd-integer filling factors testifies to the high quality of our sample. This enables us to identify several phase transitions between correlated quantum Hall states at intermediate magnetic fields, in agreement with the calculated evolution of the Landau level spectrum. The observed evolution of the degeneracies, therefore, reveals the presence of a Lifshitz transition in our system.

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.116602

Characterizing wave functions in graphene nanodevices: Electronic transport through ultrashort graphene constrictions on a boron nitride substrate

'D. Bischoff, F. Libisch, J. Burgdörfer, T. Ihn, and K. Ensslin'

We present electronic transport measurements through short and narrow (30×30nm) single-layer graphene constrictions on a hexagonal boron nitride substrate. While the general observation of Coulomb blockade is compatible with earlier work, the details are not: We show that the area on which charge is localized can be significantly larger than the area of the constriction, suggesting that the localized states responsible for the Coulomb blockade leak out into the graphene bulk. The high bulk mobility of graphene on hexagonal boron nitride, however, seems to be inconsistent with the short bulk localization length required to see Coulomb blockade. To explain these findings, charge must instead be primarily localized along the imperfect edges of the devices and extend along the edge outside of the constriction. In order to better understand the mechanisms, we compare the experimental findings with tight-binding simulations of such constrictions with disordered edges. Finally, we discuss previous experiments in the light of our findings.

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.115405

Twist-controlled resonant tunnelling in graphene/boron nitride/graphene heterostructures

'A. Mishchenko, J. S. Tu, Y. Cao, R. V. Gorbachev, J. R. Wallbank, M. T. Greenaway, V. E. Morozov, S. V. Morozov, M. J. Zhu, S. L. Wong, F. Withers, C. R. Woods, Y-J. Kim, K. Watanabe, T. Taniguchi, E. E. Vdovin, O. Makarovsky, T. M. Fromhold, V. I. Fal'ko, A. K. Geim, L. Eaves & K. S. Novoselov'

Recent developments in the technology of van der Waals heterostructures1, 2 made from two-dimensional atomic crystals3, 4 have already led to the observation of new physical phenomena, such as the metal–insulator transition5 and Coulomb drag6, and to the realization of functional devices, such as tunnel diodes7, 8, tunnel transistors9, 10 and photovoltaic sensors11. An unprecedented degree of control of the electronic properties is available not only by means of the selection of materials in the stack12, but also through the additional fine-tuning achievable by adjusting the built-in strain and relative orientation of the component layers13, 14, 15, 16, 17. Here we demonstrate how careful alignment of the crystallographic orientation of two graphene electrodes separated by a layer of hexagonal boron nitride in a transistor device can achieve resonant tunnelling with conservation of electron energy, momentum and, potentially, chirality. We show how the resonance peak and negative differential conductance in the device characteristics induce a tunable radiofrequency oscillatory current that has potential for future high-frequency technology.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.187.html

Resistance-voltage dependence of nanojunctions during electromigration in ultrahigh vacuum

'D. Stöffler, M. Marz, B. Kießig, T. Tomanic, R. Schäfer, H. v. Löhneysen, and R. Hoffmann-Vogel'

The electrical resistance R of metallic nanocontacts subjected to controlled cyclic electromigration in ultrahigh vacuum has been investigated in situ as a function of applied voltage V. For sufficiently small contacts, i.e., large resistance, a decrease of R(V) while increasing V is observed. This effect is tentatively attributed to the presence of contacts separated by thin vacuum barriers in parallel to ohmic nanocontacts. Simple model calculations indicate that both thermal activation or tunneling can lead to this unusual behavior. We describe our data by a tunneling model whose key parameter, i.e., the tunneling distance, changes because of thermal expansion due to Joule heating and/or electrostatic strain arising from the applied voltage. Oxygen exposure during electromigration prevents the formation of negative R(V) slopes, and at the same time enhances the probability of uncontrolled melting, while other gases show little effects. In addition, indication for field emission has been observed in some samples.

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.115406

Quantum interference in off-resonant transport through single molecules

'Kim G. L. Pedersen, Mikkel Strange, Martin Leijnse, Per Hedegård, Gemma C. Solomon, and Jens Paaske'

We provide a simple set of rules for predicting interference effects in off-resonant transport through single molecule junctions. These effects fall into two classes, showing, respectively, an odd or an even number of nodes in the linear conductance within a given molecular charge state, and we demonstrate how to decide the interference class directly from the contacting geometry. For neutral alternant hydrocarbons, we employ the Coulson-Rushbrooke-McLachlan pairing theorem to show that the interference class is decided simply by tunneling on and off the molecule from same or different sublattices. More generally, we investigate a range of smaller molecules by means of exact diagonalization combined with a perturbative treatment of the molecule-lead tunnel coupling. While these results generally agree well with GW calculations, they are shown to be at odds with simpler mean-field treatments. For molecules with spin-degenerate ground states, we show that for most junctions interference causes no transmission nodes, but we argue that it may lead to a nonstandard gate dependence of the zero-bias Kondo resonance.

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.125413

All-optical control of ferromagnetic thin films and nanostructures

'C-H. Lambert, S. Mangin, B. S. D. Ch. S. Varaprasad, Y. K. Takahashi, M. Hehn, M. Cinchetti, G. Malinowski, K. Hono, Y. Fainman, M. Aeschlimann, E. E. Fullerton'

The interplay of light and magnetism allowed light to be used as a probe of magnetic materials. Now the focus has shifted to use polarized light to alter or manipulate magnetism. Here, we demonstrate optical control of ferromagnetic materials ranging from magnetic thin films to multilayers and even granular films being explored for ultra-high-density magnetic recording. Our finding shows that optical control of magnetic materials is a much more general phenomenon than previously assumed and may have a major impact on data memory and storage industries through the integration of optical control of ferromagnetic bits.

http://www.sciencemag.org/content/345/6202/1337

Ultrafast optical control of orbital and spin dynamics in a solid-state defect

'Lee C. Bassett, F. Joseph Heremans, David J. Christle, Christopher G. Yale, Guido Burkard, Bob B. Buckley, David D. Awschalom'

Atom-scale defects in semiconductors are promising building blocks for quantum devices, but our understanding of their material-dependent electronic structure, optical interactions, and dissipation mechanisms is lacking. Using picosecond resonant pulses of light, we study the coherent orbital and spin dynamics of a single nitrogen-vacancy center in diamond over time scales spanning six orders of magnitude. We develop a time-domain quantum tomography technique to precisely map the defect’s excited-state Hamiltonian and exploit the excited-state dynamics to control its ground-state spin with optical pulses alone. These techniques generalize to other optically addressable nanoscale spin systems and serve as powerful tools to characterize and control spin qubits for future applications in quantum technology.

http://www.sciencemag.org/content/345/6202/1333

Environment-assisted quantum control of a solid-state spin via coherent dark states

'Jack Hansom, Carsten H. H. Schulte, Claire Le Gall, Clemens Matthiesen, Edmund Clarke, Maxime Hugues, Jacob M. Taylor & Mete Atatüre'

Understanding the interplay between a quantum system and its environment lies at the heart of quantum science and its applications. So far most efforts have focused on circumventing decoherence induced by the environment by either protecting the system from the associated noise1, 2, 3, 4, 5 or by manipulating the environment directly6, 7, 8, 9. Recently, parallel efforts using the environment as a resource have emerged, which could enable dissipation-driven quantum computation and coupling of distant quantum bits10, 11, 12, 13, 14. Here, we realize the optical control of a semiconductor quantum-dot spin by relying on its interaction with an adiabatically evolving spin environment. The emergence of hyperfine-induced, quasi-static optical selection rules enables the optical generation of coherent spin dark states without an external magnetic field. We show that the phase and amplitude of the lasers implement multi-axis manipulation of the basis spanned by the dark and bright states, enabling control via projection into a spin-superposition state. Our approach can be extended, within the scope of quantum control and feedback15, 16, to other systems interacting with an adiabatically evolving environment.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3077.html

Graphene nanoribbon heterojunctions

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.184.html

Generation and electric control of spin–valley-coupled circular photogalvanic current in WSe2

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.183.html

Sensitive room-temperature terahertz detection via the photothermoelectric effect in graphene

'Xinghan Cai, Andrei B. Sushkov, Ryan J. Suess, Mohammad M. Jadidi, Gregory S. Jenkins, Luke O. Nyakiti, Rachael L. Myers-Ward, Shanshan Li, Jun Yan, D. Kurt Gaskill, Thomas E. Murphy, H. Dennis Drew & Michael S. Fuhrer'

Terahertz radiation has uses in applications ranging from security to medicine1. However, sensitive room-temperature detection of terahertz radiation is notoriously difficult2. The hot-electron photothermoelectric effect in graphene is a promising detection mechanism; photoexcited carriers rapidly thermalize due to strong electron–electron interactions3, 4, but lose energy to the lattice more slowly3, 5. The electron temperature gradient drives electron diffusion, and asymmetry due to local gating6, 7 or dissimilar contact metals8 produces a net current via the thermoelectric effect. Here, we demonstrate a graphene thermoelectric terahertz photodetector with sensitivity exceeding 10 V W–1 (700 V W–1) at room temperature and noise-equivalent power less than 1,100 pW Hz–1/2 (20 pW Hz–1/2), referenced to the incident (absorbed) power. This implies a performance that is competitive with the best room-temperature terahertz detectors9 for an optimally coupled device, and time-resolved measurements indicate that our graphene detector is eight to nine orders of magnitude faster than those7, 10. A simple model of the response, including contact asymmetries (resistance, work function and Fermi-energy pinning) reproduces the qualitative features of the data, and indicates that orders-of-magnitude sensitivity improvements are possible.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.182.html

Polycrystalline Graphene with Single Crystalline Electronic Structure

http://pubs.acs.org/doi/abs/10.1021/nl502445j

Angle-dependent van Hove singularities and their breakdown in twisted graphene bilayers

'Wei Yan, Lan Meng, Mengxi Liu, Jia-Bin Qiao, Zhao-Dong Chu, Rui-Fen Dou, Zhongfan Liu, Jia-Cai Nie, Donald G. Naugle, and Lin He'

The creation of van der Waals heterostructures based on a graphene monolayer and other two-dimensional crystals has attracted great interest because the atomic registry of the two-dimensional crystals can modify the electronic spectra and properties of graphene. A twisted graphene bilayer can be viewed as a special van der Waals structure composed of two mutually misoriented graphene layers, where the sublayer graphene not only plays the role of a substrate, but also acts in an equivalent role as the top graphene layer in the structure. Here we report the electronic spectra of slightly twisted graphene bilayers studied by scanning tunneling microscopy and spectroscopy. Our experiment demonstrates that twist-induced van Hove singularities are ubiquitously present for rotation angles θ of less than about 3.5°, corresponding to moiré-pattern periods D longer than 4 nm. However, they totally vanish for θ>5.5° (D<2.5nm). Such a behavior indicates that the continuum models, which capture moiré-pattern periodicity more accurately at small rotation angles, are no longer applicable at large rotation angles.

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.115402

Excitation of complex spin dynamics patterns in a quantum-dot electron spin ensemble

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.121301

Tunable Floquet Majorana fermions in driven coupled quantum dots

'Yantao Li, Arijit Kundu, Fan Zhong, and Babak Seradjeh'

We propose a system of coupled quantum dots in proximity to a superconductor and driven by separate ac potentials to realize and detect Floquet Majorana fermions. We show that the appearance of Floquet Majorana fermions can be finely controlled in the expanded parameter space of the drive frequency, amplitude, and phase difference across the two dots. While these Majorana fermions are not topologically protected, the highly tunable setup provides a realistic system for observing the exotic physics associated with Majorana fermions as well as their dynamical generation and manipulation.

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.121401

Unrelated:

Inventing a Modern Periscope

http://www.tested.com/science/464266-inventing-modern-periscope/

Recent interesting articles

From NanoWiki

Latest revision as of 16:27, 11 October 2021

Contents

December 8.

November 3.

Szeptember 8-15.

Aug. 25. - szept. 7.

Március 24-31.

Március 24.

Nov. 1-21.

Szept.19-30.

Szept.13-19.

Szept.2-12.

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@@ Line 1: / Line 1: @@
-[http://www.sciencemag.org/current.dtl#twis Science this week issue]
+[https://www.science.org/toc/science/current Science this week issue]
-[http://www.nature.com/search/executeSearch?sp-advanced=true&include-collections=journals_nature%2Ccrawled_content&exclude-collections=journals_palgrave%2Clab_animal&sp-m=0&sp-q=&sp-p=all&sp-q-9%5BNATURE%5D=1&sp-q-2=&sp-p-2=all&sp-q-3=&sp-p-3=all&sp-q-4=&sp-q-5=&sp-q-6=&pub-date-mode=range&sp-date-range=7&sp-q-8=&sp-s=date_descending&sp-c=25 Nature last week]
+[https://www.nature.com/nature/current-issue Nature last week]
-[http://www.nature.com/search/executeSearch?sp-advanced=true&include-collections=journals_nature%2Ccrawled_content&exclude-collections=journals_palgrave%2Clab_animal&sp-m=0&sp-q=&sp-p=all&sp-q-9%5BNPHYS%5D=1&sp-q-2=&sp-p-2=all&sp-q-3=&sp-p-3=all&sp-q-4=&sp-q-5=&sp-q-6=&pub-date-mode=range&sp-date-range=7&sp-q-8=&sp-s=date_descending&sp-c=25 Nature Physics last week]
+[https://www.nature.com/nphys/current-issue Nature Physics last week]
-[http://www.nature.com/search/executeSearch?sp-advanced=true&include-collections=journals_nature%2Ccrawled_content&exclude-collections=journals_palgrave%2Clab_animal&sp-m=0&sp-q=&sp-p=all&sp-q-9%5BNNANO%5D=1&sp-q-2=&sp-p-2=all&sp-q-3=&sp-p-3=all&sp-q-4=&sp-q-5=&sp-q-6=&pub-date-mode=range&sp-date-range=7&sp-q-8=&sp-s=date_descending&sp-c=25 Nature Nanotechnology last week]
+[https://www.nature.com/nnano/current-issue Nature Nanotechnology last week]
 [http://pubs.acs.org/toc/nalefd/0/0 Nano Letters ASAP]
@@ Line 17: / Line 17: @@
 [http://prb.aps.org/browse Physical Review B]
+[http://magnetooptics.phy.bme.hu]
-== Máj. 11. - Jun. 7. (2012) ==
+[[Recent interesting articles - archive 2013 - | Link to the archive of this series before 2014]]
-''Válogatta: Makk Péter''
-'''Transport Properties of a Single-Molecule Diode'''
-Emanuel Lörtscher†*, Bernd Gotsmann†, Youngu Lee‡, Luping Yu§, Charles Rettner, and Heike Riel†
-Charge transport through single diblock dipyrimidinyl diphenyl molecules consisting of a donor and acceptor moiety was measured in the low-bias regime and as a function of bias at different temperatures using the mechanically controllable break-junction technique. Conductance histograms acquired at 10 mV reveal two distinct peaks, separated by a factor of 1.5, representing the two orientations of the single molecule with respect to the applied bias. The current–voltage characteristics exhibit a temperature-independent rectification of up to a factor of 10 in the temperature range between 300 and 50 K with single-molecule currents of 45–70 nA at ±1.5 V. The current–voltage characteristics are discussed using a semiempirical model assuming a variable coupling of the molecular energy levels as well as a nonsymmetric voltage drop across the molecular junction, thus shifting the energy levels accordingly. The excellent agreement of the data with the proposed model suggests that the rectification originates from an asymmetric Coulomb blockade in combination with an electric-field-induced level shifting.
-http://pubs.acs.org/doi/abs/10.1021/nn300438h
 ----
-'''Integration of Hexagonal Boron Nitride with Quasi-freestanding Epitaxial Graphene: Toward Wafer-Scale High-Performance Devices'''
-Michael S. Bresnehan,†,‡ Matthew J. Hollander,‡,§ Maxwell Wetherington,†,‡ Michael LaBella,‡
-Kathleen A. Trumbull,‡ Randal Cavalero,‡ David W. Snyder,‡,^ and Joshua A. Robinson†,*
-Hexagonal boron nitride (h-BN) is a promising dielectric material for graphene-based electronic devices. Here we investigate the potential of h-BN gate dielectrics, grown by chemical vapor deposition (CVD), for integration with quasi-freestanding epitaxial graphene (QFEG). We discuss the large scale growth of h-BN on copper foil via a catalytic thermal CVD process and the subsequent transfer of h-BN to a 75 mm QFEG wafer. X-ray photoelectron spectroscopy (XPS) measurements confirm the absence of h-BN/graphitic domains and indicate that the film is chemically stable throughout the transfer process, while Raman spectroscopy indicates a 42% relaxation of compressive stress following removal of the copper substrate and subsequent transfer of h-BN to QFEG. Despite stress-induced wrinkling observed in the films, Hall effect measurements show little degradation (<10%) in carrier mobility for h-BN coated QFEG. Temperature dependent Hall measurements indicate little contribution from remote surface optical phonon scattering and suggest that, compared to HfO2 based dielectrics, h-BN can be an excellent material for preserving electrical transport properties. Graphene transistors utilizing h-BN gates exhibit peak intrinsic cutoff frequencies >30 GHz (2.4× that of HfO2-based devices).
-http://pubs.acs.org/doi/abs/10.1021/nn300996t
 ----
-'''Terahertz and Infrared Spectroscopy of Gated Large-Area Graphene'''
-Lei Ren†, Qi Zhang†, Jun Yao#, Zhengzong Sun‡, Ryosuke Kaneko§, Zheng Yan‡, Sébastien Nanot†, Zhong Jin‡, Iwao Kawayama§, Masayoshi Tonouchi§, James M. Tour‡, and Junichiro Kono*†¶
+'''Mindig a legfrissebb bejegyzés van legfelül.'''
-We have fabricated a centimeter-size single-layer graphene device with a gate electrode, which can modulate the transmission of terahertz and infrared waves. Using time-domain terahertz spectroscopy and Fourier-transform infrared spectroscopy in a wide frequency range (10–10 000 cm–1), we measured the dynamic conductivity change induced by electrical gating and thermal annealing. Both methods were able to effectively tune the Fermi energy, EF, which in turn modified the Drude-like intraband absorption in the terahertz as well as the “2EF onset” for interband absorption in the mid-infrared. These results not only provide fundamental insight into the electromagnetic response of Dirac fermions in graphene but also demonstrate the key functionalities of large-area graphene devices that are desired for components in terahertz and infrared optoelectronics.
+== December 8. ==
+''Boros Csanád Örs''
-http://pubs.acs.org/doi/abs/10.1021/nl301496r
+'''The 100th anniversary of the four-point probe technique: the role of probe geometries in isotropic and anisotropic systems'''
-----
-'''Patterning of graphene'''
-Ji Feng ,  Wenbin Li ,  Xiaofeng Qian ,  Jingshan Qi ,  Liang Qi and Ju Li
+The electrical conductivity of solid-state matter is a fundamental physical property and can be
+precisely derived from the resistance measured via the four-point probe technique excluding
+contributions from parasitic contact resistances. Over time, this method has become an
+interdisciplinary characterization tool in materials science, semiconductor industries, geology,
+physics, etc, and is employed for both fundamental and application-driven research. However, the
+correct derivation of the conductivity is a demanding task which faces several difficulties, e.g. the
+homogeneity of the sample or the isotropy of the phases. In addition, these sample-specific
+characteristics are intimately related to technical constraints such as the probe geometry and size of
+the sample. In particular, the latter is of importance for nanostructures which can now be probed
+technically on very small length scales. On the occasion of the 100th anniversary of the four-point
+probe technique, introduced by Frank Wenner, in this review we revisit and discuss various
+correction factors which are mandatory for an accurate derivation of the resistivity from the
+measured resistance. Among others, sample thickness, dimensionality, anisotropy, and the relative
+size and geometry of the sample with respect to the contact assembly are considered. We are also
+able to derive the correction factors for 2D anisotropic systems on circular finite areas with variable
+probe spacings. All these aspects are illustrated by state-of-the-art experiments carried out using a
+four-tip STM/SEM system. We are aware that this review article can only cover some of the most
+important topics. Regarding further aspects, e.g. technical realizations, the influence of
+inhomogeneities or different transport regimes, etc, we refer to other review articles in this field.
-Two-dimensional atomic sheets of carbon (graphene, graphane, etc.) are amenable to unique patterning schemes such as cutting, bending, folding and fusion that are predicted to lead to interesting properties. In this review, we present theoretical understanding and processing routes for patterning graphene and highlight potential applications. With more precise and scalable patterning, the prospects of integrating flat carbon (graphene) with curved carbon (nanotubes & half nanotubes) and programmable graphene folding are envisioned.
+http://iopscience.iop.org/article/10.1088/0953-8984/27/22/223201/pdf
-http://pubs.rsc.org/en/content/articlelanding/2012/nr/c2nr30790a
+== November 3. ==
-----
+''Boros Csanád Örs''
-'''Spin polarization of the quantum spin Hall edge states'''
-Christoph Brüne, Andreas Roth, Hartmut Buhmann, Ewelina M. Hankiewicz, Laurens W. Molenkamp, Joseph Maciejko, Xiao-Liang Qi & Shou-Cheng Zhang
+'''Margination of micro- and nano-particles in blood flow and its effect on drug delivery'''
-The prediction and experimental verification of the quantum spin Hall state marked the discovery of a new state of matter now known as topological insulators. Two-dimensional topological insulators exhibit the quantum spin-Hall effect, characterized by gapless spin-polarized counter-propagating edge channels. Whereas the helical character of these edge channels is now well established, experimental confirmation that the transport in the edge channels is spin polarized is still outstanding. We report experiments on nanostructures fabricated from HgTe quantum wells with an inverted band structure, in which a split gate technique allows us to combine both quantum spin Hall and metallic spin Hall transport in a single device. In these devices, the quantum spin Hall effect can be used as a spin current injector and detector for the metallic spin Hall effect, and vice versa, allowing for an all-electrical detection of spin polarization.
+Kathrin Müller, Dmitry A. Fedosov & Gerhard Gompper
-http://www.nature.com/nphys/journal/v8/n6/full/nphys2322.html
+Drug delivery by micro- and nano-carriers enables controlled transport of pharmaceuticals to targeted sites.
-----
+Even though carrier fabrication has made much progress recently, the delivery including controlled particle
-'''Mechanical Annealing of Metallic Electrodes at the Atomic Scale'''
+distribution and adhesion within the body remains a great challenge. The adhesion of carriers is strongly
+affected by their margination properties (migration toward walls) in the microvasculature. To investigate
+margination characteristics of carriers of different shapes and sizes and to elucidate the relevant physical
+mechanisms, we employ mesoscopic hydrodynamic simulations of blood flow. Particle margination is
+studied for a wide range of hematocrit values, vessel sizes, and flow rates, using two- and three-dimensional
+models. The simulations show that the margination properties of particles improve with increasing carrier
+size. Spherical particles yield slightly better margination than ellipsoidal carriers; however, ellipsoidal
+particles exhibit a slower rotational dynamics near a wall favoring their adhesion. In conclusion,
+micron-sized ellipsoidal particles are favorable for drug delivery in comparison with sub-micron spherical
+particles.
-C. Sabater1, C. Untiedt1, J. J. Palacios2, and M. J. Caturla1,*
+http://www.nature.com/articles/srep04871
-The process of creating an atomically defined and robust metallic tip is described and quantified using measurements of contact conductance between gold electrodes and numerical simulations. Our experiments show how the same conductance behavior can be obtained for hundreds of cycles of formation and rupture of the nanocontact by limiting the indentation depth between the two electrodes up to a conductance value of approximately 5G0 in the case of gold. This phenomenon is rationalized using molecular dynamics simulations together with density functional theory transport calculations which show how, after repeated indentations (mechanical annealing), the two metallic electrodes are shaped into tips of reproducible structure. These results provide a crucial insight into fundamental aspects relevant to nanotribology or scanning probe microscopies.
+== Szeptember 8-15. ==
+''Scherübl Zoltán''
-http://prl.aps.org/abstract/PRL/v108/i20/e205502
+'''Guiding of Electrons in a Few Mode Ballistic Graphene Channel'''
-----
-== Máj. 4. - Máj. 11. (2012) ==
+Peter Rickhaus, Ming-Hao Liu, Péter Makk, Romain Maurand, Samuel Hess, Simon Zihlmann, Markus Weiss, Klaus Richter, Christian Schönenberger
-''Válogatta: Csontos Miklós''
+http://arxiv.org/abs/1509.02653
-'''Spectroscopy of non-local superconducting correlations in a double quantum dot'''
+'''Hexagonal boron nitride as an atomically thin oxidation barrier for ferromagnetic nano structures'''
-L. G. Herrmann, P. Burset, W. J. Herrera, F. Portier, P. Roche, C. Strunk, A. Levy Yeyati, T. Kontos
+Simon Zihlmann, Péter Makk, C. A. F. Vaz, Christian Schönenberger
-We investigate non-linear transport in a double quantum dot connected to two normal electrodes and a central superconducting finger. By this means, we perform a transport spectroscopy of such a system which implements a Cooper pair splitter. The non-linear conductance exhibits strong subgap features which can be associated with the coherence of the injected Cooper pairs. Our findings are well accounted for by the recently developed microscopic theory of Cooper pairs splitters made in SWNTs.
+http://arxiv.org/abs/1509.03087
-http://arxiv.org/abs/1205.1972
+'''Giga-Hertz quantized charge pumping in bottom gate defined InAs nanowire quantum dots'''
-----
-'''Microwave spectroscopy of a Cooper pair beam splitter'''
-Audrey Cottet
+S. d'Hollosy, M. Jung, A. Baumgartner, V.A. Guzenko, M.H. Madsen, J. Nygård, C. Schönenberger
-This article discusses how to demonstrate the entanglement of the split Cooper pairs produced in a double-quantum-dot based Cooper pair beam splitter (CPS), by performing the microwave
+http://arxiv.org/abs/1509.01574
-spectroscopy of the CPS. More precisely, one can study the DC current response of such a CPS to two on-phase microwave gate irradiations applied to the two CPS dots. Some of the current
-peaks caused by the microwaves show a strongly nonmonotonic variation with the amplitude of the irradiation applied individually to one dot. This effect is directly due to a subradiance property
-caused by the coherence of the split pairs. Using realistic parameters, one finds that this effect has a measurable amplitude.
-http://arxiv.org/abs/1205.2252
+'''Coherent coupling of a single spin to microwave cavity photons'''
-----
-'''Direct Observation of Interband Spin-Orbit Coupling in a Two-Dimensional Electron System'''
-Hendrik Bentmann, Samir Abdelouahed, Mattia Mulazzi, Jurgen Henk, and Friedrich Reinert
+J. J. Viennot, M. C. Dartiailh, A. Cottet, T. Kontos
-We report the direct observation of interband spin-orbit (SO) coupling in a two-dimensional (2D) surface electron system, in addition to the anticipated Rashba spin splitting. Using angle-resolved
+http://arxiv.org/abs/1509.03839
-photoemission experiments and first-principles calculations on Bi-Ag-Au heterostructures, we show that the effect strongly modifies the dispersion as well as the orbital and spin character of the 2D
-electronic states, thus giving rise to considerable deviations from the Rashba model. The strength of the interband SO coupling is tuned by the thickness of the thin film structures.
-http://prl.aps.org/abstract/PRL/v108/i19/e196801
+'''Negative local resistance due to viscous electron backflow in graphene'''
-----
-'''Distinguishing Spontaneous Quantum Hall States in Bilayer Graphene'''
-Fan Zhang and A. H. MacDonald
+D. A. Bandurin, I. Torre, R. Krishna Kumar, M. Ben Shalom, A. Tomadin, A. Principi, G. H. Auton, E. Khestanova, K. S. Novoselov, I. V. Grigorieva, L. A. Ponomarenko, A. K. Geim, M. Polini
-Chirally stacked N-layer graphene with N > 2 is susceptible to a variety of distinct broken symmetry states in which each spin-valley flavor spontaneously transfers charge between layers. In mean-field
+http://arxiv.org/abs/1509.04165
-theory, one of the likely candidate ground states for a neutral bilayer is the layer antiferromagnet that has opposite spin polarizations in opposite layers. In this Letter, we analyze how the layer antiferromagnet and other competing states are influenced by Zeeman fields that couple to spin and by interlayer electric fields that couple to layer pseudospin, and comment on the possibility of using Zeeman responses and edge state signatures to identify the character of the bilayer ground state experimentally.
-http://prl.aps.org/abstract/PRL/v108/i18/e186804
+'''Point contacts in encapsulated graphene'''
-----
-'''Spin-Torque Switching with the Giant Spin Hall Effect of Tantalum'''
-Luqiao Liu, Chi-Feng Pai, Y. Li, H. W. Tseng, D. C. Ralph, R. A. Buhrman1
+Clevin Handschin, Balint Fülöp, Péter Makk, Sofya Blanter, Markus Weiss, K. Watanabe, T. Taniguchi, Szabolcs Csonka, Christian Schönenberger
-Spin currents can apply useful torques in spintronic devices. The spin Hall effect has been proposed as a source of spin current, but its modest strength has limited its usefulness. We
+http://arxiv.org/abs/1509.04137
-report a giant spin Hall effect (SHE) in b-tantalum that generates spin currents intense enough to induce efficient spin-torque switching of ferromagnets at room temperature. We quantify this
-SHE by three independent methods and demonstrate spin-torque switching of both out-of-plane and in-plane magnetized layers. We furthermore implement a three-terminal device that uses
-current passing through a tantalum-ferromagnet bilayer to switch a nanomagnet, with a magnetic tunnel junction for read-out. This simple, reliable, and efficient design may eliminate the main
-obstacles to the development of magnetic memory and nonvolatile spin logic technologies.
-http://www.sciencemag.org/content/336/6081/555.full
+'''Coherent manipulation of Andreev states in superconducting atomic contacts'''
-----
-== Ápr. 27. - Máj. 3. (2012) ==
+C. Janvier (1), L. Tosi (1), L. Bretheau (1), Ç. Ö. Girit (1), M. Stern (1), P. Bertet (1), P. Joyez (1), D. Vion (1), D. Esteve (1), M.F. Goffman (1), H. Pothier (1), C. Urbina (1) ((1) QUANTRONICS)
-''Válogatta: Halbritter András''
+http://arxiv.org/abs/1509.03961
-'''Spin–orbital separation in the quasi-one-dimensional Mott insulator Sr2CuO3'''
+'''Realization of Microwave Quantum Circuits Using Hybrid Superconducting-Semiconducting Nanowire Josephson Elements'''
-J. Schlappa,1, 2
+G. de Lange, B. van Heck, A. Bruno, D. J. van Woerkom, A. Geresdi, S. R. Plissard, E. P. A. M. Bakkers, A. R. Akhmerov, and L. DiCarlo
-K. Wohlfeld,3
-K. J. Zhou,1, 9
-M. Mourigal,4
-M. W. Haverkort,5
-V. N. Strocov,1
-L. Hozoi,3
-C. Monney,1
-S. Nishimoto,3
-S. Singh,6, 9
-A. Revcolevschi,6
-J.-S. Caux,7
-L. Patthey,1, 8
-H. M. Rønnow,4
-J. van den Brink3
-& T. Schmitt
-When viewed as an elementary particle, the electron has spin and charge. When binding to the atomic nucleus, it also acquires an angular momentum quantum number corresponding to the quantized atomic orbital it occupies. Even if electrons in solids form bands and delocalize from the nuclei, in Mott insulators they retain their three fundamental quantum numbers: spin, charge and orbital1. The hallmark of one-dimensional physics is a breaking up of the elementary electron into its separate degrees of freedom2. The separation of the electron into independent quasi-particles that carry either spin (spinons) or charge (holons) was first observed fifteen years ago3. Here we report observation of the separation of the orbital degree of freedom (orbiton) using resonant inelastic X-ray scattering on the one-dimensional Mott insulator Sr2CuO3. We resolve an orbiton separating itself from spinons and propagating through the lattice as a distinct quasi-particle with a substantial dispersion in energy over momentum, of about 0.2 electronvolts, over nearly one Brillouin zone.
+http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.115.127002
-http://www.nature.com/nature/journal/v485/n7396/full/nature10974.html
+'''Semiconductor-Nanowire-Based Superconducting Qubit'''
-----
+T. W. Larsen, K. D. Petersson, F. Kuemmeth, T. S. Jespersen, P. Krogstrup, J. Nygård, and C. M. Marcus
-'''Revealing the Angular Symmetry of Chemical Bonds by Atomic Force Microscopy'''
-Joachim Welker,
+http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.115.127001
-Franz J. Giessibl
-We have measured the angular dependence of chemical bonding forces between a carbon monoxide molecule that is adsorbed to a copper surface and the terminal atom of the metallic tip of a combined scanning tunneling microscope and atomic force microscope. We provide tomographic maps of force and current as a function of distance that revealed the emergence of strongly directional chemical bonds as tip and sample approach. The force maps show pronounced single, dual, or triple minima depending on the orientation of the tip atom, whereas tunneling current maps showed a single minimum for all three tip conditions. We introduce an angular dependent model for the bonding energy that maps the observed experimental data for all observed orientations and distances.
+'''Charge Number Dependence of the Dephasing Rates of a Graphene Double Quantum Dot in a Circuit QED Architecture'''
-http://www.sciencemag.org/content/336/6080/444.abstract
+Guang-Wei Deng, Da Wei, J. R. Johansson, Miao-Lei Zhang, Shu-Xiao Li, Hai-Ou Li, Gang Cao, Ming Xiao, Tao Tu, Guang-Can Guo, Hong-Wen Jiang, Franco Nori, and Guo-Ping Guo
-----
-'''Emergence of superlattice Dirac points in graphene on hexagonal boron nitride'''
-Matthew Yankowitz,	 Jiamin Xue,	 Daniel Cormode,	 Javier D. Sanchez-Yamagishi,	 K. Watanabe,	 T. Taniguchi,	 Pablo Jarillo-Herrero,	 Philippe Jacquod	 & Brian J. LeRoy
+http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.115.126804
-The Schrödinger equation dictates that the propagation of nearly free electrons through a weak periodic potential results in the opening of bandgaps near points of the reciprocal lattice known as Brillouin zone boundaries1. However, in the case of massless Dirac fermions, it has been predicted that the chirality of the charge carriers prevents the opening of a bandgap and instead new Dirac points appear in the electronic structure of the material2, 3. Graphene on hexagonal boron nitride exhibits a rotation-dependent moiré pattern4, 5. Here, we show experimentally and theoretically that this moiré pattern acts as a weak periodic potential and thereby leads to the emergence of a new set of Dirac points at an energy determined by its wavelength. The new massless Dirac fermions generated at these superlattice Dirac points are characterized by a significantly reduced Fermi velocity. Furthermore, the local density of states near these Dirac cones exhibits hexagonal modulation due to the influence of the periodic potential.
+'''Spin pairs in a weakly coupled many-electron quantum dot'''
-http://www.nature.com/nphys/journal/v8/n5/abs/nphys2272.html
+S. Hellmüller, D. Bischoff, T. Müller, M. Beck, K. Ensslin, and T. Ihn
-----
-'''Experimental observation of the optical spin transfer torque'''
-P. Němec, E. Rozkotová, N. Tesařová, F. Trojánek, E. De Ranieri, K. Olejník, J. Zemen, V. Novák, M. Cukr, P. Malý & T. Jungwirth
+http://journals.aps.org/prb/abstract/10.1103/PhysRevB.92.115401
-Spin transfer torque—the transfer of angular momentum from a spin-polarized current to a ferromagnet’s magnetization—has already found commercial application in memory devices, but the underlying physics is still not fully understood. Researchers now demonstrate the crucial role played by the polarization of the laser light that generates the current; a subtle effect only evident when isolated from other influences such as heating.
+'''Observation of negative refraction of Dirac fermions in graphene'''
-http://dx.doi.org/doi:10.1038/nphys2279
+Gil-Ho Lee, Geon-Hyoung Park, Hu-Jong Lee
-----
-'''Nature Nanotechnology news and views: Nanoelectronics: Transistors arrive at the atomic limit'''
-Gabriel P. Lansbergen
+http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3460.html
-A single-atom transistor has been made by positioning a phosphorus atom between metallic electrodes, also made of phosphorus, on a silicon surface.
+'''Parity lifetime of bound states in a proximitized semiconductor nanowire'''
-[[file:Nnano.2012.23-f1.jpg|200 px]]
+A. P. Higginbotham, S. M. Albrecht, G. Kiršanskas, W. Chang, F. Kuemmeth, P. Krogstrup, T. S. Jespersen, J. Nygård, K. Flensberg, C. M. Marcus
-http://www.nature.com/nnano/journal/v7/n4/full/nnano.2012.23.html
+http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3461.html
-original paper:
+'''Reliable Exfoliation of Large-Area High-Quality Flakes of Graphene and Other Two-Dimensional Materials'''
-'''A single-atom transistor'''
+Yuan Huang, Eli Sutter, Norman N. Shi, Jiabao Zheng, Tianzhong Yang, Dirk Englund, Hong-Jun Gao, and Peter Sutter
-Martin Fuechsle,	 Jill A. Miwa,	 Suddhasatta Mahapatra,	 Hoon Ryu,	 Sunhee Lee,	 Oliver Warschkow, Lloyd C. L. Hollenberg,	 Gerhard Klimeck	 & Michelle Y. Simmons
+http://pubs.acs.org/doi/abs/10.1021/acsnano.5b04258
-The ability to control matter at the atomic scale and build devices with atomic precision is central to nanotechnology. The scanning tunnelling microscope1 can manipulate individual atoms2 and molecules on surfaces, but the manipulation of silicon to make atomic-scale logic circuits has been hampered by the covalent nature of its bonds. Resist-based strategies have allowed the formation of atomic-scale structures on silicon surfaces3, but the fabrication of working devices—such as transistors with extremely short gate lengths4, spin-based quantum computers5, 6, 7, 8 and solitary dopant optoelectronic devices9—requires the ability to position individual atoms in a silicon crystal with atomic precision. Here, we use a combination of scanning tunnelling microscopy and hydrogen-resist lithography to demonstrate a single-atom transistor in which an individual phosphorus dopant atom has been deterministically placed within an epitaxial silicon device architecture with a spatial accuracy of one lattice site. The transistor operates at liquid helium temperatures, and millikelvin electron transport measurements confirm the presence of discrete quantum levels in the energy spectrum of the phosphorus atom. We find a charging energy that is close to the bulk value, previously only observed by optical spectroscopy10
+== Aug. 25. - szept. 7. ==
-http://www.nature.com/nnano/journal/v7/n4/abs/nnano.2012.21.html
+'''Coupling Two Distant Double Quantum Dots with a Microwave Resonator'''
-----
-'''Signatures of Cooperative Effects and Transport Mechanisms in Conductance Histograms'''
-Matthew G. Reuter*†§, Mark C. Hersam†‡, Tamar Seideman†, and Mark A. Ratner†
+Guang-Wei Deng, Da Wei, Shu-Xiao Li, J. R. Johansson, Wei-Cheng Kong, Hai-Ou Li, Gang Cao, Ming Xiao, Guang-Can Guo, Franco Nori, Hong-Wen Jiang, and Guo-Ping Guo
-We present a computational investigation into the line shapes of peaks in conductance histograms, finding that they possess high information content. In particular, the histogram peak associated with conduction through a single molecule elucidates the electron transport mechanism and is generally well-described by beta distributions. A statistical analysis of the peak corresponding to conduction through two molecules reveals the presence of cooperative effects between the molecules and also provides insight into the underlying conduction channels. This work describes tools for extracting additional interpretations from experimental statistical data, helping us better understand electron transport processes.
+We fabricated a hybrid device with two distant graphene double quantum dots (DQDs) and a microwave resonator. A nonlinear response is observed in the resonator reflection amplitude when the two DQDs are jointly tuned to the vicinity of the degeneracy points. This observation can be well fitted by the Tavis–Cummings (T–C) model which describes two two-level systems coupling with one photonic field. Furthermore, the correlation between the DC currents in the two DQDs is studied. A nonzero cross-current correlation is observed which has been theoretically predicted to be an important sign of nonlocal coupling between two distant systems. Our results explore T–C physics in electronic transport and also contribute to the study of nonlocal transport and future implementations of remote electronic entanglement.
-[[file:Nl-2011-04379j_0001.gif|200 px]]
+http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b02400
-http://pubs.acs.org/doi/abs/10.1021/nl204379j
+'''Reliable Exfoliation of Large-Area High-Quality Flakes of Graphene and Other Two-Dimensional Materials'''
-----
-'''Transport Properties of Graphene Nanoroads in Boron Nitride Sheets'''
-Jeil Jung†, Zhenhua Qiao*†, Qian Niu†‡, and Allan H. MacDonald†
+Yuan Huang, Eli Sutter, Norman N. Shi, Jiabao Zheng, Tianzhong Yang, Dirk Englund, Hong-Jun Gao, and Peter Sutter
-We demonstrate that the one-dimensional (1D) transport channels that appear in the gap when graphene nanoroads are embedded in boron nitride (BN) sheets are more robust when they are inserted at AB/BA grain boundaries. Our conclusions are based on ab initio electronic structure calculations for a variety of different crystal orientations and bonding arrangements at the BN/C interfaces. This property is related to the valley Hall conductivity present in the BN band structure and to the topologically protected kink states that appear in continuum Dirac models with position-dependent masses.
+Mechanical exfoliation has been a key enabler of the exploration of the properties of two-dimensional materials, such as graphene, by providing routine access to high-quality material. The original exfoliation method, which remained largely unchanged during the past decade, provides relatively small flakes with moderate yield. Here, we report a modified approach for exfoliating thin monolayer and few-layer flakes from layered crystals. Our method introduces two process steps that enhance and homogenize the adhesion force between the outermost sheet in contact with a substrate: Prior to exfoliation, ambient adsorbates are effectively removed from the substrate by oxygen plasma cleaning, and an additional heat treatment maximizes the uniform contact area at the interface between the source crystal and the substrate. For graphene exfoliation, these simple process steps increased the yield and the area of the transferred flakes by more than 50 times compared to the established exfoliation methods. Raman and AFM characterization shows that the graphene flakes are of similar high quality as those obtained in previous reports. Graphene field-effect devices were fabricated and measured with back-gating and solution top-gating, yielding mobilities of ∼4000 and 12 000 cm2/(V s), respectively, and thus demonstrating excellent electrical properties. Experiments with other layered crystals, e.g., a bismuth strontium calcium copper oxide (BSCCO) superconductor, show enhancements in exfoliation yield and flake area similar to those for graphene, suggesting that our modified exfoliation method provides an effective way for producing large area, high-quality flakes of a wide range of 2D materials.
-http://pubs.acs.org/doi/abs/10.1021/nl300610w
+http://pubs.acs.org/doi/abs/10.1021/acsnano.5b04258
-----
-== Ápr. 12. - Ápr. 26 (2012) ==
+'''Influence of Impurity Spin Dynamics on Quantum Transport in Epitaxial Graphene'''
-''Válogatta: Makk Péter''
+Samuel Lara-Avila, Sergey Kubatkin, Oleksiy Kashuba, Joshua A. Folk, Silvia Lüscher, Rositza Yakimova, T. J. B. M. Janssen, Alexander Tzalenchuk, and Vladimir Fal’ko
-'''Surface conduction of topological Dirac electrons in bulk insulating Bi2Se3 '''
+Experimental evidence from both spin-valve and quantum transport measurements points towards unexpectedly fast spin relaxation in graphene. We report magnetotransport studies of epitaxial graphene on SiC in a vector magnetic field showing that spin relaxation, detected using weak-localization analysis, is suppressed by an in-plane magnetic field B∥, and thereby proving that it is caused at least in part by spinful scatterers. A nonmonotonic dependence of the effective decoherence rate on B∥ reveals the intricate role of the scatterers’ spin dynamics in forming the interference correction to the conductivity, an effect that has gone unnoticed in earlier weak localization studies.
-Dohun Kim, Sungjae Cho, Nicholas P. Butch, Paul Syers, Kevin Kirshenbaum, Shaffique Adam, Johnpierre Paglione & Michael S. Fuhrer
+http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.115.106602
-The newly discovered three-dimensional strong topological insulators (STIs) exhibit topologically protected Dirac surface states. Although the STI surface state has been studied spectroscopically, for example, by photoemission and scanned probes, transport experiments have failed to demonstrate the most fundamental signature of the STI: ambipolar metallic electronic transport in the topological surface of an insulating bulk. Here we show that the surfaces of thin (~ 10 nm), low-doped Bi2Se3 (≈1017 cm−3) crystals are strongly electrostatically coupled, and a gate electrode can completely remove bulk charge carriers and bring both surfaces through the Dirac point simultaneously. We observe clear surface band conduction with a linear Hall resistivity and a well-defined ambipolar field effect, as well as a charge-inhomogeneous minimum conductivity region. A theory of charge disorder in a Dirac band19, 20, 21 explains well both the magnitude and the variation with disorder strength of the minimum conductivity (2 to 5 e2/h per surface) and the residual (puddle) carrier density (0.4×1012 to 4×1012 cm−2). From the measured carrier mobilities 320–1,500 cm2 V−1 s−1, the charged impurity densities 0.5×1013 to 2.3×1013 cm−2 are inferred. They are of a similar magnitude to the measured doping levels at zero gate voltage (1×1013 to 3×1013 cm−2), identifying dopants as the charged impurities.
+'''Highly thermally conductive and mechanically strong graphene fibers'''
-http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2286.html
+Guoqing Xin, Tiankai Yao, Hongtao Sun, Spencer Michael Scott, Dali Shao, Gongkai Wang, Jie Lian
-----
+Graphene, a single layer of carbon atoms bonded in a hexagonal lattice, is the thinnest, strongest, and stiffest known material and an excellent conductor of heat and electricity. However, these superior properties have yet to be realized for graphene-derived macroscopic structures such as graphene fibers. We report the fabrication of graphene fibers with high thermal and electrical conductivity and enhanced mechanical strength. The inner fiber structure consists of large-sized graphene sheets forming a highly ordered arrangement intercalated with small-sized graphene sheets filling the space and microvoids. The graphene fibers exhibit a submicrometer crystallite domain size through high-temperature treatment, achieving an enhanced thermal conductivity up to 1290 watts per meter per kelvin. The tensile strength of the graphene fiber reaches 1080 megapascals.
+https://www.sciencemag.org/content/349/6252/1083.abstract
-'''Observation of the fractional ac Josephson effect: the signature of Majorana particles'''
+'''A quantum circuit rule for interference effects in single-molecule electrical junctions'''
-Leonid P. Rokhinson, Xinyu Liu, and Jacek K. Furdyna
+David Zsolt Manrique, Cancan Huang, Masoud Baghernejad, Xiaotao Zhao, Oday A. Al-Owaedi, Hatef Sadeghi, Veerabhadrarao Kaliginedi, Wenjing Hong, Murat Gulcur, Thomas Wandlowski, Martin R. Bryce, Colin J. Lambert
-We report the observation of the fractional ac Josephson effect in hybrid semiconductor/superconductor InSb/Nb nanowire junctions, a hallmark of topological matter. When the junction is irradiated with rf frequency f0 at zero external magnetic field, quantized voltage steps (Shapiro steps) with a height �V = hf0=2e are observed, as is expected for conventional superconductor junctions where the super current is carried by charge-2e Cooper pairs. At high fields the height of the first Shapiro step is doubled to hf0=e. The supercurrent carried by charge-e quasiparticles is a unique signature of Majorana fermions, elusive particles predicted ca. 80 years ago.
+A quantum circuit rule for combining quantum interference (QI) effects in the conductive properties of oligo(phenyleneethynylene) (OPE)-type molecules possessing three aromatic rings was investigated both experimentally and theoretically. Molecules were of the type X-Y-X, where X represents pyridyl anchors with para (p), meta (m) or ortho (o) connectivities and Y represents a phenyl ring with p and m connectivities. The conductances GXmX (GXpX) of molecules of the form X-m-X (X-p-X), with meta (para) connections in the central ring were predominantly lower (higher), irrespective of the meta, para, or ortho nature of the anchor groups X, demonstrating that conductance is dominated by the nature of QI in the central ring Y. The single-molecule conductances were found to satisfy the quantum circuit rule Gppp/Gpmp = Gmpm/Gmmm. This demonstrates that the contribution to the conductance from the central ring is independent of the para versus meta nature of the anchor groups.
-http://arxiv.org/abs/1204.4212
+http://xxx.lanl.gov/abs/1509.00990
-----
-'''Observation of Majorana Fermions in a Nb-InSb Nanowire-Nb Hybrid Quantum Device'''
+'''Spawning rings of exceptional points out of Dirac cones'''
-M. T. Deng, C. L. Yu, G. Y. Huang, M. Larsson, P. Caroff, and H. Q. Xu
+Bo Zhen, Chia Wei Hsu, Yuichi Igarashi, Ling Lu, Ido Kaminer, Adi Pick, Song-Liang Chua, John D. Joannopoulos & Marin Soljacˇic
-...  Here, we report on the observation of excitation of Majorana fermions in a Nb-InSb nanowire quantum dot-Nb hybrid system. The InSb nanowire quantum dot is formed between the two Nb contacts by weak Schottky barriers and is thus in the regime of strong couplings to the contacts. Due to the proximity effect, the InSb nanowire segments covered by superconductor Nb contacts turn to superconductors with a superconducting energy gap. Under an applied magnetic field larger than a critical value for which the Zeeman energy in the InSb nanowire is Ez � ��, the entire InSb nanowire is found to be in a nontrivial topological superconductor phase, supporting a pair of Majorana fermions, and Cooper pairs can transport between the superconductor Nb contacts via the Majorana fermion states. This transport process will be suppressed when the applied magnetic field becomes larger than a second critical value at which the transition to a trivial topological superconductor phase occurs in the system. This physical scenario has been observed in our experiment. We have found that the measured zero-bias conductance for our hybrid device shows a conductance plateau in a range of the applied magnetic field in quasi-particle Coulomb
+The Dirac cone underlies many unique electronic properties of
-blockade regions. This work provides a simple, solid way of detecting Majorana fermions in solid state systems and should greatly stimulate Majorana fermion research and applications.
+graphene1 and topological insulators, and its band structure—
+two conical bands touching at a single point—has also been realized
+for photons in waveguide arrays2
+, atoms in optical lattices3
+,
+and through accidental degeneracy4,5. Deformation of the Dirac cone often reveals intriguing properties; an example is the quantum Hall effect, where a constant magnetic field breaks the
+Dirac cone into isolated Landau levels. A seemingly unrelated phenomenon is the exceptional point6,7, also known as the parity–time symmetry breaking point8–11, where two resonances coincide in both their positions and widths. Exceptional points lead to counter-intuitive phenomena such as loss-induced transparency12, unidirectional transmission or reflection11,13,14, and lasers with reversed pump dependence15 or single-mode operation16,17. Dirac cones and exceptional points are connected: it was theoretically suggested that certain non-Hermitian perturbations can deform a Dirac cone and spawn a ring of exceptional points18–20. Here we experimentally demonstrate such an ‘exceptional ring’ in a photonic crystal slab. Angle-resolved reflection measurements of the photonic crystal slab reveal that the peaks of reflectivity follow the conical band structure of a Dirac cone resulting from accidental degeneracy, whereas the complex eigenvalues of the system are deformed into a two-dimensional flat band enclosed by an exceptional ring. This deformation arises from the dissimilar radiation rates of dipole and quadrupole resonances, which play a role analogous to the loss and gain in parity–time symmetric systems. Our results indicate that the radiation existing in any open system can
+fundamentally alter its physical properties in ways previously expected only in the presence of material loss and gain.
-http://arxiv.org/abs/1204.4130v1
+http://www.nature.com/nature/journal/vaop/ncurrent/pdf/nature14889.pdf
-----
+'''A Short Course on Topological Insulators: Band-structure topology and edge states in one and two dimensions'''
-'''Coherent quantum phase slip'''
-O. V. Astafiev, L. B. Ioffe, S. Kafanov, Yu. A. Pashkin, K. Yu. Arutyunov, D. Shahar, O. Cohen & J. S. Tsai
+János K. Asbóth, László Oroszlány, András Pályi
-A hundred years after the discovery of superconductivity, one fundamental prediction of the theory, coherent quantum phase slip (CQPS), has not been observed. CQPS is a phenomenon exactly dual1 to the Josephson effect; whereas the latter is a coherent transfer of charges between superconducting leads, the former is a coherent transfer of vortices or fluxes across a superconducting
+This course-based primer provides newcomers to the field with a concise introduction to some of the core topics in the emerging field of topological band insulators in one and two dimensions. The aim is to provide a basic understanding of edge states, bulk topological invariants, and of the bulk--boundary correspondence with as simple mathematical tools as possible. We use noninteracting lattice models of topological insulators, building gradually on these to arrive from the simplest one-dimensional case (the Su-Schrieffer-Heeger model for polyacetylene) to two-dimensional time-reversal invariant topological insulators (the Bernevig-Hughes-Zhang model for HgTe). In each case the model is introduced first and then its properties are discussed and subsequently generalized. The only prerequisite for the reader is a working knowledge in quantum mechanics, the relevant solid state physics background is provided as part of this self-contained text, which is complemented by end-of-chapter problems.
-wire. In contrast to previously reported observations of incoherent phase slip, CQPS has been only a subject of theoretical study. Its experimental demonstration is made difficult by quasiparticle dissipation due to gapless excitations in nanowires or in vortex cores. This difficulty might be overcome by using certain strongly disordered superconductors near the superconductor–insulator transition. Here we report direct observation of CQPS in a narrow segment of a superconducting loop made of strongly disordered indium oxide; the effect is made manifest through the superposition of quantum states with different numbers of flux quanta. As with the Josephson effect, our observation should lead to new applications in superconducting electronics and quantum
-metrology.
-http://www.nature.com/nature/journal/v484/n7394/full/nature10930.html
+http://xxx.lanl.gov/abs/1509.02295
-----
-'''Spintronics reviews in Nat. Materials:'''
+== Március 24-31. ==
+''Fülöp Gergő''
-Current-induced torques in magnetic materials (Arne Brataas, Andrew D. Kent and Hideo Ohno)
+'''Graphene-templated directional growth of an inorganic nanowire'''
-http://www.nature.com/nmat/journal/v11/n5/full/nmat3311.html
-Spin Hall effect devices (Tomas Jungwirth, Jörg Wunderlich and Kamil Olejník)
+''Won Chul Lee,	Kwanpyo Kim,	Jungwon Park,	Jahyun Koo,	Hu Young Jeong,	Hoonkyung Lee,	David A. Weitz,	Alex Zettl	& Shoji Takeuchi''
-http://www.nature.com/nmat/journal/v11/n5/full/nmat3279.html
-Spintronics and pseudospintronics in graphene and topological insulators (Dmytro Pesin and Allan H. MacDonald)
+http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2015.36.html
-http://www.nature.com/nmat/journal/v11/n5/full/nmat3305.html
-New moves of the spintronics tango (Jairo Sinova and Igor Žutić)
+'''Coaxial lithography'''
-http://www.nature.com/nmat/insight/spintronics/index.html
-----
-'''Quantum Hall Effect in Graphene with Superconducting Electrodes'''
-Peter Rickhaus, Markus Weiss,* Laurent Marot, and Christian Schönenberger
+''Tuncay Ozel,	Gilles R. Bourret	& Chad A. Mirkin''
-We have realized an integer quantum Hall system with superconducting contacts by connecting graphene
+http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2015.33.html
-to niobium electrodes. Below their upper critical field of 4 T, an integer quantum Hall effect coexists with superconductivity in the leads but with a plateau conductance that is larger than
-in the normal state. We ascribe this enhanced quantum Hall plateau conductance to Andreev processes at the graphene−superconductor interface leading to the formation of so-called Andreev edge-states. The enhancement depends strongly on the filling-factor and is less pronounced on the first plateau due to the special nature of the zero energy Landau level in monolayer graphene.
-http://dx.doi.org/10.1021/nl204415s
+'''Coupling between electrons and optical phonons in suspended bilayer graphene'''
-----
-'''Prospects for Spin-Based Quantum Computing'''
-Christoph Kloeffel and Daniel Loss
+''Antti Laitinen, Manohar Kumar, Mika Oksanen, Bernard Plaçais, Pauli Virtanen, and Pertti Hakonen''
-Experimental and theoretical progress toward quantum computation with spins in quantum dots (QDs) is
+http://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.121414
-reviewed, with particular focus on QDs formed in GaAs heterostructures, on nanowire-based QDs, and on
-self-assembled QDs. We report on a remarkable evolution of the field where decoherence – one of the main challenges for realizing quantum computers – no longer seems to be the stumbling block it had originally been considered. General concepts, relevant quantities, and basic requirements for spin-based quantum computing are explained; opportunities and challenges of spin-orbit interaction and nuclear spins are reviewed. We discuss recent achievements, present current theoretical proposals, and make several suggestions for further experiments.
-http://arxiv.org/abs/1204.5917v1
+'''Nonlocal electromagnetic response of graphene nanostructures'''
-----
-'''Electronic properties of graphene: a perspective from scanning tunneling microscopy and magneto-transport.'''
+''Arya Fallahi, Tony Low, Michele Tamagnone, and Julien Perruisseau-Carrier''
-Eva Y. Andrei, Guohong Li and Xu Du
-This review covers recent experimental progress in probing the electronic properties of graphene and how they are influenced by various substrates, by the presence of a magnetic field and by the proximity to a superconductor. The focus is on results obtained using scanning tunneling microscopy, spectroscopy, transport and magneto-transport techniques.
+http://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.121405
-http://arxiv.org/abs/1204.4532
+'''Nonequilibrium spin transport in Zeeman-split superconductors'''
-----
-'''Spectroscopy of Spin-Orbit Quantum Bits in Indium Antimonide Nanowires'''
-S. Nadj-Perge, V. S. Pribiag, J.W. G. van den Berg, K. Zuo, S. R. Plissard, E. P. A. M. Bakkers,
-S. M. Frolov, and L. P. Kouwenhoven
-A double quantum dot in the few-electron regime is achieved using local gating in an InSb nanowire.
+''Tatiana Krishtop, Manuel Houzet, and Julia S. Meyer''
-The spectrum of two-electron eigenstates is investigated using electric dipole spin resonance. Singlettriplet level repulsion caused by spin-orbit interaction is observed. The size and the anisotropy of singlettriplet repulsion are used to determine the magnitude and the orientation of the spin-orbit effective field in an InSb nanowire double dot. The obtained results are confirmed using spin blockade leakage current anisotropy and transport spectroscopy of individual quantum dots.
-http://prl.aps.org/abstract/PRL/v108/i16/e166801
+http://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.121407
-----
-'''First-Order 0-Pi Quantum Phase Transition in the Kondo Regime of a Superconducting Carbon-Nanotube Quantum Dot'''
+'''Finite-frequency noise in a quantum dot with normal and superconducting leads'''
-Romain Maurand,1 Tobias Meng,2 Edgar Bonet,1 Serge Florens,1 Lae¨titia Marty,1,* and Wolfgang Wernsdorfer1
+''Stephanie Droste, Janine Splettstoesser, and Michele Governale''
-We study a carbon-nanotube quantum dot embedded in a superconducting-quantum-interference-device
+http://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.125401
-loop in order to investigate the competition of strong electron correlations with a proximity
-effect. Depending on whether local pairing or local magnetism prevails, a superconducting quantum
-dot will exhibit a positive or a negative supercurrent, referred to as a 0 or � Josephson junction,
-respectively. In the regime of a strong Coulomb blockade, the 0-to-� transition is typically controlled by a change in the discrete charge state of the dot, from even to odd. In contrast, at a larger tunneling amplitude, the Kondo effect develops for an odd-charge (magnetic) dot in the normal state, and quenches magnetism. In this situation, we find that a first-order 0-to-� quantum phase transition can be triggered at a fixed valence when superconductivity is brought in, due to the competition of the superconducting gap and the Kondo temperature. The superconducting-quantum-interference-device geometry together with the tunability of our device allows the exploration of the associated phase diagram predicted by recent theories. We also report on the observation of anharmonic behavior of the current-phase relation in the transition regime, which we associate with the two accessible superconducting states. Our results finally demonstrate that the spin-singlet nature of the Kondo state helps to enhance the stability of the 0 phase far from the mixed-valence regime in odd-charge superconducting quantum dots.
-http://prx.aps.org/abstract/PRX/v2/i1/e011009
+== Március 24. ==
-----
+''Boros Csanád Örs''
-'''Demonstration of Entanglement of Electrostatically Coupled Singlet-Triplet Qubits'''
-M. D. Shulman, O. E. Dial, S. P. Harvey, H. Bluhm, V. Umansky, A. Yacoby
+'''Spin and charge transport in graphene-based spin transport devices with Co/MgO spin injection and spin detection electrodes'''
-Quantum computers have the potential to solve certain problems faster than classical computers. To exploit their power, it is necessary to perform interqubit operations and generate entangled states. Spin qubits are a promising candidate for implementing a quantum processor because of their potential for scalability and miniaturization. However, their weak interactions with the environment, which lead to their long coherence times, make interqubit operations challenging. We performed a controlled two-qubit operation between singlet-triplet qubits using a dynamically decoupled sequence that maintains the two-qubit coupling while decoupling each qubit from its fluctuating environment. Using state tomography, we measured the full density matrix of the system and determined the concurrence and the fidelity of the generated state, providing proof of entanglement.
+'F. Volmer, M. Drögeler, G. Güntherodt, C. Stampfer, B. Beschoten'
-http://www.sciencemag.org/content/336/6078/202.full
+In this review we discuss spin and charge transport properties in graphene-based single-layer and few-layer spin-valve devices. We give an overview of challenges and recent advances in the field of device fabrication and discuss two of our fabrication methods in more detail which result in distinctly different device performances. In the first class of devices, Co/MgO electrodes are directly deposited onto graphene which results in rough MgO-to-Co interfaces and favor the formation of conducting pinholes throughout the MgO layer. We show that the contact resistance area product (RcA) is a benchmark for spin transport properties as it scales with the measured spin lifetime in these devices indicating that contact-induced spin dephasing is the bottleneck for spin transport even in devices with large RcA values. In a second class of devices, Co/MgO electrodes are first patterned onto a silicon substrate. Subsequently, a graphene-hBN heterostructure is directly transferred onto these prepatterned electrodes which provides improved interface properties. This is seen by a strong enhancement of both charge and spin transport properties yielding charge carrier mobilities exceeding 20000 cm2/(Vs) and spin lifetimes up to 3.7 ns at room temperature. We discuss several shortcomings in the determination of both quantities which complicates the analysis of both extrinsic and intrinsic spin scattering mechanisms. Furthermore, we show that contacts can be the origin of a second charge neutrality point in gate dependent resistance measurements which is influenced by the quantum capacitance of the underlying graphene layer.
-----
-'''Revealing the Angular Symmetry of Chemical Bonds by Atomic Force Microscopy'''
-Joachim Welker, Franz J. Giessibl
-We have measured the angular dependence of chemical bonding forces between a carbon monoxide molecule that is adsorbed to a copper surface and the terminal atom of the metallic tip of a combined scanning tunneling microscope and atomic force microscope. We provide tomographic maps of force and current as a function of distance that revealed the emergence of strongly directional chemical bonds as tip and sample approach. The force maps show pronounced single, dual, or triple minima depending on the orientation of the tip atom, whereas tunneling current maps showed a single minimum for all three tip conditions. We introduce an angular dependent model for the bonding energy that maps the observed experimental data for all observed orientations and distances.
-http://www.sciencemag.org/content/336/6080/444.full
-----
-[[File:Cheers.jpg‎]]
+http://arxiv.org/abs/1503.01735v1
-'''Electrically controlled quantum dot based spin current injector'''
-Szabolcs Csonka ,  I Weymann and Gergely Zarand
+== Nov. 1-21. ==
+''Scherübl Zoltán
+'''Unusual resistance-voltage dependence of nanojunctions during electromigration in ultra-high vacuum'''
-We present a proposal for a fully electrically controllable quantum dot based spin current injector. The device consists of a quantum dot that is strongly coupled to a ferromagnetic electrode on one side and weakly coupled to a nonmagnetic electrode on the other side. The presence of ferromagnetic electrode results in an exchange field that splits the dot level. We show that this exchange-induced splitting can lead to almost full spin polarization of the current owing through the device. Moreover, we also demonstrate that the sign of the polarization can be changed by the gate or the bias voltage within a switching time in the nanosecond range. Thus the proposed device can operate as an electrically controlled, fast switchable spin current source, which can be realized in various state-of-the-art nanostructures.
+''D. Stöffler, M. Marz, B. Kießig, T. Tomanic, R. Schäfer, H. v. Löhneysen, R. Hoffmann-Vogel''
-http://pubs.rsc.org/en/content/articlelanding/2012/nr/c2nr30399j
+The electrical resistance R of metallic nanocontacts subjected to controlled cyclic electromigration in ultra-high vacuum has been investigated in-situ as a function of applied voltage V. For sufficiently small contacts, i.e., large resistance, a decrease of R(V) while increasing V is observed. This effect is tentatively attributed to the presence of contacts separated by thin vacuum barriers in parallel to ohmic nanocontacts. Simple model calculations indicate that both thermal activation or tunneling can lead to this unusual behavior. We describe our data by a tunneling model whose key parameter, i.e., the tunneling distance, changes because of thermal expansion due to Joule heating and/or electrostatic strain arising from the applied voltage. Oxygen exposure during electromigration prevents the formation of negative R(V) slopes, and at the same time enhances the probability of uncontrolled melting, while other gases show little effects. In addition, indication for field emission has been observed in some samples
-== Ápr. 6. - Ápr. 12 (2012) ==
+http://arxiv.org/abs/1411.0105
-''Válogatta: Sárkány Lőrinc''
 ----
-'''Compensation of Coulomb Blocking and Energy Transfer in the Current Voltage Characteristic of Molecular Conduction Junctions'''
+'''Pb/InAs nanowire Josephson junction with high critical current and magnetic flux focusing'''
-Guangqi Li, Manmohan S. Shishodia, Boris D. Fainberg, Boris Apter, Michal Oren, Abraham Nitzan, and Mark A. Ratner
+''J. Paajaste, M. Amado, S. Roddaro, F. S. Bergeret, D. Ercolani, L. Sorba, F. Giazotto''
-We have studied the influence of both exciton effects and Coulomb repulsion on current in molecular nanojunctions. We show that dipolar energy-transfer interactions between the sites in the wire can at high voltage compensate Coulomb blocking for particular relationships between their values. Tuning this relationship may be achieved by using the effect of plasmonic nanostructure on dipolar energy-transfer interactions.
+We have studied mesoscopic Josephson junctions formed by highly n-doped InAs nanowires and superconducting Ti/Pb source and drain leads. The current-voltage properties of the system are investigated by varying temperature and external out-of-plane magnetic field. Superconductivity in the Pb electrodes persists up to ∼7 K and with magnetic field values up to 0.4 T. Josephson coupling at zero backgate voltage is observed up to 4.5 K and the critical current is measured to be as high as 615 nA. The supercurrent suppression as a function of the magnetic field reveals a diffraction pattern that is explained by a strong magnetic flux focusing provided by the superconducting electrodes forming the junction.
-http://pubs.acs.org/doi/abs/10.1021/nl204130d
+http://xxx.lanl.gov/abs/1411.0990
 ----
-'''Ultrafast Relaxation Dynamics via Acoustic Phonons in Carbon Nanotubes'''
+'''Room temperature magnetic order on zigzag edges of narrow graphene nanoribbons'''
-Olga A. Dyatlova, Christopher Köhler, Ermin Malic, Jordi Gomis-Bresco, Janina Maultzsch, Andrey Tsagan-Mandzhiev, Tobias Watermann, Andreas Knorr, and Ulrike Woggon
+''Gabor Zsolt Magda, Xiaozhan Jin, Imre Hagymasi, Peter Vancso, Zoltan Osvath, Peter Nemes-Incze, Chanyong Hwang, Laszlo P. Biro, Levente Tapaszto''
-Carbon nanotubes as one-dimensional nanostructures are ideal model systems to study relaxation channels of excited charged carriers. The understanding of the ultrafast scattering processes is the key for exploiting the huge application potential that nanotubes offer, e.g., for light-emitting and detecting nanoscale electronic devices. In a joint study of two-color pump–probe experiments and microscopic calculations based on the density matrix formalism, we extract, both experimentally and theoretically, a picosecond carrier relaxation dynamics, and ascribe it to the intraband scattering of excited carriers with acoustic phonons. The calculated picosecond relaxation times show a decrease for smaller tube diameters. The best agreement between experiment and theory is obtained for the (8,7) nanotubes with the largest investigated diameter and chiral angle for which the applied zone-folded tight-binding wave functions are a good approximation.
+Magnetic order emerging in otherwise non-magnetic materials as carbon is a paradigmatic example of a novel type of s-p electron magnetism predicted to be of exceptional high-temperature stability. It has been demonstrated that atomic scale structural defects of graphene can host unpaired spins. However, it is still unclear under which conditions long-range magnetic order can emerge from such defect-bound magnetic moments. Here we propose that in contrast to random defect distributions, atomic scale engineering of graphene edges with specific crystallographic orientation, comprising edge atoms only from one sub-lattice of the bipartite graphene lattice, can give rise to a robust magnetic order. We employ a nanofabrication technique based on Scanning Tunneling Microscopy to define graphene nanoribbons with nanometer precision and well-defined crystallographic edge orientations. While armchair ribbons display quantum confinement gap, zigzag ribbons narrower than 7 nm reveal a bandgap of about 0.2 - 0.3 eV, which can be identified as a signature of interaction induced spin ordering along their edges. Moreover, a semiconductor to metal transition is revealed upon increasing the ribbon width, indicating the switching of the magnetic coupling between opposite ribbon edges from antiferromagnetic to ferromagnetic configuration. We found that the magnetic order on graphene edges of controlled zigzag orientation can be stable even at room temperature, raising hope for graphene-based spintronic devices operating under ambient conditions.
-http://pubs.acs.org/doi/abs/10.1021/nl2043997
+http://xxx.lanl.gov/abs/1411.1196
 ----
-'''Quantum Hall effect in exfoliated graphene affected by charged impurities: Metrological measurements'''
+'''Using polymer electrolyte gates to set-and-freeze threshold voltage and local potential in nanowire-based devices and thermoelectrics'''
-J. Guignard, D. Leprat, D. C. Glattli, F. Schopfer, and W. Poirier
+''Sofia Fahlvik Svensson, Adam M. Burke, Damon J. Carrad, Martin Leijnse, Heiner Linke, Adam P. Micolich''
-Metrological investigations of the quantum Hall effect (QHE) completed by transport measurements at low magnetic field are carried out in a-few-μm-wide Hall bars made of monolayer (ML) or bilayer (BL) exfoliated graphene transferred on Si/SiO2 substrate. From the charge carrier density dependence of the conductivity and from the measurement of the quantum corrections at low magnetic field, we deduce that transport properties in these devices are mainly governed by the Coulomb interaction of carriers with a large concentration of charged impurities. In the QHE regime, at high magnetic field and low temperature (T<1.3 K), the Hall resistance is measured by comparison with a GaAs-based quantum resistance standard using a cryogenic current comparator. In the low-dissipation limit, it is found quantized within 5 parts in 107 (one standard deviation, 1σ) at the expected rational fractions of the von Klitzing constant, respectively, RK/2 and RK/4 in the ML and BL devices. These results constitute the most accurate QHE quantization tests to date in monolayer and bilayer exfoliated graphene. It turns out that a main limitation to the quantization accuracy, which is found well above the 10−9 accuracy usually achieved in GaAs, is the low value of the QHE breakdown current being no more than 1 μA. The current dependence of the longitudinal conductivity investigated in the BL Hall bar shows that dissipation occurs through quasielastic inter-Landau-level scattering, assisted by large local electric fields. We propose that charged impurities are responsible for an enhancement of such inter-Landau-level transition rate and cause small breakdown currents.
+We use the strongly temperature-dependent ionic mobility in polymer electrolytes to 'freeze in' specific ionic charge environments around a nanowire using a local wrap-gate geometry. This enables us to set both the threshold voltage for a conventional doped substrate gate and the local disorder potential at temperatures below 200 Kelvin, which we characterize in detail by combining conductance and thermovoltage measurements with modeling. Our results demonstrate that local polymer electrolyte gates are compatible with nanowire thermoelectrics, where they offer the advantage of a very low thermal conductivity, and hold great potential towards setting the optimal operating point for solid-state cooling applications.
-http://prb.aps.org/abstract/PRB/v85/i16/e165420
+http://xxx.lanl.gov/abs/1411.2727
 ----
-'''Silicene: Compelling Experimental Evidence for Graphenelike Two-Dimensional Silicon'''
+'''Revisiting the measurement of the spin relaxation time in graphene-based devices'''
-Patrick Vogt, Paola De Padova, Claudio Quaresima, Jose Avila, Emmanouil Frantzeskakis, Maria Carmen Asensio, Andrea Resta, Bénédicte Ealet, and Guy Le Lay
+''H. Idzuchi, A. Fert, Y. Otani''
-Because of its unique physical properties, graphene, a 2D honeycomb arrangement of carbon atoms, has attracted tremendous attention. Silicene, the graphene equivalent for silicon, could follow this trend, opening new perspectives for applications, especially due to its compatibility with Si-based electronics. Silicene has been theoretically predicted as a buckled honeycomb arrangement of Si atoms and having an electronic dispersion resembling that of relativistic Dirac fermions. Here we provide compelling evidence, from both structural and electronic properties, for the synthesis of epitaxial silicene sheets on a silver (111) substrate, through the combination of scanning tunneling microscopy and angular-resolved photoemission spectroscopy in conjunction with calculations based on density functional theory.
+A long spin relaxation time (tausf) is the key for the applications of graphene to spintronics but the experimental values of tausf have been generally much shorter than expected. We show that the usual determination by the Hanle method underestimates tausf if proper account of the spin absorption by contacts is lacking. By revisiting series of experimental results, we find that the corrected tausf are longer and less dispersed, which leads to a more unified picture of tausf derived from experiments. We also discuss how the correction depends on the parameters of the graphene and contacts.
-http://prl.aps.org/abstract/PRL/v108/i15/e155501
+http://xxx.lanl.gov/abs/1411.2949
 ----
-'''Direct observation of nuclear field fluctuations in single quantum dots'''
+'''Detecting nonlocal Cooper pair entanglement by optical Bell inequality violation'''
-R. Kaji, S. Adachi, H. Sasakura, and S. Muto
+''Simon E. Nigg, Rakesh P. Tiwari, Stefan Walter, Thomas L. Schmidt''
-The spin interaction between an electron and nuclei was investigated optically in a single self-assembled InAlAs quantum dot (QD). In spin dynamics at the initial stage, the fluctuation of nuclear field and the resulting electron spin relaxation time play a crucial role. We examined a positively charged exciton in a QD to evaluate the key physical quantities directly via the temporal evolution measurements of the Overhauser shift and the degree of circular polarization. In addition, the validity of our used spin dynamics model was discussed in the context of the experimentally obtained key parameters.
+Based on the Bardeen Cooper Schrieffer (BCS) theory of superconductivity, the coherent splitting of Cooper pairs from a superconductor to two spatially separated quantum dots has been predicted to generate nonlocal pairs of entangled electrons. In order to test this hypothesis, we propose a scheme to transfer the spin state of a split Cooper pair onto the polarization state of a pair of optical photons. We show that the produced photon pairs can be used to violate a Bell inequality, unambiguously demonstrating the entanglement of the split Cooper pairs.
-http://prb.aps.org/abstract/PRB/v85/i15/e155315
+http://arxiv.org/abs/1411.3945
 ----
-'''Lagrange formalism of memory circuit elements: Classical and quantum formulations'''
+'''Current noise cross correlation mediated by Majorana bound states'''
-Guy Z. Cohen, Yuriy V. Pershin, and Massimiliano Di Ventra
+''Hai-Feng Lu, Hai-Zhou Lu, Shun-Qing Shen''
-The general Lagrange-Euler formalism for the three memory circuit elements, namely, memristive, memcapacitive, and meminductive systems, is introduced. In addition, mutual meminductance, i.e., mutual inductance with a state depending on the past evolution of the system, is defined. The Lagrange-Euler formalism for a general circuit network, the related work-energy theorem, and the generalized Joule's first law are also obtained. Examples of this formalism applied to specific circuits are provided, and the corresponding Hamiltonian and its quantization for the case of nondissipative elements are discussed. The notion of memory quanta, the quantum excitations of the memory degrees of freedom, is presented. Specific examples are used to show that the coupling between these quanta and the well-known charge quanta can lead to a splitting of degenerate levels and to other experimentally observable quantum effects.
+We study the transport properties of a quantum dot-Majorana hybrid system, in which each of paired Majorana bound states is connected to one quantum dot. With the help of non-equilibrium Green's function method, we obtain an exact solution of the Green's functions and calculate the currents through the quantum dots and nonlocal noise cross correlation between the currents. As a function of dot energy levels ϵ1 and ϵ2, we find that for the symmetric level configuration ϵ1=ϵ2, the noise cross correlation is negative in the low lead voltage regime, while it becomes positive with the increase of the lead voltages. Due to the particle-hole symmetry, the cross correlation is always positive in the anti-symmetric case ϵ1=−ϵ2. In contrast, the cross correlation of non-Majorana setups is always positive. For comparison, we also perform the diagonalized master equation calculation to check its applicability. It is found that the diagonalized master equations work well in most regimes of system parameters. Nevertheless, it shows an obvious deviation from the exact solution by the non-equilibrium Green's function method when all eigenenergies of the dot-Majorana hybrid system and simultaneously the energy intervals are comparable to the dot-lead coupling strength.
-http://prb.aps.org/abstract/PRB/v85/i16/e165428
+http://arxiv.org/abs/1411.4260
 ----
-'''Optical detection of single non-absorbing molecules using the surface plasmon resonance of a gold nanorod'''
+'''Detecting bit-flip errors in a logical qubit using stabilizer measurements'''
-Peter Zijlstra, Pedro M. R. Paulo and Michel Orrit
+''D. Ristè, S. Poletto, M.-Z. Huang, A. Bruno, V. Vesterinen, O.-P. Saira, L. DiCarlo''
-Existing methods for the optical detection of single molecules require the molecules to absorb light to produce fluorescence or direct absorption signals. This limits the range of species that can be detected, because most molecules are purely refractive. Metal nanoparticles or dielectric resonators can be used to detect non-absorbing molecules because local changes in the refractive index produce a resonance shift. However, current approaches only detect single molecules when the resonance shift is amplified by a highly polarizable label, or by a localized precipitation reaction on the surface of a nanoparticle. Without such amplification, single-molecule events can only be identified in a statistical way. Here, we report the plasmonic detection of single molecules in real time without the need for labelling or amplification. Our sensor consists of a single gold nanorod coated with biotin receptors, and the binding of single proteins is detected by monitoring the plasmon resonance of the nanorod with a sensitive photothermal assay. The sensitivity of our device is ~700 times higher than state-of-the-art plasmon sensors and is intrinsically limited by spectral diffusion of the surface plasmon resonance.
+Quantum data is susceptible to decoherence induced by the environment and to errors in the hardware processing it. A future fault-tolerant quantum computer will use quantum error correction (QEC) to actively protect against both. In the smallest QEC codes, the information in one logical qubit is encoded in a two-dimensional subspace of a larger Hilbert space of multiple physical qubits. For each code, a set of non-demolition multi-qubit measurements, termed stabilizers, can discretize and signal physical qubit errors without collapsing the encoded information. Experimental demonstrations of QEC to date, using nuclear magnetic resonance, trapped ions, photons, superconducting qubits, and NV centers in diamond, have circumvented stabilizers at the cost of decoding at the end of a QEC cycle. This decoding leaves the quantum information vulnerable to physical qubit errors until re-encoding, violating a basic requirement for fault tolerance. Using a five-qubit superconducting processor, we realize the two parity measurements comprising the stabilizers of the three-qubit repetition code protecting one logical qubit from physical bit-flip errors. We construct these stabilizers as parallelized indirect measurements using ancillary qubits, and evidence their non-demolition character by generating three-qubit entanglement from superposition states. We demonstrate stabilizer-based quantum error detection (QED) by subjecting a logical qubit to coherent and incoherent bit-flip errors on its constituent physical qubits. While increased physical qubit coherence times and shorter QED blocks are required to actively safeguard quantum information, this demonstration is a critical step toward larger codes based on multiple parity measurements.
-http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2012.51.html
+http://arxiv.org/abs/1411.5542
 ----
-'''Scalable Fabrication of Self-Aligned Graphene Transistors and Circuits on Glass'''
+'''In-situ Raman Spectroscopy of the Graphene / Water Interface of a Solution-Gated Field Effect Transistor: Electron-Phonon Coupling and Spectroelectrochemistry'''
-Lei Liao, Jingwei Bai, Rui Cheng, Hailong Zhou, Lixin Liu, Yuan Liu, Yu Huang, and Xiangfeng Duan
+''J. Binder, J. M. Urban, R. Stepniewski, W. Strupinski, A. Wysmolek''
-Graphene transistors are of considerable interest for radio frequency (rf) applications. High-frequency graphene transistors with the intrinsic cutoff frequency up to 300 GHz have been demonstrated. However, the graphene transistors reported to date only exhibit a limited extrinsic cutoff frequency up to about 10 GHz, and functional graphene circuits demonstrated so far can merely operate in the tens of megahertz regime, far from the potential the graphene transistors could offer. Here we report a scalable approach to fabricate self-aligned graphene transistors with the extrinsic cutoff frequency exceeding 50 GHz and graphene circuits that can operate in the 1–10 GHz regime. The devices are fabricated on a glass substrate through a self-aligned process by using chemical vapor deposition (CVD) grown graphene and a dielectrophoretic assembled nanowire gate array. The self-aligned process allows the achievement of unprecedented performance in CVD graphene transistors with a highest transconductance of 0.36 mS/μm. The use of an insulating substrate minimizes the parasitic capacitance and has therefore enabled graphene transistors with a record-high extrinsic cutoff frequency (> 50 GHz) achieved to date. The excellent extrinsic cutoff frequency readily allows configuring the graphene transistors into frequency doubling or mixing circuits functioning in the 1–10 GHz regime, a significant advancement over previous reports (20 MHz). The studies open a pathway to scalable fabrication of high-speed graphene transistors and functional circuits and represent a significant step forward to graphene based radio frequency devices.
+We present a novel measurement approach which combines aqueous solution-gated field effect transistors based on epitaxial bilayer graphene on 4H-SiC (0001) with simultaneous Raman spectroscopy. Since SiC is transparent in the visible wavelength range, we took advantage of the fact that one can measure graphene through the substrate. By sweeping the gate voltage, we observed Raman signatures related to the resonant electron-phonon coupling. The positions of these features allowed the extraction of the geometrical capacitance of the system and thus facilitated the accurate calculation of the Fermi levels for bilayer graphene. An intentional application of higher gate voltages allowed us to trigger electrochemical reactions, which we followed in-situ with Raman spectroscopy. The reactions showed a partially reversible character, which was monitored by an emergence / disappearance of peaks assigned to C-H and Si-H vibration modes as well as an increase / decrease of the defect-related Raman D band intensity. Our setup provides chemically specific information and electrical control by using a graphene solution-gated field-effect transistor and constitutes a highly interesting platform for future spectroelectrochemical research on electrically induced sorption processes of graphene in the micrometer scale.
-http://pubs.acs.org/doi/abs/10.1021/nl201922c
+http://arxiv.org/abs/1411.4837
 ----
-'''High-Resolution EM of Colloidal Nanocrystal Growth Using Graphene Liquid Cells'''
+'''Study of dynamical spin injection at ferromagnet-graphene interfaces'''
-Jong Min Yuk, Jungwon Park, Peter Ercius, Kwanpyo Kim, Daniel J. Hellebusch, Michael F. Crommie, Jeong Yong Lee, A. Zettl, A. Paul Alivisatos
+''S. Singh, A. Ahmadi, C.T. Cherian, E. R. Mucciolo, E. del Barco, B. Özyilmaz''
-We introduce a new type of liquid cell for in situ transmission electron microscopy (TEM) based on entrapment of a liquid film between layers of graphene. The graphene liquid cell facilitates atomic-level resolution imaging while sustaining the most realistic liquid conditions achievable under electron-beam radiation. We employ this cell to explore the mechanism of colloidal platinum nanocrystal growth. Direct atomic-resolution imaging allows us to visualize critical steps in the process, including site-selective coalescence, structural reshaping after coalescence, and surface faceting.
+We present a study of dynamical spin injection from a three-dimensional ferromagnet into two-dimensional single-layer graphene. Comparative ferromagnetic resonance (FMR) studies of ferromagnet/graphene strips buried underneath the central line of a coplanar waveguide show that the FMR linewidth broadening is the largest when the graphene layer protrudes laterally away from the ferromagnetic strip, indicating that the spin current is injected into the graphene areas away from the area directly underneath the ferromagnet being excited. Our results confirm that the observed damping is indeed a signature of dynamical spin injection, wherein a pure spin current is pumped into the single-layer graphene from the precessing magnetization of the ferromagnet. The observed spin pumping efficiency is difficult to reconcile with the expected backflow of spins according to the standard spin pumping theory and the characteristics of graphene, and constitutes an enigma for spin pumping in two-dimensional structures.
-http://www.sciencemag.org/content/336/6077/61.full
+http://arxiv.org/abs/1411.5339
 ----
-'''Fundamental temperature-dependent properties of the Si nanocrystal band gap'''
+'''Inverted singlet-triplet qubit coded on a two-electron double quantum dot'''
-A. M. Hartel, S. Gutsch, D. Hiller, and M. Zacharias
+''Sebastian Mehl and David P. DiVincenzo''
-Ever since the first reports about low-temperature photoluminescence (PL) measurements of Si nanocrystals Si (NCs), a severe deviation from the equations commonly used to describe the temperature dependence of the band-gap energy for bulk materials was reported. Our analysis reveals that the formerly observed deviation is solely attributed to too high excitation power densities that cause a preferential emission enhancement of the smaller NCs within a size distribution, due to the temperature dependence of the radiative exciton lifetime. We report on the successful fit of the temperature-dependent band-gap energy of quantum-confined silicon. By means of four size-controlled Si NC samples (1.5 to 4.5 nm), we are able to prove the validity of these equations down to liquid He temperatures, if sufficiently low excitation power densities (<500 μW/cm2) are used. Thereby, it is shown that the characteristics of the band-gap widening of quantum-confined nanocrystalline Si obeys to the same physical law as the bulk Si crystal. In addition, the previously observed decrease of the PL intensity for decreasing temperatures is demonstrated to have its origin in the same measurement artifact caused by excitation saturation.
+The sz=0 spin configuration of two electrons confined at a double quantum dot (DQD) encodes the singlet-triplet qubit (STQ). We introduce the inverted STQ (ISTQ) that emerges from the setup of two quantum dots (QDs) differing significantly in size and out-of-plane magnetic fields. The strongly confined QD has a two-electron singlet ground state, but the weakly confined QD has a two-electron triplet ground state in the sz=0 subspace. Spin-orbit interactions act nontrivially on the sz=0 subspace and provide universal control of the ISTQ together with electrostatic manipulations of the charge configuration. GaAs and InAs DQDs can be operated as ISTQs under realistic noise conditions.
-http://prb.aps.org/abstract/PRB/v85/i16/e165306
+http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.195424
 ----
-'''Injection and detection of spin in a semiconductor by tunneling via interface states'''
+'''Majorana fermions in Ge/Si hole nanowires'''
-R. Jansen, A. M. Deac, H. Saito, and S. Yuasa
+''Franziska Maier, Jelena Klinovaja, and Daniel Loss''
-Injection and detection of spin accumulation in a semiconductor having localized states at the interface is evaluated. Spin transport from a ferromagnetic contact by sequential, two-step tunneling via interface states is treated not in itself, but in parallel with direct tunneling. The spin accumulation Δμch induced in the semiconductor channel is not suppressed, as previously argued, but genuinely enhanced by the additional spin current via interface states. Spin detection with a ferromagnetic contact yields a weighted average of Δμch and the spin accumulation Δμls in the localized states. In the regime where Δμls/Δμch is largest, the detected spin signal is insensitive to Δμls and the ferromagnet probes the spin accumulation in the semiconductor channel.
+We consider Ge/Si core/shell nanowires with hole states coupled to an s-wave superconductor in the presence of electric and magnetic fields. We employ a microscopic model that takes into account material-specific details of the band structure such as strong and electrically tunable Rashba-type spin-orbit interaction and g factor anisotropy for the holes. In addition, the proximity-induced superconductivity Hamiltonian is derived starting from a microscopic model. In the topological phase, the nanowires host Majorana fermions with localization lengths that depend strongly on both the magnetic and electric fields. We identify the optimal regime in terms of the directions and magnitudes of the fields in which the Majorana fermions are the most localized at the nanowire ends. In short nanowires, the Majorana fermions hybridize and form a subgap fermion whose energy is split away from zero and oscillates as a function of the applied fields. The period of these oscillations could be used to measure the dependence of the spin-orbit interaction on the applied electric field and the g factor anisotropy.
-http://prb.aps.org/abstract/PRB/v85/i13/e134420
+http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.195421
-== Márc. 30. - Ápr. 5. (2012) ==
-''Válogatta: Scherübl Zoltán''
 ----
-'''Visualizing Electrical Breakdown and ON/OFF States in Electrically Switchable Suspended Graphene Break Junctions'''
+'''Revealing Topological Superconductivity in Extended Quantum Spin Hall Josephson Junctions'''
-Hang Zhang†, Wenzhong Bao†, Zeng Zhao†, Jhao-Wun Huang†, Brian Standley‡, Gang Liu†, Fenglin Wang†, Philip Kratz†, Lei Jing†, Marc Bockrath*†‡, and Chun Ning Lau*†
+''Shu-Ping Lee, Karen Michaeli, Jason Alicea, and Amir Yacoby''
-Narrow gaps are formed in suspended single- to few-layer graphene devices using a pulsed electrical breakdown technique. The conductance of the resulting devices can be programmed by the application of voltage pulses, with voltages of 2.5 to 4.5 V, corresponding to an ON pulse, and 8 V, corresponding to an OFF pulse. Electron microscope imaging of the devices shows that the graphene sheets typically remain suspended and that the device conductance tends to zero when the observed gap is large. The switching rate is strongly temperature dependent, which rules out a purely electromechanical switching mechanism. This observed switching in suspended graphene devices strongly suggests a switching mechanism via atomic movement and/or chemical rearrangement and underscores the potential of all-carbon devices for integration with graphene electronics.
+Quantum spin Hall–superconductor hybrids are promising sources of topological superconductivity and Majorana modes, particularly given recent progress on HgTe and InAs/GaSb. We propose a new method of revealing topological superconductivity in extended quantum spin Hall Josephson junctions supporting “fractional Josephson currents.” Specifically, we show that as one threads magnetic flux between the superconductors, the critical current traces an interference pattern featuring sharp fingerprints of topological superconductivity—even when noise spoils parity conservation.
-http://pubs.acs.org/doi/abs/10.1021/nl203160x
+http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.197001
 ----
-'''Detection of Vibration-Mode Scattering in Electronic Shot Noise'''
+'''Axially Tunable Carbon Nanotube Resonators Using Co-integrated Microactuators'''
-Manohar Kumar1, Rémi Avriller2,3,4, Alfredo Levy Yeyati2, and Jan M. van Ruitenbeek1
+''Stuart Truax , Shih-Wei Lee , Matthias Muoth , and Christofer Hierold *''
-We present shot noise measurements on Au nanowires showing very pronounced vibration-mode features. In accordance to recent theoretical predictions the sign of the inelastic signal, i.e., the signal due to vibration excitations, depends on the transmission probability becoming negative below a certain transmission value. We argue that the negative contribution to noise arises from coherent two-electron processes mediated by electron-phonon scattering and the Pauli exclusion principle. These signals can provide unique information on the local phonon population and lattice temperature of the nanoscale system.
+Tuning of the mechanical resonance frequency of single-walled carbon nanotubes (SWCNTs) is achieved by application of uniaxial strain by purely mechanical means, utilizing both directly grown and dry-transferred SWCNTs. The induction of a beam-to-string transition is achieved, resulting in an axial tension sensitivity of 9.4 × 1010 Hz/ε in the vibrating string regime. Increases in the resonant Q-factor, removal of residual slack, and resonance frequency changes from 10 to 60 MHz are affected.
-http://prl.aps.org/abstract/PRL/v108/i14/e146602
+http://pubs.acs.org/doi/abs/10.1021/nl501853w
 ----
-'''Pressure Tuning of the Optical Properties of GaAs Nanowires'''
+'''Giant Magnetoconductance Oscillations in Hybrid Superconductor−Semiconductor Core/Shell Nanowire Devices'''
-Ilaria Zardo†‡*, Sara Yazji†, Carlo Marini§, Emanuele Uccelli†, Anna Fontcuberta i Morral, Gerhard Abstreiter†‡, and Paolo Postorino§
+''Ö. Gül †, H. Y. Günel *†‡, H. Lüth †, T. Rieger †, T. Wenz †, F. Haas †, M. Lepsa †, G. Panaitov §, D. Grützmacher †, and Th. Schäpers *†''
-The tuning of the optical and electronic properties of semiconductor nanowires can be achieved by crystal phase engineering. Zinc-blende and diamond semiconductors exhibit pressure-induced structural transitions as well as a strong pressure dependence of the band gaps. When reduced to nanoscale dimensions, new phenomena may appear. We demonstrate the tuning of the optical properties of GaAs nanowires and the induction of a phase transition by applying an external pressure. The dependence of the E0 gap on the applied pressure was measured, and a direct-to-indirect transition was found. Resonant Raman scattering was obtained by pressure tuning of the E0 and the E0 + ΔSO gaps with respect to the excitation energy. The resonances of the longitudinal optical modes LO and 2LO indicate the presence of electron–phonon Fröhlich interactions. These measurements show for the first time a variation of ionicity in GaAs when in nanowire form. Furthermore, the dependence of the lattice constant on applied pressure was estimated. Finally, we found a clear indication of a structural transition above 16 GPa.
+The magnetotransport of GaAs/InAs core/shell nanowires contacted by two superconducting Nb electrodes is investigated, where the InAs shell forms a tube-like conductive channel around the highly resistive GaAs core. By applying a magnetic field along the nanowire axis, regular magnetoconductance oscillations with an amplitude in the order of e2/h are observed. The oscillation amplitude is found to be larger by 2 orders of magnitude compared to the measurements of a reference sample with normal metal contacts. For the Nb-contacted core/shell nanowire the oscillation period corresponds to half a flux quantum Φ0/2 = h/2e in contrast to the period of Φ0 of the reference sample. The strongly enhanced magnetoconductance oscillations are explained by phase-coherent resonant Andreev reflections at the Nb-core/shell nanowire interface.
-http://pubs.acs.org/doi/abs/10.1021/nn300228u
+http://pubs.acs.org/doi/abs/10.1021/nl502598s
 ----
+'''Quantum Noise and Asymmetric Scattering of Electrons and Holes in Graphene'''
+''Atikur Rahman , Janice Wynn Guikema , and Nina Marković *''
-'''Transport gap in suspended bilayer graphene at zero magnetic field'''
+We present measurements of quantum interference noise in double-gated single layer graphene devices at low temperatures. The noise characteristics show a nonmonotonic dependence on carrier density, which is related to the interplay between charge inhomogeneity and different scattering mechanisms. Linearly increasing 1/f noise at low carrier densities coincides with the observation of weak localization, suggesting the importance of short-range disorder in this regime. Using perpendicular and parallel p–n junctions, we find that the observed asymmetry of the noise with respect to the Dirac point can be related to asymmetric scattering of electrons and holes on the disorder potential.
-A. Veligura1,*, H. J. van Elferen2, N. Tombros1, J. C. Maan2, U. Zeitler2, and B. J. van Wees1
+http://pubs.acs.org/doi/abs/10.1021/nl503276s
-We report a change of three orders of magnitude in the resistance of a suspended bilayer graphene flake which varies from a few kΩ in the high-carrier-density regime to several MΩ around the charge neutrality point (CNP). The corresponding transport gap is 8 meV at 0.3 K. The sequence of quantum Hall plateaus appearing at filling factor ν=2 followed by ν=1 suggests that the observed gap is caused by the symmetry breaking of the lowest Landau level. Investigation of the gap in a tilted magnetic fields indicates that the resistance at the CNP shows a weak linear decrease for increasing total magnetic field. Those observations are in agreement with a spontaneous valley splitting at zero magnetic field followed by splitting of the spins originating from different valleys with increasing magnetic field. Both the transport gap and B field response point toward the spin-polarized layer-antiferromagnetic state as the ground state in the bilayer graphene sample. The observed nontrivial dependence of the gap value on the normal component of B suggests possible exchange mechanisms in the system.
-http://prb.aps.org/abstract/PRB/v85/i15/e155412
 ----
-'''Intertwining of Zeeman and Coulomb interactions on excitons in highly symmetric semiconductor quantum dots'''
+'''Multimode Silicon Nanowire Transistors'''
-D. Y. Oberli
+''Sebastian Glassner †, Clemens Zeiner †, Priyanka Periwal ‡§, Thierry Baron ‡§, Emmerich Bertagnolli †, and Alois Lugstein *†''
-We present an experimental study and develop a group theoretical analysis of the Zeeman effect on excitons in pyramidal semiconductor quantum dots possessing the symmetries of the C3v point group. The magnetic field dependence of the emission pattern originating from neutral exciton states is investigated in both the Faraday and Voigt configurations. The Zeeman doublet splitting of the “bright” exciton states varies linearly with the magnetic field strength in each configuration while the intensity of the “dark” exciton transitions exhibit a nonlinear dependence. We demonstrate that these observations originate from the intertwining of the Zeeman and Coulomb interactions, which provides clear spectral signatures of this effect for highly symmetric quantum dots. We uncover a large anisotropy of the Zeeman doublet splittings for longitudinal and transverse magnetic fields, revealing the ubiquitous role of a symmetry elevation in our pyramidal quantum dots. These results suggest that the common description of the Zeeman effect based on effective g factors for electrons and holes must be revised when dealing with exciton complexes.
+The combined capabilities of both a nonplanar design and nonconventional carrier injection mechanisms are subject to recent scientific investigations to overcome the limitations of silicon metal oxide semiconductor field effect transistors. In this Letter, we present a multimode field effect transistors device using silicon nanowires that feature an axial n-type/intrinsic doping junction. A heterostructural device design is achieved by employing a self-aligned nickel-silicide source contact. The polymorph operation of the dual-gate device enabling the configuration of one p- and two n-type transistor modes is demonstrated. Not only the type but also the carrier injection mode can be altered by appropriate biasing of the two gate terminals or by inverting the drain bias. With a combined band-to-band and Schottky tunneling mechanism, in p-type mode a subthreshold swing as low as 143 mV/dec and an ON/OFF ratio of up to 104 is found. As the device operates in forward bias, a nonconventional tunneling transistor is realized, enabling an effective suppression of ambipolarity. Depending on the drain bias, two different n-type modes are distinguishable. The carrier injection is dominated by thermionic emission in forward bias with a maximum ON/OFF ratio of up to 107 whereas in reverse bias a Schottky tunneling mechanism dominates the carrier transport.
-http://prb.aps.org/abstract/PRB/v85/i15/e155305
+http://pubs.acs.org/doi/abs/10.1021/nl503476t
 ----
-'''Fast Hybrid Silicon Double-Quantum-Dot Qubit'''
+'''Realization of the Meminductor'''
-Zhan Shi1, C. B. Simmons1, J. R. Prance1, John King Gamble1, Teck Seng Koh1, Yun-Pil Shim1, Xuedong Hu2, D. E. Savage1, M. G. Lagally1, M. A. Eriksson1, Mark Friesen1, and S. N. Coppersmith1
+''Jiahao Han , Cheng Song *, Shuang Gao , Yuyan Wang , Chao Chen , and Feng Pan''
-We propose a quantum dot qubit architecture that has an attractive combination of speed and fabrication simplicity. It consists of a double quantum dot with one electron in one dot and two electrons in the other. The qubit itself is a set of two states with total spin quantum numbers S2=3/4 (S=1/2) and Sz=-1/2, with the two different states being singlet and triplet in the doubly occupied dot. Gate operations can be implemented electrically and the qubit is highly tunable, enabling fast implementation of one- and two-qubit gates in a simpler geometry and with fewer operations than in other proposed quantum dot qubit architectures with fast operations. Moreover, the system has potentially long decoherence times. These are all extremely attractive properties for use in quantum information processing devices.
+The meminductor was proposed to be a fundamental circuit memdevice parallel with the memristor, linking magnetic flux and current. However, a clear material model or experimental realization of a meminductor has been challenging. Here we demonstrate pinched hysteretic magnetic flux–current signals at room temperature based on the spin Hall magnetoresistance effect in several-nanometer-thick thin films, exhibiting the nonvolatile memorizing property and magnetic energy storage ability of the meminductor. Similar to the parameters of the capacitor, resistor, and inductor, meminductance, LM, is introduced to characterize the capability of the prepared meminductor. Our findings present an indispensable element of memdevices and open an avenue for nanoscale meminductor design and manufacture, which might contribute to low-power electronic circuits, information storage, and artificial intelligence.
-http://prl.aps.org/abstract/PRL/v108/i14/e140503
+http://pubs.acs.org/doi/abs/10.1021/nn502655u
 ----
-'''Spin Polarization of the 12/5 Fractional Quantum Hall Effect'''
+== Szept.19-30.==
+''Márton Attila''
-Chi Zhang, Chao Huan, J. S. Xia, N. S. Sullivan, W. Pan, K.W. Baldwin, K. W. West, L. N. Pfeiffer, D. C. Tsuiű
+Előadás: SNS junctions in nanowires with spin-orbit coupling: role of confinement and helicity on the sub-gap spectrum (http://arxiv.org/abs/1410.6074) [[:file:Jc_141028_MA.pptx |pptx (pptx)]]'''
-We have carried out tilt magnetic field (B) studies of the \nu=12/5 fractional quantum Hall state in an ultra-high quality GaAs quantum well specimen. Its diagonal magneto-resistance Rxx shows a non-monotonic dependence on tilt angle (\theta). It first increases sharply with increasing \theta, reaches a maximal value of ~ 70 ohms at \theta ~ 14^o, and then decreases at higher tilt angles. Correlated with this dependence of Rxx on \theta, the 12/5 activation energy (\Delta_{12/5}) also shows a non-monotonic tilt dependence. \Delta_{12/5} first decreases with increasing \theta. Around \theta = 14^{o}, \Delta_{12/5} disappears as Rxx becomes non-activated. With further increasing tilt angles, \Delta_{12/5} reemerges and increases with \theta. This tilt B dependence at \nu=12/5 is strikingly different from that of the well-documented 5/2 state and calls for more investigations on the nature of its ground state.
+'''Photons made to dance together
-http://arxiv.org/abs/1204.0560
+Physicists have made two beams of light interact at the level of individual photons.
-----
+Getting photons to interact is important for all-optical computation and for producing new quantum states of light. Kristin Beck at the Massachusetts Institute of Technology in Cambridge and her colleagues crossed two beams of light inside a cavity filled with trapped and cooled caesium atoms. When photons from each beam tried to pass through the system at the same time, the trapped atoms changed their internal state, allowing only one photon to be transmitted, while the other one was reflected or scattered.
-'''Transport Properties of Graphene Nanoribbon Transistors on Transport Properties of Graphene Nanoribbon Transistors on Chemical-Vapor-Deposition Grown Wafer-Scale Graphene'''
+The interaction creates two entangled beams of light, which the authors say could eventually be used to improve the accuracy of measurements, such as of a gyroscope's rotation, that would otherwise be limited by the laws of quantum mechanics.
-Wan Sik Hwang, Kristof Tahy, Xuesong Li, Huili (Grace) Xing, Alan C. Seabaugh, Chun-Yung Sung, Debdeep Jena
+http://www.nature.com/nature/journal/v513/n7519/full/513463a.html
-Graphene nanoribbon (GNR) field-effect transistors (FETs) with widths down to 12 nm have been fabricated by electron beam lithography using a wafer-scale chemical vapor deposition (CVD) process to form the graphene. The GNR FETs show drain-current modulation of approximately 10 at 300 K, increasing to nearly 106 at 4 K. The strong temperature dependence of the minimum current indicates the opening of a bandgap for CVD-grown GNR-FETs. The extracted bandgap is estimated to be around 0.1 eV by differential conductance methods. This work highlights the development of CVD-grown large-area graphene and demonstrates the opening of a bandgap in nanoribbon transistors.
+-----
-http://arxiv.org/abs/1204.0499
+'''Quantum bits get their first compression
-----
+Without algorithms that compress data to encode information into fewer bits, hard drives would clog up and Internet traffic would slow to a snail's pace. Now, a group of physicists in Canada has shown for the first time that it is possible to compress the kind of data that might be used in the computers of tomorrow — known as quantum bits, or qubits.
-'''Epitaxial Growth of a Silicene Sheet'''
-Boubekeur Lalmi, Hamid Oughaddoub, Hanna Enriquezb, Abdelkader Karae, Sébastien Vizzini, Bénidicte Ealet, Bernard Aufray
+http://www.nature.com/news/quantum-bits-get-their-first-compression-1.15961
-Using atomic resolved scanning tunneling microscopy, we present here the experimental evidence of a silicene sheet (graphene like structure) epitaxially grown on a close-packed silver surface (Ag(111)). This has been achieved via direct condensation of a silicon atomic flux onto the single-crystal substrate in ultra-high vacuum conditions. A highly ordered silicon structure, arranged within a honeycomb lattice is synthesized and presenting two silicon sub-lattices occupying positions at different heights (0.02 nm) indicating possible sp2-sp3 hybridizations.
+-----
-http://arxiv.org/abs/1204.0523
+'''Transmission Phase in the Kondo Regime Revealed in a Two-Path Interferometer
-----
+''S. Takada, C. Bäuerle, M. Yamamoto, K. Watanabe, S. Hermelin, T. Meunier, A. Alex, A. Weichselbaum, J. von Delft, A. Ludwig, A. D. Wieck, and S. Tarucha
-'''Electronic and transport properties of azobenzene monolayer junctions as molecular switches'''
+We report on the direct observation of the transmission phase shift through a Kondo correlated quantum dot by employing a new type of two-path interferometer. We observed a clear π/2-phase shift, which persists up to the Kondo temperature TK. Above this temperature, the phase shifts by more than π/2 at each Coulomb peak, approaching the behavior observed for the standard Coulomb blockade regime. These observations are in remarkable agreement with two-level numerical renormalization group calculations. The unique combination of experimental and theoretical results presented here fully elucidates the phase evolution in the Kondo regime.
-Yan Wang, Hai-Ping Cheng
+http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.126601
-We investigate from first-principles the change in transport properties of a two-dimensional azobenzene monolayer sandwiched between two Au electrodes that undergoes molecular switching. We focus on transport differences between a chemisorbed and physisorbed top monolayer-electrode contact. The conductance of the monolayer junction with a chemisorbed top contact is higher in \textit{trans} configuration, in agreement with the previous theoretical predictions of one-dimensional single molecule junctions. However, with a physisorbed top contact, the "ON" state with larger conductance is associated with the \textit{cis} configuration due to a reduced effective tunneling pathway by switching from \textit{trans} to \textit{cis}, which successfully explains recently experimental measurements of azobenzene monolayer junctions. A simple model is developed to explain electron transmission across subsystems in the molecular junction. We also discuss the effects of monolayer packing density, molecule tilt angle, and contact geometry on the calculated transmission functions. In particular, we find that a tip-like contact with chemisorption significantly affects the electric current through the \textit{cis} monolayer, leading to highly asymmetric current-voltage characteristics as well as large negative differential resistance behavior.
+-----
-http://arxiv.org/abs/1203.6862
+'''Entanglement Entropy in Fermi Gases and Anderson’s Orthogonality Catastrophe
-----
+''A. Ossipov
-'''Quantized Charge Pumping through a Carbon Nanotube Double Quantum Dot'''
+We study the ground-state entanglement entropy of a finite subsystem of size L of an infinite system of noninteracting fermions scattered by a potential of finite range a. We derive a general relation between the scattering matrix and the overlap matrix and use it to prove that for a one-dimensional symmetric potential the von Neumann entropy, the Rényi entropies, and the full counting statistics are robust against potential scattering, provided that L/a≫1. The results of numerical calculations support the validity of this conclusion for a generic potential.
-S. J. Chorley, J. Frake, C. G. Smith, G. A. C. Jones, M. R. Buitelaar
+http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.130402
-We demonstrate single-electron pumping in a gate-defined carbon nanotube double quantum dot. By periodic modulation of the potentials of the two quantum dots we move the system around charge triple points and transport exactly one electron or hole per cycle. We investigate the pumping as a function of the modulation frequency and amplitude and observe good current quantization up to frequencies of 18 MHz where rectification effects cause the mechanism to break down.
+-----
-http://arxiv.org/abs/1204.1044
+'''Experimental realization of a Coulomb blockade refrigerator
-----
+''A. V. Feshchenko, J. V. Koski, J. P. Pekola
-'''Temperature dependence of dynamic nuclear polarization and its effect on electron spin relaxation and dephasing in InAs/GaAs quantum dots'''
+We present an experimental realization of a Coulomb blockade refrigerator (CBR) based on a single - electron transistor (SET). In the present structure, the SET island is interrupted by a superconducting inclusion to permit charge transport while preventing heat flow. At certain values of the bias and gate voltages, the current through the SET cools one of the junctions. The measurements follow theoretical model down to about 80 mK, which was the base temperature of the current measurements. The observed cooling increases rapidly with decreasing temperature in agreement with the theory, reaching about 15 mK drop at the base temperature. CBR appears as a promising electronic cooler at temperatures well below 100 mK.
-J. Beyer1, Y. Puttisong1, I. A. Buyanova1, S. Suraprapapich2, C. W. Tu2, and W. M. Chen1
+http://xxx.lanl.gov/abs/1409.5637
-Electron spin dephasing and relaxation due to hyperfine interaction with nuclear spins is studied in an InAs/GaAs quantum dot ensemble as a function of temperature up to 85 K, in an applied longitudinal magnetic field. The extent of hyperfine-induced dephasing is found to decrease, whereas dynamic nuclear polarization increases with increasing temperature. We attribute both effects to an accelerating electron spin relaxation through phonon-assisted electron-nuclear spin flip-flops driven by hyperfine interactions, which could become the dominating contribution to electron spin depolarization at high temperatures.
+-----
-http://apl.aip.org/resource/1/applab/v100/i14/p143105_s1?isAuthorized=no
+'''Klein-tunneling transistor with ballistic graphene
-----
+''Quentin Wilmart, Salim Berada, David Torrin, V. Hung Nguyen, Gwendal Fève, Jean-Marc Berroir, Philippe Dollfus, Bernard Plaçais
-'''Readout of carbon nanotube vibrations based on spin-phonon coupling '''
+Today the availability of high mobility graphene up to room temperature makes ballistic transport in nanodevices achievable. In particular, p-n-p transistor in the ballistic regime gives access to the Klein tunneling physics and allows the realization of devices exploiting the optics-like behavior of Dirac Fermions (DF) as in the Vesalego lens or the Fabry P\'erot cavity. Here we propose a Klein tunneling transistor based on geometrical optics of DF. We consider the case of a prismatic active region delimited by a triangular gate, where total internal reflection may occur, which leads to the tunable suppression of the transistor transmission. We calculate the transmission and the current by means of scattering theory and the finite bias properties using Non Equilibrium Green's Function(NEGF) simulation.
-C. Ohm1,2, C. Stampfer2,3,4, J. Splettstoesser1,2, and M. R. Wegewijs1,2,4
+http://xxx.lanl.gov/abs/1409.6170
-We propose a scheme for spin-based detection of the bending motion in suspended carbon-nanotubes, using the curvature-induced spin-orbit interaction. We show that the resulting effective spin-phonon coupling can be used to down-convert the high-frequency vibration-modulated spin-orbit field to spin-flip processes at a much lower frequency. This vibration-induced spin-resonance can be controlled with an axial magnetic field. We propose a Pauli spin blockade readout scheme and predict that the leakage current shows pronounced peaks as a function of the external magnetic field. Whereas the resonant peaks allow for frequency readout, the slightly off-resonant current is sensitive to the vibration amplitude.
+-----
-http://apl.aip.org/resource/1/applab/v100/i14/p143103_s1?isAuthorized=no
+'''Directly accessible entangling gates for capacitively coupled singlet-triplet qubits
-----
+''Fernando A. Calderon-Vargas, Jason P. Kestner
-'''Molecular motors: Myosin shifts into reverse gear'''
+The recent experimental advances in capacitively coupled singlet-triplet qubits, particularly the demonstration of entanglement, opens the question of what type of entangling gates the system's Hamiltonian can produce directly via a single square pulse. We address this question by considering the system's Hamiltonian from first principles and using the representation of its nonlocal properties in terms of local invariants. In the analysis we include the three different ways in which the system can be biased and their effect on the generation of entangling gates. We find that, in one of the possible biasing modes, the Hamiltonian has an especially simple form, which can directly generate a wide range of different entangling gates including the iSWAP gate. Moreover, using the complete form of the Hamiltonian we find that, for any biasing mode, a CNOT gate can be generated directly.
-Wilhelm J. Walter & Stefan Diez
+http://xxx.lanl.gov/abs/1409.6292
-A motor protein can be made to walk in either direction along a filamentous track by adjusting the concentration of calcium ions in the surrounding solution.
+-----
-http://www.nature.com/nnano/journal/v7/n4/full/nnano.2012.49.html
+'''Zeeman splitting spin filter in a single quantum dot electron transport with Coulomb blockade effect
-----
+''Wenxi Lai
-'''Tunable Spin–Orbit Interaction in Trilayer Graphene Exemplified in Electric-Double-Layer Transistors'''
+Electron spin filter induced by Zeeman splitting in a few-electron quantum dot coupled to two normal electrodes is studied considering Coulomb blockade effect. Based on the Anderson model and Liouville-von Neumann equation, equation of motion of the system is derived and analytical solutions are achieved. Transport windows for perfectly polarized current, partially polarized current and non-polarized current induced by the Zeeman splitting energy and Coulomb blockade potential are exploited. We will give the relations of voltage, magnetic field and temperature for high quality spin filtering.
-Zhuoyu Chen†, Hongtao Yuan*†, Yanfeng Zhang‡, Kentaro Nomura§, Teng Gao‡, Yabo Gao‡, Hidekazu Shimotani†, Zhongfan Liu‡, and Yoshihiro Iwasa*†§
+http://xxx.lanl.gov/abs/1409.6389
-Taking advantage of ultrahigh electric field generated in electric-double-layer transistors (EDLTs), we investigated spin–orbit interaction (SOI) and its modulation in epitaxial trilayer graphene. It was found in magnetotransport that the dephasing length L and spin relaxation length Lso of carriers can be effectively modulated with gate bias. As a direct result, SOI-induced weak antilocalization (WAL), together with a crossover from WAL to weak localization (WL), was observed at near-zero magnetic field. Interestingly, among existing localization models, only the Iordanskii–Lyanda-Geller–Pikus theory can successfully reproduce the obtained magnetoconductance well, serving as evidence for gate tuning of the weak but distinct SOI in graphene. Realization of SOI and its large tunability in the trilayer graphene EDLTs provides us with a possibility to electrically manipulate spin precession in graphene systems without ferromagnetics.
+-----
-http://pubs.acs.org/doi/abs/10.1021/nl204012c
+'''Majorana bound states without topological superconductivity
-----
+''Pablo San-Jose, Jorge Cayao, Elsa Prada, Ramón Aguado
-'''Raman sensitivity to crystal structure in InAs nanowires '''
+Recent experimental efforts towards the detection of Majorana bound states have focused on creating the conditions for topological superconductivity. Here we demonstrate an alternative route, which achieves fully localised zero-energy Majorana bound states when a topologically trivial superconductor is opened to a normal region. The emergence of Majorana states is a consequence of non-hermitian degeneracies of the resulting open quantum system, while arbitrarily large Majorana lifetimes follow from high junction transparency and helicity of the normal side. At these degeneracies, also known as `exceptional points', both the eigenvalues and the eigenstates coalesce, and acquire Majorana properties (zero-energy, self-conjugation, 4π-periodic braiding...) despite the trivial band topology. Exceptional Majoranas are thus the open-system counterparts of conventional Majorana bound states, to which they are continuously connected, and exhibit all their phenomenology while not requiring topological superconductivity.
-Jaya Kumar Panda1, Anushree Roy1, Achintya Singha2, Mauro Gemmi3, Daniele Ercolani4, Vittorio Pellegrini4, and Lucia Sorba4
+http://xxx.lanl.gov/abs/1409.7306
-We report electron transmission and Raman spectroscopy study of InAs nanowires. We demonstrate that the temperature dependent behavior of optical phonon energies can be used to determine the relative wurtzite fraction in the InAs nanowires. Furthermore, we propose that the interfacial strain between zincblende and wurtzite phases along the length of the wires manifests in the temperature-evolution of the phonon linewidths. From these studies, temperature-dependent Raman measurements emerge as a non-invasive method to study polytypism in such nanowires.
+-----
-http://apl.aip.org/resource/1/applab/v100/i14/p143101_s1?isAuthorized=no
+'''Majorana Fermions in Ge/Si Hole Nanowires
-----
+''Franziska Maier, Jelena Klinovaja, Daniel Loss
-'''Theory of ferromagnetic unconventional superconductors with spin-triplet electron pairing'''
+We consider Ge/Si core/shell nanowires with hole states coupled to an s-wave superconductor in the presence of electric and magnetic fields. We employ a microscopic model that takes into account material-specific details of the band structure such as strong and electrically tunable Rashba-type spin-orbit interaction and g factor anisotropy for the holes. In addition, the proximity-induced superconductivity Hamiltonian is derived starting from a microscopic model. In the topological phase, the nanowires host Majorana fermions with localization lengths that depend strongly on both the magnetic and electric fields. We identify the optimal regime in terms of the directions and magnitudes of the fields in which the Majorana fermions are the most localized at the nanowire ends. In short nanowires, the Majorana fermions hybridize and form a subgap fermion whose energy is split away from zero and oscillates as a function of the applied fields. The period of these oscillations could be used to measure the dependence of the spin-orbit interaction on the applied electric field and the g factor anisotropy.
-Dimo I. Uzunov
+http://xxx.lanl.gov/abs/1409.8645
-A general phenomenological theory is presented for the phase behavior of ferromagnetic superconductors with spin-triplet electron Cooper pairing. The theory describes in details the temperature-pressure phase diagrams of real inter-metallic compounds exhibiting the remarkable phenomenon of coexistence of spontaneous magnetic moment of the itinerant electrons and spin-triplet superconductivity. The quantum phase transitions which may occur in these systems are also described. The theory allows for a classification of these itinerant ferromagnetic superconductors in two types: type I and type II. The classification is based on quantitative criteria.The comparison of theory and experiment is performed and outstanding problems are discussed.
+== Szept.13-19.==
+''Tóvári Endre''
-http://arxiv.org/abs/1204.1007
+'''Crossover from Josephson Effect to Single Interface Andreev Reflection in Asymmetric Superconductor/Nanowire Junctions'''
-== Márc. 1. - Márc. 22. (2012) ==
+'H. Y. Günel, N. Borgwardt, I. E. Batov, H. Hardtdegen, K. Sladek, G. Panaitov, D. Grützmacher, and Th. Schäpers'
-''Válogatta: Magyarkuti András''
+We report on the fabrication and characterization of symmetric nanowire-based Josephson junctions, that is, Al- and Nb-based junctions, and asymmetric junctions employing superconducting Al and Nb. In the symmetric junctions, a clear and pronounced Josephson supercurrent is observed. These samples also show clear signatures of subharmonic gap structures. At zero magnetic field, a Josephson coupling is found for the asymmetric Al/InAs-nanowire/Nb junctions as well. By applying a magnetic field above the critical field of Al or by raising the temperature above the critical temperature of Al the junction can be switched to an effective single-interface superconductor/nanowire structure. In this regime, a pronounced zero-bias conductance peak due to reflectionless tunneling has been observed.
-----
-'''Dynamical Coulomb Blockade Observed in Nanosized Electrical Contacts'''
-Christophe Brun1,2, Konrad H. Müller3, I-Po Hong1,4, François Patthey1, Christian Flindt3, and Wolf-Dieter Schneider1
-Electrical contacts between nanoengineered systems are expected to constitute the basic building blocks of future nanoscale electronics. However, the accurate characterization and understanding of electrical contacts at the nanoscale is an experimentally challenging task. Here, we employ low-temperature scanning tunneling spectroscopy to investigate the conductance of individual nanocontacts formed between flat Pb islands and their supporting substrates. We observe a suppression of the differential tunnel conductance at small bias voltages due to dynamical Coulomb blockade effects. The differential conductance spectra allow us to determine the capacitances and resistances of the electrical contacts which depend systematically on the island-substrate contact area. Calculations based on the theory of environmentally assisted tunneling agree well with the measurements.
+http://pubs.acs.org/doi/abs/10.1021/nl501350v
-http://prl.aps.org/abstract/PRL/v108/i12/e126802
 ----
-'''Relativistic Hall Effect'''
+'''Robust Electron Pairing in the Integer Quantum Hall Effect Regime'''
-Konstantin Y. Bliokh1,2 and Franco Nori1,3
+'Hyungkook Choi, Itamar Sivan, Amir Rosenblatt, Moty Heiblum, Vladimir Umansky, Diana Mahalu'
-We consider the relativistic deformation of quantum waves and mechanical bodies carrying intrinsic angular momentum (AM). When observed in a moving reference frame, the centroid of the object undergoes an AM-dependent transverse shift. This is the relativistic analogue of the spin-Hall effect, which occurs in free space without any external fields. Remarkably, the shifts of the geometric and energy centroids differ by a factor of 2, and both centroids are crucial for the Lorentz transformations of the AM tensor. We examine manifestations of the relativistic Hall effect in quantum vortices and mechanical flywheels and also discuss various fundamental aspects of this phenomenon. The perfect agreement of quantum and relativistic approaches allows applications at strikingly different scales, from elementary spinning particles, through classical light, to rotating black holes.
+Electron pairing is a rare phenomenon appearing only in a few unique physical systems; e.g., superconductors and Kondo-correlated quantum dots. Here, we report on an unexpected, but robust, electron "pairing" in the integer quantum Hall effect (IQHE) regime. The pairing takes place within an interfering edge channel circulating in an electronic Fabry-Perot interferometer at a wide range of bulk filling factors, 2<νB<5. The main observations are: (a) High visibility Aharonov-Bohm conductance oscillations with magnetic flux periodicity Δϕ=φ0/2=h/2e (instead of the ubiquitous h/e), with e the electron charge and h the Planck constant; (b) An interfering quasiparticle charge e∗∼2e - revealed by quantum shot noise measurements; and (c) Full dephasing of the h/2e periodicity by induced dephasing of the adjacent edge channel (while keeping the interfering edge channel intact) : a clear realization of inter-channel entanglement. While this pairing phenomenon clearly results from inter-channel interaction, the exact mechanism that leads to e-e attraction within a single edge channel is not clear.
-http://prl.aps.org/abstract/PRL/v108/i12/e120403
+http://xxx.lanl.gov/abs/1409.4427
-----
-'''Enhanced Carrier Transport along Edges of Graphene Devices'''
-Jungseok Chae†§, Suyong Jung‡§, Sungjong Woo, Hongwoo Baek†, Jeonghoon Ha†, Young Jae Song‡§, Young-Woo Son, Nikolai B. Zhitenev‡, Joseph A. Stroscio‡, and Young Kuk†
-The relation between macroscopic charge transport properties and microscopic carrier distribution is one of the central issues in the physics and future applications of graphene devices (GDs). We find strong conductance enhancement at the edges of GDs using scanning gate microscopy. This result is explained by our theoretical model of the opening of an additional conduction channel localized at the edges by depleting accumulated charge by the tip.
-http://pubs.acs.org/doi/abs/10.1021/nl2041222
 ----
-'''Highly Efficient Charge Separation and Collection across in Situ Doped Axial VLS-Grown Si Nanowire p–n Junctions'''
+'''Robust 2D Topological Insulators in van der Waals Heterostructures'''
-A. D. Mohite*†§, D. E. Perea†‡, S. Singh†§, S. A. Dayeh†‡, I. H. Campbell‡, S. T. Picraux*†‡, and H. Htoon
-VLS-grown semiconductor nanowires have emerged as a viable prospect for future solar-based energy applications. In this paper, we report highly efficient charge separation and collection across in situ doped Si p–n junction nanowires with a diameter <100 nm grown in a cold wall CVD reactor. Our photoexcitation measurements indicate an internal quantum efficiency of 50%, whereas scanning photocurrent microscopy measurements reveal effective minority carrier diffusion lengths of 1.0 μm for electrons and 0.66 μm for holes for as-grown Si nanowires (dNW ≈ 65–80 nm), which are an order of magnitude larger than those previously reported for nanowires of similar diameter. Further analysis reveals that the strong suppression of surface recombination is mainly responsible for these relatively long diffusion lengths, with surface recombination velocities (S) calculated to be 2 orders of magnitude lower than found previously for as-grown nanowires, all of which used hot wall reactors. The degree of surface passivation achieved in our as-grown nanowires is comparable to or better than that achieved for nanowires in prior studies at significantly larger diameters. We suggest that the dramatically improved surface recombination velocities may result from the reduced sidewall reactions and deposition in our cold wall CVD reactor.
-http://pubs.acs.org/doi/abs/10.1021/nl204505p
-----
-'''Scanning probe microscopy: Seeing the charge within'''
+'Liangzhi Kou, Shu-Chun Wu, Claudia Felser, Thomas Frauenheim, Changfeng Chen, and Binghai Yan'
-Peter Grutter
+We predict a family of robust two-dimensional (2D) topological insulators in van der Waals heterostructures comprising graphene and chalcogenides BiTeX (X = Cl, Br, and I). The layered structures of both constituent materials produce a naturally smooth interface that is conducive to proximity-induced topological states. First-principles calculations reveal intrinsic topologically nontrivial bulk energy gaps as large as 70–80 meV, which can be further enhanced up to 120 meV by compression. The strong spin–orbit coupling in BiTeX has a significant influence on the graphene Dirac states, resulting in the topologically nontrivial band structure, which is confirmed by calculated nontrivial Z2 index and an explicit demonstration of metallic edge states. Such heterostructures offer a unique Dirac transport system that combines the 2D Dirac states from graphene and 1D Dirac edge states from the topological insulator, and it offers ideas for innovative device designs.
-The distribution of electric charge within a single naphthalocyanine molecule has been revealed by researchers using a combination of three types of microscopy and theoretical modelling.
+http://pubs.acs.org/doi/abs/10.1021/nn503789v
-http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2012.43.html
 ----
-'''Quantum Hall Effect in Graphene with Superconducting Electrodes'''
+'''Pseudospin-driven spin relaxation mechanism in graphene'''
-Peter Rickhaus, Markus Weiss*, Laurent Marot, and Christian Schönenberger
+'Dinh Van Tuan,	Frank Ortmann,	David Soriano,	Sergio O. Valenzuela	& Stephan Roche'
-We have realized an integer quantum Hall system with superconducting contacts by connecting graphene to niobium electrodes. Below their upper critical field of 4 T, an integer quantum Hall effect coexists with superconductivity in the leads but with a plateau conductance that is larger than in the normal state. We ascribe this enhanced quantum Hall plateau conductance to Andreev processes at the graphene–superconductor interface leading to the formation of so-called Andreev edge-states. The enhancement depends strongly on the filling-factor and is less pronounced on the first plateau due to the special nature of the zero energy Landau level in monolayer graphene.
+The prospect of transporting spin information over long distances in graphene, possible because of its small intrinsic spin–orbit coupling (SOC) and vanishing hyperfine interaction, has stimulated intense research exploring spintronics applications. However, measured spin relaxation times are orders of magnitude smaller than initially predicted, while the main physical process for spin dephasing and its charge-density and disorder dependences remain unconvincingly described by conventional mechanisms. Here, we unravel a spin relaxation mechanism for non-magnetic samples that follows from an entanglement between spin and pseudospin driven by random SOC, unique to graphene. The mixing between spin and pseudospin-related Berry’s phases results in fast spin dephasing even when approaching the ballistic limit, with increasing relaxation times away from the Dirac point, as observed experimentally. The SOC can be caused by adatoms, ripples or even the substrate, suggesting novel spin manipulation strategies based on the pseudospin degree of freedom.
-http://pubs.acs.org/doi/abs/10.1021/nl204415s
+http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3083.html
-----
-'''Direct Measurement of the Fermi Energy in Graphene Using a Double-Layer Heterostructure'''
-Seyoung Kim1, Insun Jo2, D. C. Dillen1, D. A. Ferrer1, B. Fallahazad1, Z. Yao2, S. K. Banerjee1, and E. Tutuc1
-We describe a technique which allows a direct measurement of the relative Fermi energy in an electron system by employing a double-layer heterostructure. We illustrate this method by using a graphene double layer to probe the Fermi energy as a function of carrier density in monolayer graphene, at zero and in high magnetic fields. This technique allows us to determine the Fermi velocity, Landau level spacing, and Landau level broadening. We find that the N=0 Landau level broadening is larger by comparison to the broadening of upper and lower Landau levels.
-http://prl.aps.org/abstract/PRL/v108/i11/e116404
 ----
-'''Majorana Fermions and Exotic Surface Andreev Bound States in Topological Superconductors: Application to CuxBi2Se3'''
+'''Observation of topological surface state quantum Hall effect in an intrinsic three-dimensional topological insulator'''
-Timothy H. Hsieh and Liang Fu
+'Yang Xu, Ireneusz Miotkowski, Chang Liu, Jifa Tian, Hyoungdo Nam, Nasser Alidoust, Jiuning Hu, Chih-Kang Shih, M. Zahid Hasan, Yong P. Chen'
-The recently discovered superconductor CuxBi2Se3 is a candidate for three-dimensional time-reversal-invariant topological superconductors, which are predicted to have robust surface Andreev bound states hosting massless Majorana fermions. In this work, we analytically and numerically find the linearly dispersing Majorana fermions at k=0, which smoothly evolve into a new branch of gapless surface Andreev bound states near the Fermi momentum. The latter is a new type of Andreev bound states resulting from both the nontrivial band structure and the odd-parity pairing symmetry. The tunneling spectra of these surface Andreev bound states agree well with a recent point-contact spectroscopy experiment [ S. Sasaki et al. Phys. Rev. Lett. 107 217001 (2011)] and yield additional predictions for low temperature tunneling and photoemission experiments.
+A three-dimensional (3D) topological insulator (TI) is a quantum state of matter with a gapped insulating bulk yet a conducting surface hosting topologically-protected gapless surface states. One of the most distinct electronic transport signatures predicted for such topological surface states (TSS) is a well-defined half-integer quantum Hall effect (QHE) in a magnetic field, where the surface Hall conductivities become quantized in units of (1/2)e2/h (e being the electron charge, h the Planck constant) concomitant with vanishing resistance. Here, we observe well-developed QHE arising from TSS in an intrinsic TI of BiSbTeSe2. Our samples exhibit surface dominated conduction even close to room temperature, while the bulk conduction is negligible. At low temperatures and high magnetic fields perpendicular to the top and bottom surfaces, we observe well-developed integer quantized Hall plateaus, where the two parallel surfaces each contributing a half integer e2/h quantized Hall (QH) conductance, accompanied by vanishing longitudinal resistance. When the bottom surface is gated to match the top surface in carrier density, only odd integer QH plateaus are observed, representing a half-integer QHE of two degenerate Dirac gases. This system provides an excellent platform to pursue a plethora of exotic physics and novel device applications predicted for TIs, ranging from magnetic monopoles and Majorana particles to dissipationless electronics and fault-tolerant quantum computers.
-http://prl.aps.org/abstract/PRL/v108/i10/e107005
+http://xxx.lanl.gov/abs/1409.3778
-----
-'''Direct Observation of Inhomogeneous Solid Electrolyte Interphase on MnO Anode with Atomic Force Microscopy and Spectroscopy'''
-Jie Zhang†§, Rui Wang‡, Xiaocheng Yang†, Wei Lu*†, Xiaodong Wu†, Xiaoping Wang§, Hong Li‡, and Liwei Chen
-Solid electrolyte interphase (SEI) is an in situ formed thin coating on lithium ion battery (LIB) electrodes. The mechanical property of SEI largely defines the cycling performance and the safety of LIBs but has been rarely investigated. Here, we report quantitatively the Young’s modulus of SEI films on MnO anodes. The inhomogeneity of SEI film in morphology, structure, and mechanical properties provides new insights to the evolution of SEI on electrodes. Furthermore, the quantitative methodology established in this study opens a new approach to direct investigation of SEI properties in various electrode materials systems.
-http://pubs.acs.org/doi/abs/10.1021/nl300570d
 ----
-== Febr. 24. - Febr. 31. (2012) ==
+'''MoS2: a Choice Substrate for Accessing and Tuning the Electronic Properties of Graphene'''
-''Válogatta: Fülöp Gergő''
+'Chih-Pin Lu, Guohong Li, K. Watanabe, T. Taniguchi, Eva Y. Andrei'
-----
-'''Field-Effect Tunneling Transistor Based on Vertical Graphene Heterostructures'''
-L. Britnell, R. V. Gorbachev2, R. Jalil2, B. D. Belle2, F. Schedin2, A. Mishchenko1, T. Georgiou1,  M. I. Katsnelson3, L. Eaves4,  S. V. Morozov5,  N. M. R. Peres6,7,  J. Leist8, A. K. Geim1,2,*,    K. S. Novoselov1,*,  L. A. Ponomarenko1,*
+One of the enduring challenges in graphene research and applications is the extreme sensitivity of its charge carriers to external perturbations, especially those introduced by the substrate. The best available substrates to date, graphite and hBN, still pose limitations: graphite being metallic does not allow gating, while both hBN and graphite having lattice structures closely matched to that of graphene, may cause significant band structure reconstruction. Here we show that the atomically smooth surface of exfoliated MoS2 provides access to the intrinsic electronic structure of graphene without these drawbacks. Using scanning tunneling microscopy and Landau-level spectroscopy in a device configuration which allows tuning the carrier concentration, we find that graphene on MoS2 is ultra-flat producing long mean free paths, while avoiding band structure reconstruction. Importantly, the screening of the MoS2 substrate can be tuned by changing the position of the Fermi energy with relatively low gate voltages. We show that shifting the Fermi energy from the gap to the edge of the conduction band gives rise to enhanced screening and to a substantial increase in the mean-free-path and quasiparticle lifetime. MoS2 substrates thus provide unique opportunities to access the intrinsic electronic properties of graphene and to study in situ the effects of screening on electron-electron interactions and transport.
-An obstacle to the use of graphene as an alternative to silicon electronics has been the absence of an energy gap between its conduction and valence bands, which makes it difficult to achieve low power dissipation in the OFF state. We report a bipolar field-effect transistor that exploits the low density of states in graphene and its one-atomic-layer thickness. Our prototype devices are graphene heterostructures with atomically thin boron nitride or molybdenum disulfide acting as a vertical transport barrier. They exhibit room-temperature switching ratios of ≈50 and ≈10,000, respectively. Such devices have potential for high-frequency operation and large-scale integration.
+http://xxx.lanl.gov/abs/1409.5179
-http://www.sciencemag.org/content/335/6071/947.abstract
 ----
-''' Imaging the charge distribution within a single molecule'''
+'''Aharonov-Bohm Oscillations in a Quasi-Ballistic 3D Topological Insulator Nanowire'''
-Fabian Mohn, Leo Gross, Nikolaj Moll & Gerhard Meyer
+'S. Cho, B. Dellabetta, R. D. Zhong, J. Schneeloch, T. S. Liu, G. Gu, Matthew J. Gilbert, Nadya Mason'
-[...] Here, we use a combination of scanning tunnelling microscopy, atomic force microscopy and Kelvin probe force microscopy to examine naphthalocyanine molecules (which have been used as molecular switches13) on a thin insulating layer of NaCl on Cu(111). We show that Kelvin probe force microscopy can map the local contact potential difference of this system with submolecular resolution, and we use density functional theory calculations to verify that these maps reflect the intramolecular distribution of charge. This approach could help to provide fundamental insights into single-molecule switching and bond formation, processes that are usually accompanied by the redistribution of charge within or between molecules
+In three-dimensional topological insulators (3D TI) nanowires, transport occurs via gapless surface states where the spin is fixed perpendicular to the momentum[1-6]. Carriers encircling the surface thus acquire a \pi Berry phase, which is predicted to open up a gap in the lowest-energy 1D surface subband. Inserting a magnetic flux ({\Phi}) of h/2e through the nanowire should cancel the Berry phase and restore the gapless 1D mode[7-8]. However, this signature has been missing in transport experiments reported to date[9-11]. Here, we report measurements of mechanically-exfoliated 3D TI nanowires which exhibit Aharonov-Bohm oscillations consistent with topological surface transport. The use of low-doped, quasi-ballistic devices allows us to observe a minimum conductance at {\Phi} = 0 and a maximum conductance reaching e^2/h at {\Phi} = h/2e near the lowest subband (i.e. the Dirac point), as well as the carrier density dependence of the transport.
-http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2012.20.html
+http://xxx.lanl.gov/abs/1409.5095
-----
-'''A Chemically-Responsive Nanojunction within a Silver Nanowire'''
-Wendong Xing, Jun Hu, Sheng-Chin Kung, Keith C. Donavan†, Wenbo Yan†, Ruqian Wu, and Reginald M. Penner
-The formation of a nanometer-scale chemically responsive junction (CRJ) within a silver nanowire is described. A silver nanowire was first prepared on glass using the lithographically patterned nanowire electrodeposition method. A 1–5 nm gap was formed in this wire by electromigration. Finally, this gap was reconnected by applying a voltage ramp to the nanowire resulting in the formation of a resistive, ohmic CRJ. Exposure of this CRJ-containing nanowire to ammonia (NH3) induced a rapid (<30 s) and reversible resistance change that was as large as ΔR/R0 = (+)138% in 7% NH3 and observable down to 500 ppm NH3. Exposure to water vapor produced a weaker resistance increase of ΔR/R0,H2O = (+)10–15% (for 2.3% water) while nitrogen dioxide (NO2) exposure induced a stronger concentration-normalized resistance decrease of ΔR/R0,NO2 = (−)10–15% (for 500 ppm NO2). The proposed mechanism of the resistance response for a CRJ, supported by temperature-dependent measurements of the conductivity for CRJs and density functional theory calculations, is that semiconducting p-type AgxO is formed within the CRJ and the binding of molecules to this AgxO modulates its electrical resistance.
-http://pubs.acs.org/doi/full/10.1021/nl300427w
 ----
-'''Topographic and Spectroscopic Characterization of Electronic Edge States in CVD Grown Graphene Nanoribbons'''
-Minghu Pan, E. Costa Girão, Xiaoting Jia, Sreekar Bhaviripudi, Qing Li, Jing Kong, V. Meunier, and Mildred S. Dresselhaus
+'''Experimental Realization of a Three-Dimensional Dirac Semimetal'''
-We used scanning tunneling microscopy and spectroscopy (STM/S) techniques to analyze the relationships between the edge shapes and the electronic structures in as-grown chemical vapor deposition (CVD) graphene nanoribbons (GNRs). A rich variety of single-layered graphene nanoribbons exhibiting a width of several to 100 nm and up to 1 μm long were studied. High-resolution STM images highlight highly crystalline nanoribbon structures with well-defined and clean edges. Theoretical calculations indicate clear spin-split edge states induced by electron–electron Coulomb repulsion. The edge defects can significantly modify these edge states, and different edge structures for both sides of a single ribbon produce asymmetric electronic edge states, which reflect the more realistic features of CVD grown GNRs. Three structural models are proposed and analyzed to explain the observations. By comparing the models with an atomic resolution image at the edge, a pristine (2,1) structure was ruled out in favor of a reconstructed edge structure composed of 5–7 member rings, showing a better match with experimental results, and thereby suggesting the possibility of a defective morphology at the edge of CVD grown nanoribbons.
+'Sergey Borisenko, Quinn Gibson, Danil Evtushinsky, Volodymyr Zabolotnyy, Bernd Büchner, and Robert J. Cava'
-http://pubs.acs.org/doi/full/10.1021/nl204392s
+We report the direct observation of the three-dimensional (3D) Dirac semimetal phase in cadmium arsenide (Cd3As2) by means of angle-resolved photoemission spectroscopy. We identify two momentum regions where electronic states that strongly disperse in all directions form narrow conelike structures, and thus prove the existence of the long sought 3D Dirac points. This electronic structure naturally explains why Cd3As2 has one of the highest known bulk electron mobilities. This realization of a 3D Dirac semimetal in Cd3As2 not only opens a direct path to a wide spectrum of applications, but also offers a robust platform for engineering topologically nontrivial phases including Weyl semimetals and quantum spin Hall systems.
-----
-== Febr. 16. - Febr. 23. (2012) ==
+http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.027603
-''Válogatta: Gubicza Ági''
 ----
-'''A single-atom transistor'''
+'''Terahertz Generation by Dynamical Photon Drag Effect in Graphene Excited by Femtosecond Optical Pulses'''
-Martin Fuechsle, Jill A. Miwa, Suddhasatta Mahapatra, Hoon Ryu, Sunhee Lee, Oliver Warschkow, Lloyd C. L. Hollenberg, Gerhard Klimeck & Michelle Y. Simmons
+'J. Maysonnave, S. Huppert, F. Wang, S. Maero, C. Berger, W. de Heer, T. B. Norris, L. A. De Vaulchier, S. Dhillon, J. Tignon, R. Ferreira, and J. Mangeney'
-doi:10.1038/nnano.2012.21
-Published online 19 February 2012
+Graphene has been proposed as a particularly attractive material for the achievement of strong optical nonlinearities, in particular generation of terahertz radiation. However, owing to the particular symmetries of the C-lattice, second-order nonlinear effects such as difference-frequency or rectification processes are predicted to vanish in a graphene layer for optical excitations (ℏω ≫ 2EF) involving the two relativistic dispersion bands. Here we experimentally demonstrate that graphene excited by femtosecond optical pulses generate a coherent THz radiation ranging from 0.1 to 4 THz via a second-order nonlinear effect. We fully interpret its characteristics with a model describing the electron and hole states beyond the usual massless relativistic scheme. This second-order nonlinear effect is dynamical photon drag, which relies on the transfer of light momentum to the carriers by the ponderomotive electric and magnetic forces. The model highlights the key roles of next-C-neighbor couplings and of unequal electron and hole lifetimes in the observed second-order response. Finally, our results indicate that dynamical photon drag effect in graphene can provide emission up to 60 THz, opening new routes for the generation of ultrabroadband terahertz pulses.
-The ability to control matter at the atomic scale and build devices with atomic precision is central to nanotechnology. The scanning tunnelling microscope can manipulate individual atoms and molecules on surfaces, but the manipulation of silicon to make atomic-scale logic circuits has been hampered by the covalent nature of its bonds. Resist-based strategies have allowed the formation of atomic-scale structures on silicon surfaces, but the fabrication of working devices—such as transistors with extremely short gate lengths, spin-based quantum computers and solitary dopant optoelectronic devices—requires the ability to position individual atoms in a silicon crystal with atomic precision. Here, we use a combination of scanning tunnelling microscopy and hydrogen-resist lithography to demonstrate a single-atom transistor in which an individual phosphorus dopant atom has been deterministically placed within an epitaxial silicon device architecture with a spatial accuracy of one lattice site. The transistor operates at liquid helium temperatures, and millikelvin electron transport measurements confirm the presence of discrete quantum levels in the energy spectrum of the phosphorus atom. We find a charging energy that is close to the bulk value, previously only observed by optical spectroscopy.
-http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2012.21.html
+http://pubs.acs.org/doi/abs/10.1021/nl502684j
-----
-'''Thermal Stability and Surface Passivation of Ge Nanowires Coated by Epitaxial SiGe Shells'''
-Shu Hu†, Yoko Kawamura, Kevin C. Y. Huang, Yanying Li, Ann F. Marshall, Kohei M. Itoh, Mark L. Brongersma, and Paul C. McIntyre
-DOI: 10.1021/nl204053w
-Publication Date (Web): February 24, 2012
-Epitaxial growth of a highly strained, coherent SiGe alloy shell around a Ge nanowire core is investigated as a method to achieve surface passivation and carrier confinement, important in realizing nanowire devices. The high photoluminescence intensity observed from the core–shell nanowires with spectral features similar to that of bulk Ge indicates effective surface passivation. Thermal stability of these core–shell heterostructures has been systematically investigated, with a method demonstrated to avoid misfit strain relaxation during postgrowth annealing.
-http://pubs.acs.org/doi/abs/10.1021/nl204053w
 ----
-'''Low-Temperature Chemical Vapor Deposition Growth of Graphene from Toluene on Electropolished Copper Foils'''
+'''Exceptional Charge Transport Properties of Graphene on Germanium'''
-Bin Zhang, Wi Hyoung Lee, Richard Piner, Iskandar Kholmanov, Yaping Wu, Huifeng Li, Hengxing Ji, and Rodney S Ruoff
+'Francesca Cavallo, Richard Rojas Delgado, Michelle M. Kelly, José R. Sánchez Pérez, Daniel P. Schroeder, Huili Grace Xing, Mark A. Eriksson, and Max G. Lagally'
-DOI: 10.1021/nn204827h
-Publication Date (Web): February 17, 2012
+The excellent charge transport properties of graphene suggest a wide range of application in analog electronics. While most practical devices will require that graphene be bonded to a substrate, such bonding generally degrades these transport properties. In contrast, when graphene is transferred to Ge(001) its conductivity is extremely high and the charge carrier mobility derived from the relevant transport measurements is, under some circumstances, higher than that of freestanding, edge-supported graphene. We measure a mobility of ∼5 × 105 cm2 V–1 s–1 at 20 K, and ∼103 cm2 V–1 s–1 at 300 K. These values are close to the theoretical limit for doped graphene. Carrier densities in the graphene are as high as 1014 cm–2 at 300 K.
-A two-step CVD route with toluene as the carbon precursor was used to grow continuous large-area monolayer graphene films on a very flat, electropolished Cu foil surface at 600 °C, lower than any temperature reported to date for growing continuous monolayer graphene. Graphene coverage is higher on the surface of electropolished Cu foil than that on the unelectropolished one under the same growth conditions. The measured hole and electron mobilities of the monolayer graphene grown at 600 °C were 811 and 190 cm2/(V·s), respectively, and the shift of the Dirac point was 18 V. The asymmetry in carrier mobilities can be attributed to extrinsic doping during the growth or transfer. The optical transmittance of graphene at 550 nm was 97.33%, confirming it was a monolayer, and the sheet resistance was ~8.02 × 103 Ω/□.
+http://pubs.acs.org/doi/abs/10.1021/nn503381m
-http://pubs.acs.org/doi/abs/10.1021/nn204827h
 ----
-'''Controlled Synthesis of Compositionally Tunable Ternary PbSexS1–x as Well as Binary PbSe and PbS Nanowires'''
+'''Direct Laser Writing of Graphene Electronics'''
-Anthony C. Onicha, Nattasamon Petchsang, Thomas H. Kosel, and Masaru Kuno
-DOI: 10.1021/nn300373w
-Publication Date (Web): February 16, 2012
-High-quality compositionally tunable ternary PbSexS1–x (x = 0.23, 0.39, 0.49, 0.68, and 0.90) nanowires (NWs) and their binary analogues have been grown using solution–liquid–solid growth with lead(II) diethyldithiocarbamate, Pb(S2CNEt2)2, and lead(II) imido(bis(selenodiisopropylphosphinate)), Pb((SePiPr2)2N)2, as single-source precursors. The alloyed nature of PbSexS1–x wires was confirmed using ensemble X-ray diffraction and energy dispersive X-ray spectroscopy (EDXS). Single NW EDXS line scans taken along the length of individual wires show no compositional gradients. NW compositions were independently confirmed using inductively coupled plasma atomic emission spectroscopy. Slight stoichiometric deviations occur but never exceed 13.3% of the expected composition, based on the amount of introduced precursor. In all cases, resulting nanowires have been characterized using transmission electron microscopy. Mean diameters are between 9 and 15 nm with accompanying lengths that range from 4 to 10 μm. Associated selected area electron diffraction patterns indicate that the PbSexS1–x, PbSe, and PbS NWs all possess the same <002> growth direction, with diffraction patterns consistent with an underlying rock salt crystal structure.
+http://pubs.acs.org/doi/abs/10.1021/nn504946k
-http://pubs.acs.org/doi/abs/10.1021/nn300373w
 ----
-'''Tunable Band Gaps and p-Type Transport Properties of Boron-Doped Graphenes by Controllable Ion Doping Using Reactive Microwave Plasma'''
+'''Room-temperature coupling between electrical current and nuclear spins in OLEDs'''
-Yong-Bing Tang, Li-Chang Yin, Yang Yang, Xiang-Hui Bo, Yu-Lin Cao, Hong-En Wang, Wen-Jun Zhang, Igor Bello, Shuit-Tong Lee, Hui-Ming Cheng, and Chun-Sing Lee
+'H. Malissa, M. Kavand, D. P. Waters, K. J. van Schooten, P. L. Burn, Z. V. Vardeny, B. Saam, J. M. Lupton, C. Boehme'
-DOI: 10.1021/nn3005262
-Publication Date (Web): February 21, 2012
+The effects of external magnetic fields on the electrical conductivity of organic semiconductors have been attributed to hyperfine coupling of the spins of the charge carriers and hydrogen nuclei. We studied this coupling directly by implementation of pulsed electrically detected nuclear magnetic resonance spectroscopy in organic light-emitting diodes (OLEDs). The data revealed a fingerprint of the isotope (protium or deuterium) involved in the coherent spin precession observed in spin-echo envelope modulation. Furthermore, resonant control of the electric current by nuclear spin orientation was achieved with radiofrequency pulses in a double-resonance scheme, implying current control on energy scales one-millionth the magnitude of the thermal energy.
-We report tunable band gaps and transport properties of B-doped graphenes that were achieved via controllable doping through reaction with the ion atmosphere of trimethylboron decomposed by microwave plasma. Both electron energy loss spectroscopy and X-ray photoemission spectroscopy analyses of the graphene reacted with ion atmosphere showed that B atoms are substitutionally incorporated into graphenes without segregation of B domains. The B content was adjusted over a range of 0–13.85 atom % by controlling the ion reaction time, from which the doping effects on transport properties were quantitatively evaluated. Electrical measurements from graphene field-effect transistors show that the B-doped graphenes have a distinct p-type conductivity with a current on/off ratio higher than 102. Especially, the band gap of graphenes is tuned from 0 to ~0.54 eV with increasing B content, leading to a series of modulated transport properties. We believe the controllable doping for graphenes with predictable transport properties may pave a way for the development of graphene-based devices.
+http://www.sciencemag.org/content/345/6203/1487
-http://pubs.acs.org/doi/abs/10.1021/nn3005262
 ----
-'''Dynamic Tuning and Symmetry Lowering of Fano Resonance in Plasmonic Nanostructure'''
+'''Large, non-saturating magnetoresistance in WTe2'''
-Yonghao Cui, Jianhong Zhou, Venkata A. Tamma, and Wounjhang Park
+'Mazhar N. Ali,	Jun Xiong,	Steven Flynn,	Jing Tao,	Quinn D. Gibson,	Leslie M. Schoop,	Tian Liang,	Neel Haldolaarachchige,	Max Hirschberger,	N. P. Ong	& R. J. Cava'
-DOI: 10.1021/nn204647b
-Publication Date (Web): February 16, 2012
+Magnetoresistance is the change in a material’s electrical resistance in response to an applied magnetic field. Materials with large magnetoresistance have found use as magnetic sensors1, in magnetic memory2, and in hard drives3 at room temperature, and their rarity has motivated many fundamental studies in materials physics at low temperatures4. Here we report the observation of an extremely large positive magnetoresistance at low temperatures in the non-magnetic layered transition-metal dichalcogenide WTe2: 452,700 per cent at 4.5 kelvins in a magnetic field of 14.7 teslas, and 13 million per cent at 0.53 kelvins in a magnetic field of 60 teslas. In contrast with other materials, there is no saturation of the magnetoresistance value even at very high applied fields. Determination of the origin and consequences of this effect, and the fabrication of thin films, nanostructures and devices based on the extremely large positive magnetoresistance of WTe2, will represent a significant new direction in the study of magnetoresistivity.
-We present dynamic tuning and symmetry lowering of Fano resonances in gold heptamers accomplished by applying uniaxial mechanical stress. The flexible heptamer structure was obtained by embedding the seven-gold-nanocylinder complex in a polydimethylsiloxane membrane. Under uniaxial stress, the Fano resonance exhibited opposite spectral shifts for the two orthogonal polarizations parallel and perpendicular to the mechanical stress. Furthermore, a new resonance was observed for polarization parallel to the mechanical stress but not for the perpendicular polarization. The experimental results showed good agreement with the numerical simulations. A detailed group theoretical analysis showed that the symmetry lowering caused by the mechanical stress not only splits the originally degenerate mode but also modifies the originally optically inactive mode into an optically active mode, which then interacts strongly with a closely spaced mode and exhibits anticrossing behavior. The symmetry tuning enabled by applying mechanical stress is a simple and efficient way to engineer the nature of coupled plasmon resonances in complex nanostructures. The mechanically tunable plasmonic nanostructures also provide an excellent platform for dynamically tunable nanophotonic devices such as tunable filters and sensors.
+http://www.nature.com/nature/journal/vaop/ncurrent/full/nature13763.html
-http://pubs.acs.org/doi/abs/10.1021/nn204647b
 ----
-'''Wetting and Interfacial Properties of Water Nanodroplets in Contact with Graphene and Monolayer Boron–Nitride Sheets'''
+'''Fast non-thermal switching between macroscopic charge-ordered quantum states induced by charge injection'''
-Hui Li and Xiao Cheng Zeng
+'I. Vaskivskyi, I. A. Mihailovic, S. Brazovskii, J. Gospodaric, T. Mertelj, D. Svetin, P. Sutar, D. Mihailovic'
-DOI: 10.1021/nn204661d
-Publication Date (Web): February 22, 2012
+The functionality of logic and memory elements in current electronics is based on multi-stability, driven either by manipulating local concentrations of electrons in transistors, or by switching between equivalent states of a material with a degener- ate ground state in magnetic or ferroelectric materials. Another possibility is offered by phase transitions with switching between metallic and insulating phases, but classical phase transitions are limited in speed by slow nucleation, proliferation of domains and hysteresis. We can in principle avoid these problems by using quantum states for switching, but microscopic systems suffer from decoherence which prohibits their use in everyday devices. Macroscopic quantum states, such as the superconducting ground state have the advantage that on a fundamental level they do not suffer from decoherence plaguing microscopic systems. Here we demonstrate for the first time ultrafast non-thermal switching between different metastable electronically ordered states by pulsed electrical charge injection. The macroscopic nature of the many-body quantum states(1-4) - which are not part of the equilibrium phase diagram - gives rise to unprecedented stability and remarka- bly sharp switching thresholds. Fast sub-50 ps switching, large associated re- sistance changes, 2-terminal operation and demonstrable high fidelity of bi-stability control suggest new opportunities for the use of macroscopic quantum states in electronics, particularly for an ultrafast non-volatile quantum charge-order resistive random access memory (QCOR-RAM).
-Born–Oppenheim quantum molecular dynamics (QMD) simulations are performed to investigate wetting, diffusive, and interfacial properties of water nanodroplets in contact with a graphene sheet or a monolayer boron–nitride (BN) sheet. Contact angles of the water nanodroplets on the two sheets are computed for the first time using QMD simulations. Structural and dynamic properties of the water droplets near the graphene or BN sheet are also studied to gain insights into the interfacial interaction between the water droplet and the substrate. QMD simulation results are compared with those from previous classic MD simulations and with the experimental measurements. The QMD simulations show that the graphene sheet yields a contact angle of 87°, while the monolayer BN sheet gives rise to a contact angle of 86°. Hence, like graphene, the monolayer BN sheet is also weakly hydrophobic, even though the BN bonds entail a large local dipole moment. QMD simulations also show that the interfacial water can induce net positive charges on the contacting surface of the graphene and monolayer BN sheets, and such charge induction may affect electronic structure of the contacting graphene in view that graphene is a semimetal. Contact angles of nanodroplets of water in a supercooled state on the graphene are also computed. It is found that under the supercooled condition, water nanodroplets exhibit an appreciably larger contact angle than under the ambient condition.
+http://xxx.lanl.gov/abs/1409.3794
-http://pubs.acs.org/doi/abs/10.1021/nn204661d
 ----
-'''Second and higher harmonics generation with memristive systems'''
+'''Imaging the two-component nature of Dirac–Landau levels in the topological surface state of Bi2Se3'''
-Guy Z. Cohen, Yuriy V. Pershin, Massimiliano Di Ventra
+'Ying-Shuang Fu,	M. Kawamura,	K. Igarashi,	H. Takagi,	T. Hanaguri	& T. Sasagawa'
-Submitted on 21 Feb 2012
+Massless Dirac electrons in condensed matter1, 2, 3, 4, 5, 6 are, unlike conventional electrons, described by two-component wavefunctions associated with the spin degrees of freedom in the surface state of topological insulators5, 6. Hence, the ability to observe the two-component wavefunction is useful for exploring novel spin phenomena. Here we show that the two-component nature is manifest in Landau levels, the degeneracy of which is lifted by a Coulomb potential. Using spectroscopic-imaging scanning tunnelling microscopy, we visualize energy and spatial structures of Landau levels in Bi2Se3, a prototypical topological insulator. The observed Landau-level splitting and internal structures of Landau orbits are distinct from those in a conventional electron system7 and are well reproduced by a two-component model Dirac Hamiltonian. Our model further predicts energy-dependent spin-magnetization textures in a potential variation and provides a way for manipulating spins in the topological surface state.
-We show that memristive systems can be used very efficiently to generate passively both double and higher frequency harmonics. A technique for maximizing the power conversion efficiency into any given harmonic is developed and applied to a single memristive system and memristive bridge circuits. We find much higher rates of power conversion compared to the standard diode bridge, with the memristive bridge more efficient for second and higher harmonics generation compared to the single memristive system. The memristive bridge circuit optimized for second harmonic generation behaves as a two-quarter-wave rectifier.
+http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3084.html
-http://arxiv.org/abs/1202.4727
 ----
-'''Nuclear spin physics in quantum dots: an optical investigation'''
+'''Ultrafast non-local control of spontaneous emission'''
-Bernhard Urbaszek, Xavier Marie, Thierry Amand, Olivier Krebs, Paul Voisin, Patrick Maletinsky, Alexander Hogele, Atac Imamoglu
+http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.190.html
-Submitted on 21 Feb 2012
-The mesoscopic spin system formed by the 104 − 106 nuclear spins in a semiconductor quantum dot offers a unique setting for the study of many-body spin physics in the condensed matter. The dynamics of this system and its coupling to electron spins is fundamentally different from its bulk counter-part as well as that of atoms due to increased fluctuations that result from reduced dimensions. In recent years, the interest in studying quantum dot nuclear spin systems and their coupling to confined electron spins has been fueled by its direct implication for possible applications of such systems in quantum information processing as well as by the fascinating nonlinear (quantum-)dynamics of the coupled electron-nuclear spin system. In this article, we review experimental work performed over the last decades in studying this mesoscopic, coupled electron-nuclear spin system and discuss how optical addressing of electron spins can be exploited to manipulate and read-out quantum dot nuclei. We discuss how such techniques have been applied in quantum dots to efficiently establish a non-zero mean nuclear spin polarization and, most recently, were used to reduce fluctuations of the average quantum dot nuclear spin orientation. Both results in turn have important implications for the preservation of electron spin coherence in quantum dots, which we discuss. We conclude by speculating how this recently gained understanding of the quantum dot nuclear spin system could in the future enable experimental observation of quantum-mechanical signatures or possible collective behavior of mesoscopic nuclear spin ensembles.
-http://arxiv.org/abs/1202.4637
 ----
-'''Dynamics of superconducting nanowires shunted with an external resistor'''
+http://www.sciencedirect.com/science/article/pii/S0008622314007465
-Matthew W. Brenner, Dibyendu Roy, Nayana Shah, Alexey Bezryadin
+http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.125428
-Submitted on 21 Feb 2012
-We present the first study of superconducting nanowires shunted with an external resistor, geared towards understanding and controlling coherence and dissipation in nanowires. The dynamics is probed by measuring the evolution of the V -I characteristics and the distributions of switching and retrapping currents upon varying the shunt resistor and temperature. Theoretical analysis of the experiments indicates that as the value of the shunt resistance is decreased, the dynamics turns more coherent presumably due to stabilization of phase-slip centers in the wire and furthermore the switching current approaches the Bardeen’s prediction for equilibrium depairing current. By a detailed comparison between theory and experimental, we make headway into identifying regimes in which the quasi-one-dimensional wire can effectively be described by a zero-dimensional circuit model analogous to the RCSJ (resistively and capacitively shunted Josephson junction) model of Stewart and McCumber. Besides its fundamental significance, our study has implications for a range of promising technological applications.
-http://arxiv.org/abs/1202.4526
 ----
-'''Counting statistics in an InAs nanowire quantum dot with a vertically coupled charge detector'''
+For fun:
-Theodore Choi, Thomas Ihn, Silke Schön, Klaus Ensslin
+'''Spraying Quantum Dot Conjugates in the Colon of Live Animals Enabled Rapid and Multiplex Cancer Diagnosis Using Endoscopy'''
-Submitted on 20 Feb 2012
+http://pubs.acs.org/doi/abs/10.1021/nn5009269
-A gate-defined quantum dot in an InAs nanowire is fabricated on top of a quantum point contact realized in a two-dimensional electron gas. The strong coupling between these two quantum devices is used to perform time-averaged as well as time-resolved charge detection experiments for electron flow through the quantum dot. We demonstrate that the Fano factor describing shot noise or time- correlations in single-electron transport depends in the theoretically expected way on the asymmetry of the tunneling barriers even in a regime where the thermal energy kB T is comparable to the single- particle level spacing in the dot.
-http://arxiv.org/abs/1202.4273
 ----
-'''Stability of Spinmotive Force in Perpendicularly Magnetized Nanowires under High Magnetic Fields'''
+== Szept.2-12. ==
+''Tóvári Endre''
-Yuta Yamane, Jun'ichi Ieda, Sadamichi Maekawa
+'''Fabry-Pérot Interference in Gapped Bilayer Graphene with Broken Anti-Klein Tunneling'''
-Submitted on 20 Feb 2012
+'Anastasia Varlet, Ming-Hao Liu (劉明豪), Viktor Krueckl, Dominik Bischoff, Pauline Simonet, Kenji Watanabe, Takashi Taniguchi, Klaus Richter, Klaus Ensslin, and Thomas Ihn'
-Spinmotive force induced by domain wall motion in perpendicularly magnetized nanowires is numerically demonstrated. We show that by use of nanowires with large magnetic anisotropy, high stability of spinmotive force can be achieved under strong magnetic fields. We observe tens of μV of the spinmotive force in a multilayered Co/Ni nanowire, and even a several hundred μV in a L10 -ordered FePt nanowire, which is two orders of magnitude larger than that reported so far in permalloy nanowires. The narrow structure and low mobility of a domain wall under magnetic fields in perpendicularly magnetized nanowires permits downsizing of spinmotive force devices.
+We report the experimental observation of Fabry-Pérot interference in the conductance of a gate-defined cavity in a dual-gated bilayer graphene device. The high quality of the bilayer graphene flake, combined with the device’s electrical robustness provided by the encapsulation between two hexagonal boron nitride layers, allows us to observe ballistic phase-coherent transport through a 1−μm-long cavity. We confirm the origin of the observed interference pattern by comparing to tight-binding calculations accounting for the gate-tunable band gap. The good agreement between experiment and theory, free of tuning parameters, further verifies that a gap opens in our device. The gap is shown to destroy the perfect reflection for electrons traversing the barrier with normal incidence (anti-Klein tunneling). The broken anti-Klein tunneling implies that the Berry phase, which is found to vary with the gate voltages, is always involved in the Fabry-Pérot oscillations regardless of the magnetic field, in sharp contrast with single-layer graphene.
-http://arxiv.org/abs/1202.4256
+http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.116601
-----
-'''Angle dependent conductance in graphene'''
-C. H. Fuentevilla and J.D. Lejarreta , C. Cobaleda and E. Diez
-Submitted on 19 Feb 2012
-In this paper, we study a theoretical method to calculate the conductance across a square barrier potential in monolayer graphene. We have obtained an analytical expression for the transmission coefficient across a potential barrier for monolayer graphene. Using the transmission coefficient obtained we have an analytical expression for the conductance. This expression will be used to calculate the conductance in the case in which there is a potential barrier, which in our case will modelise the behaviour of a top gate voltage of a field effect transistor. Once this analysis has been performed we study the scenario in which carriers scatter with the potential barrier with different incidence angles and we have found that for any incident angle an effective gap is induced.
-http://arxiv.org/abs/1202.4145
 ----
+'''Anomalous Sequence of Quantum Hall Liquids Revealing a Tunable Lifshitz Transition in Bilayer Graphene'''
-'''Competition for Graphene: Graphynes with Direction-Dependent Dirac Cones'''
+'Anastasia Varlet, Dominik Bischoff, Pauline Simonet, Kenji Watanabe, Takashi Taniguchi, Thomas Ihn, Klaus Ensslin, Marcin Mucha-Kruczyński, and Vladimir I. Fal’ko'
-Daniel Malko, Christian Neiss, Francesc Viñes, and Andreas Görling
-Phys. Rev. Lett. 108, 086804 (2012)
+Bilayer graphene is a unique system where both the Fermi energy and the low-energy electron dispersion can be tuned. This is brought about by an interplay between trigonal warping and the band gap opened by a transverse electric field. Here, we drive the Lifshitz transition in bilayer graphene to experimentally controllable carrier densities by applying a large transverse electric field to a h-BN-encapsulated bilayer graphene structure. We perform magnetotransport measurements and investigate the different degeneracies in the Landau level spectrum. At low magnetic fields, the observation of filling factors −3 and −6 quantum Hall states reflects the existence of three maxima at the top of the valence-band dispersion. At high magnetic fields, all integer quantum Hall states are observed, indicating that deeper in the valence band the constant energy contours are singly connected. The fact that we observe ferromagnetic quantum Hall states at odd-integer filling factors testifies to the high quality of our sample. This enables us to identify several phase transitions between correlated quantum Hall states at intermediate magnetic fields, in agreement with the calculated evolution of the Landau level spectrum. The observed evolution of the degeneracies, therefore, reveals the presence of a Lifshitz transition in our system.
-The existence of Dirac cones in the band structure of two-dimensional materials accompanied by unprecedented electronic properties is considered to be a unique feature of graphene related to its hexagonal symmetry. Here, we present other two-dimensional carbon materials, graphynes, that also possess Dirac cones according to first-principles electronic structure calculations. One of these materials, 6,6,12-graphyne, does not have hexagonal symmetry and features two self-doped nonequivalent distorted Dirac cones suggesting electronic properties even more amazing than that of graphene.
+http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.116602
-http://prl.aps.org/abstract/PRL/v108/i8/e086804
 ----
-'''Spin-Selective Kondo Insulator: Cooperation of Ferromagnetism and the Kondo Effect'''
+'''Characterizing wave functions in graphene nanodevices: Electronic transport through ultrashort graphene constrictions on a boron nitride substrate'''
-Robert Peters and Norio Kawakami
+'D. Bischoff, F. Libisch, J. Burgdörfer, T. Ihn, and K. Ensslin'
-Phys. Rev. Lett. 108, 086402 (2012)
+We present electronic transport measurements through short and narrow (30×30nm) single-layer graphene constrictions on a hexagonal boron nitride substrate. While the general observation of Coulomb blockade is compatible with earlier work, the details are not: We show that the area on which charge is localized can be significantly larger than the area of the constriction, suggesting that the localized states responsible for the Coulomb blockade leak out into the graphene bulk. The high bulk mobility of graphene on hexagonal boron nitride, however, seems to be inconsistent with the short bulk localization length required to see Coulomb blockade. To explain these findings, charge must instead be primarily localized along the imperfect edges of the devices and extend along the edge outside of the constriction. In order to better understand the mechanisms, we compare the experimental findings with tight-binding simulations of such constrictions with disordered edges. Finally, we discuss previous experiments in the light of our findings.
-We propose the notion of a spin-selective Kondo insulator, which provides a fundamental mechanism to describe the ferromagnetic phase of the Kondo lattice model with antiferromagnetic coupling. This unveils a remarkable feature of the ferromagnetic metallic phase: the majority-spin conduction electrons show metallic while the minority-spin electrons show insulating behavior. The resulting Kondo gap in the minority-spin sector, which is due to the cooperation of ferromagnetism and partial Kondo screening, evidences a dynamically induced commensurability for a combination of minority-spin electrons and parts of localized spins. Furthermore, this mechanism predicts a nontrivial relation between the macroscopic quantities such as electron magnetization, spin polarization, and electron filling.
+http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.115405
-http://prl.aps.org/abstract/PRL/v108/i8/e086402
 ----
-'''Redshift of Excitons in Carbon Nanotubes Caused by the Environment Polarizability'''
+'''Twist-controlled resonant tunnelling in graphene/boron nitride/graphene heterostructures'''
-Michael Rohlfing
+'A. Mishchenko,	J. S. Tu,	Y. Cao,	R. V. Gorbachev,	J. R. Wallbank,	M. T. Greenaway,	V. E. Morozov,	S. V. Morozov,	M. J. Zhu,	S. L. Wong,	F. Withers,	C. R. Woods,	Y-J. Kim,	K. Watanabe,	T. Taniguchi,	E. E. Vdovin,	O. Makarovsky,	T. M. Fromhold,	V. I. Fal'ko,	A. K. Geim,	L. Eaves	& K. S. Novoselov'
-Phys. Rev. Lett. 108, 087402 (2012)
+Recent developments in the technology of van der Waals heterostructures1, 2 made from two-dimensional atomic crystals3, 4 have already led to the observation of new physical phenomena, such as the metal–insulator transition5 and Coulomb drag6, and to the realization of functional devices, such as tunnel diodes7, 8, tunnel transistors9, 10 and photovoltaic sensors11. An unprecedented degree of control of the electronic properties is available not only by means of the selection of materials in the stack12, but also through the additional fine-tuning achievable by adjusting the built-in strain and relative orientation of the component layers13, 14, 15, 16, 17. Here we demonstrate how careful alignment of the crystallographic orientation of two graphene electrodes separated by a layer of hexagonal boron nitride in a transistor device can achieve resonant tunnelling with conservation of electron energy, momentum and, potentially, chirality. We show how the resonance peak and negative differential conductance in the device characteristics induce a tunable radiofrequency oscillatory current that has potential for future high-frequency technology.
-Optical excitations of molecular systems can be modified by their physical environment. We analyze the underlying mechanisms within many-body perturbation theory, which is particularly suited to study nonlocal polarizability effects on the electronic structure. Here we focus on the example of a semiconducting carbon nanotube, which observes redshifts of its excitons when the tube is touched by another nanotube or other physisorbates. We show that the redshifts mostly result from the polarizability of the attached ad system. Electronic coupling may enhance the redshifts, but depends very sensitively on the structural details of the contact.
+http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.187.html
-http://prl.aps.org/abstract/PRL/v108/i8/e087402
 ----
-'''Twisting graphene nanoribbons into carbon nanotubes'''
+'''Resistance-voltage dependence of nanojunctions during electromigration in ultrahigh vacuum'''
-O. O. Kit, T. Tallinen, L. Mahadevan, J. Timonen, and P. Koskinen
+'D. Stöffler, M. Marz, B. Kießig, T. Tomanic, R. Schäfer, H. v. Löhneysen, and R. Hoffmann-Vogel'
-Phys. Rev. B 85, 085428 (2012)
+The electrical resistance R of metallic nanocontacts subjected to controlled cyclic electromigration in ultrahigh vacuum has been investigated in situ as a function of applied voltage V. For sufficiently small contacts, i.e., large resistance, a decrease of R(V) while increasing V is observed. This effect is tentatively attributed to the presence of contacts separated by thin vacuum barriers in parallel to ohmic nanocontacts. Simple model calculations indicate that both thermal activation or tunneling can lead to this unusual behavior. We describe our data by a tunneling model whose key parameter, i.e., the tunneling distance, changes because of thermal expansion due to Joule heating and/or electrostatic strain arising from the applied voltage. Oxygen exposure during electromigration prevents the formation of negative R(V) slopes, and at the same time enhances the probability of uncontrolled melting, while other gases show little effects. In addition, indication for field emission has been observed in some samples.
-Although carbon nanotubes consist of honeycomb carbon, they have never been fabricated from graphene directly. Here, it is shown by quantum molecular-dynamics simulations and classical continuum-elasticity modeling, that graphene nanoribbons can, indeed, be transformed into carbon nanotubes by means of twisting. The chiralities of the tubes thus fabricated can be not only predicted but also externally controlled. This twisting route is an opportunity for nanofabrication, and is easily generalizable to ribbons made of other planar nanomaterials.
+http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.115406
-http://prb.aps.org/abstract/PRB/v85/i8/e085428
 ----
+'''Quantum interference in off-resonant transport through single molecules'''
-== Febr. 9. - Febr. 16. (2012) ==
+'Kim G. L. Pedersen, Mikkel Strange, Martin Leijnse, Per Hedegård, Gemma C. Solomon, and Jens Paaske'
-''Válogatta: Tóvári Endre''
+We provide a simple set of rules for predicting interference effects in off-resonant transport through single molecule junctions. These effects fall into two classes, showing, respectively, an odd or an even number of nodes in the linear conductance within a given molecular charge state, and we demonstrate how to decide the interference class directly from the contacting geometry. For neutral alternant hydrocarbons, we employ the Coulson-Rushbrooke-McLachlan pairing theorem to show that the interference class is decided simply by tunneling on and off the molecule from same or different sublattices. More generally, we investigate a range of smaller molecules by means of exact diagonalization combined with a perturbative treatment of the molecule-lead tunnel coupling. While these results generally agree well with GW calculations, they are shown to be at odds with simpler mean-field treatments. For molecules with spin-degenerate ground states, we show that for most junctions interference causes no transmission nodes, but we argue that it may lead to a nonstandard gate dependence of the zero-bias Kondo resonance.
-----
-'''Nonlinear detection of spin currents in graphene with non-magnetic electrodes'''
-Ivan J. Vera-Marun,Vishal Ranjan & Bart J. van Wees
-doi:10.1038/nphys2219
-Published online: 12 February 2012
-The abilities to inject and detect spin carriers are fundamental for research on transport and manipulation of spin information1, 2. Pure electronic spin currents have been recently studied in nanoscale electronic devices using a non-local lateral geometry, both in metallic systems 3 and in semiconductors4. To unlock the full potential of spintronics we must understand the interactions of spin with other degrees of freedom. Such interactions have been explored recently, for example, by using spin Hall5, 6, 7 or spin thermoelectric effects6, 8,9. Here we present the detection of non-local spin signals using non-magnetic detectors, through an as-yet-unexplored nonlinear interaction between spin and charge. In analogy to the Seebeck effect10, where a heat current generates a charge potential, we demonstrate that a spin current in a paramagnet leads to a charge potential, if the conductivity is energy dependent. We use graphene11 as a model system to study this effect, as recently proposed12. The physical concept demonstrated here is generally valid, opening new possibilities for spintronics.
+http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.125413
-http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2219.html
 ----
-'''Long Spin Relaxation Times in Wafer Scale Epitaxial Graphene on SiC(0001)'''
+'''All-optical control of ferromagnetic thin films and nanostructures'''
-Thomas Maassen*†, J. Jasper van den Berg†, Natasja IJbema†, Felix Fromm‡, Thomas Seyller‡, Rositza Yakimova§, and Bart J. van Wees†
+'C-H. Lambert, S. Mangin, B. S. D. Ch. S. Varaprasad, Y. K. Takahashi, M. Hehn, M. Cinchetti, G. Malinowski, K. Hono, Y. Fainman, M. Aeschlimann, E. E. Fullerton'
-Nano Lett., Article ASAP
+The interplay of light and magnetism allowed light to be used as a probe of magnetic materials. Now the focus has shifted to use polarized light to alter or manipulate magnetism. Here, we demonstrate optical control of ferromagnetic materials ranging from magnetic thin films to multilayers and even granular films being explored for ultra-high-density magnetic recording. Our finding shows that optical control of magnetic materials is a much more general phenomenon than previously assumed and may have a major impact on data memory and storage industries through the integration of optical control of ferromagnetic bits.
-DOI: 10.1021/nl2042497
+http://www.sciencemag.org/content/345/6202/1337
-Publication Date (Web): February 10, 2012
-We developed an easy, upscalable process to prepare lateral spin-valve devices on epitaxially grown monolayer graphene on SiC(0001) and perform nonlocal spin transport measurements. We observe the longest spin relaxation times τS in monolayer graphene, while the spin diffusion coefficient DS is strongly reduced compared to typical results on exfoliated graphene. The increase of τS is probably related to the changed substrate, while the cause for the small value of DS remains an open question.
-http://pubs.acs.org/doi/abs/10.1021/nl2042497
------------------------------------------------------------------------------------------
-'''Remote Catalyzation for Direct Formation of Graphene Layers on Oxides'''
-Po-Yuan Teng†, Chun-Chieh Lu†, Kotone Akiyama-Hasegawa‡, Yung-Chang Lin†, Chao-Hui Yeh†, Kazu Suenaga‡, and Po-Wen Chiu*†
-DOI: 10.1021/nl204024k
-Publication Date (Web): February 14, 2012
-Direct deposition of high-quality graphene layers on insulating substrates such as SiO2 paves the way toward the development of graphene-based high-speed electronics. Here, we describe a novel growth technique that enables the direct deposition of graphene layers on SiO2 with crystalline quality potentially comparable to graphene grown on Cu foils using chemical vapor deposition (CVD). Rather than using Cu foils as substrates, our approach uses them to provide subliming Cu atoms in the CVD process. The prime feature of the proposed technique is remote catalyzation using floating Cu and H atoms for the decomposition of hydrocarbons. This allows for the direct graphitization of carbon radicals on oxide surfaces, forming isolated low-defect graphene layers without the need for postgrowth etching or evaporation of the metal catalyst. The defect density of the resulting graphene layers can be significantly reduced by tuning growth parameters such as the gas ratios, Cu surface areas, and substrate-to-Cu distance. Under optimized conditions, graphene layers with nondiscernible Raman D peaks can be obtained when predeposited graphite flakes are used as seeds for extended growth.
-http://pubs.acs.org/doi/abs/10.1021/nl204024k
 ----
+'''Ultrafast optical control of orbital and spin dynamics in a solid-state defect'''
-'''Van der Waals Epitaxy of InAs Nanowires Vertically Aligned on Single-Layer Graphene'''
+'Lee C. Bassett, F. Joseph Heremans, David J. Christle, Christopher G. Yale, Guido Burkard, Bob B. Buckley, David D. Awschalom'
-Young Joon Hong*†, Wi Hyoung Lee‡, Yaping Wu‡, Rodney S. Ruoff*‡, and Takashi Fukui*†
+Atom-scale defects in semiconductors are promising building blocks for quantum devices, but our understanding of their material-dependent electronic structure, optical interactions, and dissipation mechanisms is lacking. Using picosecond resonant pulses of light, we study the coherent orbital and spin dynamics of a single nitrogen-vacancy center in diamond over time scales spanning six orders of magnitude. We develop a time-domain quantum tomography technique to precisely map the defect’s excited-state Hamiltonian and exploit the excited-state dynamics to control its ground-state spin with optical pulses alone. These techniques generalize to other optically addressable nanoscale spin systems and serve as powerful tools to characterize and control spin qubits for future applications in quantum technology.
-DOI: 10.1021/nl204109t
+http://www.sciencemag.org/content/345/6202/1333
-Publication Date (Web): February 10, 2012
-Semiconductor nanowire arrays integrated vertically on graphene films offer significant advantages for many sophisticated device applications. We report on van der Waals (VDW) epitaxy of InAs nanowires vertically aligned on graphene substrates using metal–organic chemical vapor deposition. The strong correlation between the growth direction of InAs nanowires and surface roughness of graphene substrates was investigated using various graphene films with different numbers of stacked layers. Notably, vertically well-aligned InAs nanowire arrays were obtained easily on single-layer graphene substrates with sufficiently strong VDW attraction. This study presents a considerable advance toward the VDW heteroepitaxy of inorganic nanostructures on chemical vapor-deposited large-area graphenes. More importantly, this work demonstrates the thinnest epitaxial substrate material that yields vertical nanowire arrays by the VDW epitaxy method.
-http://pubs.acs.org/doi/abs/10.1021/nl204109t
---------------------------------------------------------------------------------------------------------
-'''Ambipolar MoS2 Thin Flake Transistors'''
-Yijin Zhang†, Jianting Ye*†, Yusuke Matsuhashi†, and Yoshihiro Iwasa*†‡
-DOI: 10.1021/nl2021575
-Publication Date (Web): January 25, 2012
-Field effect transistors (FETs) made of thin flake single crystals isolated from layered materials have attracted growing interest since the success of graphene. Here, we report the fabrication of an electric double layer transistor (EDLT, a FET gated by ionic liquids) using a thin flake of MoS2, a member of the transition metal dichalcogenides, an archetypal layered material. The EDLT of the thin flake MoS2 unambiguously displayed ambipolar operation, in contrast to its commonly known bulk property as an n-type semiconductor. High-performance transistor operation characterized by a large “ON” state conductivity in the order of  mS and a high on/off ratio >102 was realized for both hole and electron transport. Hall effect measurements revealed mobility of 44 and 86 cm2 V–1 s–1 for electron and hole, respectively. The hole mobility is twice the value of the electron mobility, and the density of accumulated carrier reached 1 × 1014 cm–2, which is 1 order of magnitude larger than conventional FETs with solid dielectrics. The high-density carriers of both holes and electrons can create metallic transport in the MoS2 channel. The present result is not only important for device applications with new functionalities, but the method itself would also act as a protocol to study this class of material for a broader scope of possibilities in accessing their unexplored properties.
-http://pubs.acs.org/doi/abs/10.1021/nl2021575
---------------------------------------------------------------
-'''Three-Terminal Graphene Negative Differential Resistance Devices'''
-Yanqing Wu, Damon B. Farmer*, Wenjuan Zhu, Shu-Jen Han, Christos D. Dimitrakopoulos, Ageeth A. Bol, Phaedon Avouris*, and Yu-Ming Lin*
-IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598, United States
-DOI: 10.1021/nn205106z
-Publication Date (Web): February 10, 2012
-A new mechanism for negative differential resistance (NDR) is discovered in three-terminal graphene devices based on a field-effect transistor configuration. This NDR effect is a universal phenomenon for graphene and is demonstrated in devices fabricated with different types of graphene materials and gate dielectrics. Operation of conventional NDR devices is usually based on quantum tunneling or intervalley carrier transfer, whereas the NDR behavior observed here is unique to the ambipolar behavior of zero-bandgap graphene and is associated with the competition between electron and hole conduction as the drain bias increases. These three terminal graphene NDR devices offer more operation flexibility than conventional two-terminal devices based on tunnel diodes, Gunn diodes, or molecular devices, and open up new opportunities for graphene in microwave to terahertz applications.
-http://pubs.acs.org/doi/abs/10.1021/nn205106z
---------------------------------------------------------------
-'''Spin-selective transport through helical molecular systems'''
-R. Gutierrez1, E. Díaz1,2, R. Naaman3, and G. Cuniberti1,4
-Received 1 February 2012; published 16 February 2012
-Highly spin-selective transport of electrons through a helically shaped electrostatic potential is demonstrated in the frame of a minimal model approach. The effect is significant even for weak spin-orbit coupling. Two main factors determine the selectivity: an unconventional Rashba-like spin-orbit interaction, reflecting the helical symmetry of the system, and a weakly dispersive electronic band of the helical system. The weak electronic coupling, associated with the small dispersion, leads to a low mobility of the charges in the system and allows even weak spin-orbit interactions to be effective. The results are expected to be generic for chiral molecular systems displaying low spin-orbit coupling and low conductivity.
-DOI:10.1103/PhysRevB.85.081404
-http://prb.aps.org/abstract/PRB/v85/i8/e081404
--------------------------------------------------------------------------------------------------------------------
-'''Signatures of Majorana Fermions in Hybrid Superconductor-Topological Insulator Devices'''
-J. R. Williams, A. J. Bestwick, P. Gallagher, Seung Sae Hong, Y. Cui, Andrew S. Bleich, J. G. Analytis, I. R. Fisher, D. Goldhaber-Gordon
-(Submitted on 10 Feb 2012)
-The ability to measure and manipulate complex particles in the solid state is a cornerstone of modern condensed-matter physics. Typical excitations of a sea of electrons, called quasiparticles, have properties similar to those of free electrons. However, in recent years exotic excitations with very different properties have been created in designer quantum materials, including Dirac fermions in graphene [1] and fractionally charged quasiparticles in fractional quantum Hall systems [2]. Here we report signatures of a new quasiparticle -- the Majorana fermion -- in Josephson junctions consisting of two superconducting leads coupled through a three-dimensional topological insulator [3]. We observe two striking departures from the common transport properties of Josephson junctions: a characteristic energy that scales inversely with the width of the junction, and a low characteristic magnetic field for suppressing supercurrent. To explain these effects, we propose a phenomenological model in which a one-dimensional wire of Majorana fermions is present along the width of the junction, similar to a theoretical prediction by Fu and Kane [4]. These results present an opening into the investigation of Majorana fermions in the solid state and their exotic properties, including non-Abelian statistics [5], a suggested basis for fault-tolerant quantum computation [6].
-http://arxiv.org/abs/1202.2323
 ----
-'''Non-linear resistivity and heat dissipation in monolayer graphene'''
+'''Environment-assisted quantum control of a solid-state spin via coherent dark states'''
-A. S. Price, S. M. Hornett, A. V. Shytov, E. Hendry, D. W. Horsell
-(Submitted on 15 Feb 2012)
-We have experimentally studied the nonlinear nature of electrical conduction in monolayer graphene devices on silica substrates. This nonlinearity manifests itself as a nonmonotonic dependence of the differential resistance on applied DC voltage bias across the sample. At temperatures below ~70K, the differential resistance exhibits a peak near zero bias that can be attributed to self-heating of the charge carriers. We show that the shape of this peak arises from a combination of different energy dissipation mechanisms of the carriers. The energy dissipation at higher carrier temperatures depends critically on the length of the sample. For samples longer than 10um the heat loss is shown to be determined by optical phonons at the silica-graphene interface.
-http://xxx.lanl.gov/abs/1202.3394
-------------------------------------------------------------------------------------------------------------------
-'''The Integration of High-k Dielectric on Two-Dimensional Crystals by Atomic Layer Deposition'''
-Han Liu, Kun Xu, Xujie Zhang, Peide D. Ye
-(Submitted on 15 Feb 2012)
-We investigate the integration of Al2O3 high-k dielectric on two-dimensional (2D) crystals of boron nitride (BN) and molybdenum disulfide (MoS2) by atomic layer deposition (ALD). We demonstrate the feasibility of direct ALD growth with trimethylaluminum(TMA) and water as precursors on both 2D crystals. Through theoretical and experimental studies, we found that the initial ALD cycles play the critical role, during which physical adsorption dominates precursor adsorption at the semiconductor surface. We model the initial ALD growth stages at the 2D surface by analyzing Lennard-Jones Potentials, which could guide future optimization of the ALD process on 2D crystals.
-http://xxx.lanl.gov/abs/1202.3391
---------------------------------------------------------------------------------------------------------
-'''Precision comparison of the quantum Hall effect in graphene and gallium arsenide'''
-T.J.B.M. Janssen, J.M. Williams, N.E. Fletcher, R. Goebel, A. Tzalenchuk, R. Yakimova, S. Lara-Avila, S. Kubatkin, V.I. Fal'ko
-(Submitted on 14 Feb 2012)
-The half-integer quantum Hall effect in epitaxial graphene is compared with high precision to the well known integer effect in a GaAs/AlGaAs heterostructure. We find no difference between the quantised resistance values within the relative standard uncertainty of our measurement of $8.7\times 10^{-11}$. The result places new tighter limits on any possible correction terms to the simple relation $R_{\rm K}=h/e^2$, and also demonstrates that epitaxial graphene samples are suitable for application as electrical resistance standards of the highest metrological quality. We discuss the characterisation of the graphene sample used in this experiment and present the details of the cryogenic current comparator bridge and associated uncertainty budget.
-http://xxx.lanl.gov/abs/1202.2985
--------------------------------------------------------------------------------------------
-'''Graphene radio: Detecting radiowaves with a single atom sheet'''
-Mircea Dragoman, Dan Neculoiu, Alina Cismaru, George Deligeorgis, George Konstantinidis, Daniela Dragoman
-(Submitted on 9 Feb 2012)
-We present the experimental evidence of RF demodulation by a graphene monolayer embedded in a coplanar structure. The demodulator was tested in the frequency range from 100 MHz to 25 GHz using amplitude modulated input signals. An input power of 0 dBm (1 mW) was used which is the typical power emitted for short range wireless communication systems, such as Bluetooth. The graphene demodulator exhibits good signal response in the frequency range associated to industrial, scientific and medical (ISM) radio band (2.4 GHz).
+'Jack Hansom,	Carsten H. H. Schulte,	Claire Le Gall,	Clemens Matthiesen,	Edmund Clarke, Maxime Hugues,	Jacob M. Taylor	& Mete Atatüre'
-http://xxx.lanl.gov/abs/1202.1968
+Understanding the interplay between a quantum system and its environment lies at the heart of quantum science and its applications. So far most efforts have focused on circumventing decoherence induced by the environment by either protecting the system from the associated noise1, 2, 3, 4, 5 or by manipulating the environment directly6, 7, 8, 9. Recently, parallel efforts using the environment as a resource have emerged, which could enable dissipation-driven quantum computation and coupling of distant quantum bits10, 11, 12, 13, 14. Here, we realize the optical control of a semiconductor quantum-dot spin by relying on its interaction with an adiabatically evolving spin environment. The emergence of hyperfine-induced, quasi-static optical selection rules enables the optical generation of coherent spin dark states without an external magnetic field. We show that the phase and amplitude of the lasers implement multi-axis manipulation of the basis spanned by the dark and bright states, enabling control via projection into a spin-superposition state. Our approach can be extended, within the scope of quantum control and feedback15, 16, to other systems interacting with an adiabatically evolving environment.
-== Febr. 2. - Febr. 9. (2012) ==
+http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3077.html
-''Válogatta: Tóvári Endre''
 ----
-'''Shallow pockets and very strong coupling superconductivity in FeSe<sub>x</sub>Te<sub>1−x</sub>'''
+'''Graphene nanoribbon heterojunctions'''
-Y. Lubashevsky, E. Lahoud, K. Chashka, D. Podolsky, A. Kanigel
+http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.184.html
-The celebrated Bardeen–Cooper–Schrieffer (BCS) theory has been successful in explaining metallic superconductors, yet many believe that it must be modified to deal with the newer high-temperature superconductors. A possible extension is provided by the BCS–Bose–Einstein condensate (BEC) theory, describing a smooth evolution from a system of weakly interacting pairs to a BEC of molecules of strongly bound fermions. Despite its appeal, spectroscopic evidence for the BCS–BEC crossover has never been observed in solids. Here we report electronic structure measurements in FeSe<sub>x</sub>Te<sub>1−x</sub> showing that these materials are in the BCS–BEC crossover regime. Above the superconducting transition temperature, ''T<sub>c</sub>'', we find multiple bands with remarkably small values for the Fermi energy ε<sub>F</sub>. Yet, in the superconducting state, the gap Δ is comparable to ε<sub>F</sub>. The ratio Δ/ε<sub>F</sub>≈0.5 is much larger than found in any previously studied superconductor, resulting in an anomalous dispersion of the coherence peak very similar to that found in cold Fermi gas experiments, in agreement with the predictions of the BCS–BEC crossover theory.
-http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2216.html
 ----
-'''Extremely Bendable, High-Performance Integrated Circuits Using Semiconducting Carbon Nanotube Networks for Digital, Analog, and Radio-Frequency Applications'''
+'''Generation and electric control of spin–valley-coupled circular photogalvanic current in WSe2'''
-Chuan Wang, Jun-Chau Chien, Kuniharu Takei, Toshitake Takahashi, Junghyo Nah, Ali M. Niknejad, and Ali Javey
+http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.183.html
-Solution-processed thin-films of semiconducting carbon nanotubes as the channel material for flexible electronics simultaneously offers high performance, low cost, and ambient stability, which significantly outruns the organic semiconductor materials. In this work, we report the use of semiconductor-enriched carbon nanotubes for high-performance integrated circuits on mechanically flexible substrates for digital, analog and radio frequency applications. The as-obtained thin-film transistors (TFTs) exhibit highly uniform device performance with on-current and transconductance up to 15 μA/μm and 4 μS/μm. By performing capacitance–voltage measurements, the gate capacitance of the nanotube TFT is precisely extracted and the corresponding peak effective device mobility is evaluated to be around 50 cm<sup>2</sup>V<sup>–1</sup>s<sup>–1</sup>. Using such devices, digital logic gates including inverters, NAND, and NOR gates with superior bending stability have been demonstrated. Moreover, radio frequency measurements show that cutoff frequency of 170 MHz can be achieved in devices with a relatively long channel length of 4 μm, which is sufficient for certain wireless communication applications. This proof-of-concept demonstration indicates that our platform can serve as a foundation for scalable, low-cost, high-performance flexible electronics.
-http://pubs.acs.org/doi/full/10.1021/nl2043375
 ----
-'''Magnetoelectric Charge Trap Memory'''
+'''Sensitive room-temperature terahertz detection via the photothermoelectric effect in graphene'''
-Uwe Bauer, Marek Przybylski, Jürgen Kirschner, and Geoffrey S. D. Beach
-It is demonstrated that a charge-trapping layer placed in proximity to a ferromagnetic metal enables efficient electrical and optical control of the metal’s magnetic properties. Retention of charge trapped inside the charge-trapping layer provides nonvolatility to the magnetoelectric effect and enhances its efficiency by an order of magnitude. As such, an engineered charge-trapping layer can be used to realize the magnetoelectric equivalent to today’s pervasive charge trap flash memory technology. Moreover, by supplying trapped charges optically instead of electrically, a focused laser beam can be used to imprint the magnetic state into a continuous metal film.
-http://pubs.acs.org/doi/full/10.1021/nl204114t
-----
-'''Number of observable features in the acoustic Raman spectra of nanocrystals'''
+'Xinghan Cai,	Andrei B. Sushkov,	Ryan J. Suess,	Mohammad M. Jadidi,	Gregory S. Jenkins, Luke O. Nyakiti,	Rachael L. Myers-Ward,	Shanshan Li,	Jun Yan,	D. Kurt Gaskill,	Thomas E. Murphy,	H. Dennis Drew	& Michael S. Fuhrer'
-Lucien Saviot, Nicolas Combe, and Adnen Mlayah
+Terahertz radiation has uses in applications ranging from security to medicine1. However, sensitive room-temperature detection of terahertz radiation is notoriously difficult2. The hot-electron photothermoelectric effect in graphene is a promising detection mechanism; photoexcited carriers rapidly thermalize due to strong electron–electron interactions3, 4, but lose energy to the lattice more slowly3, 5. The electron temperature gradient drives electron diffusion, and asymmetry due to local gating6, 7 or dissimilar contact metals8 produces a net current via the thermoelectric effect. Here, we demonstrate a graphene thermoelectric terahertz photodetector with sensitivity exceeding 10 V W–1 (700 V W–1) at room temperature and noise-equivalent power less than 1,100 pW Hz–1/2 (20 pW Hz–1/2), referenced to the incident (absorbed) power. This implies a performance that is competitive with the best room-temperature terahertz detectors9 for an optimally coupled device, and time-resolved measurements indicate that our graphene detector is eight to nine orders of magnitude faster than those7, 10. A simple model of the response, including contact asymmetries (resistance, work function and Fermi-energy pinning) reproduces the qualitative features of the data, and indicates that orders-of-magnitude sensitivity improvements are possible.
-Low-frequency Raman-scattering spectra are presented for gold nanocrystals with diameters 3.5 and 13 nm. The frequencies of the Raman peaks but also their number are shown to vary with the nanocrystal size. These results are analyzed using both the continuous elastic medium approximation and an atomistic approach. We show that the number of atoms in the nanocrystal determines an upper limit of the number of observable Raman features. The frequency range in which the continuous elastic medium approximation is valid is defined by comparison with the calculations based on the atomistic approach.
+http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.182.html
-http://prb.aps.org/abstract/PRB/v85/i7/e075405
 ----
-'''Electric Field Confinement Effect on Charge Transport in Organic Field-Effect Transistors'''
+'''Polycrystalline Graphene with Single Crystalline Electronic Structure'''
-Xiaoran Li, Andrey Kadashchuk, Ivan I. Fishchuk, Wiljan T. T. Smaal, Gerwin Gelinck, Dirk J. Broer, Jan Genoe, Paul Heremans, and Heinz Bässler
+http://pubs.acs.org/doi/abs/10.1021/nl502445j
-While it is known that the charge-carrier mobility in organic semiconductors is only weakly dependent on the electric field at low fields, the experimental mobility in organic field-effect transistors using silylethynyl-substituted pentacene is found to be surprisingly field dependent at low source-drain fields. Corroborated by scanning Kelvin probe measurements, we explain this observation by the severe difference between local conductivities within grains and at grain boundaries. Redistribution of accumulated charges creates very strong local lateral fields in the latter regions. We further confirm this picture by verifying that the charge mobility in channels having no grain boundaries, made from the same organic semiconductor, is not significantly field dependent. We show that our model allows us to quantitatively model the source-drain field dependence of the mobility in polycrystalline organic transistors.
-http://prl.aps.org/abstract/PRL/v108/i6/e066601
 ----
-'''Excitation of collective modes in a quantum flute'''
+'''Angle-dependent van Hove singularities and their breakdown in twisted graphene bilayers'''
-Kristinn Torfason, Andrei Manolescu, Valeriu Molodoveanu, Vidar Gudmundsson
+'Wei Yan, Lan Meng, Mengxi Liu, Jia-Bin Qiao, Zhao-Dong Chu, Rui-Fen Dou, Zhongfan Liu, Jia-Cai Nie, Donald G. Naugle, and Lin He'
-We use a generalized master equation (GME) formalism to describe the non-equilibrium time-dependent transport of Coulomb interacting electrons through a short quantum wire connected to semi-infinite biased leads. The contact strength between the leads and the wire is modulated by out-of-phase time-dependent potentials which simulate a turnstile device. We explore this setup by keeping the contact with one lead at a fixed location at one end of the wire whereas the contact with the other lead is placed on various sites along the length of the wire. We study the propagation of sinusoidal and rectangular pulses. We find that the current profiles in both leads depend not only on the shape of the pulses, but also on the position of the second contact. The current reflects standing waves created by the contact potentials, like in a wind musical instrument (for example a flute), but occurring on the background of the equilibrium charge distribution. The number of electrons in our quantum "flute" device varies between two and three. We find that for rectangular pulses the currents in the leads may flow against the bias for short time intervals, due to the higher harmonics of the charge response. The GME is solved numerically in small time steps without resorting to the traditional Markov and rotating wave approximations. The Coulomb interaction between the electrons in the sample is included via the exact diagonalization method. The system (leads plus sample wire) is described by a lattice model.
+The creation of van der Waals heterostructures based on a graphene monolayer and other two-dimensional crystals has attracted great interest because the atomic registry of the two-dimensional crystals can modify the electronic spectra and properties of graphene. A twisted graphene bilayer can be viewed as a special van der Waals structure composed of two mutually misoriented graphene layers, where the sublayer graphene not only plays the role of a substrate, but also acts in an equivalent role as the top graphene layer in the structure. Here we report the electronic spectra of slightly twisted graphene bilayers studied by scanning tunneling microscopy and spectroscopy. Our experiment demonstrates that twist-induced van Hove singularities are ubiquitously present for rotation angles θ of less than about 3.5°, corresponding to moiré-pattern periods D longer than 4 nm. However, they totally vanish for θ>5.5° (D<2.5nm). Such a behavior indicates that the continuum models, which capture moiré-pattern periodicity more accurately at small rotation angles, are no longer applicable at large rotation angles.
-http://xxx.lanl.gov/abs/1202.0566
+http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.115402
-----
-'''Atomically thin boron nitride: a tunnelling barrier for graphene devices'''
-Liam Britnell, Roman V. Gorbachev, Rashid Jalil, Branson D. Belle, Fred Schedin, Mikhail I. Katsnelson, Laurence Eaves, Sergey V. Morozov, Alexander S. Mayorov, Nuno M. R. Peres, Antonio H. Castro Neto, Jon Leist, Andre K. Geim, Leonid A. Ponomarenko, Kostya S. Novoselov
-We investigate the electronic properties of heterostructures based on ultrathin hexagonal boron nitride (h-BN) crystalline layers sandwiched between two layers of graphene as well as other conducting materials (graphite, gold). The tunnel conductance depends exponentially on the number of h-BN atomic layers, down to a monolayer thickness. Exponential behaviour of I-V characteristics for graphene/BN/graphene and graphite/BN/graphite devices is determined mainly by the changes in the density of states with bias voltage in the electrodes. Conductive atomic force microscopy scans across h-BN terraces of different thickness reveal a high level of uniformity in the tunnel current. Our results demonstrate that atomically thin h-BN acts as a defect-free dielectric with a high breakdown field; it offers great potential for applications in tunnel devices and in field-effect transistors with a high carrier density in the conducting channel.
-http://xxx.lanl.gov/abs/1202.0735
 ----
-'''Gate Defined Quantum Confinement in Suspended Bilayer Graphene'''
+'''Excitation of complex spin dynamics patterns in a quantum-dot electron spin ensemble'''
-Monica T. Allen, Jens Martin, Amir Yacoby
-Quantum confined devices that manipulate single electrons in graphene are emerging as attractive candidates for nanoelectronics applications. Previous experiments have employed etched graphene nanostructures, but edge and substrate disorder severely limit device functionality. Here we present a technique that builds quantum confined structures in suspended bilayer graphene with tunnel barriers defined by external electric fields that break layer inversion symmetry, thereby eliminating both edge and substrate disorder. We report clean quantum dot formation in two regimes: at zero magnetic field B using the single particle energy gap induced by a perpendicular electric field and at B > 0 using the quantum Hall ferromagnet &nu; = 0 gap for confinement. Coulomb blockade oscillations exhibit periodicity consistent with electrostatic simulations based on local top gate geometry, a direct demonstration of local control over the band structure of graphene. This technology integrates single electron transport with high device quality and access to vibrational modes, enabling broad applications from electromechanical sensors to quantum bits.
+http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.121301
-http://xxx.lanl.gov/abs/1202.0820
 ----
-'''Transport through side-coupled double quantum dots: from weak to strong interdot coupling'''
+'''Tunable Floquet Majorana fermions in driven coupled quantum dots'''
-D. Y. Baines, T. Meunier, D. Mailly, A. D. Wieck, C. Bäuerle, L. Saminadayar, Pablo S. Cornaglia, Gonzalo Usaj, C. A. Balseiro, D. Feinberg
+'Yantao Li, Arijit Kundu, Fan Zhong, and Babak Seradjeh'
-We report low-temperature transport measurements through a double quantum dot device in a configuration where one of the quantum dots is coupled directly to the source and drain electrodes, and a second (side-coupled) quantum dot interacts electrostatically and via tunneling to the first one. As the interdot coupling increases, a crossover from weak to strong interdot tunneling is observed in the charge stability diagrams that present a complex pattern with mergings and apparent crossings of Coulomb blockade peaks. While the weak coupling regime can be understood by considering a single level on each dot, in the intermediate and strong coupling regimes, the multi-level nature of the quantum dots needs to be taken into account. Surprisingly, both in the strong and weak coupling regimes, the double quantum dot states are mainly localized on each dot for most values of the parameters. Only in an intermediate coupling regime the device presents a single dot-like molecular behavior as the molecular wavefunctions weight is evenly distributed between the quantum dots. At temperatures larger than the interdot coupling energy scale, a loss of coherence of the molecular states is observed.
+We propose a system of coupled quantum dots in proximity to a superconductor and driven by separate ac potentials to realize and detect Floquet Majorana fermions. We show that the appearance of Floquet Majorana fermions can be finely controlled in the expanded parameter space of the drive frequency, amplitude, and phase difference across the two dots. While these Majorana fermions are not topologically protected, the highly tunable setup provides a realistic system for observing the exotic physics associated with Majorana fermions as well as their dynamical generation and manipulation.
-http://xxx.lanl.gov/abs/1202.1580
+http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.121401
-----
-'''Giant negative magnetoresistance in high-mobility two-dimensional electron systems'''
-A. T. Hatke, M. A. Zudov, J. L. Reno, L. N. Pfeiffer, and K. W. West
-We report on a giant negative magnetoresistance in very high mobility GaAs/AlGaAs heterostructures and quantum wells. The effect is the strongest at B≃1 kG, where the magnetoresistivity develops a minimum emerging at T≲2 K. Unlike the zero-field resistivity which saturates at T≃2 K, the resistivity at this minimum continues to drop at an accelerated rate to much lower temperatures and becomes ''several times smaller'' than the zero-field resistivity. Unexpectedly, we also find that the effect is destroyed not only by increasing temperature but also by modest in-plane magnetic fields. The analysis shows that giant negative magnetoresistance ''cannot'' be explained by existing theories considering interaction-induced or disorder-induced corrections.
-http://prb.aps.org/abstract/PRB/v85/i8/e081304
 ----
-'''Resonance-hybrid states in a triple quantum dot'''
+Unrelated:
-S. Amaha, T. Hatano, H. Tamura, S. Teraoka, T. Kubo, Y. Tokura, D. G. Austing, and S. Tarucha
-Delocalization by resonance between contributing structures explains the enhanced stability of ''resonance-hybrid molecules''. Here we report the realization of resonance-hybrid states in a few-electron triple quantum dot (TQD) obseved by excitation spectroscopy. The stabilization of the resonance-hybrid state and the bond between contributing states are achieved through access to the intermediate states with double occupancy of the dots. This explains why the energy of the hybridized singlet state is signiﬁcantly lower than that of the triplet state. The properties of the three-electron doublet states can also be understood with the resonance-hybrid picture and geometrical phase. As well as for fundamental TQD physics, our results are useful for the investigation of materials such as quantum dot arrays, quantum information processors, and chemical reaction and quantum simulators.
-http://prb.aps.org/pdf/PRB/v85/i8/e081301
-----
-----
-== Jan. 25. - Feb. 1. (2012) ==
-''Válogatta: Balogh Zoltán''
-----
-'''A local optical probe for measuring motion and stress in a nanoelectromechanical system'''
-Antoine Reserbat-Plantey, Lae¨titia Marty, Olivier Arcizet, Nedjma Bendiab and Vincent Bouchiat
-Nanoelectromechanical systems can be operated as ultrasensitive
-mass sensors and ultrahigh-frequency resonators, and
-can also be used to explore fundamental physical phenomena
-such as nonlinear damping and quantum effects in macroscopic
-objects. Various dissipation mechanisms are known to
-limit the mechanical quality factors of nanoelectromechanical
-systems and to induce aging due to material degradation, so
-there is a need for methods that can probe the motion of
-these systems, and the stresses within them, at the nanoscale.
-Here, we report a non-invasive local optical probe for the
-quantitative measurement of motion and stress within a nanoelectromechanical
-system, based on Fizeau interferometry and
-Raman spectroscopy. The system consists of a multilayer graphene
-resonator that is clamped to a gold film on an oxidized
-silicon surface. The resonator and the surface both act as
-mirrors and therefore define an optical cavity. Fizeau interferometry
-provides a calibrated measurement of the motion of the
-resonator, while Raman spectroscopy can probe the strain
-within the system and allows a purely spectral detection of
-mechanical resonance at the nanoscale.
-http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2011.250.html
-----
-'''How Does a Single Pt Nanocatalyst Behave in Two Different Reactions? A Single-Molecule Study'''
-Kyu Sung Han, Guokun Liu, Xiaochun Zhou, Rita E. Medina, and Peng Chen
-Using single-molecule microscopy of fluorogenic reactions we studied Pt nanoparticle catalysis at single-particle, single-turnover resolution for two reactions: one an oxidative N-deacetylation and the other a reductive N-deoxygenation. These Pt nanoparticles show distinct catalytic kinetics in these two reactions: one following noncompetitive reactant adsorption and the other following competitive reactant adsorption. In both reactions, single nanoparticles exhibit temporal activity fluctuations attributable to dominantly spontaneous surface restructuring. Depending on the reaction sequence, single Pt nanoparticles may or may not show activity correlations in catalyzing both reactions, reflecting the structure insensitivity of the N-deacetylation reaction and the structure sensitivity of the N-deoxygenation reaction.
-http://pubs.acs.org/doi/abs/10.1021/nl203677b
-----
-'''Single-Molecule Force-Clamp Experiments Reveal Kinetics of Mechanically Activated Silyl Ester Hydrolysis'''
-Sebastian W. Schmidt, Pavel Filippov, Alfred Kersch, Martin K. Beyer, and Hauke Clausen-Schaumann
-We have investigated the strength of silyl ester bonds formed between carboxymethylated amylose (CMA) molecules and silane-functionalized silicon oxide surfaces using AFM-based single-molecule force spectroscopy in the force-clamp mode. Single tethered CMA molecules were picked up, and bond lifetimes were determined at constant clamp forces of 0.8, 1.0, and 1.2 nN at seven temperatures between 295 and 320 K at pH 2.0. The results reveal biexponential rupture kinetics. To obtain the reaction rate constants for each force and temperature individually, the results were analyzed with a biexponential kinetic model using the maximum likelihood estimation (MLE) method. The force-independent kinetic and structural parameters of the underlying bond rupture mechanisms were extracted by fitting the entire data set with a parallel MLE fit procedure using the Zhurkov/Bell model and, alternatively, an Arrhenius kinetics model combined with a Morse potential as an analytic representation of the binding potential. With activation energies between 37 and 40 kJ mol–1, and with Arrhenius prefactors between 5 × 104 and 2 × 106 s–1, the results point to the hydrolysis of the silyl ester bond.
-http://pubs.acs.org/doi/abs/10.1021/nn204111w
-----
-'''A surface-anchored molecular four-level conductance switch based on single proton transfer'''
-Willi Auwa¨rter, Knud Seufert, Felix Bischoff, David Ecija, Saranyan Vijayaraghavan, Sushobhan Joshi, Florian Klappenberger, Niveditha Samudrala and Johannes V. Barth
-The development of a variety of nanoscale applications
-requires the fabrication and control of atomic or molecular
-switches that can be reversibly operated by light, a shortrange
-force electric current or other external stimuli.
-For such molecules to be used as electronic components, they
-should be directly coupled to a metallic support and the switching
-unit should be easily connected to other molecular species
-without suppressing switching performance. Here, we show
-that a free-base tetraphenyl-porphyrin molecule, which is
-anchored to a silver surface, can function as a molecular conductance
-switch. The saddle-shaped molecule has two hydrogen
-atoms in its inner cavity that can be flipped between two
-states with different local conductance levels using the electron
-current through the tip of a scanning tunnelling microscope.
-Moreover, by deliberately removing one of the hydrogens, a
-four-level conductance switch can be created. The resulting
-device, which could be controllably integrated into the
-surrounding nanoscale environment, relies on the transfer of a
-single proton and therefore contains the smallest possible
-atomistic switching unit.
-http://www.nature.com/nnano/journal/v7/n1/full/nnano.2011.211.html
-----
-'''Transport spectroscopy of symmetry-broken insulating states in bilayer graphene'''
-J. Velasco Jr, L. Jing, W. Bao, Y. Lee, P. Kratz, V. Aji, M. Bockrath, C. N. Lau, C. Varma, R. Stillwell, D. Smirnov, Fan Zhang, J. Jung & A. H. MacDonald
-Bilayer graphene is an attractive platform for studying new two-dimensional electron physics, because its flat energy bands are sensitive to out-of-plane electric fields and these bands magnify electron–electron interaction effects. Theory predicts a variety of interesting broken symmetry states when the electron density is at the carrier neutrality point, and some of these states are characterized by spontaneous mass gaps, which lead to insulating behaviour. These proposed gaps are analogous to the masses generated by broken symmetries in particle physics, and they give rise to large Berry phase effects accompanied by spontaneous quantum Hall effects. Although recent experiments have provided evidence for strong electronic correlations near the charge neutrality point, the presence of gaps remains controversial. Here, we report transport measurements in ultraclean double-gated bilayer graphene and use source–drain bias as a spectroscopic tool to resolve a gap of ~2 meV at the charge neutrality point. The gap can be closed by a perpendicular electric field of strength ~15 mV nm−1, but it increases monotonically with magnetic field, with an apparent particle–hole asymmetry above the gap. These data represent the first spectroscopic mapping of the ground states in bilayer graphene in the presence of both electric and magnetic fields.
-http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2011.251.html
-----
-'''Plasmonic Systems Unveiled by Fano Resonances'''
-Yan Francescato, Vincenzo Giannini, and Stefan A. Maier
-We show in detail how a derivation of Fano theory can serve as a new paradigm to study, understand, and control the interaction of nano-objects with light. Examples include a plasmonic crystal, a dolmen-type structure sustaining dark and bright plasmon modes, and a nanoshell heptamer. On the basis of only three coupling factors, a straightforward analytical formula is obtained, only assuming a plasmonic resonance for the continuum, and retaining the nonclassical character of the original formalism. It allows one to predict, reproduce, or decompose Fano interferences solely in terms of the physical properties of the uncoupled nanostructures when available, without the need of additional fitting parameters.
-http://pubs.acs.org/doi/abs/10.1021/nn2050533
-----
-'''Electromagnetic Energy Transport in Nanoparticle Chains via Dark Plasmon Modes'''
-David Solis, Jr., Britain Willingham, Scott L. Nauert, Liane S. Slaughter, Jana Olson, Pattanawit Swanglap, Aniruddha Paul, Wei-Shun Chang, and Stephan Link
-Using light to exchange information offers large bandwidths and high speeds, but the miniaturization of optical components is limited by diffraction. Converting light into electron waves in metals allows one to overcome this problem. However, metals are lossy at optical frequencies and large-area fabrication of nanometer-sized structures by conventional top-down methods can be cost-prohibitive. We show electromagnetic energy transport with gold nanoparticles that were assembled into close-packed linear chains. The small interparticle distances enabled strong electromagnetic coupling causing the formation of low-loss subradiant plasmons, which facilitated energy propagation over many micrometers. Electrodynamic calculations confirmed the dark nature of the propagating mode and showed that disorder in the nanoparticle arrangement enhances energy transport, demonstrating the viability of using bottom-up nanoparticle assemblies for ultracompact opto-electronic devices.
-http://pubs.acs.org/doi/abs/10.1021/nl2039327
-----
-'''Flexible Gigahertz Transistors Derived from Solution-Based Single-Layer Graphene'''
-Cédric Sire, Florence Ardiaca, Sylvie Lepilliet, Jung-Woo T. Seo, Mark C. Hersam, Gilles Dambrine, Henri Happy, and Vincent Derycke
-Flexible electronics mostly relies on organic semiconductors but the limited carrier velocity in polymers and molecular films prevents their use at frequencies above a few megahertz. Conversely, the high potential of graphene for high-frequency electronics on rigid substrates was recently demonstrated. We conducted the first study of solution-based graphene transistors at gigahertz frequencies, and we show that solution-based single-layer graphene ideally combines the required properties to achieve high speed flexible electronics on plastic substrates. Our graphene flexible transistors have current gain cutoff frequencies of 2.2 GHz and power gain cutoff frequencies of 550 MHz. Radio frequency measurements directly performed on bent samples show remarkable mechanical stability of these devices and demonstrate the advantages of solution-based graphene field-effect transistors over other types of flexible transistors based on organic materials.
-http://pubs.acs.org/doi/abs/10.1021/nl203316r
-----
-'''Atomically localized plasmon enhancement in monolayer graphene'''
-Wu Zhou, Jaekwang Lee, Jagjit Nanda, Sokrates T. Pantelides, Stephen J. Pennycook & Juan-Carlos Idrobo
-Plasmons in graphene can be tuned by using electrostatic gating or chemical doping, and the ability to confine plasmons in very small regions could have applications in optoelectronics, plasmonics and transformation optics12. However, little is known about how atomic-scale defects influence the plasmonic properties of graphene. Moreover, the smallest localized plasmon resonance observed in any material to date has been limited to around 10 nm. Here, we show that surface plasmon resonances in graphene can be enhanced locally at the atomic scale. Using electron energy-loss spectrum imaging in an aberration-corrected scanning transmission electron microscope, we find that a single point defect can act as an atomic antenna in the petahertz (1015 Hz) frequency range, leading to surface plasmon resonances at the subnanometre scale.
-http://www.nature.com/nnano/journal/vaop/ncurrent/abs/nnano.2011.252.html
-----
-'''Measurement of Quantum Noise in a Carbon Nanotube Quantum Dot in the Kondo Regime'''
-J. Basset, A. Yu. Kasumov, C. P. Moca, G. Zaránd, P. Simon, H. Bouchiat, and R. Deblock
-he current emission noise of a carbon nanotube quantum dot in the Kondo regime is measured at frequencies ν of the order or higher than the frequency associated with the Kondo effect kBTK/h, with TK the Kondo temperature. The carbon nanotube is coupled via an on-chip resonant circuit to a quantum noise detector, a superconductor-insulator-superconductor junction. We find for hν≈kBTK a Kondo effect related singularity at a voltage bias eV≈hν, and a strong reduction of this singularity for hν≈3kBTK, in good agreement with theory. Our experiment constitutes a new original tool for the investigation of the nonequilibrium dynamics of many-body phenomena in nanoscale devices.
-http://prl.aps.org/abstract/PRL/v108/i4/e046802
-----
-'''Complete Optical Absorption in Periodically Patterned Graphene'''
-Sukosin Thongrattanasiri, Frank H. L. Koppens, and F. Javier García de Abajo
-We demonstrate that 100% light absorption can take place in a single patterned sheet of doped graphene. General analysis shows that a planar array of small particles with losses exhibits full absorption under critical-coupling conditions provided the cross section of each individual particle is comparable to the area of the lattice unit cell. Specifically, arrays of doped graphene nanodisks display full absorption when supported on a substrate under total internal reflection and also when lying on a dielectric layer coating a metal. Our results are relevant for infrared light detectors and sources, which can be made tunable via electrostatic doping of graphene.
-http://prl.aps.org/abstract/PRL/v108/i4/e047401
-----
-== Nov. 17. - Nov. 23. (2011) ==
-''Válogatta: Oroszlány  László''
-----
-== Nov. 17. - Nov. 23. (2011) ==
-''Válogatta: Csontos Miklós''
-----
-'''Single-Electron Capacitance Spectroscopy of Individual Dopants in Silicon'''
-M. Gasseller, M. DeNinno, R. Loo, J. F. Harrison, M. Caymax, S. Rogge and S. H. Tessmer
-Motivated by recent transport experiments and proposed atomic-scale semiconductor devices, we present measurements that extend the reach of scanned-probe methods to discern the properties of individual dopants tens of nanometers below the surface of a silicon sample. Using a capacitance-based approach, we have both spatially resolved individual subsurface boron acceptors and detected spectroscopically single holes
-entering and leaving these minute systems of atoms. A resonance identified as the B+ state is shown to shift in energy from acceptor to acceptor. We examine this behavior with respect to nearestneighbor distances. By directlymeasuring the quantum levels and testing the effect of dopant-dopant interactions, this method represents a valuable tool for the development of future atomicscale semiconductor devices.
-http://pubs.acs.org/doi/abs/10.1021/nl2025163
-----
-'''Coulomb Blockade in an Open Quantum Dot'''
-S. Amasha, I. G. Rau, M. Grobis, R. M. Potok, H. Shtrikman and D. Goldhaber-Gordon
-We report the observation of Coulomb blockade in a quantum dot contacted by two quantum point contacts each with a single fully transmitting mode, a system thought to be well described without invoking Coulomb interactions. Below 50 mK we observe a periodic oscillation in the conductance of the dot with gate voltage, corresponding to a residual quantization of charge. From the temperature and magnetic field dependence, we infer the oscillations are mesoscopic Coulomb blockade, a type of Coulomb blockade caused by electron interference in an otherwise open system.
-http://prl.aps.org/abstract/PRL/v107/i21/e216804
-----
-'''Majorana fermions in a topological-insulator nanowire proximity-coupled to an s-wave superconductor'''
-A. Cook and M. Franz
-A finite-length topological-insulator nanowire, proximity-coupled to an ordinary bulk s-wave superconductor and subject to a longitudinal applied magnetic field, is shown to realize a one-dimensional topological
-superconductor with unpaired Majorana fermions localized at both ends. This situation occurs under a wide range of conditions and constitutes an easily accessible physical realization of the elusive Majorana particle in a solid-state system.
-http://prb.aps.org/abstract/PRB/v84/i20/e201105
-----
-'''Josephson and Andreev transport through quantum dots'''
-A. Martin-Rodero and A. Levy Yeyati
-In this article we review the state of the art on the transport properties of quantum dot systems connected to superconducting and normal electrodes. The review is mainly focused on the theoretical achievements although a summary of the most relevant experimental results is also given. A large part of the discussion is devoted to the single level Anderson type models generalized to include superconductivity in the leads, which already contains most of the interesting physical phenomena. Particular attention is paid to the competition between pairing and Kondo correlations, the emergence of pi-junction behavior, the interplay of Andreev and resonant tunneling, and the important role of Andreev bound states which characterized the spectral properties of most of these systems. We give technical details on the several different analytical and numerical methods which have been developed for describing these properties. We further discuss the recent theoretical efforts devoted to extend this analysis to more complex situations like multidot, multilevel or multiterminal configurations in which novel phenomena is expected to emerge. These include control of the localized spin states by a Josephson current and also the possibility of creating entangled electron pairs by means of non-local Andreev processes.
-http://xxx.lanl.gov/abs/1111.4939
-----
-'''Time scales in the dynamics of an interacting quantum dot'''
-L. Debora Contreras-Pulido, Janine Splettstoesser, Michele Governale, Jurgen Konig and Markus Buttiker
-We analyze the dynamics of a single-level quantum dot with Coulomb interaction, weakly tunnel coupled to an electronic reservoir, after it has been brought out of equilibrium, e.g. by a step-pulse potential. We investigate the exponential decay towards the equilibrium state, which is governed by three time scales. In addition to the charge and spin relaxation time there is a third time scale which is independent of the level position and the Coulomb interaction. This time scale emerges in the time evolution of physical quantities sensitive to two-particle processes.
-http://xxx.lanl.gov/abs/1111.4135
-----
-'''Spin-half paramagnetism in graphene induced by point defects'''
-R. R. Nair, M. Sepioni, I-Ling Tsai, O. Lehtinen, J. Keinonen, A. V. Krasheninnikov, T. Thomson, A. K. Geim and I. V. Grigorieva
-Using magnetization measurements, we show that point defects in graphene – fluorine adatoms and irradiation defects (vacancies) – carry magnetic moments with spin 1/2. Both types of defects lead to notable
-paramagnetism but no magnetic ordering could be detected down to liquid helium temperatures. The induced paramagnetism dominates graphene’s low-temperature magnetic properties despite the fact that maximum response we could achieve was limited to one moment per approximately 1000 carbon atoms. This limitation is explained by clustering of adatoms and, for the case of vacancies, by losing graphene’s structural stability.
-http://xxx.lanl.gov/abs/1111.3775
-----
-'''Majorana Fermions in Semiconductor Nanowires'''
-Tudor D. Stanescu, Roman M. Lutchyn and S. Das Sarma
-We study multiband semiconducting nanowires proximity-coupled with an s-wave superconductor and calculate the topological phase diagram as a function of the chemical potential and magnetic field. The non-trivial topological state corresponds to a superconducting phase supporting an odd number of pairs of Majorana modes localized at the ends of the wire, whereas the non-topological state corresponds to a superconducting phase with no Majoranas or with an even number of pairs of Majorana modes. Our key finding is that multiband occupancy not only lifts the stringent constraint of one-dimensionality, but also allows having higher carrier density in the nanowire. Consequently, multiband nanowires are better-suited for stabilizing the topological superconducting phase and for observing the Majorana physics. We present a detailed study of the parameter space for multiband semiconductor nanowires focusing on understanding the key experimental conditions required for the realization and detection of Majorana fermions in solid-state systems. We include various sources of disorder and characterize their effects on the stability of the topological phase. Finally, we calculate the local density of states as well as the differential tunneling conductance as functions of external parameters and predict the experimental signatures that would establish the existence of emergent Majorana zero-energy modes in solid-state systems.
-http://xxx.lanl.gov/abs/1106.3078
-----
-== Nov. 3. - Nov. 10. (2011) ==
-''Válogatta: Makk Péter''
-----
-'''Spatially resolved electronic inhomogeneities of graphene due to subsurface charges'''
-Andres Castellanos-Gomez, Roel H. M. Smit, Nicolás Agraït, Gabino Rubio-Bollinger
-We probe the local inhomogeneities in the electronic properties of exfoliated graphene due to the presence of charged impurities in the SiO2 substrate using a combined scanning tunneling and electrostatic force microscope. Contact potential difference measurements using electrostatic force microscopy permit us to obtain the average charge density but it does not provide enough resolution to identify individual charges. We find that the tunneling current decay constant, which is related to the local tunneling barrier height, enables one to probe the electronic properties of graphene distorted at the nanometer scale by individual charged impurities. We observe that such inhomogeneities do not show long range ordering and their surface density obtained by direct counting is consistent with the value obtained by macroscopic charge density measurements. These microscopic perturbations of the carrier density significantly alter the electronic properties of graphene, and their characterization is essential for improving the performance of graphene based devices.
-http://arxiv.org/abs/1111.0840
-----
-'''Switching and Rectification of a Single Light-sensitive Diarylethene Molecule Sandwiched between Graphene Nanoribbons'''
-Yongqing Cai, Aihua Zhang, Yuan Ping Feng, Chun Zhang
-The 'open' and 'closed' isomers of the diarylethene molecule that can be converted between each other upon photo-excitation are found to have drastically different current-voltage characteristics when sandwiched between two graphene nanoribbons (GNRs). More importantly, when one GNR is metallic and another one is semiconducting, strong rectification behavior of the 'closed' diarylethene isomer with the rectification ratio >10^3 is observed. The surprisingly high rectification ratio originates from the band gap of GNR and the bias-dependent variation of the lowest unoccupied molecular orbital (LUMO) of the diarylethene molecule, the combination of which completely shuts off the current at positive biases. Results presented in this paper may form the basis for a new class of molecular electronic devices.
-http://arxiv.org/abs/1111.1811
-----
-'''Majorana fermions in superconducting nanowires without spin-orbit coupling'''
-Authors: Morten Kjærgaard, Konrad Wölms, Karsten Flensberg
-We show that confined Majorana fermions can exist in nanowires with proximity induced s-wave superconducting pairing if the direction of an external magnetic field rotates along the wire. The system is equivalent to nanowires with Rashba-type spin-orbit coupling, with strength proportional to the derivative of the field angle. For realistic parameters, we demonstrate that a set of permanent magnets can bring a nearby nanowire into the topologically non-trivial phase with localized Majorana modes at its ends. Without the requirement of spin-orbit coupling this opens up for a new route for demonstration and design of Majorana fermion systems.
-http://arxiv.org/abs/1111.2129
-----
-'''Role of Polytetrahedral Structures in the Elongation and Rupture of Gold Nanowires'''
-Christopher R. Iacovella†, William R. French†, Brandon G. Cook‡, Paul R. C. Kent§, and Peter T. Cummings†§*
-We report comprehensive high-accuracy molecular dynamics simulations using the ReaxFF force field to explore the structural changes that occur as Au nanowires are elongated, establishing trends as a function of both temperature and nanowire diameter. Our simulations and subsequent quantitative structural analysis reveal that polytetrahedral structures (e.g., icosahedra) form within the “amorphous” neck regions, most prominently for systems with small diameter at high temperature. We demonstrate that the formation of polytetrahedra diminishes the conductance quantization as compared to systems without this structural motif. We demonstrate that use of the ReaxFF force field, fitted to high-accuracy first-principles calculations of Au, combines the accuracy of quantum calculations with the speed of semiempirical methods.
-http://pubs.acs.org/doi/abs/10.1021/nn203941r
-----
-'''Spin Relaxation in InGaN Quantum Disks in GaN Nanowires'''
-Animesh Banerjee†, Fatih Doğan‡, Junseok Heo†, Aurelien Manchon‡, Wei Guo†, and Pallab Bhattacharya
-The spin relaxation time of photoinduced conduction electrons has been measured in InGaN quantum disks in GaN nanowires as a function of temperature and In composition in the disks. The relaxation times are of the order of 100 ps at 300 K and are weakly dependent on temperature. Theoretical considerations show that the Elliott–Yafet scattering mechanism is essentially absent in these materials and the results are interpreted in terms of the D’yakonov–Perel’ relaxation mechanism in the presence of Rashba spin–orbit coupling of the wurtzite structure. The calculated spin relaxation times are in good agreement with the measured values.
-http://pubs.acs.org/doi/abs/10.1021/nl203091f
-----
-'''Electronic transport through single noble gas atoms '''
-L. A. Zotti1, M. Bürkle2, Y. J. Dappe3, F. Pauly2, and J. C. Cuevas1
-We present a theoretical study of the conductance of atomic junctions comprising single noble gas atoms (He, Ne, Ar, Kr, and Xe) coupled to gold electrodes. The aim is to elucidate how the presence of noble gas atoms affects the electronic transport through metallic atomic-size contacts. Our analysis, based on density functional theory and including van der Waals interactions, shows that for the lightest elements (He and Ne) no significant current flows through the noble gas atoms and their effect is to reduce the conductance of the junctions by screening the interaction between the gold electrodes. This explains the observations reported in metallic atomic-size contacts with adsorbed He atoms. Conversely, the heaviest atoms (Kr and Xe) increase the conductance because of the additional current path provided by their valence p states.
-http://prb.aps.org/abstract/PRB/v84/i19/e193404
-----
-'''Andreev nanoprobe of half-metallic CrO2 films using superconducting cuprate tips '''
-C. S. Turel, I. J. Guilaran, P. Xiong, and J. Y. T. Wei
-Superconducting tips of YBa2Cu3O7−x were used to perform point-contact Andreev reflection spectroscopy on half-metallic CrO2 thin films. At 4.2 K, strong suppression of the d-wave Andreev reflection characteristics was observed, consistent with the high spin polarization of CrO2. Our technique was validated by comparison with data taken on non-magnetic Au films and with data taken by superconducting Pb tips. The point contacts were estimated to be ≲10  nm in size, attesting to their ballistic and microscopic nature. Our results demonstrate the feasibility of using superconducting cuprate tips as spin-sensitive nanoprobes of ferromagnets.
-http://apl.aip.org/resource/1/applab/v99/i19/p192508_s1
-----
-'''Spin-filter Josephson junctions'''
-Kartik Senapati,Mark G. Blamire & Zoe H. Barber
-Josephson junctions with ferromagnetic barriers have been intensively investigated in recent years1. Of particular interest has been the realization of so called π-junctions with a built-in phase difference2, and induced triplet pairing3, 4. Such experiments have so far been limited to systems containing metallic ferromagnets. Although junctions incorporating a ferromagnetic insulator (IF) have been predicted to show a range of unique properties including π-shifts with intrinsically low dissipation5, 6 and an unconventional temperature dependence7 of the critical current Ic, difficulties with the few known IF materials have prevented experimental tests. Here we report supercurrents through magnetic GdN barriers and show that the field and temperature dependence of Icis strongly modified by the IF. In particular we show that the strong suppression of Cooper pair tunnelling by the spin filtering of the IF barrier can be modified by magnetic inhomogeneity in the barrier.
-http://www.nature.com/nmat/journal/v10/n11/pdf/nmat3116.pdf
-----
-'''Fast control of nuclear spin polarization in an optically pumped single quantum dot'''
-M. N. Makhonin, K. V. Kavokin, P. Senellart, A. Lemaître,A. J. Ramsay, M. S. Skolnick & A. I. Tartakovskii
-Highly polarized nuclear spins within a semiconductor quantum dot induce effective magnetic (Overhauser) fields of up to several Tesla acting on the electron spin1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or up to a few hundred mT for the hole spin13, 14. Recently this has been recognized as a resource for intrinsic control of quantum-dot-based spin quantum bits. However, only static long-lived Overhauser fields could be used10, 11. Here we demonstrate fast redirection on the microsecond timescale of Overhauser fields on the order of 0.5 T experienced by a single electron spin in an optically pumped GaAs quantum dot. This has been achieved using coherent control of an ensemble of 105 optically polarized nuclear spins by sequences of short radiofrequency pulses. These results open the way to a new class of experiments using radiofrequency techniques to achieve highly correlated nuclear spins in quantum dots, such as adiabatic demagnetization in the rotating frame15 leading to sub-μK nuclear spin temperatures, rapid adiabatic passage15, and spin squeezing16.
-http://www.nature.com/nmat/journal/v10/n11/pdf/nmat3102.pdf
-----
-== Okt. 28. - Nov. 3. (2011) ==
-''Válogatta: Márton Attila''
-----
-'''Positive and negative Coulomb drag in vertically integrated one-dimensional quantum wires'''
-D. Laroche, G. Gervais, M. P. Lilly& J. L. Reno
-DOI:10.1038/nnano.2011.182
-Received 03 August 2011; Accepted 23 September 2011; Published online 30 October 2011
-Electron interactions in and between wires become increasingly complex and important as circuits are scaled to nanometre sizes, or use reduced-dimensional conductors1 such as carbon nanotubes2, 3, 4, 5, 6, nanowires7, 8, 9,10 and gated high-mobility two-dimensional electron systems11, 12, 13. This is because the screening of the long-range Coulomb potential of individual carriers is weakened in these systems, which can lead to phenomena such as Coulomb drag, where a current in one wire induces a voltage in a second wire through Coulomb interactions alone. Previous experiments have demonstrated Coulomb electron drag in wires separated by a soft electrostatic barrier of width ≳80 nm (ref. 12), which was interpreted as resulting entirely from momentum transfer. Here, we measure both positive and negative drag between adjacent vertical quantum wires that are separated by ~15 nm and have independent contacts, which allows their electron densities to be tuned independently. We map out the drag signal versus the number of electron sub-bands occupied in each wire, and interpret the results both in terms of momentum-transfer and charge-fluctuation induced transport models. For wires of significantly different sub-band occupancies, the positive drag effect can be as large as 25%.
-http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2011.182.html
-+1: http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2011.197.html
-----
-'''Spin Relaxation in InGaN Quantum Disks in GaN Nanowires'''
-Animesh Banerjee†, Fatih Doğan‡, Junseok Heo†, Aurelien Manchon‡, Wei Guo†, and Pallab Bhattacharya*†
-DOI: 10.1021/nl203091f
-The spin relaxation time of photoinduced conduction electrons has been measured in InGaN quantum disks in GaN nanowires as a function of temperature and In composition in the disks. The relaxation times are of the order of  100 ps at 300 K and are weakly dependent on temperature. Theoretical considerations show that the Elliott–Yafet scattering mechanism is essentially absent in these materials and the results are interpreted in terms of the D’yakonov–Perel’ relaxation mechanism in the presence of Rashba spin–orbit coupling of the wurtzite structure. The calculated spin relaxation times are in good agreement with the measured values.
-http://pubs.acs.org/doi/abs/10.1021/nl203091f
-----
-'''Joule Heating and Spin-Transfer Torque Investigated on the Atomic Scale Using a Spin-Polarized Scanning Tunneling Microscope'''
-S. Krause*, G. Herzog, A. Schlenhoff, A. Sonntag, and R. Wiesendanger
-DOI: 10.1103/PhysRevLett.107.186601
-The influence of a high spin-polarized tunnel current onto the switching behavior of a superparamagnetic nanoisland on a nonmagnetic substrate is investigated by means of spin-polarized scanning tunneling microscopy. A detailed lifetime analysis allows for a quantification of the effective temperature rise of the nanoisland and the modification of the activation energy barrier for magnetization reversal, thereby using the nanoisland as a local thermometer and spin-transfer torque analyzer. Both the Joule heating and spin-transfer torque are found to scale linearly with the tunnel current. The results are compared to experiments performed on lithographically fabricated magneto-tunnel junctions, revealing a very high spin-transfer torque switching efficiency in our experiments.
-http://link.aps.org/doi/10.1103/PhysRevLett.107.186601
-----
-'''Coulomb stability of the 4π-periodic Josephson effect of Majorana fermions'''
-B. van Heck, F. Hassler, A. R. Akhmerov, and C. W. J. Beenakker
-DOI: 10.1103/PhysRevB.84.180502
-The Josephson energy of two superconducting islands containing Majorana fermions is a 4π-periodic function of the superconducting phase difference. If the islands have a small capacitance, their ground state energy is governed by the competition of Josephson and charging energies. We calculate this ground-state energy in a ring geometry, as a function of the flux Φ enclosed by the ring, and show that the dependence on the Aharonov-Bohm phase 2eΦ/ℏ remains 4π periodic regardless of the ratio of charging and Josephson energies—provided that the entire ring is in a topologically nontrivial state. If part of the ring is topologically trivial, then the charging energy induces quantum phase slips that restore the usual 2π periodicity.
-http://link.aps.org/doi/10.1103/PhysRevB.84.180502
-----
-'''Spin-orbit coupling induced enhancement of superconductivity in a two-dimensional repulsive gas of fermions'''
-Oskar Vafek and Luyang Wang
-DOI: 10.1103/PhysRevB.84.172501
-We study a model of a two-dimensional repulsive Fermi gas with Rashba spin-orbit coupling αR and investigate the superconducting instability using the renormalization-group approach. We find that, in general, superconductivity is enhanced as the dimensionless ratio 1/2mαR2/EF increases, resulting in unconventional superconducting states which break time-reversal symmetry.
-http://link.aps.org/doi/10.1103/PhysRevB.84.172501
-----
-'''Reading and writing charge on graphene devices'''
-M. R. Connolly, E. D. Herbschleb, R. K. Puddy, M. Roy, D. Anderson, G. A. C. Jones, P. Maksym, C. G. Smith
-We use a combination of charge writing and scanning gate microscopy to map and modify the local charge neutrality point of graphene field-effect devices. We give a demonstration of the technique by writing remote charge in a thin dielectric layer over the graphene-metal interface and detecting the resulting shift in local charge neutrality point. We perform electrostatic simulations to characterize the gating effect of a realistic scanning probe tip on a graphene bilayer and find a good agreement with the experimental results.
-http://xxx.lanl.gov/abs/1111.0560
-----
-'''The influence of anisotropic gate potentials on the phonon induced spin-flip rate in GaAs quantum dots'''
-Sanjay Prabhakar, Roderick V. N. Melnik, Luis L. Bonilla
-We study the anisotropic orbital effect in the electric field tunability of the phonon induced spin-flip rate in quantum dots (QDs). Our study shows that anisotropic gate potential enhances the spin-flip rate and reduces the level crossing point to a lower quantum dot radius due to the suppression of the Land$\acute{e}$ g-factor towards bulk crystal. In the range of $10^4-10^6$ V/cm, the electric field tunability of the phonon induced spin-flip rate can be manipulated through strong Dresselhaus spin-orbit coupling. These results might assist the development of a spin based solid state quantum computer by manipulating phonon induced spin-flip rate through spin-orbit coupling with the application of anisotropic gate potential in a regime where the g-factor changes its sign.
-http://xxx.lanl.gov/abs/1111.0558
-----
-'''Enhancement of shot noise due to the fluctuation of Coulomb interaction'''
-Duo Li, Lei Zhang, Fuming Xu, Jian Wang
-We have developed a theoretical formalism to investigate the contribution of fluctuation of Coulomb interaction to the shot noise based on Keldysh non-equilibrium Green's function method. We have applied our theory to study the behavior of dc shot noise of atomic junctions using the method of nonequilibrium Green's function combined with the density functional theory (NEGF-DFT). In particular, for atomic carbon wire consisting 4 carbon atoms in contact with two Al(100) electrodes, first principles calculation within NEGF-DFT formalism shows a negative differential resistance (NDR) region in I-V curve at finite bias due to the effective band bottom of the Al lead. We have calculated the shot noise spectrum using the conventional gauge invariant transport theory with Coulomb interaction considered explicitly on the Hartree level along with exchange and correlation effect. Although the Fano factor is enhanced from 0.6 to 0.8 in the NDR region, the expected super-Poissonian behavior in the NDR regionis not observed. When the fluctuation of Coulomb interaction is included in the shot noise, our numerical results show that the Fano factor is greater than one in the NDR region indicating a super-Poissonian behavior.
-http://xxx.lanl.gov/abs/1111.0112
-----
-'''Datta-Das transistor in the quantum Hall regime'''
-Luca Chirolli, D. Venturelli, F. Taddei, Rosario Fazio, V. Giovannetti
-We propose a mechanism to couple spin-resolved edge states in the integer quantum Hall effect by employing an array of voltage-controlled top gates. Strong enhancement of the coupling is achieved when the array periodicity matches the inverse of the wave-vector difference of the two states involved. Well known techniques of separately contacting the edge states make possible to selectively populate and read-out the edge states, allowing full spin read-out. Our device represents the quantum Hall version of the all-electrical Datta-Das spin-field effect transistor.
-http://xxx.lanl.gov/abs/1111.0675
-----
-== Okt. 21. - Okt. 27. (2011) ==
-''Válogatta: Csonka Szabolcs''
-----
-'''Charge detection in a bilayer graphene quantum dot'''
-Stefan Fringes, Christian Volk, Caroline Norda, Bernat Terrés, Jan Dauber, Stephan Engels, Stefan Trellenkamp, Christoph Stampfer
-http://arxiv.org/abs/1110.5811
-----
-'''Tunable capacitive inter-dot coupling in a bilayer graphene double quantum dot'''
-Stefan Fringes, Christian Volk, Bernat Terrés, Jan Dauber, Stephan Engels, Stefan Trellenkamp, Christoph Stampfer
-http://arxiv.org/abs/1110.5803
-----
-'''Joule-assisted silicidation for short-channel silicon nanowire devices'''
-Massimo Mongillo, Panayotis Spathis, Georgios Katsaros, Pascal Gentile, Marc Sanquer, Silvano De Franceschi
-http://arxiv.org/abs/1110.5668
-----
-'''Majorana fermions in superconducting helical magnets'''
-Ivar Martin, Alberto F. Morpurgo
-http://arxiv.org/abs/1110.5637
-----
-'''Magnetoresistance of individual ferromagnetic GaAs/(Ga,Mn)As core-shell nanowires'''
-Christian H. Butschkow, Elisabeth Reiger, Stefan Geißler, Andreas Rudolph, Marcello Soda, Dieter Schuh, Georg Woltersdorf, Werner Wegscheider, Dieter Weiss
-http://arxiv.org/abs/1110.5507
-----
-'''Voltage induced conformational changes and current control in charge transfer through molecules'''
-Lars Kecke, Joachim Ankerhold
-http://arxiv.org/abs/1110.5505
-----
-'''Design of new superconducting materials, and point contact spectroscopy as a probe of strong electron correlations'''
-Laura H. Greene, Hamood Z. Arham, Cassandra R. Hunt, Wan Kyu Park
-http://arxiv.org/abs/1110.4742
-----
-'''Quantum confined electronic states in atomically well-defined graphene nanostructures'''
-Sampsa Hämäläinen, Zhixiang Sun, Mark P. Boneschanscher, Andreas Uppstu, Mari Ijäs, Ari Harju, Daniël Vanmaekelbergh, Peter Liljeroth
-http://arxiv.org/abs/1110.4208
-----
-'''How to distinguish specular from retro Andreev reflection in graphene rings'''
-Jörg Schelter, Björn Trauzettel, Patrik Recher
-http://arxiv.org/abs/1110.4383
-----
-'''Au40: A Large Tetrahedral Magic Cluster'''
-De-en Jiang, Michael Walter
-is a magic number for tetrahedral symmetry predicted in both nuclear physics and the electronic jellium model. We show that Au40 could be such a a magic cluster from density functional theory-based basin hopping for global minimization. The putative global minimum found for Au40 has a twisted pyramid structure, reminiscent of the famous tetrahedral Au20, and a sizable HOMO-LUMO gap of 0.69 eV, indicating its molecular nature. Analysis of the electronic states reveals that the gap is related to shell closings of the metallic electrons in a tetrahedrally distorted effective potential.
-http://arxiv.org/abs/1110.4556
-----
-'''The Kondo effect in the presence of the Rashba spin-orbit interaction'''
-Rok Zitko, Janez Bonca
-We study the temperature scale of the Kondo screening of a magnetic impurity which hybridizes with a two-dimensional electron gas in the presence of the Rashba spin-orbit interaction. The problem is mapped to an effective single-band impurity model with a hybridization function having an inverse-square-root divergence at the bottom of the band. We study the effect of this divergence on the Kondo screening. The problem is solved numerically without further approximations using the numerical renormalization group technique. We find that the Rashba interaction leads to a small variation of the Kondo temperature (increase or decrease) which depends on the values of the impurity parameters.
-http://arxiv.org/abs/1110.4566
-----
-'''Joule Heating and Spin-Transfer Torque Investigated on the Atomic Scale Using a Spin-Polarized Scanning Tunneling Microscope'''
-S. Krause, G. Herzog, A. Schlenhoff, A. Sonntag, and R. Wiesendanger
-The influence of a high spin-polarized tunnel current onto the switching behavior of a superparamagnetic nanoisland on a nonmagnetic substrate is investigated by means of spin-polarized scanning tunneling microscopy. A detailed lifetime analysis allows for a quantification of the effective temperature rise of the nanoisland and the modification of the activation energy barrier for magnetization reversal, thereby using the nanoisland as a local thermometer and spin-transfer torque analyzer. Both the Joule heating and spin-transfer torque are found to scale linearly with the tunnel current. The results are compared to experiments performed on lithographically fabricated magneto-tunnel junctions, revealing a very high spin-transfer torque switching efficiency in our experiments.
-http://prl.aps.org/abstract/PRL/v107/i18/e186601
-----
-'''Gate-Dependent Orbital Magnetic Moments in Carbon Nanotubes'''
-T. S. Jespersen, K. Grove-Rasmussen, K. Flensberg, J. Paaske, K. Muraki, T. Fujisawa, and J. Nygård
-We investigate how the orbital magnetic moments of electron and hole states in a carbon nanotube quantum dot depend on the number of carriers on the dot. Low temperature transport measurements are carried out in a setup where the device can be rotated in an applied magnetic field, thus enabling accurate alignment with the nanotube axis. The field dependence of the level structure is measured by excited state spectroscopy and excellent correspondence with a single-particle calculation is found. In agreement with band structure calculations we find a decrease of the orbital magnetic moment with increasing electron or hole occupation of the dot, with a scale given by the band gap of the nanotube.
-http://prl.aps.org/abstract/PRL/v107/i18/e186802
-----
-'''Magneto-Coulomb Effect in Carbon Nanotube Quantum Dots Filled with Magnetic Nanoparticles'''
-S. Datta, L. Marty, J. P. Cleuziou, C. Tilmaciu, B. Soula, E. Flahaut, and W. Wernsdorfer
-Electrical transport measurements of carbon nanotubes filled with magnetic iron nanoparticles are reported. Low-temperature (40 mK) magnetoresistance measurements showed conductance hysteresis with sharp jumps at the switching fields of the nanoparticles. Depending on the gate voltage, positive or negative hysteresis was observed. The results are explained in terms of a magneto-Coulomb effect: The spin flip of the iron island at a nonzero magnetic field causes a shift of the chemical potential induced by the change of Zeeman energy; i.e., an effective charge variation is detected by the nanotube quantum dot.
-http://prl.aps.org/abstract/PRL/v107/i18/e186804
-----
-'''Enhanced NMR Relaxation of Tomonaga-Luttinger Liquids and the Magnitude of the Carbon Hyperfine Coupling in Single-Wall Carbon Nanotubes'''
-A. Kiss, A. Pályi, Y. Ihara, P. Wzietek, P. Simon, H. Alloul, V. Zólyomi, J. Koltai, J. Kürti, B. Dóra, and F. Simon
-Recent transport measurements [Churchill et al. Nature Phys. 5 321 (2009)] found a surprisingly large, 2–3 orders of magnitude larger than usual 13C hyperfine coupling (HFC) in 13C enriched single-wall carbon nanotubes. We formulate the theory of the nuclear relaxation time in the framework of the Tomonaga-Luttinger liquid theory to enable the determination of the HFC from recent data by Ihara et al. [ Europhys. Lett. 90 17 004 (2010)]. Though we find that 1/T1 is orders of magnitude enhanced with respect to a Fermi-liquid behavior, the HFC has its usual, small value. Then, we reexamine the theoretical description used to extract the HFC from transport experiments and show that similar features could be obtained with HFC-independent system parameters.
-http://prl.aps.org/abstract/PRL/v107/i18/e187204
-----
-== Okt. 13. - Okt. 20. (2011) ==
-''Válogatta: Pályi András''
-----
-'''Field-induced polarization of Dirac valleys in bismuth'''
-Zengwei Zhu, Aurélie Collaudin, Benoît Fauqué, Woun Kang, Kamran Behnia
-The electronic structure of certain crystal lattices can contain multiple degenerate ’valleys’ for their charge carriers to occupy. This valley degree of freedom could be useful in the development of electronic devices. The principal challenge in the development of ’valleytronics’ is to lift the valley degeneracy of charge carriers in a controlled way. Here we show that in semi-metallic bismuth the flow of Dirac fermions along the trigonal axis is extremely sensitive to the orientation of in-plane magnetic field. Thus, a rotatable magnetic field can be used as a valley valve to tune the contribution of each valley to the total conductivity. At high temperature and low magnetic field, bismuth’s three valleys are interchangeable and the three-fold symmetry of its lattice is maintained. As the temperature is decreased or the magnetic field increased, this symmetry is spontaneously lost. This loss may be an experimental manifestation of the recently proposed valley-nematic Fermi liquid state.
-http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2111.html
-----
-'''Efficient quantum computing using coherent photon conversion'''
-N. K. Langford, S. Ramelow, R. Prevedel, W. J. Munro, G. J. Milburn and A. Zeilinger
-Single photons are excellent quantum information carriers: they were used in the earliest demonstrations of entanglement and in the production of the highest-quality entanglement reported so far. However, current schemes for preparing, processing and measuring them are inefficient. For example, down-conversion provides heralded, but randomly timed, single photons, and linear optics gates are inherently probabilistic. Here we introduce a deterministic process—coherent photon conversion (CPC)—that provides a new way to generate and process complex, multiquanta states for photonic quantum information applications. The technique uses classically pumped nonlinearities to induce coherent oscillations between orthogonal states of multiple quantum excitations. One example of CPC, based on a pumped four-wave-mixing interaction, is shown to yield a single, versatile process that provides a full set of photonic quantum processing tools. This set satisfies the DiVincenzo criteria for a scalable quantum computing architecture, including deterministic multiqubit entanglement gates (based on a novel form of photon–photon interaction), high-quality heralded single- and multiphoton states free from higher-order imperfections, and robust, high-efficiency detection. It can also be used to produce heralded multiphoton entanglement, create optically switchable quantum circuits and implement an improved form of down-conversion with reduced higher-order effects. Such tools are valuable building blocks for many quantum-enabled technologies. Finally, using photonic crystal fibres we experimentally demonstrate quantum correlations arising from a four-colour nonlinear process suitable for CPC and use these measurements to study the feasibility of reaching the deterministic regime with current technology. Our scheme, which is based on interacting bosonic fields, is not restricted to optical systems but could also be implemented in optomechanical, electromechanical and superconducting systems with extremely strong intrinsic nonlinearities. Furthermore, exploiting higher-order nonlinearities with multiple pump fields yields a mechanism for multiparty mediation of the complex, coherent dynamics.
-http://www.nature.com/nature/journal/v478/n7369/full/nature10463.html
-----
-'''Electrical probe for mechanical vibrations in suspended carbon nanotubes'''
-N. Traverso Ziani, G. Piovano, F. Cavaliere, and M. Sassetti
-The transport properties of a suspended carbon nanotube probed by means of a scanning tunnel microscope (STM) tip are investigated. A microscopic theory of the coupling between electrons and mechanical vibrations is developed. It predicts a position-dependent coupling constant, sizable only in the region where the vibron is located. This fact has profound consequences on the transport properties, which allow to extract information on the location and size of the vibrating portions of the nanotube.
-http://link.aps.org/doi/10.1103/PhysRevB.84.155423
-----
-'''Ultraefficient Cooling of Resonators: Beating Sideband Cooling with Quantum Control'''
-Xiaoting Wang, Sai Vinjanampathy, Frederick W. Strauch, and Kurt Jacobs
-The present state of the art in cooling mechanical resonators is a version of sideband cooling. Here we present a method that uses the same configuration as sideband cooling—coupling the resonator to be cooled to a second microwave (or optical) auxiliary resonator—but will cool significantly colder. This is achieved by varying the strength of the coupling between the two resonators over a time on the order of the period of the mechanical resonator. As part of our analysis, we also obtain a method for fast, high-fidelity quantum information transfer between resonators.
-http://link.aps.org/doi/10.1103/PhysRevLett.107.177204
-----
-'''Coherent Control of Two Nuclear Spins Using the Anisotropic Hyperfine Interaction'''
-Yingjie Zhang, Colm A. Ryan, Raymond Laflamme, and Jonathan Baugh
-We demonstrate coherent control of two nuclear spins mediated by the magnetic resonance of a hyperfine-coupled electron spin. This control is used to create a double-nuclear coherence in one of the two electron spin manifolds, starting from an initial thermal state, in direct analogy to the creation of an entangled (Bell) state from an initially pure unentangled state. We identify challenges and potential solutions to obtaining experimental gate fidelities useful for quantum information processing in this type of system.
-http://link.aps.org/doi/10.1103/PhysRevLett.107.170503
-----
-'''Experimentally Faking the Violation of Bell’s Inequalities'''
-Ilja Gerhardt, Qin Liu, Antía Lamas-Linares, Johannes Skaar, Valerio Scarani, Vadim Makarov, andChristian Kurtsiefer
-Entanglement witnesses such as Bell inequalities are frequently used to prove the nonclassicality of a light source and its suitability for further tasks. By demonstrating Bell inequality violations using classical light in common experimental arrangements, we highlight why strict locality and efficiency conditions are not optional, particularly in security-related scenarios.
-http://link.aps.org/doi/10.1103/PhysRevLett.107.170404
-----
-'''Current-induced switching in transport through anisotropic magnetic molecules'''
-Niels Bode, Liliana Arrachea, Gustavo Lozano, Tamara S. Nunner, Felix von Oppen
-Anisotropic single-molecule magnets may be thought of as molecular switches, with possible applications to molecular spintronics. In this paper, we consider current-induced switching in single-molecule junctions containing an anisotropic magnetic molecule. We assume that the carriers interact with the magnetic molecule through the exchange interaction and focus on the regime in which the molecular spin dynamics is slow compared to the electronic tunneling rates. In this limit, the molecular spin obeys a non-equilibrium Langevin equation which takes the form of a generalized Landau-Lifshitz-Gilbert equation and which we derive microscopically by means of a non-equilibrium Born-Oppenheimer approximation. We exploit this Langevin equation to identify the relevant switching mechanisms and to derive the current-induced switching rates. As a byproduct, we also derive S-matrix expressions for the various torques entering into the Landau-Lifshitz-Gilbert equation which generalize previous expressions in the literature to non-equilibrium situations.
-http://arxiv.org/abs/1110.4270
-----
-'''Effects of Interface Disorder on Valley Splitting in SiGe/Si/SiGe Quantum Wells'''
-Zhengping Jiang, Neerav Kharche, Timothy Boykin, Gerhard Klimeck
-A sharp potential barrier at the Si/SiGe interface introduces valley splitting (VS), which lifts the 2-fold valley degeneracy in strained SiGe/Si/SiGe quantum wells (QWs). This work examines in detail the effects of Si/SiGe interface disorder on the VS in an atomistic tight binding approach based on statistical sampling. VS is analyzed as a function of electric field, QW thickness, and simulation domain size. Strong electric fields push the electron wavefunctions into the SiGe buffer and introduce significant VS fluctuations from device to device. A Gedankenexperiment with ordered alloys sheds light on the importance of different bonding configurations on VS. We conclude that a single SiGe band offset and effective mass cannot comprehend the complex Si/SiGe interface interactions that dominate VS.
-http://arxiv.org/abs/1110.4097
-== Okt. 07. - Okt. 13. (2011) ==
-''Válogatta: Tóvári Endre''
-----
-'''Tunable metal-insulator transition in double-layer graphene heterostructures'''
-L. A. Ponomarenko, A. A. Zhukov, R. Jalil, S. V. Morozov, K. S. Novoselov, V.V. Cheianov, V.I. Fal'ko, K. Watanabe, T. Taniguchi, A. K. Geim, R. V. Gorbachev
-We report a double-layer electronic system made of two closely-spaced but electrically isolated graphene monolayers sandwiched in boron nitride. For large carrier densities in one of the layers, the adjacent layer no longer exhibits a minimum metallic conductivity at the neutrality point, and its resistivity diverges at low temperatures. This divergence can be suppressed by magnetic field or by reducing the carrier density in the adjacent layer. We believe that the observed localization is intrinsic for neutral graphene with generic disorder if metallic electron-hole puddles are screened out.
-http://arxiv.org/abs/1107.0115
-Published in Nature Physics (09 October 2011)
-----
-'''Mapping the Density of Scattering Centers Limiting the Electron Mean Free Path in Graphene'''
-Filippo Giannazzo, Sushant Sonde, Raffaella Lo Nigro, Emanuele Rimini, and Vito Raineri
-Recently, giant carrier mobility μ (>105 cm2 V–1 s–1) and micrometer electron mean free path (l) have been measured in suspended graphene or in graphene encapsulated between inert and ultraflat BN layers. Much lower μ values (10000–20000 cm2 V–1 s–1) are typically reported in graphene on common substrates (SiO2, SiC) used for device fabrication. The debate on the factors limiting graphene electron mean free path is still open with charged impurities (CI) and resonant scatterers (RS) indicated as the most probable candidates. As a matter of fact, the inhomogeneous distribution of such scattering sources in graphene is responsible of nanoscale lateral inhomogeneities in the electronic properties, which could affect the behavior of graphene nanodevices. Hence, high resolution two-dimensional (2D) mapping of their density is very important. Here, we used scanning capacitance microscopy/spectroscopy to obtain 2D maps of l in graphene on substrates with different dielectric permittivities, that is, SiO2 (κSiO2 = 3.9), 4H-SiC (0001) (κSiC = 9.7) and the very-high-κ perovskite strontium titanate, SrTiO3 (001), briefly STO (κSTO = 330). After measuring l versus the gate bias Vg on an array of points on graphene, maps of the CI density (NCI) have been determined by the neutrality point shift from Vg = 0 V in each curve, whereas maps of the RS density (NRS) have been extracted by fitting the dependence of l on the carrier density (n). Laterally inhomogeneous densities of CI and RS have been found. The RS distribution exhibits an average value 3 × 1010 cm–2 independently on the substrate. For the first time, a clear correlation between the minima in the l map and the maxima in the NCI map is obtained for graphene on SiO2 and 4H-SiC, indicating that CI are the main source of the lateral inhomogeneity of l. On the contrary, the l and NCI maps are uncorrelated in graphene on STO, while a clear correlation is found between l and NRS maps. This demonstrates a very efficient dielectric screening of CI in graphene on STO and the role of RS as limiting factor for electron mean free path.
-http://pubs.acs.org/doi/abs/10.1021/nl2020922
-----
-'''Raman Signature of Graphene Superlattices'''
-Victor Carozo, Clara M. Almeida, Erlon H. M. Ferreira, Luiz Gustavo Cançado, Carlos Alberto Achete, and Ado Jorio
-When two identical two-dimensional periodic structures are superposed, a mismatch rotation angle between the structures generates a superlattice. This effect is commonly observed in graphite, where the rotation between graphene layers generates Moiré patterns in scanning tunneling microscopy images. Here, a study of intravalley and intervalley double-resonance Raman processes mediated by static potentials in rotationally stacked bilayer graphene is presented. The peak properties depend on the mismatch rotation angle and can be used as an optical signature for superlattices in bilayer graphene. An atomic force microscopy system is used to produce and identify specific rotationally stacked bilayer graphenes that demonstrate the validity of our model.
-http://pubs.acs.org/doi/abs/10.1021/nl201370m
-----
-'''Optical Force Stamping Lithography'''
-Spas Nedev, Alexander S. Urban, Andrey A. Lutich, and Jochen Feldmann
-Here we introduce a new paradigm of far-field optical lithography, optical force stamping lithography. The approach employs optical forces exerted by a spatially modulated light field on colloidal nanoparticles to rapidly stamp large arbitrary patterns comprised of single nanoparticles onto a substrate with a single-nanoparticle positioning accuracy well beyond the diffraction limit. Because the process is all-optical, the stamping pattern can be changed almost instantly and there is no constraint on the type of nanoparticle or substrates used.
-http://pubs.acs.org/doi/abs/10.1021/nl203214n
-----
-'''Anomalous Optoelectronic Properties of Chiral Carbon Nanorings...and One Ring to Rule Them All'''
-Bryan M. Wong, Jonathan W. Lee
-Carbon nanorings are hoop-shaped, {\pi}-conjugated macrocycles which form the fundamental annular segments of single-walled carbon nanotubes (SWNTs). In a very recent report, the structures of chiral carbon nanorings (which may serve as chemical templates for synthesizing chiral nanotubes) were experimentally synthesized and characterized for the first time. Here, in our communication, we show that the excited-state properties of these unique chiral nanorings exhibit anomalous and extremely interesting optoelectronic properties, with excitation energies growing larger as a function of size (in contradiction with typical quantum confinement effects). While the first electronic excitation in armchair nanorings is forbidden with a weak oscillator strength, we find that the same excitation in chiral nanorings is allowed due to a strong geometric symmetry breaking. Most importantly, among all the possible nanorings synthesized in this fashion, we show that only one ring, corresponding to a SWNT with chiral indices (n+3,n+1), is extremely special with large photoinduced transitions that are most readily observable in spectroscopic experiments.
-http://xxx.lanl.gov/abs/1110.2756
-----
-'''Electron-Electron scattering and resistivity of ballistic multimode channels'''
-K. E. Nagaev, N. Yu. Sergeeva
-We show that electron--electron scattering gives a positive contribution to the resistivity of ballistic multimode wires whose width is much smaller than their length. This contribution is not exponentially small at low temperatures and therefore may be experimentally observable. It scales with temperature as $T^2$ for three-dimensional channels and as $T^{5/2}$ for two-dimensional ones.
-http://xxx.lanl.gov/abs/1110.2607
-----
-'''Spin Manipulation and Relaxation in Spin-Orbit Qubits'''
-Massoud Borhani, Xuedong Hu
-We derive a generalized form of the Electric Dipole Spin Resonance (EDSR) Hamiltonian in the presence of the spin-orbit interaction for single spins in an elliptic quantum dot (QD) subject to an arbitrary (in both direction and magnitude) applied magnetic field. We predict a nonlinear behavior of the Rabi frequency as a function of the magnetic field for sufficiently large Zeeman energies, and present a microscopic expression for the anisotropic electron g-tensor. Similarly, an EDSR Hamiltonian is devised for two spins confined in a double quantum dot (DQD), where coherent Rabi oscillations between the singlet and triplet states are induced by jittering the inter-dot distance at the resonance frequency. Finally, we calculate two-electron-spin relaxation rates due to phonon emission, for both in-plane and perpendicular magnetic fields. Our results have immediate applications to current EDSR experiments on nanowire QDs, g-factor optimization of confined carriers, and spin decay measurements in DQD spin-orbit qubits.
-http://xxx.lanl.gov/abs/1110.2193
-----
-'''Room-temperature gating of molecular junctions using few-layer graphene nanogap electrodes'''
-Ferry Prins, Amelia Barreiro, Justus W. Ruitenberg, Johannes S. Seldenthuis, Nuria Aliaga-Alcalde, Lieven M. K. Vandersypen, Herre S. J. van der Zant
-We report on a method to fabricate and measure gateable molecular junctions which are stable at room temperature. The devices are made by depositing molecules inside a few-layer graphene nanogap, formed by feedback controlled electroburning. The gaps have separations on the order of 1-2 nm as estimated from a Simmons model for tunneling. The molecular junctions display gateable IV-characteristics at room temperature.
-http://xxx.lanl.gov/abs/1110.2335
-----
-'''A spin quantum bit architecture with coupled donors and quantum dots in silicon'''
-T. Schenkel, C. C. Lo, C. D. Weis, J. Bokor, A. M. Tyryshkin, S. A. Lyon
-Spins of donor electrons and nuclei in silicon are promising quantum bit (qubit) candidates which combine long coherence times with the fabrication finesse of the silicon nanotechnology industry. We outline a potentially scalable spin qubit architecture where donor nuclear and electron spins are coupled to spins of electrons in quantum dots and discuss requirements for donor placement aligned to quantum dots by single ion implantation.
-http://xxx.lanl.gov/abs/1110.2228
-----
-'''High-resolution spatial mapping of the temperature distribution of a Joule self-heated graphene nanoribbon'''
-Young-Jun Yu, Melinda Y. Han, Stephane Berciaud, Alexandru B. Georgescu, Tony F. Heinz, Louis E. Brus, Kwang S. Kim, Philip Kim
-We investigate the temperature distributions of Joule self-heated graphene nanoribbons (GNRs) with a spatial resolution finer than 100 nm by scanning thermal microscopy (SThM). The SThM probe is calibrated using the Raman G mode Stokes/anti-Stokes intensity ratio as a function of electric power applied to the GNR devices. From a spatial map of the temperature distribution, heat dissipation and transport pathways are investigated. By combining SThM and scanning gate microscopy data from a defected GNR, we observe hot spot formation at well-defined, localized sites.
-http://xxx.lanl.gov/abs/1110.2984
-----
-'''Kinetics of spin relaxation in quantum wires and channels: Boundary spin echo and formation of a persistent spin helix'''
-Valeriy A. Slipko and Yuriy V. Pershin
-In this paper we use a spin kinetic equation to study spin-polarization dynamics in one-dimensional (1D) wires and 2D channels. The spin kinetic equation is valid in both diffusive and ballistic spin transport regimes and therefore is more general than the usual spin drift-diffusion equations. In particular, we demonstrate that in infinite 1D wires with Rashba spin-orbit interaction the exponential spin-relaxation decay can be modulated by an oscillating function. In the case of spin relaxation in finite length 1D wires, it is shown that an initially homogeneous spin polarization spontaneously transforms into a persistent spin helix. We find that a propagating spin-polarization profile reflects from a system boundary and returns back to its initial position similarly to the reflectance of sound waves from an obstacle. The Green’s function of the spin kinetic equation is derived for both finite and infinite 1D systems. Moreover, we demonstrate explicitly that the spin relaxation in specifically oriented 2D channels with Rashba and Dresselhaus spin-orbit interactions of equal strength occurs similarly to that in 1D wires of finite length. Finally, a simple transformation mapping 1D spin kinetic equation into the Klein-Gordon equation with an imaginary mass is found thus establishing an interesting connection between semiconductor spintronics and relativistic quantum mechanics.
-http://prb.aps.org/abstract/PRB/v84/i15/e155306
-----
-'''Landau levels, edge states, and strained magnetic waveguides in graphene monolayers with enhanced spin-orbit interaction'''
-Alessandro De Martino, Artur Hütten, and Reinhold Egger
-The electronic properties of a graphene monolayer in a magnetic and a strain-induced pseudomagnetic field are studied in the presence of spin-orbit interactions (SOIs) that are artificially enhanced (e.g., by suitable adatom deposition). For the homogeneous case, we provide analytical results for the Landau level eigenstates for arbitrary intrinsic and Rashba SOIs, including also the Zeeman field. The edge states in a semi-infinite geometry are studied in the absence of the Rashba term. For a critical value of the magnetic field, we find a quantum phase transition separating two phases with spin-filtered helical edge states at the Dirac point. These phases have opposite spin current direction. We also discuss strained magnetic waveguides with inhomogeneous field profiles that allow for chiral snake orbits. Such waveguides are practically immune to disorder-induced backscattering, and the SOI provides nontrivial spin texture to these modes.
-http://prb.aps.org/abstract/PRB/v84/i15/e155420
-== Szept. 30. - Okt. 06. (2011) ==
-''Válogatta: Balogh Zoltán''
-----
-'''From Geneva to Italy Faster Than a Speeding Photon?'''
-Adrian Cho
-When news spread last week that physicists in Europe had spotted subatomic particles called neutrinos traveling faster than light, some of their colleagues reacted with incredulity. After all, the observation would contradict Einstein's special theory of relativity, which says that nothing can travel faster than light. Jim Al-Khalili, a theorist at the University of Surrey in the United Kingdom, even vowed to eat his boxer shorts on live television if the result holds up. But if it does, physicists won't be quite as bewildered as such reactions imply. Some have already developed a theoretical framework that can handle faster-than-light neutrinos and all other potential breaches of special relativity.
-http://www.sciencemag.org/content/333/6051/1809.summary
-----
-'''Nobel Prize 2011: Perlmutter, Schmidt & Riess'''
-Alison Wright
-The 2011 Nobel Prize in Physics has been awarded to Saul Perlmutter, Brian Schmidt and Adam Riess, "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae".
-http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2131.html
-----
-'''Topology by dissipation in atomic quantum wires'''
-Sebastian Diehl, Enrique Rico, Mikhail A. Baranov & Peter Zoller
-Robust edge states and non-Abelian excitations are the trademark of topological states of matter, with promising applications such as ‘topologically protected’ quantum memory and computing. So far, topological phases have been exclusively discussed in a Hamiltonian context. Here we show that such phases and the associated topological protection and phenomena also emerge in open quantum systems with engineered dissipation. The specific system studied here is a quantum wire of spinless atomic fermions in an optical lattice coupled to a bath. The key feature of the dissipative dynamics described by a Lindblad master equation is the existence of Majorana edge modes, representing a non-local decoherence-free subspace. The isolation of the edge states is enforced by a dissipative gap in the p-wave paired bulk of the wire. We describe dissipative non-Abelian braiding operations within the Majorana subspace, and illustrate the insensitivity to imperfections. Topological protection is granted by a non-trivial winding number of the system density matrix.
-http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2106.html
-----
-'''Ambipolar field effect in the ternary topological insulator (BixSb1–x)2Te3 by composition tuning'''
-Desheng Kong, Yulin Chen, Judy J. Cha, Qianfan Zhang, James G. Analytis, Keji Lai, Zhongkai Liu,Seung Sae Hong, Kristie J. Koski, Sung-Kwan Mo, Zahid Hussain, Ian R. Fisher, Zhi-Xun Shen & Yi Cui
-Topological insulators exhibit a bulk energy gap and spin-polarized surface states that lead to unique electronic properties1, 2, 3, 4, 5, 6, 7, 8, 9, with potential applications in spintronics and quantum information processing. However, transport measurements have typically been dominated by residual bulk charge carriers originating from crystal defects or environmental doping10, 11, 12, and these mask the contribution of surface carriers to charge transport in these materials. Controlling bulk carriers in current topological insulator materials, such as the binary sesquichalcogenides Bi2Te3, Sb2Te3 and Bi2Se3, has been explored extensively by means of material doping8, 9, 11 and electrical gating13, 14, 15, 16, but limited progress has been made to achieve nanostructures with low bulk conductivity for electronic device applications. Here we demonstrate that the ternary sesquichalcogenide (BixSb1–x)2Te3 is a tunable topological insulator system. By tuning the ratio of bismuth to antimony, we are able to reduce the bulk carrier density by over two orders of magnitude, while maintaining the topological insulator properties. As a result, we observe a clear ambipolar gating effect in (BixSb1–x)2Te3 nanoplate field-effect transistor devices, similar to that observed in graphene field-effect transistor devices17. The manipulation of carrier type and density in topological insulator nanostructures demonstrated here paves the way for the implementation of topological insulators in nanoelectronics and spintronics.
-http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2011.172.html
-----
-'''Atomic Force Microscopy Based Tunable Local Anodic Oxidation of Graphene'''
-Satoru Masubuchi, Miho Arai, and Tomoki Machida
-We have fabricated graphene/graphene oxide/graphene (G/GO/G) junctions by local anodic oxidation lithography using atomic force microscopy (AFM). The conductance of the G/GO/G junction decreased with the bias voltage applied to the AFM cantilever Vtip. For G/GO/G junctions fabricated with large and small |Vtip|. GO was semi-insulating and semiconducting, respectively. AFM-based LAO lithography can be used to locally oxidize graphene with various oxidation levels and achieve tunability from semiconducting to semi-insulating GO.
-http://pubs.acs.org/doi/abs/10.1021/nl201448q
-----
-'''MoS2 Nanoplates Consisting of Disordered Graphene-like Layers for High Rate Lithium Battery Anode Materials'''
-Haesuk Hwang, Hyejung Kim, and Jaephil Cho
-MoS2 nanoplates, consisting of disordered graphene-like layers, with a thickness of 30 nm were prepared by a simple, scalable, one-pot reaction using Mo(CO)6 and S in an autoclave. The product has a interlayer distance of 0.69 nm, which is much larger than its bulk counterpart (0.62 nm). This expanded interlater distance and disordered graphene-like morphology led to an excellent rate capability even at a 50C (53.1 A/g) rate, showing a reversible capacity of 700 mAh/g. In addition, a full cell (LiCoO2/MoS2) test result also demonstrates excellent capacity retention up to 60 cycles.
-http://pubs.acs.org/doi/abs/10.1021/nl202675f
-----
-'''Mesoporous Manganese Oxide Nanowires for High-Capacity, High-Rate, Hybrid Electrical Energy Storage'''
-Wenbo Yan, Talin Ayvazian, Jungyun Kim, Yu Liu, Keith C. Donavan, Wendong Xing, Yongan Yang, John C. Hemminger, and Reginald M. Penner
-Arrays of mesoporous manganese dioxide, mp-MnO2, nanowires were electrodeposited on glass and silicon surfaces using the lithographically patterned nanowire electrodeposition (LPNE) method. The electrodeposition procedure involved the application, in a Mn(ClO4)2-containing aqueous electrolyte, of a sequence of 0.60 V (vs MSE) voltage pulses delineated by 25 s rest intervals. This “multipulse” deposition program produced mp-MnO2 nanowires with a total porosity of 43–56%. Transmission electron microscopy revealed the presence within these nanowires of a network of 3–5 nm diameter fibrils that were X-ray and electron amorphous, consistent with the measured porosity values. mp-MnO2 nanowires were rectangular in cross-section with adjustable height, ranging from 21 to 63 nm, and adjustable width ranging from 200 to 600 nm. Arrays of 20 nm × 400 nm mp-MnO2 nanowires were characterized by a specific capacitance, Csp, of 923 ± 24 F/g at 5 mV/s and 484 ± 15 F/g at 100 mV/s. These Csp values reflected true hybrid electrical energy storage with significant contributions from double-layer capacitance and noninsertion pseudocapacitance (38% for 20 nm × 400 nm nanowires at 5 mV/s) coupled with a Faradaic insertion capacity (62%). These two contributions to the total Csp were deconvoluted as a function of the potential scan rate.
-http://pubs.acs.org/doi/abs/10.1021/nn2029583#cor1
-----
-'''Low Bias Electron Scattering in Structure-Identified Single Wall Carbon Nanotubes: Role of Substrate Polar Phonons'''
-Bhupesh Chandra, Vasili Perebeinos, Stéphane Berciaud, Jyoti Katoch, Masa Ishigami, Philip Kim, Tony F. Heinz, and James Hone
-We have performed temperature-dependent electrical transport measurements on known structure single wall carbon nanotubes at low bias. The experiments show a superlinear increase in nanotube resistivity with temperature, which is in contradiction with the linear dependence expected from nanotube acoustic-phonon scattering. The measured electron mean free path is also much lower than expected, especially at medium to high temperatures (>100  K). A theoretical model that includes scattering due to surface polar phonon modes of the substrates reproduces the experiments very well. The role of surface phonons is further confirmed by resistivity measurements of nanotubes on aluminum nitride.
-http://prl.aps.org/abstract/PRL/v107/i14/e146601
-----
-'''Qubit state detection using the quantum Duffing oscillator'''
-V. Leyton, M. Thorwart, and V. Peano
-We introduce a detection scheme for the state of a qubit that is based on resonant few-photon transitions in a driven nonlinear resonator. The latter is parametrically coupled to the qubit and is used as its detector. Close to the fundamental resonator frequency, the nonlinear resonator shows sharp resonant few-photon transitions. Depending on the qubit state, these few-photon resonances are shifted to different driving frequencies. We show that this detection scheme offers the advantage of small back action, a large discrimination power with an enhanced readout fidelity, and a sufficiently large measurement efficiency. A realization of this scheme in the form of a persistent current qubit inductively coupled to a driven SQUID detector in its nonlinear regime is discussed.
-http://prb.aps.org/abstract/PRB/v84/i13/e134501
-== Szept. 23. - Szept. 29. (2011) ==
-''Válogatta: Piszter Gábor''
-----
-'''Stacking-dependent band gap and quantum transport in trilayer graphene'''
-W. Bao, L. Jing, J. Velasco Jr, Y. Lee, G. Liu, D. Tran, B. Standley, M. Ayko, S. B. Cronin, D. Smirnov, M. Koshino, E. McCann, M. Bockrath and C. N. Lau
-Graphene is an extraordinary two-dimensional (2D) system with chiral charge carriers and fascinating electronic, mechanical and thermal properties. In multilayer graphene, stacking order provides an important yet rarely explored degree of freedom for tuning its electronic properties. For instance, Bernal-stacked trilayer graphene (B-TLG) is semi-metallic with a tunable band overlap, and rhombohedral-stacked trilayer graphene (r-TLG) is predicted to be semiconducting with a tunable band gap. These multilayer graphenes are also expected to exhibit rich novel phenomena at low charge densities owing to enhanced electronic interactions and competing
-symmetries. Here we demonstrate the dramatically different transport properties in TLG with different stacking orders, and the unexpected spontaneous gap opening in charge neutral r-TLG. At the Dirac point, B-TLG remains metallic, whereas r-TLG becomes insulating with an intrinsic interaction-driven gap ~6 meV. In magnetic fields, well-developed quantum Hall (QH) plateaux in r-TLG split into three branches at higher fields. Such splitting is a signature of the Lifshitz transition, a topological change in the Fermi surface, that is found only in r-TLG. Our results underscore the rich interaction-induced phenomena in trilayer graphene with different stacking orders, and its potential towards electronic applications.
-http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2103.html
-----
-'''The experimental observation of quantum Hall effect of l = 3 chiral quasiparticles in trilayer graphene'''
-Liyuan Zhang, Yan Zhang, Jorge Camacho, Maxim Khodas and Igor Zaliznyak
-The linear dispersion of the low-energy electronic structure of monolayer graphene supports chiral quasiparticles that obey the relativistic Dirac equation and have a Berry phase of π. In bilayer graphene, the shape of the energy bands is quadratic, and its quasiparticles have a chiral degree, l = 2, and a Berry phase of 2π. These characteristics are usually determined from quantum Hall effect (QHE) measurements in which the Berry phase causes shifts in Shubnikov–de Haas (SdH) resistance oscillations. The QHE in graphene also exhibits an unconventional sequence of plateaux of Hall conductivity, SZIGMA_xy, with quantized steps of 4e^2=h, except for the first plateau, where it is governed by the Berry phase. Here, we report magnetotransport measurements in ABC-stacked trilayer graphene, and their variation with carrier density, magnetic field and temperature. Our results provide the first evidence of the presence of l = 3 chiral quasiparticles with cubic dispersion, predicted to occur in ABC-stacked trilayer graphene. The SdH oscillations we observe suggest Landau levels with four-fold degeneracy, a Berry phase of 3π, and the marked increase of cyclotron mass near charge neutrality. We also observe the predicted unconventional sequence of QHE plateaux, SZIGMA_xy = +- 6e^2/h,+-10e^2/h, and so on.
-http://www.nature.com/nphys/journal/vaop/ncurrent/abs/nphys2104.html
-----
-'''Observation of an electrically tunable band gap in trilayer graphene'''
-Chun Hung Lui, Zhiqiang Li, Kin Fai Mak, Emmanuele Cappelluti and Tony F. Heinz
-A striking feature of bilayer graphene is the induction of a significant band gap in the electronic states by the application of a perpendicular electric field. Thicker graphene layers are also highly attractive materials. The ability to produce a band gap in these systems is of great fundamental and practical interest. Both experimental and theoretical investigations of graphene trilayers with the typical ABA layer stacking have, however, revealed the lack of any appreciable induced gap. Here we contrast this behaviour with that exhibited by graphene trilayers with ABC crystallographic stacking. The symmetry of this structure is similar to that of AB-stacked graphene bilayers and, as shown by infrared conductivity measurements, permits a large band gap to be formed by
-an applied electric field. Our results demonstrate the critical and hitherto neglected role of the crystallographic stacking sequence on the induction of a band gap in few-layer graphene.
-http://www.nature.com/nphys/journal/vaop/ncurrent/abs/nphys2102.html
-----
-'''Exchange-Induced Electron Transport in Heavily Phosphorus-Doped Si Nanowires'''
-Tae-Eon Park, Byoung-Chul Min, Ilsoo Kim, Jee-Eun Yang, Moon-Ho Jo, Joonyeon Chang, and Heon-Jin Choi
-Heavily phosphorus-doped silicon nanowires (Si NWs) show intriguing transport phenomena at low temperature. As we decrease the temperature, the resistivity of the Si NWs initially decreases, like metals, and starts to increase logarithmically below a resistivity minimum temperature (Tmin), which is accompanied by (i) a zero-bias dip in the differential conductance and (ii) anisotropic negative magnetoresistance (MR), depending on the angle between the applied magnetic field and current flow. These results are associated with the impurity band conduction and electron scattering by the localized spins at phosphorus donor states. The analysis on the MR reveals that the localized spins are coupled antiferromagnetically at low temperature via the exchange interaction.
-http://pubs.acs.org/doi/abs/10.1021/nl202535d
-----
-'''Direct Observation of Electron Confinement in Epitaxial Graphene Nanoislands'''
-Soo-hyon Phark, Jérome Borme, Augusto León Vanegas, Marco Corbetta, Dirk Sander, and Jürgen Kirschner
-One leading question for the application of graphene in nanoelectronics is how electronic properties depend on the size at the nanoscale. Direct observation of the quantized electronic states is central to conveying the relationship between electronic structures and local geometry. Scanning tunneling spectroscopy was used to measure differential conductance dI/dV patterns of nanometer-size graphene islands on an Ir(111) surface. Energy-resolved dI/dV maps clearly show a spatial modulation, indicating a modulated local density of states due to quantum confinement, which is unaffected by the edge configuration. We establish the energy dispersion relation with the quantized electron wave vector obtained from a Fourier analysis of dI/dV maps. The nanoislands preserve the Dirac Fermion properties with a reduced Fermi velocity.
-http://pubs.acs.org/doi/abs/10.1021/nn2028105
-----
-'''Robust Au-Ag-Au Bimetallic Atom-Scale Junctions Fabricated by Self-Limited Ag Electrodeposition at Au Nanogaps'''
-Tai-Wei Hwang and Paul W. Bohn
-Atom-scale junctions (ASJs) exhibit quantum conductance behavior and have potential both for fundamental studies of adsorbate- ediated conductance in mesoscopic conductors and as chemical sensors. Electrochemically fabricated ASJs, in particular, show the stability needed for molecular detection applications. However, achieving physically robust ASJs at high yield is a challenge because it is difficult to control the direction and kinetics of metal deposition. In this work, a novel electrochemical approach is reported, in which Au-Ag-Au bimetallic ASJs are reproducibly fabricated from an initially prepared Au nanogap by sequential overgrowth and self-limited thinning. Applying a potential across specially prepared Au nanoelectrodes in the presence of aqueous Ag(I) leads to preferential galvanic reactions resulting in the deposition of Ag and the formation of an atom-scale junction between the electrodes. An external resistor is added in series with the ASJ to control self-termination, and adjusting solution chemical potential (concentration) is used to mediate self-thinning of junctions. The result is long-lived, mechanically stable ASJs that, unlike previous constructions, are stable in flowing solution, as well as to changes in solution media. These bimetallic ASJs exhibit a number of behaviors characteristic of quantum structures, including long-lived fractional conductance states, that are interpreted to arise from two or more quantized ASJs in series.
-http://pubs.acs.org/doi/abs/10.1021/nn203404k
-----
-'''Reduced Graphene Oxide (rGO)- Wrapped Fullerene (C60) Wires'''
-Jieun Yang, Mihee Heo, Hyo Joong Lee, Su-Moon Park, Jin Young Kim, and Hyeon Suk Shin
-The assembly of reduced graphene oxide (rGO) and fullerene (C60) into hybrid (rGO/C60) wires was successfully performed by employing the liquid-liquid interfacial precipitation method. The rGO sheets spontaneously wrapped C60 wires through the π-π interaction between rGO and C60. Structural characterization of the rGO/C60 wires was carried out by using UV/visible spectroscopy, scanning electron microscopy, and transmission electron microscopy. FET devices with rGO/C60 wires were fabricated to investigate their electrical properties. The Ids-Vg curves of the hybrid wires exhibited p-type semiconducting behavior both in vacuum and in air, indicating hole transport through rGO as a shell layer, whereas pure C60 wires and rGO sheets showed n-type and ambipolar behaviors, respectively, under vacuum. Possible application of the fabricated wires, such as photovoltaic devices, was also demonstrated.
-http://pubs.acs.org/doi/abs/10.1021/nn203073q
-----
-'''Interferometric and Noise Signatures of Majorana Fermion Edge States in Transport Experiments'''
-Grégory Strübi, Wolfgang Belzig, Mahn-Soo Choi, and C. Bruder
-Domain walls between superconducting and magnetic regions placed on top of a topological insulator support transport channels for Majorana fermions. We propose to study noise correlations in a Hanbury Brown–Twiss type interferometer and find three signatures of the Majorana nature of the channels. First, the average charge current in the outgoing leads vanishes. Furthermore, we predict an anomalously large shot noise in the output ports for a vanishing average current signal. Adding a quantum point contact to the setup, we find a surprising absence of partition noise which can be traced back to the Majorana nature of the carriers.
-http://prl.aps.org/abstract/PRL/v107/i13/e136403
-----
-'''Phase Diffusion in Graphene-Based Josephson Junctions'''
-I.V. Borzenets, U. C. Coskun, S. J. Jones, and G. Finkelstein
-We report on graphene-based Josephson junctions with contacts made from lead. The high transition temperature of this superconductor allows us to observe the supercurrent branch at temperatures up to ~2 K, at which point we can detect a small, but nonzero, resistance. We attribute this resistance to the phase diffusion mechanism, which has not been yet identified in graphene. By measuring the resistance as a function of temperature and gate voltage, we can further characterize the nature of the electromagnetic
-environment and dissipation in our samples.
-http://prl.aps.org/abstract/PRL/v107/i13/e137005
-----
-'''Finite-Bias Cooper Pair Splitting'''
-L. Hofstetter, Sz. Csonka, A. Baumgartner, G. Fülöp, S. d’Hollosy, J. Nygard, and C. Schönenberger
-In a device with a superconductor coupled to two parallel quantum dots (QDs) the electrical tunability of the QD levels can be used to exploit nonclassical current correlations due to the splitting of Cooper pairs. We experimentally investigate the effect of a finite potential difference across one quantum dot on the conductance through the other completely grounded QD in a Cooper pair splitter fabricated on an InAs nanowire. We demonstrate that the nonlocal electrical transport through the device can be tuned by electrical means and that the energy dependence of the effective density of states in the QDs is relevant for the rates of Cooper pair splitting (CPS) and elastic cotunneling. Such experimental tools are necessary to understand and develop CPS-based sources of entangled electrons in solid-state devices.
-http://prl.aps.org/abstract/PRL/v107/i13/e136801
-----
-== Szept. 15. - Szept. 22. (2011) ==
-''Válogatta: Fülöp Gergő''
-----
-'''On-demand single-electron transfer between distant quantum dots'''
-R. P. G. McNeil, M. Kataoka, C. J. B. Ford, C. H. W. Barnes, D. Anderson, G. A. C. Jones, I. Farrer & D. A. Ritchie
-Single-electron circuits of the future, consisting of a network of quantum dots, will require a mechanism to transport electrons from one functional part of the circuit to another. For example, in a quantum computer decoherence and circuit complexity can be reduced by separating quantum bit (qubit) manipulation from measurement and by providing a means of transporting electrons between the corresponding parts of the circuit. Highly controlled tunnelling between neighbouring dots has been demonstrated, and our ability to manipulate electrons in single- and double-dot systems is improving rapidly. For distances greater than a few hundred nanometres, neither free propagation nor tunnelling is viable while maintaining confinement of single electrons. Here we show how a single electron may be captured in a surface acoustic wave minimum and transferred from one quantum dot to a second, unoccupied, dot along a long, empty channel. The transfer direction may be reversed and the same electron moved back and forth more than sixty times—a cumulative distance of 0.25 mm—without error. Such on-chip transfer extends communication between quantum dots to a range that may allow the integration of discrete quantum information processing components and devices.
-http://www.nature.com/nature/journal/v477/n7365/full/nature10444.html
-[[media:Nature10444.pdf|pdf]]
-----
-'''Strong back-action of a linear circuit on a single electronic quantum channel'''
-F. D. Parmentier, A. Anthore, S. Jezouin, H. le Sueur, U. Gennser, A. Cavanna, D. Mailly & F. Pierre
-The question of which laws govern electricity in mesoscopic circuitsis a fundamental matter that also has direct implications for the quantum engineering of nanoelectronic devices. When a quantum-coherent conductor is inserted into a circuit, its transport properties are modified; in particular, its conductance is reduced because of the circuit back-action. This phenomenon, known as environmental Coulomb blockade, results from the granularity of charge transfers across the coherent conductor1. Although extensively studied for a tunnel junction in a linear circuit2, 3, 4, 5, it is only fully understood for arbitrary short coherent conductors in the limit of small circuit impedances and small conductance reduction6, 7, 8. Here, we investigate experimentally the strong-back-action regime, with a conductance reduction of up to 90%. This is achieved by embedding a single quantum channel of tunable transmission in an adjustable on-chip circuit of impedance comparable to the resistance quantum RK = h/e2 at microwave frequencies. The experiment reveals significant deviations from calculations performed in the weak back-action framework6, 7, and is in agreement with recent theoretical results9, 10. Based on these measurements, we propose a generalized expression for the conductance of an arbitrary quantum channel embedded in a linear circuit.
-http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2092.html
-[[media:Nphys2092.pdf|pdf]]
-----
-'''Spin Polarization Measurement of Homogeneously Doped Fe<sub>1–x</sub>Co<sub>x</sub>Si Nanowires by Andreev Reflection Spectroscopy'''
-John P. DeGrave†, Andrew L. Schmitt†, Rachel S. Selinsky†, Jeremy M. Higgins†, David J. Keavney‡, and Song Jin*†
-We report a general method for determining the spin polarization from nanowire materials using Andreev reflection spectroscopy implemented with a Nb superconducting contact and common electron-beam lithography device fabrication techniques. This method was applied to magnetic semiconducting Fe1–xCoxSi alloy nanowires with = 0.23, and the average spin polarization extracted from 6 nanowire devices is 28 ± 7% with a highest observed value of 35%. Local-electrode atom probe tomography (APT) confirms the homogeneous distribution of Co atoms in the FeSi host lattice, and X-ray magnetic circular dichroism (XMCD) establishes that the elemental origin of magnetism in this strongly correlated electron system is due to Co atoms.
-http://pubs.acs.org/doi/abs/10.1021/nl2026426
-----
-''' Thermoelectricity in Fullerene–Metal Heterojunctions '''
-Shannon K. Yee†, Jonathan A. Malen†, Arun Majumdar*‡, and Rachel A. Segalman*§
-[...] Herein, we report molecular junction thermoelectric measurements of fullerene molecules (i.e., C60, PCBM, and C70) trapped between metallic electrodes (i.e., Pt, Au, Ag). Fullerene junctions demonstrate the first strongly n-type molecular thermopower corresponding to transport through the LUMO, and the highest measured magnitude of molecular thermopower to date. While the electronic conductance of fullerenes is highly variable, due to fullerene’s variable bonding geometries with the electrodes, the thermopower shows predictable trends based on the alignment of the LUMO with the work function of the electrodes. Both the magnitude and trend of the thermopower suggest that heterostructuring organic and inorganic materials at the nanoscale can further enhance thermoelectric performance, therein providing a new pathway for designing thermoelectric materials.
-http://pubs.acs.org/doi/abs/10.1021/nl2014839
-----
-''' Mobility-Dependent Low-Frequency Noise in Graphene Field-Effect Transistors ''''
-Yan Zhang†, Emilio E. Mendez†‡, and Xu Du†
-We have investigated the low-frequency 1/f noise of both suspended and on-substrate graphene field-effect transistors and its dependence on gate voltage, in the temperature range between 300 and 30 K. We have found that the noise amplitude away from the Dirac point can be described by a generalized Hooge’s relation in which the Hooge parameter αH is not constant but decreases monotonically with the device’s mobility, with a universal dependence that is sample and temperature independent. The value of αH is also affected by the dynamics of disorder, which is not reflected in the DC transport characteristics and varies with sample and temperature. We attribute the diverse behavior of gate voltage dependence of the noise amplitude to the relative contributions from various scattering mechanisms, and to potential fluctuations near the Dirac point caused by charge carrier inhomogeneity. The higher carrier mobility of suspended graphene devices accounts for values of 1/f noise significantly lower than those observed in on-substrate graphene devices and most traditional electronic materials.
-http://pubs.acs.org/doi/abs/10.1021/nn202749z
-----
-''' Impurity effects on Fabry-Perot physics of ballistic carbon nanotubes '''
-F. Romeo, R. Citro, A. Di Bartolomeo
-We present a theoretical model accounting for the anomalous Fabry-Perot pattern observed in the ballistic conductance of a single-wall carbon nanotubes. Using the scattering field theory, it is shown that the presence of a limited number of impurities along the nanotube can be identified by a measurement of the conductance and their position determined. Impurities can be made active or silent depending on the interaction with the substrate via the back-gate. The conceptual steps for designing a bio-molecules detector are briefly discussed.
-http://lanl.arxiv.org/abs/1109.1104v1
-----
-''' Signature of Majorana Fermions in Charge Transport in Semiconductor Nanowires '''
-Chunlei Qu, Yongping Zhang, Li Mao, Chuanwei Zhang
-We investigate the charge transport in a semiconductor nanowire that is subject to a perpendicular magnetic field and in partial contact with an s-wave superconductor. We find that Majorana fermions, existing at the interface between superconducting and normal sections of the nanowire within certain parameter region, can induce resonant Andreev reflection of electrons at the interface, which yields a zero energy peak in the electrical conductance of the nanowire. The width of the zero energy conductance peak for different experimental parameters is characterized. While the zero energy peak provides a signature for Majorana fermions in one dimensional nanowires, it disappears in a two-dimensional semiconductor thin film with the same experimental setup because of the existence of other edge states in two dimensions. The proposed charge transport experiment may provide a simple and experimentally feasible method for the detection of Majorana fermions in semiconductor nanowires.
-http://xxx.lanl.gov/abs/1109.4108
-----
-''' Integer Quantum Hall Effect in Trilayer Graphene ''''
-A. Kumar1, W. Escoffier1, J. M. Poumirol1, C. Faugeras1, D. P. Arovas2, M. M. Fogler2, F. Guinea3, S. Roche4,5, M. Goiran1, and B. Raquet1
-By using high-magnetic fields (up to 60 T), we observe compelling evidence of the integer quantum Hall effect in trilayer graphene. The magnetotransport fingerprints are similar to those of the graphene monolayer, except for the absence of a plateau at a filling factor of ν=2. At a very low filling factor, the Hall resistance vanishes due to the presence of mixed electron and hole carriers induced by disorder. The measured Hall resistivity plateaus are well reproduced theoretically, using a self-consistent Hartree calculations of the Landau levels and assuming an ABC stacking order of the three layers.
-http://prl.aps.org/abstract/PRL/v107/i12/e126806
-----
-''' Shubnikov-de Haas oscillations of a single layer graphene under dc current bias '''
-Zhenbing Tan, Changling Tan, Li Ma, G. T. Liu, L. Lu, and C. L. Yang*
-Shubnikov-de Haas (SdH) oscillations under a dc current bias are experimentally studied on a Hall bar sample of single-layer graphene. In dc resistance, the bias current shows the common damping effect on the SdH oscillations and the effect can be well accounted for by an elevated electron temperature that is found to be linearly dependent on the current bias. In differential resistance, a novel phase inversion of the SdH oscillations has been observed with increasing dc bias, namely we observe the oscillation maxima develop into minima and vice versa. Moreover, it is found that the onset bias current, at which a SdH extremum is about to invert, is linearly dependent on the magnetic field of the SdH extrema. These observations are quantitatively explained with the help of a general SdH formula.
-http://prb.aps.org/abstract/PRB/v84/i11/e115429
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-Az alábbiak régiek.
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-'''High Current Density Esaki Tunnel Diodes Based on GaSb-InAsSb Heterostructure Nanowires'''
-http://pubs.acs.org/doi/abs/10.1021/nl202180b
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-''' Electronic Double Slit Interferometers Based on Carbon Nanotubes '''
-http://pubs.acs.org/doi/abs/10.1021/nl202360h
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-''' Observation of Raman G-Peak Split for Graphene Nanoribbons with Hydrogen-Terminated Zigzag Edges '''
-http://pubs.acs.org/doi/abs/10.1021/nl201387x
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-''' Magnetic Proximity Effect as a Pathway to Spintronic Applications of Topological Insulators '''
-http://pubs.acs.org/doi/abs/10.1021/nl201275q
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-''' Synthesis of Graphene Nanoribbons Encapsulated in Single-Walled Carbon Nanotubes '''
-http://pubs.acs.org/doi/abs/10.1021/nl2024678
-== Szept. 08. - Szept. 14. (2011) ==
-''Válogatta: Scherübl Zoltán''
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-'''In situ tunable g factor for a single electron confined inside an InAs quantum dot'''
-W. Liu1, S. Sanwlani1, R. Hazbun2, J. Kolodzey2, A. S. Bracker3, D. Gammon3, and M. F. Doty1
-Tailoring the properties of single spins confined in self-assembled quantum dots (QDs) is critical to the development of new optoelectronic logic devices. However, the range of heterostructure engineering techniques that can be used to control spin properties is severely limited by the requirements of QD self-assembly. We demonstrate a new strategy for rationally engineering the spin properties of single confined electrons or holes by adjusting the composition of the barrier between a stacked pair of InAs QDs coupled by coherent tunneling to form a quantum dot molecule (QDM). We demonstrate this strategy by designing, fabricating, and characterizing a QDM in which the g-factor for a single confined electron can be tuned in situ by over 50% with a minimal change in applied voltage.
-http://prb.aps.org/abstract/PRB/v84/i12/e121304
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-'''Hole-spin initialization and relaxation times in InAs/GaAs quantum dots'''
-F. Fras, B. Eble, P. Desfonds, F. Bernardot, C. Testelin, and M. Chamarro
-We study, at low temperature and zero magnetic field, the hole-spin dynamics in InAs/GaAs quantum dots. We measure the hole-spin relaxation time at a time scale longer than the dephasing time (about ten nanoseconds), imposed by the hole-nuclear hyperfine coupling. We use a pump-probe configuration and compare two experimental techniques based on differential absorption. The first one works in the time domain, and the second one is a new experimental method, the dark-bright time-scanning spectroscopy (DTS), working in the frequency domain. The measured hole-spin relaxation times, using these two techniques, are very similar, in the order of TNh≈1 μs. It is mainly imposed by the inhomogeneous hole hyperfine coupling in the hole localization volume. The DTS technique allows us also to measure the hole-spin initialization time τi. The hole spin is initialized by a periodic train of circularly polarized pulses at 76 MHz; we have observed that τi decreases as the power density increases, and we have measured a minimum value of τi≈100 ns in good agreement with a simple model [see B. Eble, P. Desfonds, F. Fras, F. Bernardot, C. Testelin, M. Chamarro, A. Miard and A. Lemaître Phys. Rev. B 81 045322 (2010)].
-http://prb.aps.org/abstract/PRB/v84/i12/e125431
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-'''Efficient terahertz emission from InAs nanowires'''
-Denis V. Seletskiy1,4,*, Michael P. Hasselbeck1, Jeffrey G. Cederberg2, Aaron Katzenmeyer3, Maria E. Toimil-Molares3, François Léonard3, A. Alec Talin3,†, and Mansoor Sheik-Bahae1
-We observe intense pulses of far-infrared electromagnetic radiation emitted from arrays of InAs nanowires. The terahertz radiation power efficiency of these structures is ∼15 times higher than a planar InAs substrate. This is explained by the preferential orientation of coherent plasma motion to the wire surface, which overcomes radiation trapping by total-internal reflection. We present evidence that this radiation originates from a low-energy acoustic surface plasmon mode of the nanowire. This is supported by independent measurements of electronic transport on individual nanowires, ultrafast terahertz spectroscopy, and theoretical analysis. Our combined experiments and analysis further indicate that these plasmon modes are specific to high aspect ratio geometries.
-http://prb.aps.org/abstract/PRB/v84/i11/e115421
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-'''Magnetic Proximity Effect as a Pathway to Spintronic Applications of Topological Insulators'''
-Ivana Vobornik*†, Unnikrishnan Manju‡, Jun Fujii†, Francesco Borgatti§, Piero Torelli†, Damjan Krizmancic†, Yew San Hor, Robert J. Cava, and Giancarlo Panaccione†
-We complete our recently introduced theoretical framework treating the double-quantum-dot system with a generalized form of Hubbard model. The effects of all quantum parameters involved in our model on the charge-stability diagram are discussed in detail. A general formulation of the microscopic theory is presented and, truncating at one orbital per site, we study the implication of different choices of the model confinement potential on the Hubbard parameters as well as the charge-stability diagram. We calculate the charge-stability diagram keeping three orbitals per site and find that the effect of additional higher-lying orbitals on the subspace with lowest-energy orbitals only can be regarded as a small renormalization of Hubbard parameters, thereby justifying our practice of keeping only the lowest orbital in all other calculations. The role of the harmonic-oscillator frequency in the implementation of the Gaussian model potential is discussed, and the effect of an external magnetic field is identified to be similar to choosing a more localized electron wave function in microscopic calculations. The full matrix form of the Hamiltonian, including all possible exchange terms and several peculiar charge-stability diagrams due to unphysical parameters, is presented in the Appendices, thus emphasizing the critical importance of a reliable microscopic model in obtaining the system parameters defining the Hamiltonian.
-http://prb.aps.org/abstract/PRB/v84/i11/e115301
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-'''Direct measurement of quantum phases in graphene via photoemission spectroscopy'''
-Choongyu Hwang1, Cheol-Hwan Park2, David A. Siegel1,2, Alexei V. Fedorov3, Steven G. Louie1,2,*, and Alessandra Lanzara1,2,
-Quantum phases provide us with important information for understanding the fundamental properties of a system. However, the observation of quantum phases, such as Berry's phase and the sign of the matrix element of the Hamiltonian between two nonequivalent localized orbitals in a tight-binding formalism, has been challenged by the presence of other factors, e.g. , dynamic phases and spin or valley degeneracy, and the absence of methodology. Here, we report a way to directly access these quantum phases, through polarization-dependent angle-resolved photoemission spectroscopy (ARPES), using graphene as a prototypical two-dimensional material. We show that the momentum- and polarization-dependent spectral intensity provides direct measurements of (i) the phase of the band wavefunction and (ii) the sign of matrix elements for nonequivalent orbitals. Upon rotating light polarization by π/2, we found that graphene with a Berry's phase of nπ (n=1 for single- and n=2 for double-layer graphene for Bloch wavefunction in the commonly used form) exhibits the rotation of ARPES intensity by π/n, and that ARPES signals reveal the signs of the matrix elements in both single- and double-layer graphene. The method provides a technique to directly extract fundamental quantum electronic information on a variety of materials.
-http://prb.aps.org/abstract/PRB/v84/i12/e125422
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-'''Current correlations in the interacting Cooper-pair beam-splitter'''
-J. Rech, D. Chevallier, T. Jonckheere, T. Martin
-Using a conserving many-body treatment, we propose an approach allowing the computation of currents and their correlations in interacting multi-terminal mesoscopic systems involving quantum dots coupled to normal and/or superconducting leads. We illustrate our method with the Cooper-pair beam-splitter setup recently proposed, which we model as a double quantum dot with weak interactions, connected to a superconducting lead and two normal ones. Our results suggest that even a weak Coulomb repulsion tends to favor positive current cross-correlations.
-http://xxx.lanl.gov/abs/1109.2476
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-'''Quantum Hall effect and semimetallic behavior in dual-gated ABA trilayer graphene'''
-E. A. Henriksen, D. Nandi, J. P. Eisenstein
-The electronic structure of multilayer graphenes depends strongly on the number of layers as well as the stacking order. Here we explore the electronic transport of purely ABA-stacked trilayer graphenes in a dual-gated field effect device configuration. We find both the quantum Hall effect (QHE) and low-field transport to be distinctly different from the mono- and bilayer graphenes, showing electron-hole asymmetries that are strongly suggestive of a semimetallic band overlap. When subject to an electric field perpendicular to the sheet, Landau level splittings due to breaking of the lattice mirror symmetry are clearly observed.
-http://xxx.lanl.gov/abs/1109.2385
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-'''Measuring the complex admittance of a carbon nanotube double quantum dot
-S.J. Chorley, J. Wabnig, Z.V. Penfold-Fitch, K.D. Petersson, J. Frake, C.G. Smith, M.R. Buitelaar
-We investigate radio-frequency (rf) reflectometry in a tunable carbon nanotube double quantum dot coupled to a resonant circuit. By measuring the in-phase and quadrature components of the reflected rf signal, we are able to determine the complex admittance of the double quantum dot as a function of the energies of the single-electron states. The measurements are found to be in good agreement with a theoretical model of the device in the incoherent limit. Besides being of fundamental interest, our results present an important step forward towards non-invasive charge and spin state readout in carbon nanotube quantum dots.
-http://xxx.lanl.gov/abs/1109.1827
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-'''Imaging the lateral shift of a quantum-point contact using scanning-gate microscopy'''
-S. Schnez, C. Rössler, T. Ihn, K. Ensslin, C. Reichl, W. Wegscheider
-We perform scanning-gate microscopy on a quantum-point contact. It is defined in a high-mobility two-dimensional electron gas of an AlGaAs/GaAs heterostructure, giving rise to a weak disorder potential. The lever arm of the scanning tip is significantly smaller than that of the split gates defining the conducting channel of the quantum-point contact. We are able to observe that the conducting channel is shifted in real space when asymmetric gate voltages are applied. The observed shifts are consistent with transport data and numerical estimations.
-http://xxx.lanl.gov/abs/1109.1544
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-'''Carbon tips as electrodes for single-molecule junctions'''
-Andres Castellanos-Gomez, Stefan Bilan, Linda A. Zotti, Carlos R. Arroyo, Nicolas Agrait, Juan Carlos Cuevas, Gabino Rubio-Bollinger
-We study electron transport through single-molecule junctions formed by an octanethiol molecule bonded with the thiol anchoring group to a gold electrode and the opposing methyl endgroup to a carbon tip. Using the scanning tunneling microscope based break junction technique, we measure the electrical conductance of such molecular junctions. We observe the presence of well-defined conductance plateaus during the stretching of the molecular bridge, which is the signature of the formation of a molecular junction.
-http://xxx.lanl.gov/abs/1109.2089
-== Szept. 01. - Szept. 07. (2011) ==
-''Válogatta: Pósa László''
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-[[File:Suggestion.jpg‎]]'''Cím'''
-Authors: ???
+Inventing a Modern Periscope
-Abstract
+http://www.tested.com/science/464266-inventing-modern-periscope/
-http://???
 ----