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Cond-mat Mesoscale and Nanoscale Physics - recent papers Note: all the papers in a certain month can be listed as e.g. , where 11 stands for 2011 and 04 for April.

Physical Review Letters

Physical Review B


Link to the archive of this series before 2014

Mindig a legfrissebb bejegyzés van legfelül.


December 8.

Boros Csanád Örs

The 100th anniversary of the four-point probe technique: the role of probe geometries in isotropic and anisotropic systems

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.

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.

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

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

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

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

Coherent coupling of a single spin to microwave cavity photons

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

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

Point contacts in encapsulated graphene

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

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)

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

Semiconductor-Nanowire-Based Superconducting Qubit

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

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

Spin pairs in a weakly coupled many-electron quantum dot

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

Observation of negative refraction of Dirac fermions in graphene

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

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

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

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.

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.

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.

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.

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.

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.

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.

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

Coaxial lithography

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

Coupling between electrons and optical phonons in suspended bilayer graphene

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

Nonlocal electromagnetic response of graphene nanostructures

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

Nonequilibrium spin transport in Zeeman-split superconductors

Tatiana Krishtop, Manuel Houzet, and Julia S. Meyer

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

Stephanie Droste, Janine Splettstoesser, and Michele Governale

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.


Márton Attila

Előadás: SNS junctions in nanowires with spin-orbit coupling: role of confinement and helicity on the sub-gap spectrum ( 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.


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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Direct Laser Writing of Graphene Electronics

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.

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.

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).

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.

Ultrafast non-local control of spontaneous emission

For fun:

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


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.

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.

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.

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.

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.

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.

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.

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.

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.

Graphene nanoribbon heterojunctions

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

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.

Polycrystalline Graphene with Single Crystalline Electronic Structure

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.

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

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.


Inventing a Modern Periscope

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