Recent interesting articles
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Nature Nanotechnology last week
Cond-mat Mesoscale and Nanoscale Physics - recent papers Note: all the papers in a certain month can be listed as e.g. http://xxx.lanl.gov/list/cond-mat.mes-hall/1104 , where 11 stands for 2011 and 04 for April.
Contents |
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
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
Spin Polarization Measurement of Homogeneously Doped Fe1–xCoxSi 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
Az alábbiak régiek.
High Current Density Esaki Tunnel Diodes Based on GaSb-InAsSb Heterostructure Nanowires
http://pubs.acs.org/doi/abs/10.1021/nl202180b
Electronic Double Slit Interferometers Based on Carbon Nanotubes
http://pubs.acs.org/doi/abs/10.1021/nl202360h
Observation of Raman G-Peak Split for Graphene Nanoribbons with Hydrogen-Terminated Zigzag Edges
http://pubs.acs.org/doi/abs/10.1021/nl201387x
Magnetic Proximity Effect as a Pathway to Spintronic Applications of Topological Insulators
http://pubs.acs.org/doi/abs/10.1021/nl201275q
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
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
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
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
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
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
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
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
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
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
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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