Electron Dynamics in a Coupled Quantum Point Contact Structure with a Local Magnetic Moment

We develop a theoretical model for the description of electron dynamics in coupled quantum wires when the local magnetic moment is formed in one of the wires. We employ a single-particle Hamiltonian that takes account of the specific geometry of potentials defining the structure as well as electron scattering on the local magnetic moment. The equations for the wave functions in both wires are derived and the approach for their solution is discussed. We determine the transmission coefficient and conductance of the wire having the local magnetic moment and show that our description reproduces the experimentally observed features.Comment: Based on work presented at 2004 IEEE NTC Quantum Device Technology Worksho

Mechanism of electron localization in a quantum wire

We show that quasi-bound electron states are formed in a quantum wire as a result of electron backscattering in the transition regions between the wire and the electron reservoirs, to which the wire is coupled. The backscattering mechanism is caused by electron density oscillations arising even in smooth transitions due to the reflection of electrons not transmitting through the wire. The quasi-bound states reveal themselves in resonances of the electron transmission probability through the wire. The calculations were carried out within the Hartree-Fock approximation using quasiclassic wavefunctions.Comment: 7 pages, IOP style, 4 figures, typos corrected, published versio

Andreev transport in two-dimensional normal-superconducting systems in strong magnetic fields

The conductance in two-dimensional (2D) normal-superconducting (NS) systems is analyzed in the limit of strong magnetic fields when the transport is mediated by the electron-hole states bound to the sample edges and NS interface, i.e., in the Integer Quantum Hall Effect regime.The Andreev-type process of the conversion of the quasiparticle current into the superflow is shown to be strongly affected by the mixing of the edge states localized at the NS and insulating boundaries. The magnetoconductance in 2D NS structures is calculated for both quadratic and Dirac-like normal state spectra. Assuming a random scattering of the edge modes we analyze both the average value and fluctuations of conductance for an arbitrary number of conducting channels.Comment: 5 pages, 1 figur

Influence of Magnetic Moment Formation on the Conductance of Coupled Quantum Wires

In this report, we develop a model for the resonant interaction between a pair of coupled quantum wires, under conditions where self-consistent effects lead to the formation of a local magnetic moment in one of the wires. Our analysis is motivated by the experimental results of Morimoto et al. [Appl. Phys. Lett. \bf{82}, 3952 (2003)], who showed that the conductance of one of the quantum wires exhibits a resonant peak at low temperatures, whenever the other wire is swept into the regime where local-moment formation is expected. In order to account for these observations, we develop a theoretical model for the inter-wire interaction that calculated the transmission properties of one (the fixed) wire when the device potential is modified by the presence of an extra scattering term, arising from the presence of the local moment in the swept wire. To determine the transmission coefficients in this system, we derive equations describing the dynamics of electrons in the swept and fixed wires of the coupled-wire geometry. Our analysis clearly shows that the observation of a resonant peak in the conductance of the fixed wire is correlated to the appearance of additional structure (near $0.75\cdot$ or $0.25\cdot 2e^2/h$) in the conductance of the swept wire, in agreement with the experimental results of Morimoto et al

Detection of local-moment formation using the resonant interaction between coupled quantum wires

We study the influence of many-body interactions on the transport characteristics of a novel device structure, consisting of a pair of quantum wires that are coupled to each other by means of a quantum dot. Under conditions where a local magnetic moment is formed in one of the wires, we show that tunnel coupling to the other gives rise to an associated peak in its density of states, which can be detected directly in a conductance measurement. Our theory is therefore able to account for the key observations in the recent study of T. Morimoto et al. [Appl. Phys. Lett. {\bf 82}, 3952 (2003)], and demonstrates that coupled quantum wires may be used as a system for the detection of local magnetic-moment formation

Phonon assisted dynamical Coulomb blockade in a thin suspended graphite sheet

The differential conductance in a suspended few layered graphene sample is fou nd to exhibit a series of quasi-periodic sharp dips as a function of bias at l ow temperature. We show that they can be understood within a simple model of dyn amical Coulomb blockade where energy exchanges take place between the charge carriers transmitted trough the sample and a dissipative electromagnetic envir onment with a resonant phonon mode strongly coupled to the electrons

Critical currents in graphene Josephson junctions

We study the superconducting correlations induced in graphene when it is placed between two superconductors, focusing in particular on the supercurrents supported by the 2D system. For this purpose we make use of a formalism placing the emphasis on the many-body aspects of the problem, with the aim of investigating the dependence of the critical currents on relevant variables like the distance L between the superconducting contacts, the temperature, and the doping level. Thus we show that, despite the vanishing density of states at the Fermi level in undoped graphene, supercurrents may exist at zero temperature with a natural 1/L^3 dependence at large L. When temperature effects are taken into account, the supercurrents are further suppressed beyond the thermal length L_T (~ v_F / k_B T, in terms of the Fermi velocity v_F of graphene), entering a regime where the decay is given by a 1/L^5 dependence. On the other hand, the supercurrents can be enhanced upon doping, as the Fermi level is shifted by a chemical potential \mu from the charge neutrality point. This introduces a new crossover length L* ~ v_F / \mu, at which the effects of the finite charge density start being felt, marking the transition from the short-distance 1/L^3 behavior to a softer 1/L^2 decay of the supercurrents at large L. It turns out that the decay of the critical currents is given in general by a power-law behavior, which can be seen as a consequence of the perfect scaling of the Dirac theory applied to the low-energy description of graphene.Comment: 11 pages, 6 figures, to appear in J. Phys.: Condens. Matte

Effects of Electron-Electron and Electron-Phonon Interactions in Weakly Disordered Conductors and Heterostuctures

We investigate quantum corrections to the conductivity due to the interference of electron-electron (electron-phonon) scattering and elastic electron scattering in weakly disordered conductors. The electron-electron interaction results in a negative $T^2 \ln T$-correction in a 3D conductor. In a quasi-two-dimensional conductor, $d < v_F/T$ ($d$ is the thickness, $v_F$ is the Fermi velocity), with 3D electron spectrum this correction is linear in temperature and differs from that for 2D electrons (G. Zala et. al., Phys. Rev.B {\bf 64}, 214204 (2001)) by a numerical factor. In a quasi-one-dimensional conductor, temperature-dependent correction is proportional to $T^2$. The electron interaction via exchange of virtual phonons also gives $T^2$-correction. The contribution of thermal phonons interacting with electrons via the screened deformation potential results in $T^4$-term and via unscreened deformation potential results in $T^2$-term. The interference contributions dominate over pure electron-phonon scattering in a wide temperature range, which extends with increasing disorder.Comment: 6 pages, 2figure

Proximity Induced Superconductivity and Multiple Andreev Reflections in Few-Layer-Graphene

We have investigated electronic transport of few-layer-graphene (FLG) connected to superconducting electrodes. The device is prepared by mechanical exfoliation of graphite. A small mesa of FLG is placed on the surface of an insulating Alumina layer over silicon substrate, and is connected with two tungsten electrodes, separated by 2.5 microns, grown by focused ion beam. While tungsten electrodes are superconducting below 4 K, proximity induced superconductivity in FLG is observed below 1K with a large differential resistance drop at low bias. Signatures of multiple Andreev reflections are observed as peaks located at voltages corresponding to sub-multiple values of the superconducting gap of the electrodes