24 research outputs found
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 or
) 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 -correction in a 3D conductor. In
a quasi-two-dimensional conductor, ( is the thickness, 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 . The electron interaction via exchange of virtual phonons
also gives -correction. The contribution of thermal phonons interacting
with electrons via the screened deformation potential results in -term and
via unscreened deformation potential results in -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