72 research outputs found
Magnetic field influence on the proximity effect in semiconductor - superconductor hybrid structures and their thermal conductance
We show that a magnetic field can influnce the proximity effect in NS
junctions via diamagnetic screening current flowing in the superconductor.
Using ballistic quasi-one-dimensional (Q1D) electron channels as an example, we
show that the supercurrent flow shifts the proximity-induced minigap in the
excitation spectrum of a Q1D system from the Fermi level to higher
quasiparticle energies. Thermal conductance of a Q1D channel (normalized by
that of a normal Q1D ballistic system) is predicted to manifest such a spectral
feature as a nonmonotonic behavior at temperatures corresponding to the energy
of excitation into the gapful part of the spectrum.Comment: 5 pages, 3 figures, revised version with a new titl
Energy spectroscopy of Andreev levels between two superconductors
We perform energy spectroscopy of Andreev reflection processes occurring at two superconducting electrodes connected in series via a ballistic two-dimensional channel, by measuring the voltage dependence of the part of the conductance which is modulated by the macroscopic phase difference. The modulation amplitude oscillates as a function of energy and the phase exhibits an abrupt shift close to π at the energy for which the amplitude is minimum. We discuss how our findings are related to the properties of bound states formed between the two superconductors.
Electron-phonon and electron-electron interactions in organic field effect transistors
Recent experiments have demonstrated that the performances of organic FETs
strongly depend on the dielectric properties of the gate insulator. In
particular, it has been shown that the temperature dependence of the mobility
evolves from a metallic-like to an insulating behavior upon increasing the
dielectric constant of the gate material. This phenomenon can be explained in
terms of the formation of small polarons, due to the polar interaction of the
charge carriers with the phonons at the organic/dielectric interface. Building
on this model, the possible consequences of the Coulomb repulsion between the
carriers at high concentrations are analyzed.Comment: proceedings of the SMEC'07 conference, submitted to the Journal of
Physics and Chemistry of Solid
Statistical significance of fine structure in the frequency spectrum of Aharonov-Bohm conductance oscillations
We discuss a statistical analysis of Aharonov-Bohm conductance oscillations
measured in a two-dimensional ring, in the presence of Rashba spin-orbit
interaction. Measurements performed at different values of gate voltage are
used to calculate the ensemble-averaged modulus of the Fourier spectrum and, at
each frequency, the standard deviation associated to the average. This allows
us to prove the statistical significance of a splitting that we observe in the
h/e peak of the averaged spectrum. Our work illustrates in detail the role of
sample specific effects on the frequency spectrum of Aharonov-Bohm conductance
oscillations and it demonstrates how fine structures of a different physical
origin can be discriminated from sample specific features.Comment: accepted for publication in PR
Magnetoconductance Oscillations in Ballistic Semiconductor-Superconductor Junctions
The mechanism of the magnetoconductance oscillations in junctions of a
ballistic semiconductor and a superconductor is discussed. The oscillations
appear when both the normal and the Andreev reflection occur at the interface.
The interplay between the classical cyclotron motion of a quasiparticle and the
phase shift caused by the magnetic field is the origin of the conductance
oscillations.Comment: 4 pages, 4 figure
Nonequilibrium Josephson effect in short-arm diffusive SNS interferometers
We study non-equilibrium Josephson effect and phase-dependent conductance in
three-terminal diffusive interferometers with short arms. We consider strong
proximity effect and investigate an interplay of dissipative and Josephson
currents co-existing within the same proximity region. In junctions with
transparent interfaces, the suppression of the Josephson current appears at
rather large voltage, , and the current vanishes at
. Josephson current inversion becomes possible in junctions with
resistive interfaces, where the inversion occurs within a finite interval of
the applied voltage. Due to the presence of considerably large and
phase-dependent injection current, the critical current measured in a current
biased junction does not coincide with the maximum Josephson current, and
remains finite when the true Josephson current is suppressed. The voltage
dependence of the conductance shows two pronounced peaks, at the bulk gap
energy, and at the proximity gap energy; the phase oscillation of the
conductance exhibits qualitatively different form at small voltage ,
and at large voltage .Comment: 11 pages, 9 figures, revised version, to be published in Phys. Rev.
Contact resistance in graphene-based devices
We report a systematic study of the contact resistance present at the
interface between a metal (Ti) and graphene layers of different, known
thickness. By comparing devices fabricated on 11 graphene flakes we demonstrate
that the contact resistance is quantitatively the same for single-, bi-, and
tri-layer graphene (), and is in all cases
independent of gate voltage and temperature. We argue that the observed
behavior is due to charge transfer from the metal, causing the Fermi level in
the graphene region under the contacts to shift far away from the charge
neutrality point
Direct demonstration of circulating currents in a controllable -SQUID generated by a 0 to transition of the weak links
A controllable -SQUID is a DC SQUID with two controllable
-junctions as weak links. A controllable -junction consists of a
superconducting - normal metal - superconducting Josephson junction with two
additional contacts to the normal region of the junction. By applying a voltage
over these contacts it is possible to control the sate of the junction,
i.e. a conventional (0) state or a -state, depending on the magnitude of
. We demonstrate experimentally that, by putting one junction into a
-state, a screening current is generated around the SQUID loop at integer
external flux. To be able to do this, we have fabricated controllable
-junctions, based on Cu-Nb or Ag-Nb, in a new geometry. We show that at
1.4 K only the Nb-Ag device shows the transition to a -state as a function
of consistent with theoretical predictions. In a controllable SQUID
based on Nb-Ag we observe, a part from a screening current at integer external
flux, a phase shift of of the oscillations under suitable
current bias, depending on the magnitude of .Comment: 11 pages, 12 figures, subm. to Phys. Rev.
Conditions for Adiabatic Spin Transport in Disordered Systems
We address the controversy concerning the necessary conditions for the
observation of Berry phases in disordered mesoscopic conductors. For this
purpose we calculate the spin-dependent conductance of disordered
two-dimensional structures in the presence of inhomogeneous magnetic fields.
Our numerical results show that for both, the overall conductance and quantum
corrections, the relevant parameter defining adiabatic spin transport scales
with the square root of the number of scattering events, in generalization of
Stern's original proposal [Phys. Rev. Lett. 68, 1022 (1992)]. This could hinder
a clear-cut experimental observation of Berry phase effects in diffusive
metallic rings.Comment: 5 pages, 4 figures. To appear in Phys. Rev. B (Rapid Communications
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