163 research outputs found
Energy dependence of current noise in superconducting/normal metal junctions
Interference of electronic waves undergoing Andreev reflection in diffusive
conductors determines the energy profile of the conductance on the scale of the
Thouless energy. A similar dependence exists in the current noise, but its
behavior is known only in few limiting cases. We consider a metallic diffusive
wire connected to a superconducting reservoir through an interface
characterized by an arbitrary distribution of channel transparencies. Within
the quasiclassical theory for current fluctuations we provide a general
expression for the energy dependence of the current noise.Comment: 5 pages, 1 Figur
Theory of microwave spectroscopy of Andreev bound states with a Josephson junction
We present a microscopic theory for the current through a tunnel Josephson
junction coupled to a non-linear environment, which consists of an Andreev
two-level system coupled to a harmonic oscillator. It models a recent
experiment [Bretheau, Girit, Pothier, Esteve, and Urbina, Nature (London) 499,
312 (2013)] on photon spectroscopy of Andreev bound states in a superconducting
atomic-size contact. We find the eigenenergies and eigenstates of the
environment and derive the current through the junction due to inelastic Cooper
pair tunneling. The current-voltage characteristic reveals the transitions
between the Andreev bound states, the excitation of the harmonic mode that
hybridizes with the Andreev bound states, as well as multi-photon processes.
The calculated spectra are in fair agreement with the experimental data.Comment: 8 pages, 6 figure
Temperature-dependent Ginzburg-Landau parameter
Taking into account both the orbital and the paramagnetic depairing effects
we derive a simple analytic formula for the temperature dependence of the
Ginzburg-Landau parameter valid in vicinity of field dependent critical
temperature in a type-II superconductor.Comment: 3 pages, no figure
Critical fluorescence of a transmon at the Schmid transition
We investigate inelastic microwave photon scattering by a transmon qubit
embedded in a high-impedance circuit. The transmon undergoes a
charge-localization (Schmid) transition upon the impedance reaching the
critical value. Due to the unique transmon level structure, the fluorescence
spectrum carries a signature of the transition point. At higher circuit
impedance, quasielastic photon scattering may account for the main part of the
inelastic scattering cross-section; we find its dependence on the qubit and
circuit parameters.Comment: 6 pages, 3 figure
Distribution function of persistent current
We introduce a variant of the replica trick within the nonlinear sigma model
that allows calculating the distribution function of the persistent current. In
the diffusive regime, a Gaussian distribution is derived. This result holds in
the presence of local interactions as well. Breakdown of the Gaussian
statistics is predicted for the tails of the distribution function at large
deviations
New superconducting phases in field-induced organic superconductor lambda-(BETS)2FeCl4
We derive the parallel upper critical field, Hc2, as a function of the
temperature T in quasi-2D organic compound lambda-(BETS)2FeCl4, accounting for
the formation of the nonuniform LOFF state. To further check the 2D LOFF model
we propose to study the Hc2(T) curve at low T in tilted fields, where the
vortex state is described by the high Landau level functions characterized by
the index n. We predict a cascade of first order transitions between vortex
phases with different n, between phases with different types of the symmetry at
given n and the change of the superconducting transition from the second order
to the first order as FeCl4 ions are replaced partly by GaCl4 ions.Comment: 4 pages, 3 figures, to be published in PR
Quantum Charge Fluctuations in a Superconducting Grain
We consider charge quantization in a small superconducting grain that is
contacted by a normal-metal electrode and is controlled by a capacitively
coupled gate. At zero temperature and zero conductance between the grain
and the electrode, the charge as a function of the gate voltage
changes in steps. The step height is if , where and
are, respectively, the superconducting gap and the charging energy of the
grain. Quantum charge fluctuations at finite conductance remove the
discontinuity in the dependence of on and lead to a finite step width
. The resulting shape of the Coulomb blockade staircase is
of a novel type. The grain charge is a continuous function of while the
differential capacitance, , has discontinuities at certain values of
the gate voltage. We determine analytically the shape of the Coulomb blockade
staircase also at non-zero temperatures.Comment: 12 pages, 3 figure
Single-dopant resonance in a single-electron transistor
Single dopants in semiconductor nanostructures have been studied in great
details recently as they are good candidates for quantum bits, provided they
are coupled to a detector. Here we report coupling of a single As donor atom to
a single-electron transistor (SET) in a silicon nanowire field-effect
transistor. Both capacitive and tunnel coupling are achieved, the latter
resulting in a dramatic increase of the conductance through the SET, by up to
one order of magnitude. The experimental results are well explained by the rate
equations theory developed in parallel with the experiment.Comment: 16 pages, 8 figure
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