275 research outputs found
Current responses and voltage fluctuations in Josephson-junction systems
We consider arrays of Josephson junctions as well as single junctions in both
the classical and quantum-mechanical regimes, and examine the generalized
(frequency-dependent) resistance, which describes the dynamic responses of such
Josephson-junction systems to external currents. It is shown that the
generalized resistance and the power spectrum of voltage fluctuations are
related via the fluctuation-dissipation theorem. Implications of the obtained
relations are also discussed in various experimental situations.Comment: 4 pages, To appear in Europhys. Let
Incoherent multiple Andreev reflection in an array of SNS junctions
Last years many interesting effects related to incoherent MAR have been
experimentally found, but only few of them were theoretically explained. It was
shown, for example, that if the voltage at the edges of a linear array is
then subgarmonic structures in the current -voltage characteristics appear not
only at usual for nonstationary Josephson effect positions, ,
where is integer, but also at voltages other than . A step towards
description of electron transport in a dirty array of SNS junctions is done in
this letter. It is shown that subgarmonic structures may indeed appear at
``unusual'' voltages
Considerable enhancement of the critical current in a superconducting film by magnetized magnetic strip
We show that a magnetic strip on top of a superconducting strip magnetized in
a specified direction may considerably enhance the critical current in the
sample. At fixed magnetization of the magnet we observed diode effect - the
value of the critical current depends on the direction of the transport
current. We explain these effects by a influence of the nonuniform magnetic
field induced by the magnet on the current distribution in the superconducting
strip. The experiment on a hybrid Nb/Co structure confirmed the predicted
variation of the critical current with a changing value of magnetization and
direction of the transport current.Comment: 6 pages, 7 figure
Observation of a New Fluxon Resonant Mechanism in Annular Josephson Tunnel Structures
A novel dynamical state has been observed in the dynamics of a perdurbed
sine-Gordon system. This resonant state, has been experimentally observed as a
singularity in the dc current voltage characteristic of an annular Josephson
tunnel junction, excited in the presence of a magnetic field. With this
respect, it can be assimilated to self-resonances known as Fiske steps.
Differently from these, however, we demonstrate, on the basis of numerical
simulations, that its detailed dynamics involves rotating fluxon pairs, a
mechanism associated, so far, to self-resonances known as zero-field steps.Comment: 4 pages, 2 figures, submitted to Physical Review Letter
Sub-electron Charge Relaxation via 2D Hopping Conductors
We have extended Monte Carlo simulations of hopping transport in completely
disordered 2D conductors to the process of external charge relaxation. In this
situation, a conductor of area shunts an external capacitor
with initial charge . At low temperatures, the charge relaxation process
stops at some "residual" charge value corresponding to the effective threshold
of the Coulomb blockade of hopping. We have calculated the r.m.s value
of the residual charge for a statistical ensemble of capacitor-shunting
conductors with random distribution of localized sites in space and energy and
random , as a function of macroscopic parameters of the system. Rather
unexpectedly, has turned out to depend only on some parameter
combination: for negligible Coulomb interaction
and for substantial interaction. (Here
is the seed density of localized states, while is the
dielectric constant.) For sufficiently large conductors, both functions
follow the power law , but with different
exponents: for negligible and
for significant Coulomb interaction. We have been able to derive this law
analytically for the former (most practical) case, and also explain the scaling
(but not the exact value of the exponent) for the latter case. In conclusion,
we discuss possible applications of the sub-electron charge transfer for
"grounding" random background charge in single-electron devices.Comment: 12 pages, 5 figures. In addition to fixing minor typos and updating
references, the discussion has been changed and expande
Radio-frequency Bloch-transistor electrometer
A quantum-limited electrometer based on charge modulation of the Josephson
supercurrent in the Bloch transistor inserted into a superconducting ring is
proposed. As this ring is inductive coupled to a high-Q resonance tank circuit,
the variations of the charge on the transistor island (input signal) are
converted into variations of amplitude and phase of radio-frequency
oscillations in the tank. These variations are amplified and then detected. The
output noise, the back-action fluctuations and their cross-correlation are
computed. It is shown that our device enables measurements of the charge with a
sensitivity which is determined by the energy resolution of its amplifier, that
can be reduced down to the standard quantum limit of \hbar/2. On the basis of
this setup a "back-action-evading" scheme of the charge measurements is
proposed.Comment: 5 pages incl. 2 figure
Universality of transport properties of ultra-thin oxide films
We report low-temperature measurements of current-voltage characteristics for
highly conductive Nb/Al-AlOx-Nb junctions with thicknesses of the Al interlayer
ranging from 40 to 150 nm and ultra-thin barriers formed by diffusive oxidation
of the Al surface. In the superconducting state these devices have revealed a
strong subgap current leakage. Analyzing Cooper-pair and quasiparticle currents
across the devices, we conclude that the strong suppression of the subgap
resistance comparing with conventional tunnel junctions originates from a
universal bimodal distribution of transparencies across the Al-oxide barrier
proposed earlier by Schep and Bauer. We suggest a simple physical explanation
of its source in the nanometer-thick oxide films relating it to strong local
barrier-height fluctuations which are generated by oxygen vacancies in thin
aluminum oxide tunnel barriers formed by thermal oxidation.Comment: revised text and a new figur
Resistance in Superconductors
In this pedagogical review, we discuss how electrical resistance can arise in
superconductors. Starting with the idea of the superconducting order parameter
as a condensate wave function, we introduce vortices as topological excitations
with quantized phase winding, and we show how phase slips occur when vortices
cross the sample. Superconductors exhibit non-zero electrical resistance under
circumstances where phase slips occur at a finite rate. For one-dimensional
superconductors or Josephson junctions, phase slips can occur at isolated
points in space-time. Phase slip rates may be controlled by thermal activation
over a free-energy barrier, or in some circumstances, at low temperatures, by
quantum tunneling through a barrier. We present an overview of several
phenomena involving vortices that have direct implications for the electrical
resistance of superconductors, including the Berezinskii-Kosterlitz-Thouless
transition for vortex-proliferation in thin films, and the effects of vortex
pinning in bulk type II superconductors on the non-linear resistivity of these
materials in an applied magnetic field. We discuss how quantum fluctuations can
cause phase slips and review the non-trivial role of dissipation on such
fluctuations. We present a basic picture of the superconductor-to-insulator
quantum phase transitions in films, wires, and Josephson junctions. We point
out related problems in superfluid helium films and systems of ultra-cold
trapped atoms. While our emphasis is on theoretical concepts, we also briefly
describe experimental results, and we underline some of the open questions.Comment: Chapter to appear in "Bardeen, Cooper and Schrieffer: 50 Years,"
edited by Leon N. Cooper and Dmitri Feldman, to be published by World
Scientific Pres
Charge transport through weakly open one dimensional quantum wires
We consider resonant transmission through a finite-length quantum wire
connected to leads via finite transparency junctions. The coherent electron
transport is strongly modified by the Coulomb interaction. The low-temperature
current-voltage () curves show step-like dependence on the bias voltage
determined by the distance between the quantum levels inside the conductor, the
pattern being dependent on the ratio between the charging energy and level
spacing. If the system is tuned close to the resonance condition by the gate
voltage, the low-voltage curve is Ohmic. At large Coulomb energy and low
temperatures, the conductance is temperature-independent for any relationship
between temperature, level spacing, and coupling between the wire and the
leads
Coupled Superconducting Phase and Ferromagnetic Order Parameter Dynamics
Via a direct coupling between the magnetic order parameter and the singlet
Josephson supercurrent, we detect spin-wave resonances, and their dispersion,
in ferromagnetic Josephson junctions in which the usual insulating or metallic
barrier is replaced with a weak ferromagnet. The coupling arises within the
Fraunhofer interferential description of the Josephson effect, because the
magnetic layer acts as a time dependent phase plate. A spin-wave resonance at a
frequency ws implies a dissipation that is reflected as a depression in the
current-voltage curve of the Josephson junction when hbar ws = 2eV. We have
thereby performed a resonance experiment on only 10^7 Ni atoms.Comment: 4 pages, 4 figure
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