89 research outputs found
Minigap, Parity Effect and Persistent Currents in SNS Nanorings
We have evaluated a proximity-induced minigap in the density of states (DOS)
of SNS junctions and SNS nanorings at an arbitrary concentration of
non-magnetic impurities. We have demonstrated that an isotropic energy minigap
in the electron spectrum opens up already at arbitrarily weak disorder, while
angle resolved DOS at higher energies can remain strongly anisotropic. The
minigap value can be tuned by passing a supercurrent through an
SNS junction or by applying a magnetic flux to an SNS ring. A
non-monotonous dependence of on has been found at weak
disorder. We have also studied persistent currents in isolated SNS nanorings.
For odd number of electrons in the ring we have found a non-trivial
current-phase (current-flux) relation which -- at relatively high disorder --
may lead to a -junction state and spontaneous currents in the ground state
of the system.Comment: 7 pages, 8 figure
Non-adiabatic Josephson Dynamics in Junctions with in-Gap Quasiparticles
Conventional models of Josephson junction dynamics rely on the absence of low
energy quasiparticle states due to a large superconducting gap. With this
assumption the quasiparticle degrees of freedom become "frozen out" and the
phase difference becomes the only free variable, acting as a fictitious
particle in a local in time Josephson potential related to the adiabatic and
non-dissipative supercurrent across the junction. In this article we develop a
general framework to incorporate the effects of low energy quasiparticles
interacting non-adiabatically with the phase degree of freedom. Such
quasiparticle states exist generically in constriction type junctions with high
transparency channels or resonant states, as well as in junctions of
unconventional superconductors. Furthermore, recent experiments have revealed
the existence of spurious low energy in-gap states in tunnel junctions of
conventional superconductors - a system for which the adiabatic assumption
typically is assumed to hold. We show that the resonant interaction with such
low energy states rather than the Josephson potential defines nonlinear
Josephson dynamics at small amplitudes.Comment: 9 pages, 1 figur
Low-temperature characterization of Nb-Cu-Nb weak links with Ar ion-cleaned interfaces
We characterize niobium-based lateral Superconductor (S) - Normal metal (N) -
Superconductor weak links through low-temperature switching current
measurements and tunnel spectroscopy. We fabricate the SNS devices in two
separate lithography and deposition steps, combined with strong argon ion
cleaning before the normal metal deposition in the last step. Our SNS weak link
consists of high-quality sputtered Nb electrodes that are contacted with
evaporated Cu. The two-step fabrication flow enables great flexibility in the
choice of materials and pattern design. A comparison of the
temperature-dependent equilibrium critical supercurrent with theoretical
predictions indicates that the quality of the Nb-Cu interface is similar to
that of evaporated Al-Cu weak links. Aiming at increased sensitivity, range of
operation temperatures, and thermal isolation, we investigate how these SNS
structures can be combined with shadow-evaporated aluminum tunnel junctions for
sensor applications that utilize the superconducting proximity effect. To this
end, we demonstrate a hybrid magnetic flux sensor based on a Nb-Cu-Nb SNS
junction, where the phase-dependent normal metal density of states is probed
with an Al tunnel junction.Comment: 5 pages, 3 figure
Coherent quantum phase slip
A hundred years after discovery of superconductivity, one fundamental
prediction of the theory, the coherent quantum phase slip (CQPS), has not been
observed. CQPS is a phenomenon exactly dual to the Josephson effect: whilst the
latter is a coherent transfer of charges between superconducting contacts, the
former is a coherent transfer of vortices or fluxes across a superconducting
wire. In contrast to previously reported observations of incoherent phase slip,
the CQPS has been only a subject of theoretical study. Its experimental
demonstration is made difficult by quasiparticle dissipation due to gapless
excitations in nanowires or in vortex cores. This difficulty might be overcome
by using certain strongly disordered superconductors in the vicinity of the
superconductor-insulator transition (SIT). Here we report the first direct
observation of the CQPS in a strongly disordered indium-oxide (InOx)
superconducting wire inserted in a loop, which is manifested by the
superposition of the quantum states with different number of fluxes. Similarly
to the Josephson effect, our observation is expected to lead to novel
applications in superconducting electronics and quantum metrology.Comment: 14 pages, 3 figure
Mesoscopic fluctuations of the supercurrent in diffusive Josephson junctions
We study mesoscopic fluctuations and weak localization correction to the
supercurrent in Josephson junctions with coherent diffusive electron dynamics
in the normal part. Two kinds of junctions are considered: a chaotic dot
coupled to superconductors by tunnel barriers and a diffusive junction with
transparent normal--superconducting interfaces. The amplitude of current
fluctuations and the weak localization correction to the average current are
calculated as functions of the ratio between the superconducting gap and the
electron dwell energy, temperature, and superconducting phase difference across
the junction. Technically, fluctuations on top of the spatially inhomogeneous
proximity effect in the normal region are described by the replicated version
of the \sigma-model. For the case of diffusive junctions with transparent
interfaces, the magnitude of mesoscopic fluctuations of the critical current
appears to be nearly 3 times larger than the prediction of the previous theory
which did not take the proximity effect into account.Comment: 19 pages, 14 figures, 2 table
First experimental evidence of one-dimensional plasma modes in superconducting thin wires
We have studied niobium superconducting thin wires deposited onto a
SrTiO substrate. By measuring the reflection coefficient of the wires,
resonances are observed in the superconducting state in the 130 MHz to 4 GHz
range. They are interpreted as standing wave resonances of one-dimensional
plasma modes propagating along the superconducting wire. The experimental
dispersion law, versus , presents a linear dependence over the
entire wave vector range. The modes are softened as the temperature increases
close the superconducting transition temperature. Very good agreement are
observed between our data and the dispersion relation predicted by Kulik and
Mooij and Sch\"on.Comment: Submitted to Physical review Letter
Relation between Barrier Conductance and Coulomb Blockade Peak Splitting for Tunnel-Coupled Quantum Dots
We study the relation between the barrier conductance and the Coulomb
blockade peak splitting for two electrostatically equivalent dots connected by
tunneling channels with bandwidths much larger than the dot charging energies.
We note that this problem is equivalent to a well-known single-dot problem and
present solutions for the relation between peak splitting and barrier
conductance in both the weak and strong coupling limits. Results are in good
qualitative agreement with the experimental findings of F. R. Waugh et al.Comment: 19 pages (REVTeX 3.0), 3 Postscript figure
Rough Surface Effect on Meissner Diamagnetism in Normal-layer of N-S Proximity-Contact System
Rough surface effect on the Meissner diamagnetic current in the normal layer
of proximity contact N-S bi-layer is investigated in the clean limit. The
diamagnetic current and the screening length are calculated by use of
quasi-classical Green's function. We show that the surface roughness has a
sizable effect, even when a normal layer width is large compared with the
coherence length . The effect is as large as that
of the impurity scattering and also as that of the finite reflection at the N-S
interface.Comment: 12 pages, 3 figures. To be published in J. Phys. Soc. Jpn. Vol.71-
"Chain scenario" for Josephson tunneling with pi-shift in YBa2Cu3O7
We point out that all current Josephson-junction experiments probing directly
the symmetry of the superconducting state in YBa2Cu3O7, can be interpreted in
terms of the bilayer antiferromagnetic spin fluctuation model, which renders
the superconducting state with the order parameters of extended symmetry,
but with the opposite signs in the bonding and antibonding Cu-O plane bands.
The essential part of our interpretation includes the Cu-O chain band which
would have the order parameter of the same sign as antibonding plane band. We
show that in this case net Josephson currents along and perpendicular to the
chains have the phase shift equal to pi.Comment: 4 pages, revtex, 1 figure uuencoded (POSTSCRIPT figure replaced - the
previous file did not print Greek letters correctly
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