423 research outputs found
Conductance Fluctuations, Weak Localization, and Shot Noise for a Ballistic Constriction in a Disordered Wire
This is a study of phase-coherent conduction through a ballistic point
contact with disordered leads. The disorder imposes mesoscopic
(sample-to-sample) fluctuations and weak-localization corrections on the
conductance, and also leads to time-dependent fluctuations (shot noise) of the
current. These effects are computed by means of a mapping onto an unconstricted
conductor with a renormalized mean free path. The mapping holds both in the
metallic and in the localized regime, and permits a solution for arbitrary
ratio of mean free path to sample length. In the case of a single-channel
quantum point contact, the mapping is onto a one-dimensional disordered chain,
for which the complete distribution of the conductance is known. The theory is
supported by numerical simulations. ***Submitted to Physical Review B.****Comment: 15 pages, REVTeX-3.0, 9 postscript figures appended as
self-extracting archive, INLO-PUB-940309
Tail States below the Thouless Gap in SNS junctions: Classical Fluctuations
We study the tails of the density of states (DOS) in a diffusive
superconductor-normal metal-superconductor (SNS) junction below the Thouless
gap. We show that long-wave fluctuations of the concentration of impurities in
the normal layer lead to the formation of subgap quasiparticle states, and
calculate the associated subgap DOS in all effective dimensionalities. We
compare the resulting tails with those arising from mesoscopic gap
fluctuations, and determine the dimensionless parameters controlling which
contribution dominates the subgap DOS. We observe that the two contributions
are formally related to each other by a dimensional reduction.Comment: 6 pages, 1 figur
Density of States and Energy Gap in Andreev Billiards
We present numerical results for the local density of states in semiclassical
Andreev billiards. We show that the energy gap near the Fermi energy develops
in a chaotic billiard. Using the same method no gap is found in similar square
and circular billiards.Comment: 9 pages, 6 Postscript figure
Quantum-to-classical crossover for Andreev billiards in a magnetic field
We extend the existing quasiclassical theory for the superconducting
proximity effect in a chaotic quantum dot, to include a time-reversal-symmetry
breaking magnetic field. Random-matrix theory (RMT) breaks down once the
Ehrenfest time becomes longer than the mean time between
Andreev reflections. As a consequence, the critical field at which the
excitation gap closes drops below the RMT prediction as is
increased. Our quasiclassical results are supported by comparison with a fully
quantum mechanical simulation of a stroboscopic model (the Andreev kicked
rotator).Comment: 11 pages, 10 figure
A pseudointegrable Andreev billiard
A circular Andreev billiard in a uniform magnetic field is studied. It is
demonstrated that the classical dynamics is pseudointegrable in the same sense
as for rational polygonal billiards. The relation to a specific polygon, the
asymmetric barrier billiard, is discussed. Numerical evidence is presented
indicating that the Poincare map is typically weak mixing on the invariant
sets. This link between these different classes of dynamical systems throws
some light on the proximity effect in chaotic Andreev billiards.Comment: 5 pages, 5 figures, to appear in PR
Universal gap fluctuations in the superconductor proximity effect
Random-matrix theory is used to study the mesoscopic fluctuations of the
excitation gap in a metal grain or quantum dot induced by the proximity to a
superconductor. We propose that the probability distribution of the gap is a
universal function in rescaled units. Our analytical prediction for the gap
distribution agrees well with exact diagonalization of a model Hamiltonian
Commensurability effects in Andreev antidot billiards
An Andreev billiard was realized in an array of niobium filled antidots in a
high-mobility InAs/AlGaSb heterostructure. Below the critical temperature T_C
of the Nb dots we observe a strong reduction of the resistance around B=0 and a
suppression of the commensurability peaks, which are usually found in antidot
lattices. Both effects can be explained in a classical Kubo approach by
considering the trajectories of charge carriers in the semiconductor, when
Andreev reflection at the semiconductor-superconductor interface is included.
For perfect Andreev reflection, we expect a complete suppression of the
commensurability features, even though motion at finite B is chaotic.Comment: 4 pages, 4 figure
Andreev Conductance of Chaotic and Integrable Quantum Dots
We examine the voltage V and magnetic field B dependent Andreev conductance
of a chaotic quantum dot coupled via point contacts to a normal metal and a
superconductor. In the case where the contact to the superconductor dominates,
we find that the conductance is consistent with the dot itself behaving as a
superconductor-- it appears as though Andreev reflections are occurring locally
at the interface between the normal lead and the dot. This is contrasted
against the behaviour of an integrable dot, where for a similar strong coupling
to the superconductor, no such effect is seen. The voltage dependence of the
Andreev conductance thus provides an extremely pronounced quantum signature of
the nature of the dot's classical dynamics. For the chaotic dot, we also study
non-monotonic re-entrance effects which occur in both V and B.Comment: 13 pages, 9 figure
Conductance Fluctuations in a Disordered Double-Barrier Junction
We consider the effect of disorder on coherent tunneling through two barriers
in series, in the regime of overlapping transmission resonances. We present
analytical calculations (using random-matrix theory) and numerical simulations
(on a lattice) to show that strong mode-mixing in the inter-barrier region
induces mesoscopic fluctuations in the conductance of universal magnitude
for a symmetric junction. For an asymmetric junction, the
root-mean-square fluctuations depend on the ratio of the two tunnel
resistances according to ,
where in the presence (absence) of time-reversal symmetry.Comment: 12 pages, REVTeX-3.0, 2 figures, submitted to Physical Review
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