510 research outputs found
Random background charges and Coulomb blockade in one-dimensional tunnel junction arrays
We have numerically studied the behavior of one dimensional tunnel junction
arrays when random background charges are included using the ``orthodox''
theory of single electron tunneling. Random background charge distributions are
verified in both amplitude and density. The use of a uniform array as a
transistor is discussed both with and without random background charges. An
analytic expression for the gain near zero gate voltage in a uniform array with
no background charges is derived. The gate modulation with background charges
present is simulated.Comment: 10 pages, 7 figure
Quantum Transparency of Anderson Insulator Junctions: Statistics of Transmission Eigenvalues, Shot Noise, and Proximity Conductance
We investigate quantum transport through strongly disordered barriers, made
of a material with exceptionally high resistivity that behaves as an Anderson
insulator or a ``bad metal'' in the bulk, by analyzing the distribution of
Landauer transmission eigenvalues for a junction where such barrier is attached
to two clean metallic leads. We find that scaling of the transmission
eigenvalue distribution with the junction thickness (starting from the single
interface limit) always predicts a non-zero probability to find high
transmission channels even in relatively thick barriers. Using this
distribution, we compute the zero frequency shot noise power (as well as its
sample-to-sample fluctuations) and demonstrate how it provides a single number
characterization of non-trivial transmission properties of different types of
disordered barriers. The appearance of open conducting channels, whose
transmission eigenvalue is close to one, and corresponding violent mesoscopic
fluctuations of transport quantities explain at least some of the peculiar
zero-bias anomalies in the Anderson-insulator/superconductor junctions observed
in recent experiments [Phys. Rev. B {\bf 61}, 13037 (2000)]. Our findings are
also relevant for the understanding of the role of defects that can undermine
quality of thin tunnel barriers made of conventional band-insulators.Comment: 9 pages, 8 color EPS figures; one additional figure on mesoscopic
fluctuations of Fano facto
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
Anomalous density of states in a metallic film in proximity with a superconductor
We investigated the local electronic density of states in
superconductor-normal metal (Nb-Au) bilayers using a very low temperature (60
mK) STM. High resolution tunneling spectra measured on the normal metal (Au)
surface show a clear proximity effect with an energy gap of reduced amplitude
compared to the bulk superconductor (Nb) gap. Within this mini-gap, the density
of states does not reach zero and shows clear sub-gap features. We show that
the experimental spectra cannot be described with the well-established Usadel
equations from the quasi-classical theory.Comment: 4 pages, 5 figure
Effect of topical capsaicin on corticomotor control of tongue musculature
Abstract no. 1520published_or_final_versio
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
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
Superconductors with Magnetic Impurities: Instantons and Sub-gap States
When subject to a weak magnetic impurity potential, the order parameter and
quasi-particle energy gap of a bulk singlet superconductor are suppressed.
According to the conventional mean-field theory of Abrikosov and Gor'kov, the
integrity of the energy gap is maintained up to a critical concentration of
magnetic impurities. In this paper, a field theoretic approach is developed to
critically analyze the validity of the mean field theory. Using the
supersymmetry technique we find a spatially homogeneous saddle-point that
reproduces the Abrikosov-Gor'kov theory, and identify instanton contributions
to the density of states that render the quasi-particle energy gap soft at any
non-zero magnetic impurity concentration. The sub-gap states are associated
with supersymmetry broken field configurations of the action. An analysis of
fluctuations around these configurations shows how the underlying supersymmetry
of the action is restored by zero modes. An estimate of the density of states
is given for all dimensionalities. To illustrate the universality of the
present scheme we apply the same method to study `gap fluctuations' in a normal
quantum dot coupled to a superconducting terminal. Using the same instanton
approach, we recover the universal result recently proposed by Vavilov et al.
Finally, we emphasize the universality of the present scheme for the
description of gap fluctuations in d-dimensional superconducting/normal
structures.Comment: 18 pages, 9 eps figure
Quantum interference and the formation of the proximity effect in chaotic normal-metal/superconducting structures
We discuss a number of basic physical mechanisms relevant to the formation of
the proximity effect in superconductor/normal metal (SN) systems. Specifically,
we review why the proximity effect sharply discriminates between systems with
integrable and chaotic dynamics, respectively, and how this feature can be
incorporated into theories of SN systems. Turning to less well investigated
terrain, we discuss the impact of quantum diffractive scattering on the
structure of the density of states in the normal region. We consider ballistic
systems weakly disordered by pointlike impurities as a test case and
demonstrate that diffractive processes akin to normal metal weak localization
lead to the formation of a hard spectral gap -- a hallmark of SN systems with
chaotic dynamics. Turning to the more difficult case of clean systems with
chaotic boundary scattering, we argue that semiclassical approaches, based on
classifications in terms of classical trajectories, cannot explain the gap
phenomenon. Employing an alternative formalism based on elements of
quasiclassics and the ballistic -model, we demonstrate that the inverse
of the so-called Ehrenfest time is the relevant energy scale in this context.
We discuss some fundamental difficulties related to the formulation of low
energy theories of mesoscopic chaotic systems in general and how they prevent
us from analysing the gap structure in a rigorous manner. Given these
difficulties, we argue that the proximity effect represents a basic and
challenging test phenomenon for theories of quantum chaotic systems.Comment: 21 pages (two-column), 6 figures; references adde
Scaling Theory of Conduction Through a Normal-Superconductor Microbridge
The length dependence is computed of the resistance of a disordered
normal-metal wire attached to a superconductor. The scaling of the transmission
eigenvalue distribution with length is obtained exactly in the metallic limit,
by a transformation onto the isobaric flow of a two-dimensional ideal fluid.
The resistance has a minimum for lengths near l/Gamma, with l the mean free
path and Gamma the transmittance of the superconductor interface.Comment: 8 pages, REVTeX-3.0, 3 postscript figures appended as self-extracting
archive, INLO-PUB-94031
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