18 research outputs found
Model calculations of the proximity effect in finite multilayers
The proximity-effect theory developed by Takahashi and Tachiki for infinite
multilayers is applied to multilayer systems with a finite number of layers in
the growth direction. The purpose is to investigate why previous applications
to infinite multilayers fail to describe the measured data satisfactorily.
Surface superconductivity may appear, depending on the thickness of the
covering normal metallic N layers on both the top and the bottom. The
parameters used are characteristic for V/Ag and Nb/Pd systems. The nucleation
process is studied as a function of the system parameters.Comment: 12 pages, 15 figures, RevTe
The influence of the boundary resistivity on the proximity effect
We apply the theory of Takahashi and Tachiki in order to explain
theoretically the dependence of the upper critical magnetic field of a S/N
multilayer on the temperature. This problem has been already investigated in
the literature, but with a use of an unphysical scaling parameter for the
coherence length. We show explicitely that, in order to describe the data, such
an unphysical parameter is unnecessary if one takes into account the boundary
resisitivity of the S/N interface. We obtain a very good agreement with the
experiments for the multilayer systems Nb/Cu and V/Ag, with various layer
thicknesses.Comment: 12 pages, 5 figure
Andreev States in long shallow SNS constrictions
We study Andreev bound states in a long shallow normal constriction, which is
open to a superconductor at both ends. The interesting features of such setup
include the absence of electron-hole symmetry and the interference of electron
waves along the constriction. We compare results of a numerical approach based
on the Bogoliubov equations with those of a refined semiclassical description.
Three types of Andreev bound states occur in the constriction: {\it i}) one
where both electron and hole wave part of the bound state propagate through the
constriction, {\it ii}) one where neither electron nor hole wave part
propagate, and {\it iii}) one where only the electron wave propagates. We show
that in a wide energy region the spacing between the Andreev states is strongly
modulated by the interference of electron waves passing the constriction
Andreev states, supercurrents and interface effects in clean SN multilayers
We present results for the local density of states in the S and N layers of a
SN multilayer, and the supercurrent, based on a Green's function formalism, as
an extension of previous calculations on NS, SNS and SNSNS systems. The gap
function is determined selfconsistently. Our systems are chosen to have a
finite transverse width. We focus on phenomena which occur at so-called
critical transverse widths, at which a new transverse mode is starting to
contribute. It appears, that for an arbitrary width the Andreev approximation
(AA), which takes into account only Andreev reflection at the SN interfaces,
works well. We show that at a critical width the AA breaks down. An exact
treatment is required, which considers also ordinary reflections. In addition,
we study the influence of an interface barrier on the coupling between the
S-layers
Andreev Bound States and Self-Consistent Gap Functions for SNS and SNSNS Systems
Andreev bound states in clean, ballistic SNS and SNSNS junctions are
calculated exactly and by using the Andreev approximation (AA). The AA appears
to break down for junctions with transverse dimensions chosen such that the
motion in the longitudinal direction is very slow. The doubly degenerate states
typical for the traveling waves found in the AA are replaced by two standing
waves in the exact treatment and the degeneracy is lifted.
A multiple-scattering Green's function formalism is used, from which the
states are found through the local density of states. The scattering by the
interfaces in any layered system of ballistic normal metals and clean
superconducting materials is taken into account exactly. The formalism allows,
in addition, for a self-consistent determination of the gap function. In the
numerical calculations the pairing coupling constant for aluminum is used.
Various features of the proximity effect are shown
Size-effects in the Density of States in NS and SNS junctions
The quasiparticle local density of states (LDOS) is studied in clean NS and
SNS junctions with increasing transverse size, from quasi-one-dimensional to
three-dimensional. It is shown that finite transverse dimensions are related to
pronounced effects in the LDOS, such as fast oscillations superimposed on the
quasiparticle interference oscillations (for NS) and additional peaks in the
bound state spectrum in the subgap region (for SNS). Also, the validity of the
Andreev approximation is discussed. It turns out to be an acceptable
approximation in all situations tested.Comment: 9 pages, RevTex, 5 figures, accepted in Phys. Rev.
Calcualtion of the upper critical fields in Nb/Ta multilayers.
The Takahashi-Tachiki proximity-effect theory is applied to the Nb/Ta multilayer system. The diffusion coefficients of the two metals and the critical temperature of Nb are used as free parameters in fitting experimental phase diagrams. Magnetic-coherence-length scaling is used in order to obtain phase diagrams that best reproduce the measured data. Several parameter sets can compete in fitting an experimental curve. It is not always possible to decide which set gives the most realistic result