52 research outputs found
The Upper Critical Field in Disordered Two-Dimensional Superconductors
We present calculations of the upper critical field in superconducting films
as a function of increasing disorder (as measured by the normal state
resistance per square). In contradiction to previous work, we find that there
is no anomalous low-temperature positive curvature in the upper critical field
as disorder is increased. We show that the previous prediction of this effect
is due to an unjustified analytical approximation of sums occuring in the
perturbative calculation. Our treatment includes both a careful analysis of
first-order perturbation theory, and a non-perturbative resummation technique.
No anomalous curvature is found in either case. We present our results in
graphical form.Comment: 11 pages, 8 figure
Effect of Magnetic Impurities on Suppression of the Transition Temperature in Disordered Superconductors
We calculate the first-order perturbative correction to the transition
temperature in a superconductor with both non-magnetic and magnetic
impurities. We do this by first evaluating the correction to the effective
potential, , and then obtain the first-order correction to the
order parameter, , by finding the minimum of . Setting
finally allows to be evaluated. is now a function of
both the resistance per square, , a measure of the non-magnetic
disorder, and the spin-flip scattering rate, , a measure of the
magnetic disorder. We find that the effective pair-breaking rate per magnetic
impurity is virtually independent of the resistance per square of the film, in
agreement with an experiment of Chervenak and Valles. This conclusion is
supported by both the perturbative calculation, and by a non-perturbative
re-summation technique.Comment: 29 pages, 9 figure
Proximity effect in ultrathin Pb/Ag multilayers within the Cooper limit
We report on transport and tunneling measurements performed on ultra-thin
Pb/Ag (strong coupled superconductor/normal metal) multilayers evaporated by
quench condensation. The critical temperature and energy gap of the
heterostructures oscillate with addition of each layer, demonstrating the
validity of the Cooper limit model in the case of multilayers. We observe
excellent agreement with a simple theory for samples with layer thickness
larger than 30\AA . Samples with single layers thinner than 30\AA deviate from
the Cooper limit theory. We suggest that this is due to the "inverse proximity
effect" where the normal metal electrons improve screening in the
superconducting ultrathin layer and thus enhance the critical temperature.Comment: 4 pages, 4 figure
Quantum superconductor-metal transition
We consider a system of superconducting grains embedded in a normal metal. At
zero temperature this system exhibits a quantum superconductor-normal metal
phase transition. This transition can take place at arbitrarily large
conductance of the normal metal.Comment: 13 pages, 1 figure include
Disordered Bosons: Condensate and Excitations
The disordered Bose Hubbard model is studied numerically within the
Bogoliubov approximation. First, the spatially varying condensate wavefunction
in the presence of disorder is found by solving a nonlinear Schrodinger
equation. Using the Bogoliubov approximation to find the excitations above this
condensate, we calculate the condensate fraction, superfluid density, and
density of states for a two-dimensional disordered system. These results are
compared with experiments done with adsorbed in porous media.Comment: RevTeX, 26 pages and 10 postscript figures appended (Figure 9 has
three separate plots, so 12 postcript files altogether
Early stage morphology of quench condensed Ag, Pb and Pb/Ag hybrid films
Scanning Tunneling Microscopy (STM) has been used to study the morphology of
Ag, Pb and Pb/Ag bilayer films fabricated by quench condensation of the
elements onto cold (T=77K), inert and atomically flat Highly Oriented Pyrolytic
Graphite (HOPG) substrates. All films are thinner than 10 nm and show a
granular structure that is consistent with earlier studies of QC films. The
average lateral diameter, , of the Ag grains, however, depends on
whether the Ag is deposited directly on HOPG ( = 13 nm) or on a Pb
film consisting of a single layer of Pb grains ( = 26.8 nm). In
addition, the critical thickness for electrical conduction () of Pb/Ag
films on inert glass substrates is substantially larger than for pure Ag films.
These results are evidence that the structure of the underlying substrate
exerts an influence on the size of the grains in QC films. We propose a
qualitative explanation for this previously unencountered phenomenon.Comment: 11 pages, 3 figures and one tabl
Anomalous Quantum Diffusion at the Superfluid-Insulator Transition
We consider the problem of the superconductor-insulator transition in the
presence of disorder, assuming that the fermionic degrees of freedom can be
ignored so that the problem reduces to one of Cooper pair localization. Weak
disorder drives the critical behavior away from the pure critical point,
initially towards a diffusive fixed point. We consider the effects of Coulomb
interactions and quantum interference at this diffusive fixed point. Coulomb
interactions enhance the conductivity, in contrast to the situation for
fermions, essentially because the exchange interaction is opposite in sign. The
interaction-driven enhancement of the conductivity is larger than the
weak-localization suppression, so the system scales to a perfect conductor.
Thus, it is a consistent possibility for the critical resistivity at the
superconductor-insulator transition to be zero, but this value is only
approached logarithmically. We determine the values of the critical exponents
and comment on possible implications for the interpretation of
experiments
Onset of Superfluidity in 4He Films Adsorbed on Disordered Substrates
We have studied 4He films adsorbed in two porous glasses, aerogel and Vycor,
using high precision torsional oscillator and DC calorimetry techniques. Our
investigation focused on the onset of superfluidity at low temperatures as the
4He coverage is increased. Torsional oscillator measurements of the 4He-aerogel
system were used to determine the superfluid density of films with transition
temperatures as low as 20 mK. Heat capacity measurements of the 4He-Vycor
system probed the excitation spectrum of both non-superfluid and superfluid
films for temperatures down to 10 mK. Both sets of measurements suggest that
the critical coverage for the onset of superfluidity corresponds to a mobility
edge in the chemical potential, so that the onset transition is the bosonic
analog of a superconductor-insulator transition. The superfluid density
measurements, however, are not in agreement with the scaling theory of an onset
transition from a gapless, Bose glass phase to a superfluid. The heat capacity
measurements show that the non-superfluid phase is better characterized as an
insulator with a gap.Comment: 15 pages (RevTex), 21 figures (postscript
Weak Localization Effect in Superconductors by Radiation Damage
Large reductions of the superconducting transition temperature and
the accompanying loss of the thermal electrical resistivity (electron-phonon
interaction) due to radiation damage have been observed for several A15
compounds, Chevrel phase and Ternary superconductors, and in
the high fluence regime. We examine these behaviors based on the recent theory
of weak localization effect in superconductors. We find a good fitting to the
experimental data. In particular, weak localization correction to the
phonon-mediated interaction is derived from the density correlation function.
It is shown that weak localization has a strong influence on both the
phonon-mediated interaction and the electron-phonon interaction, which leads to
the universal correlation of and resistance ratio.Comment: 16 pages plus 3 figures, revtex, 76 references, For more information,
Plesse see http://www.fen.bilkent.edu.tr/~yjki
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