65 research outputs found
Dynamic Impedance of Two-Dimensional Superconducting Films Near the Superconducting Transition
The sheet impedances, Z(w,T), of several superconducting a-Mo77Ge23 films and
one In/InOx film have been measured in zero field using a two-coil mutual
inductance technique at frequencies from 100 Hz to 100 kHz. Z(w,T) is found to
have three contributions: the inductive superfluid, renormalized by nonvortex
phase fluctuations; conventional vortex-antivortex pairs, whose contribution
turns on very rapidly just below the usual Kosterlitz-Thouless-Berezinskii
unbinding temperature; and an anomalous contribution. The latter is
predominantly resistive, persists well below the KTB temperature, and is weakly
dependent on frequency down to remarkably low frequencies, at least 100 Hz. It
increases with T as e-U'(T)/kT, where the activation energy, U'(T), is about
half the energy to create a vortex-antivortex pair, indicating that the
frequency dependence is that of individual excitations, rather than critical
behavior.Comment: 10 pages, 10 figs; subm PR
Effect of thermal phase fluctuations on the superfluid density of two-dimensional superconducting films
High precision measurements of the complex sheet conductivity of
superconducting Mo77Ge23 thin films have been made from 0.4 K through Tc. A
sharp drop in the inverse sheet inductance, 1/L(T), is observed at a
temperature, Tc, which lies below the mean-field transition temperature, Tco.
Just below Tc, the suppression of 1/L(T) below its mean-field value indicates
that longitudinal phase fluctuations have nearly their full classical
amplitude, but they disappear rapidly as T decreases. We argue that there is a
quantum crossover at about 0.94 Tco, below which classical phase fluctuations
are suppressed.Comment: 14 pages, 3 figures. Subm. to PR
Experimental Study of the Inductance of Pinned Vortices in Superconducting YBa2Cu3O7-d Films
Using a two-coil mutual inductance method, we have measured the complex
resistivity, rho_v(T,Be), of pinned vortices in c-axis pulsed laser deposited
YBa2Cu3O7-d films with magnetic field Be applied perpendicular to the film. At
low frequencies, (<100 kHz), rho_v is inductive and is inversely proportional
to the Labusch parameter, the average vortex pinning force constant, kappa_exp.
The observed weakening of kappa_exp with Be is consistent with a simple model
based on linear pinning defects. Adding classical thermal fluctuations to the
model in a simple way describes the observed linear T dependence of rho_v,
below ~15 K and provides reasonable values for the effective radius (.3 nm to
>.8 nm) of the defects and the depth of the pinning potential. The success of
this model implies that thermal supercurrent (phase) fluctuations have their
full classical amplitude down to 5 K for frequencies below the characteristic
depinning frequency. To date, no sufficient theory exists to explain the data
between ~15 K and the vortex glass melting temperature.Comment: 31 pages, 8 figures. Subm. to PR
Evidence for a Nodeless Gap from the Superfluid Density of Optimally Doped Pr_{1.855}Ce_{0.145}CuO_{4-y} Films
We present measurements of the ab-plane magnetic penetration depth,
\lambda(T), in five optimally doped Pr_{1.855}Ce_{0.145}CuO_{4-y} films for 1.6
K \leq T \leq T_c \sim 24 K. Low resistivities, high superfluid densities
n_s(T)\propto \lambda^{-2}(T), high T_c's, and small transition widths are
reproducible and indicative of excellent film quality. For all five films,
\lambda^{-2}(T)/\lambda^{-2}(0) at low T is well fitted by an exponential
temperature dependence with a gap, \Delta_{min}, of 0.85 k_B T_c. This behavior
is consistent with a nodeless gap and is incompatible with d-wave
superconductivity.Comment: 5 pages, 4 figures, reorganized for clarit
Conductivity Due to Classical Phase Fluctuations in a Model For High-T_c Superconductors
We consider the real part of the conductivity, \sigma_1(\omega), arising from
classical phase fluctuations in a model for high-T_c superconductors. We show
that the frequency integral of that conductivity, \int_0^\infty \sigma_1
d\omega, is non-zero below the superconducting transition temperature ,
provided there is some quenched disorder in the system. Furthermore, for a
fixed amount of quenched disorder, this integral at low temperatures is
proportional to the zero-temperature superfluid density, in agreement with
experiment. We calculate \sigma_1(\omega) explicitly for a model of overdamped
phase fluctuations.Comment: 4pages, 2figures, submitted to Phys.Rev.
Inelastic response of silicon to shock compression
The elastic and inelastic response of [001] oriented silicon to laser compression has been a topic of considerable discussion for well over a decade, yet there has been little progress in understanding the basic behaviour of this apparently simple material. We present experimental x-ray diffraction data showing complex elastic strain profiles in laser compressed samples on nanosecond timescales. We also present molecular dynamics and elasticity code modelling which suggests that a pressure induced phase transition is the cause of the previously reported 'anomalous' elastic waves. Moreover, this interpretation allows for measurement of the kinetic timescales for transition. This model is also discussed in the wider context of reported deformation of silicon to rapid compression in the literature
Experimental implications of quantum phase fluctuations in layered superconductors
I study the effect of quantum and thermal phase fluctuations on the in-plane
and c-axis superfluid stiffness of layered d-wave superconductors. First, I
show that quantum phase fluctuations in the superconductor can be damped in the
presence of external screening of Coulomb interactions, and suggest an
experiment to test the importance of these fluctuations, by placing a metal in
close proximity to the superconductor to induce such screening. Second, I show
that a combination of quantum phase fluctuations and the linear temperature
dependence of the in-plane superfluid stiffness leads to a linear temperature
dependence of the c-axis penetration depth, below a temperature scale
determined by the magnitude of in-plane dissipation.Comment: 6 pgs, 1 figure, minor changes in comparison with c-axis expt, final
published versio
Phase fluctuations, dissipation and superfluid stiffness in d-wave superconductors
We study the effect of dissipation on quantum phase fluctuations in d-wave
superconductors. Dissipation, arising from a nonzero low frequency optical
conductivity which has been measured in experiments below , has two
effects: (1) a reduction of zero point phase fluctuations, and (2) a reduction
of the temperature at which one crosses over to classical thermal fluctuations.
For parameter values relevant to the cuprates, we show that the crossover
temperature is still too large for classical phase fluctuations to play a
significant role at low temperature. Quasiparticles are thus crucial in
determining the linear temperature dependence of the in-plane superfluid
stiffness. Thermal phase fluctuations become important at higher temperatures
and play a role near .Comment: Presentation improved, new references added (10 latex pages, 3 eps
figures). submitted to PR
Microwave Electrodynamics of Electron-Doped Cuprate Superconductors
We report microwave cavity perturbation measurements of the temperature
dependence of the penetration depth, lambda(T), and conductivity, sigma(T) of
Pr_{2-x}Ce_{x}CuO_{4-delta} (PCCO) crystals, as well as parallel-plate
resonator measurements of lambda(T) in PCCO thin films. Penetration depth
measurements are also presented for a Nd_{2-x}Ce_{x}CuO_{4-delta} (NCCO)
crystal. We find that delta-lambda(T) has a power-law behavior for T<T_c/3, and
conclude that the electron-doped cuprate superconductors have nodes in the
superconducting gap. Furthermore, using the surface impedance, we have derived
the real part of the conductivity, sigma_1(T), below T_c and found a behavior
similar to that observed in hole-doped cuprates.Comment: 4 pages, 4 figures, 1 table. Submitted to Physical Review Letters
revised version: new figures, sample characteristics added to table, general
clarification give
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