18 research outputs found
Strong-disorder renormalization group approach to the Anderson model using Raleigh-Schr\"odinger perturbation theory
Previous work proposed a strong-disorder renormalization approach for the
Anderson model, using it to calculate the density of states and the inverse
participation ratio [Johri \& Bhatt, Phys.\ Rev.\ B {\bf 90} 060205(R) (2014)].
This is interesting because of the potential for expansion to higher dimensions
and to interacting systems. The original proposal used a non-standard
perturbation theory approach which avoided degeneracies. We implemented the
same structure but with standard Rayleigh-Schr\"odinger perturbation theory.
Here degeneracies do arise, and we consider two approaches, one in which
renormalization is suppressed if degeneracy is present and a second in which
the most common form of degeneracy is handled using standard degenerate
perturbation theory. The version in which degeneracies are not handled performs
similarly to the original proposal, and the addition of degeneracies provides
some improvement.Comment: Annals of Physic
Calculated NMR T_2 relaxation due to vortex vibrations in cuprate superconductors
We calculate the rate of transverse relaxation arising from vortex motion in
the mixed state of YBa_2Cu_3O_7 with the static field applied along the c axis.
The vortex dynamics are described by an overdamped Langevin equation with a
harmonic elastic free energy. We find that the variation of the relaxation with
temperature, average magnetic field, and local field is consistent with
experiments; however, the calculated time dependence is different from what has
been measured and the value of the rates calculated is roughly two orders of
magnitude slower than what is observed. Combined with the strong experimental
evidence pointing to vortex motion as the dominant mechanism for T_2
relaxation, these results call into question a prior conclusion that vortex
motion is not significant in T_1 measurements in the vortex state.Comment: 6 pages, 5 figures, to be published in Phys. Rev.
Nonlocal Conductivity in the Vortex-Liquid Regime of a Two-Dimensional Superconductor
We have simulated the time-dependent Ginzburg-Landau equation with thermal
fluctuations, to study the nonlocal dc conductivity of a superconducting film.
Having examined points in the phase diagram at a wide range of temperatures and
fields below the mean-field upper critical field, we find a portion of the
vortex-liquid regime in which the nonlocal ohmic conductivity in real space is
negative over a distance several times the spacing between vortices. The effect
is suppressed when driven beyond linear response. Earlier work had predicted
the existence of such a regime, due to the high viscosity of a
strongly-correlated vortex liquid. This behavior is clearly distinguishable
from the monotonic spatial fall-off of the conductivity in the higher
temperature or field regimes approaching the normal state. The possibilities
for experimental study of the nonlocal transport properties are discussed.Comment: 18 pages, revtex, 6 postscript figure
Theory of NMR as a local probe for the electronic structure in the mixed state of the high- cuprates
We argue that nuclear magnetic resonance experiments are a site-sensitive
probe for the electronic spectrum in the mixed state of the high-
cuprates. Within a spin-fermion model, we show that the Doppler-shifted
electronic spectrum arising from the circulating supercurrent changes the
low-frequency behavior of the imaginary part of the spin-susceptibility. For a
hexagonal vortex lattice, we predict that these changes lead to {\it (a)} a
unique dependence of the Cu spin lattice relaxation rate, , on
resonance frequency, and {\it (b)} a temperature dependence of which
varies with frequency. We propose a nuclear quadrupole experiment to study the
effects of a uniform supercurrent on the electronic structure and predict that
varies with the direction of the supercurrent.Comment: RevTex, 5 pages, 3 figures embedded in the tex