247 research outputs found
Nature of the vortex-glass order in strongly type-II superconductors
The stability and the critical properties of the three-dimensional
vortex-glass order in random type-II superconductors with point disorder is
investigated in the unscreened limit based on a lattice {\it XY} model with a
uniform field. By performing equilibrium Monte Carlo simulations for the system
with periodic boundary conditions, the existence of a stable vortex-glass order
is established in the unscreened limit. Estimated critical exponents are
compared with those of the gauge-glass model.Comment: Error in the reported value of the exponent eta is correcte
Vortex dynamics for two-dimensional XY models
Two-dimensional XY models with resistively shunted junction (RSJ) dynamics
and time dependent Ginzburg-Landau (TDGL) dynamics are simulated and it is
verified that the vortex response is well described by the Minnhagen
phenomenology for both types of dynamics. Evidence is presented supporting that
the dynamical critical exponent in the low-temperature phase is given by
the scaling prediction (expressed in terms of the Coulomb gas temperature
and the vortex renormalization given by the dielectric constant
) both for RSJ and TDGL
and that the nonlinear IV exponent a is given by a=z+1 in the low-temperature
phase. The results are discussed and compared with the results of other recent
papers and the importance of the boundary conditions is emphasized.Comment: 21 pages including 15 figures, final versio
Defective Vortex Lattices in Layered Superconductors with Point Pins at the Extreme Type-II Limit
The mixed phase of layered superconductors with no magnetic screening is
studied through a partial duality analysis of the corresponding frustrated XY
model in the presence of weak random point pins. Isolated layers exhibit a
defective vortex lattice at low temperature that is phase coherent.
Sufficiently weak Josephson coupling between adjacent layers results in an
entangled vortex solid that exhibits weak superconductivity across layers. The
corresponding vortex liquid state shows an inverted specific heat anomaly that
we propose accounts for that seen in YBCO. A three-dimensional vortex lattice
with dislocations occurs at stronger coupling. This crossover sheds light on
the apparent discrepancy concerning the observation of a vortex-glass phase in
recent Monte Carlo simulations of the same XY model.Comment: 4 pages, 1 figure. To appear in PRB, rapid communicatio
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.
Universality of Frequency and Field Scaling of the Conductivity Measured by Ac-Susceptibility of a Ybco-Film
Utilizing a novel and exact inversion scheme, we determine the complex linear
conductivity from the linear magnetic ac-susceptibility
which has been measured from 3\,mHz to 50\,MHz in fields between 0.4\,T and
4\,T applied parallel to the c-axis of a 250\,nm thin disk. The frequency
derivative of the phase and the dynamical scaling of
above and below provide clear evidence for a
continuous phase transition at to a generic superconducting state. Based
on the vortex-glass scaling model, the resulting critical exponents and
are close to those frequently obtained on films by other means and
associated with an 'isotropic' vortex glass. The field effect on
can be related to the increase of the glass coherence length,
.Comment: 8 pages (5 figures upon request), revtex 3.0, APK.94.01.0
Superconducting Coherence and the Helicity Modulus in Vortex Line Models
We show how commonly used models for vortex lines in three dimensional
superconductors can be modified to include k=0 excitations. We construct a
formula for the k=0 helicity modulus in terms of fluctuations in the projected
area of vortex loops. This gives a convenient criterion for the presence of
superconducting coherence. We also present Monte Carlo simulations of a
continuum vortex line model for the melting of the Abrikosov vortex lattice in
pure YBCO.Comment: 4 pages RevTeX, 2 eps figures included using eps
Collapse of the vortex-lattice inductance and shear modulus at the melting transition in untwinned
The complex resistivity of the vortex lattice in an
untwinned crystal of 93-K has been measured at frequencies
from 100 kHz to 20 MHz in a 2-Tesla field ,
using a 4-probe RF transmission technique that enables continuous measurements
versus and temperature . As is increased, the inductance increases steeply to a cusp
at the melting temperature , and then undergoes a steep collapse
consistent with vanishing of the shear modulus . We discuss in detail
the separation of the vortex-lattice inductance from the `volume' inductance,
and other skin-depth effects. To analyze the spectra, we consider a weakly
disordered lattice with a low pin density. Close fits are obtained to
over 2 decades in . Values of the pinning parameter
and shear modulus obtained show that collapses by
over 4 decades at , whereas remains finite.Comment: 11 pages, 8 figures, Phys. Rev. B, in pres
Numerical study of duality and universality in a frozen superconductor
The three-dimensional integer-valued lattice gauge theory, which is also
known as a "frozen superconductor," can be obtained as a certain limit of the
Ginzburg-Landau theory of superconductivity, and is believed to be in the same
universality class. It is also exactly dual to the three-dimensional XY model.
We use this duality to demonstrate the practicality of recently developed
methods for studying topological defects, and investigate the critical behavior
of the phase transition using numerical Monte Carlo simulations of both
theories. On the gauge theory side, we concentrate on the vortex tension and
the penetration depth, which map onto the correlation lengths of the order
parameter and the Noether current in the XY model, respectively. We show how
these quantities behave near the critical point, and that the penetration depth
exhibits critical scaling only very close to the transition point. This may
explain the failure of superconductor experiments to see the inverted XY model
scaling.Comment: 17 pages, 18 figures. Updated to match the version published in PRB
(http://link.aps.org/abstract/PRB/v67/e014525) on 27 Jan 200
Critical scaling of the a.c. conductivity for a superconductor above Tc
We consider the effects of critical superconducting fluctuations on the
scaling of the linear a.c. conductivity, \sigma(\omega), of a bulk
superconductor slightly above Tc in zero applied magnetic field. The dynamic
renormalization- group method is applied to the relaxational time-dependent
Ginzburg-Landau model of superconductivity, with \sigma(\omega) calculated via
the Kubo formula to O(\epsilon^{2}) in the \epsilon = 4 - d expansion. The
critical dynamics are governed by the relaxational XY-model
renormalization-group fixed point. The scaling hypothesis \sigma(\omega) \sim
\xi^{2-d+z} S(\omega \xi^{z}) proposed by Fisher, Fisher and Huse is explicitly
verified, with the dynamic exponent z \approx 2.015, the value expected for the
d=3 relaxational XY-model. The universal scaling function S(y) is computed and
shown to deviate only slightly from its Gaussian form, calculated earlier. The
present theory is compared with experimental measurements of the a.c.
conductivity of YBCO near Tc, and the implications of this theory for such
experiments is discussed.Comment: 16 pages, submitted to Phys. Rev.
Scaling critical behavior of superconductors at zero magnetic field
We consider the scaling behavior in the critical domain of superconductors at
zero external magnetic field. The first part of the paper is concerned with the
Ginzburg-Landau model in the zero magnetic field Meissner phase. We discuss the
scaling behavior of the superfluid density and we give an alternative proof of
Josephson's relation for a charged superfluid. This proof is obtained as a
consequence of an exact renormalization group equation for the photon mass. We
obtain Josephson's relation directly in the form , that
is, we do not need to assume that the hyperscaling relation holds. Next, we
give an interpretation of a recent experiment performed in thin films of
. We argue that the measured mean field like
behavior of the penetration depth exponent is possibly associated with a
non-trivial critical behavior and we predict the exponents and
for the correlation lenght and specific heat, respectively. In the
second part of the paper we discuss the scaling behavior in the continuum dual
Ginzburg-Landau model. After reviewing lattice duality in the Ginzburg-Landau
model, we discuss the continuum dual version by considering a family of
scalings characterized by a parameter introduced such that
, where is the bare mass of the magnetic
induction field. We discuss the difficulties in identifying the renormalized
magnetic induction mass with the photon mass. We show that the only way to have
a critical regime with is having , that
is, with having the scaling behavior of the renormalized photon mass.Comment: RevTex, 15 pages, no figures; the subsection III-C has been removed
due to a mistak
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