431 research outputs found
Simulation Studies on the Stability of the Vortex-Glass Order
The stability of the three-dimensional vortex-glass order in random type-II
superconductors with point disorder is investigated by equilibrium Monte Carlo
simulations based on a lattice XY model with a uniform field threading the
system. It is found that the vortex-glass order, which stably exists in the
absence of screening, is destroyed by the screenng effect, corroborating the
previous finding based on the spatially isotropic gauge-glass model. Estimated
critical exponents, however, deviate considerably from the values reported for
the gauge-glass model.Comment: Minor modifications made, a few referenced added; to appear in J.
Phys. Soc. Jpn. Vol.69 No.1 (2000
Critical Fluctuations and Disorder at the Vortex Liquid to Crystal Transition in Type-II Superconductors
We present a functional renormalization group (FRG) analysis of a
Landau-Ginzburg model of type-II superconductors (generalized to complex
fields) in a magnetic field, both for a pure system, and in the presence of
quenched random impurities. Our analysis is based on a previous FRG treatment
of the pure case [E.Br\'ezin et. al., Phys. Rev. B, {\bf 31}, 7124 (1985)]
which is an expansion in . If the coupling functions are
restricted to the space of functions with non-zero support only at reciprocal
lattice vectors corresponding to the Abrikosov lattice, we find a stable FRG
fixed point in the presence of disorder for , identical to that of the
disordered model in dimensions. The pure system has a stable fixed
point only for and so the physical case () is likely to have a
first order transition. We speculate that the recent experimental findings that
disorder removes the apparent first order transition are consistent with these
calculations.Comment: 4 pages, no figures, typeset using revtex (v3.0
First-Order Melting of a Moving Vortex Lattice: Effects of Disorder
We study the melting of a moving vortex lattice through numerical simulations
with the current driven 3D XY model with disorder. We find that there is a
first-order phase transition even for large disorder when the corresponding
equilibrium transition is continuous. The low temperature phase is an
anisotropic moving glass.Comment: Important changes from original version. Finite size analysis of
results has been added. Figure 2 has been changed. There is a new additional
Figure. To be published in Physical Review Letter
Disorder and thermally driven vortex-lattice melting in La{2-x}Sr{x}CuO{4} crystals
Magnetization measurements in La{2-x}Sr{x}CuO{4} crystals indicate vortex
order-disorder transition manifested by a sharp kink in the second
magnetization peak. The transition field exhibits unique temperature
dependence, namely a strong decrease with temperature in the entire measured
range. This behavior rules out the conventional interpretation of a
disorder-driven transition into an entangled vortex solid phase. It is shown
that the transition in La{2-x}Sr{x}CuO{4} is driven by both thermally- and
disorder-induced fluctuations, resulting in a pinned liquid state. We conclude
that vortex solid-liquid, solid-solid and solid to pinned-liquid transitions
are different manifestations of the same thermodynamic order-disorder
transition, distinguished by the relative contributions of thermal and
disorder-induced fluctuations.Comment: To be published in phys. Rev. B Rapid Com
Longitudinal Current Dissipation in Bose-glass Superconductors
A scaling theory of vortex motion in Bose glass superconductors with currents
parallel to the common direction of the magnetic field and columnar defects is
presented. Above the Bose-glass transition the longitudinal DC resistivity
vanishes much faster than the
corresponding transverse resistivity , thus {\it reversing} the usual anisotropy of electrical transport in
the normal state of layered superconductors. In the presence of a current at an angle with the common field and columnar defect axis, the
electric field angle approaches as .
Scaling also predicts the behavior of penetration depths for the AC currents as
, and implies a {\it jump discontinuity} at in
the superfluid density describing transport parallel to the columns.Comment: 5 pages, revte
Hall Anomaly and Vortex-Lattice Melting in Superconducting Single Crystal YBa2Cu3O7-d
Sub-nanovolt resolution longitudinal and Hall voltages are measured in an
ultra pure YBa2Cu3O7-d single crystal. The Hall anomaly and the first-order
vortex-lattice melting transition are observed simultaneously. Changes in the
dynamic behavior of the vortex solid and liquid are correlated with features of
the Hall conductivity sxy. With the magnetic field oriented at an angle from
the twin-boundaries, the Hall conductivity sharply decreases toward large
negative values at the vortex-lattice melting transition.Comment: 6 pages, 2 figures included, Postscript, to appear in Phys. Rev. Let
First order phase transition of the vortex lattice in twinned YBa2Cu3O7 single crystals in tilted magnetic fields
We present an exhaustive analysis of transport measurements performed in
twinned YBa2Cu3O7 single crystals which stablishes that the vortex solid-liquid
transition is first order when the magnetic field H is applied at an angle
theta away from the direction of the twin planes. We show that the resistive
transitions are hysteretic and the V-I curves are non-linear, displaying a
characteristic s-shape at the melting line Hm(T), which scales as
epsilon(theta)Hm(T,theta). These features are gradually lost when the critical
point H*(theta) is approached. Above H*(theta) the V-I characteristics show a
linear response in the experimentally accessible V-I window, and the transition
becomes reversible. Finally we show that the first order phase transition takes
place between a highly correlated vortex liquid in the field direction and a
solid state of unknown symmetry. As a consequence, the available data support
the scenario for a vortex-line melting rather than a vortex sublimation as
recently suggested [T.Sasagawa et al. PRL 80, 4297 (1998)].Comment: 10 pages, 8 figures, submitted to PR
Is there a Phase Transition to the Flux Lattice State?
The sharp drops in the resistance and magnetization which are usually
attributed to a phase transition from the vortex liquid state to a crystal
state are explained instead as a crossover between three and two dimensional
behavior, which occurs when the phase coherence length in the liquid becomes
comparable to the sample thickness. Estimates of the width of the crossover
region and the phase coherence length scales are in agreement with experiment.Comment: 4 pages, RevTe
Plasticity and memory effects in the vortex solid phase of twinned YBa2Cu3O7 single crystals
We report on marked memory effects in the vortex system of twinned YBa2Cu3O7
single crystals observed in ac susceptibility measurements. We show that the
vortex system can be trapped in different metastable states with variable
degree of order arising in response to different system histories. The pressure
exerted by the oscillating ac field assists the vortex system in ordering,
locally reducing the critical current density in the penetrated outer zone of
the sample. The robustness of the ordered and disordered states together with
the spatial profile of the critical current density lead to the observed memory
effects
Direct observation of the washboard noise of a driven vortex lattice in a high-temperature superconductor, Bi2Sr2CaCu2Oy
We studied the conduction noise spectrum in the vortex state of a
high-temperature superconductor, Bi2Sr2CaCu2Oy, subject to a uniform driving
force. Two characteristic features, a broadband noise (BBN) and a narrow-band
noise (NBN), were observed in the vortex-solid phase. The origin of the large
BBN was determined to be plastic motion of the vortices, whereas the NBN was
found to originate from the washboard modulation of the translational velocity
of the driven vortices. We believe this to be the first observation ofComment: 4 pages, 4 figures, to appear in Phys. Rev. Let
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