398 research outputs found
Current-induced vortex dynamics in Josephson-junction arrays: Imaging experiments and model simulations
We study the dynamics of current-biased Josephson-junction arrays with a
magnetic penetration depth smaller than the lattice spacing. We compare the
dynamics imaged by low-temperature scanning electron microscopy to the vortex
dynamics obtained from model calculations based on the resistively-shunted
junction model, in combination with Maxwell's equations. We find three bias
current regions with fundamentally different array dynamics. The first region
is the subcritical region, i.e. below the array critical current I_c. The
second, for currents I above I_c, is a "vortex region", in which the response
is determined by the vortex degrees of freedom. In this region, the dynamics is
characterized by spatial domains where vortices and antivortices move across
the array in opposite directions in adjacent rows and by transverse voltage
fluctuations. In the third, for still higher currents, the dynamics is
dominated by coherent-phase motion, and the current-voltage characteristics are
linear.Comment: 10 pages, with eps figures. To appear in Phys. Rev.
Monte-Carlo calculation of longitudinal and transverse resistivities in a model Type-II superconductor
We study the effect of a transport current on the vortex-line lattice in
isotropic type-II superconductors in the presence of strong thermal
fluctuations by means of 'driven-diffusion' Monte Carlo simulations of a
discretized London theory with finite magnetic penetration depth. We calculate
the current-voltage (I-V) characteristics for various temperatures, for
transverse as well as longitudinal currents I. From these characteristics, we
estimate the linear resistivities R_xx=R_yy and R_zz and compare these with
equilibrium results for the vortex-lattice structure factor and the helicity
moduli. From this comparison a consistent picture arises, in which the melting
of the flux-line lattice occurs in two stages for the system size considered.
In the first stage of the melting, at a temperature T_m, the structure factor
drops to zero and R_xx becomes finite. For a higher temperature T_z, the second
stage takes place, in which the longitudinal superconducting coherence is lost,
and R_zz becomes finite as well. We compare our results with related recent
numerical work and experiments on cuprate superconductors.Comment: 4 pages, with eps figure
The time to extinction for an SIS-household-epidemic model
We analyse a stochastic SIS epidemic amongst a finite population partitioned
into households. Since the population is finite, the epidemic will eventually
go extinct, i.e., have no more infectives in the population. We study the
effects of population size and within household transmission upon the time to
extinction. This is done through two approximations. The first approximation is
suitable for all levels of within household transmission and is based upon an
Ornstein-Uhlenbeck process approximation for the diseases fluctuations about an
endemic level relying on a large population. The second approximation is
suitable for high levels of within household transmission and approximates the
number of infectious households by a simple homogeneously mixing SIS model with
the households replaced by individuals. The analysis, supported by a simulation
study, shows that the mean time to extinction is minimized by moderate levels
of within household transmission
Numerical studies of the phase diagram of layered type II superconductors in a magnetic field
We report on simulations of layered superconductors using the
Lawrence-Doniach model in the framework of the lowest Landau level
approximation. We find a first order phase transition with a dependence
which agrees very well with the experimental ``melting'' line in YBaCuO. The
transition is not associated with vortex lattice melting, but separates two
vortex liquid states characterised by different degrees of short-range
crystalline order and different length scales of correlations between vortices
in different layers. The transition line ends at a critical end-point at low
fields. We find the magnetization discontinuity and the location of the lower
critical magnetic field to be in good agreement with experiments in YBaCuO.
Length scales of order parameter correlations parallel and perpendicular to the
magnetic field increase exponentially as 1/T at low temperatures. The dominant
relaxation time scales grow roughly exponentially with these correlation
lengths. We find that the first order phase transition persists in the presence
of weak random point disorder but can be suppressed entirely by strong
disorder. No vortex glass or Bragg glass state is found in the presence of
disorder. The consistency of our numerical results with various experimental
features in YBaCuO, including the dependence on anisotropy, and the temperature
dependence of the structure factor at the Bragg peaks in neutron scattering
experiments is demonstrated.Comment: 25 pages (revtex), 19 figures included, submitted to PR
Effects of Electronic Correlations on the Thermoelectric Power of the Cuprates
We show that important anomalous features of the normal-state thermoelectric
power S of high-Tc materials can be understood as being caused by doping
dependent short-range antiferromagnetic correlations. The theory is based on
the fluctuation-exchange approximation applied to Hubbard model in the
framework of the Kubo formalism. Firstly, the characteristic maximum of S as
function of temperature can be explained by the anomalous momentum dependence
of the single-particle scattering rate. Secondly, we discuss the role of the
actual Fermi surface shape for the occurrence of a sign change of S as a
function of temperature and doping.Comment: 4 pages, with eps figure
1/\omega-flux-noise and dynamical critical properties of two-dimensional XY-models
We have numerically studied the dynamic correlation functions in
thermodynamic equilibrium of two-dimensional O(2)-symmetry models with either
bond (RSJ) or site (TDGL) dissipation as a function of temperature T. We find
that above the critical temperature the frequency dependent flux noise
, with
and ,
while the dynamic critical exponents and .
Contrary to expectation the TDGL results are in closer agreement with the
experiments in Josephson-junction arrays by Shaw et al., than those from the
RSJ model. We find that these results are related to anomalous vortex diffusion
through vortex clusters.Comment: 4 pages Rev-Tex, two figures in postscript. To appear In Physical
Review Letter
Nonlinear Viscous Vortex Motion in Two-Dimensional Josephson-Junction Arrays
When a vortex in a two-dimensional Josephson junction array is driven by a
constant external current it may move as a particle in a viscous medium. Here
we study the nature of this viscous motion. We model the junctions in a square
array as resistively and capacitively shunted Josephson junctions and carry out
numerical calculations of the current-voltage characteristics. We find that the
current-voltage characteristics in the damped regime are well described by a
model with a {\bf nonlinear} viscous force of the form , where is the vortex velocity,
is the velocity dependent viscosity and and are
constants for a fixed value of the Stewart-McCumber parameter. This result is
found to apply also for triangular lattices in the overdamped regime. Further
qualitative understanding of the nature of the nonlinear friction on the vortex
motion is obtained from a graphic analysis of the microscopic vortex dynamics
in the array. The consequences of having this type of nonlinear friction law
are discussed and compared to previous theoretical and experimental studies.Comment: 14 pages RevTex, 9 Postscript figure
Vortex-line liquid phases: Longitudinal superconductivity in the lattice London model
We study the vortex-line lattice and liquid phases of a clean type-II
superconductor by means of Monte Carlo simulations of the lattice London model.
Motivated by a recent controversy regarding the presence, within this model, of
a vortex-liquid regime with longitudinal superconducting coherence over long
length scales, we directly compare two different ways to calculate the
longitudinal coherence. For an isotropic superconductor, we interpret our
results in terms of a temperature regime within the liquid phase in which
longitudinal superconducting coherence extends over length scales larger than
the system thickness studied. We note that this regime disappears in the
moderately anisotropic case due to a proliferation, close to the flux-line
lattice melting temperature, of vortex loops between the layers.Comment: 8 pages, Revtex, with eps figures. To appear in Phys. Rev.
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