26 research outputs found
Disorder driven destruction of a phase transition in a superconductor
We investigate the effects of disorder on a layered superconductor. The clean
system is known to have a first order phase transition which is clearly
identified by a sharp peak in the specific heat. The peak is lost abruptly as
the strength of the disorder is increased. Hence, for strong disorder there is
no phase transition as a function of temperature but merely a crossover which
is still detectable in the IV characteristic.Comment: 3 pages REVTeX , 5 figure
Connecting the Micro-dynamics to the Emergent Macro-variables: Self-Organized Criticality and Absorbing Phase Transitions in the Deterministic Lattice Gas
We reinvestigate the Deterministic Lattice Gas introduced as a paradigmatic
model of the 1/f spectra (Phys. Rev. Lett. V26, 3103 (1990)) arising according
to the Self-Organized Criticality scenario. We demonstrate that the density
fluctuations exhibit an unexpected dependence on systems size and relate the
finding to effective Langevin equations. The low density behavior is controlled
by the critical properties of the gas at the absorbing state phase transition.
We also show that the Deterministic Lattice Gas is in the Manna universality
class of absorbing state phase transitions. This is in contrast to expectations
in the literature which suggested that the entirely deterministic nature of the
dynamics would put the model in a different universality class. To our
knowledge this is the first fully deterministic member of the Manna
universality class.Comment: 8 pages, 12 figures. Changes in the new version: Reference list has
been correcte
Vortices Clustering: The Origin of the Second Peak in the Magnetisation Loops of High Temperature Superconductors
We study vortex clustering in type II Superconductors. We demonstrate that
the ``second peak'' observed in magnetisation loops may be a dynamical effect
associated with a density driven instability of the vortex system. At the
microscopic level the instability shows up as the clustering of individual
vortices at (rare) preferential regions of the pinning potential. In the limit
of quasi-static ramping the instability is related to a phase transition in the
equilibrium vortex system.Comment: 11 pages + 3 figure
Tangled Nature: A model of emergent structure and temporal mode among co-evolving agents
Understanding systems level behaviour of many interacting agents is
challenging in various ways, here we'll focus on the how the interaction
between components can lead to hierarchical structures with different types of
dynamics, or causations, at different levels. We use the Tangled Nature model
to discuss the co-evolutionary aspects connecting the microscopic level of the
individual to the macroscopic systems level. At the microscopic level the
individual agent may undergo evolutionary changes due to mutations of
strategies. The micro-dynamics always run at a constant rate. Nevertheless, the
system's level dynamics exhibit a completely different type of intermittent
abrupt dynamics where major upheavals keep throwing the system between
meta-stable configurations. These dramatic transitions are described by a
log-Poisson time statistics. The long time effect is a collectively adapted of
the ecological network. We discuss the ecological and macroevolutionary
consequences of the adaptive dynamics and briefly describe work using the
Tangled Nature framework to analyse problems in economics, sociology,
innovation and sustainabilityComment: Invited contribution to Focus on Complexity in European Journal of
Physics. 25 page, 1 figur
Simulations of clustering of vortices in superconductors: a possible origin of the second peak
Abstract We describe by use of molecular dynamics simulations how the attraction, at short range, between the vortex cores aids the clustering of vortices. Our simulations suggest that the short range core attraction and ensuing vortex clustering is the mechanism responsible for the second peak in the magnetisation loops of superconductors.
Neutral Evolution as Diffusion in phenotype space: reproduction with mutation but without selection
The process of `Evolutionary Diffusion', i.e. reproduction with local
mutation but without selection in a biological population, resembles standard
Diffusion in many ways. However, Evolutionary Diffusion allows the formation of
local peaks with a characteristic width that undergo drift, even in the
infinite population limit. We analytically calculate the mean peak width and
the effective random walk step size, and obtain the distribution of the peak
width which has a power law tail. We find that independent local mutations act
as a diffusion of interacting particles with increased stepsize.Comment: 4 pages, 2 figures. Paper now representative of published articl
How a spin-glass remembers. Memory and rejuvenation from intermittency data: an analysis of temperature shifts
The memory and rejuvenation aspects of intermittent heat transport are
explored theoretically and by numerical simulation for Ising spin glasses with
short-ranged interactions. The theoretical part develops a picture of
non-equilibrium glassy dynamics recently introduced by the authors. Invoking
the concept of marginal stability, this theory links irreversible
`intermittent' events, or `quakes' to thermal fluctuations of record magnitude.
The pivotal idea is that the largest energy barrier surmounted prior
to by thermal fluctuations at temperature determines the rate of the intermittent events occurring near . The idea leads
to a rate of intermittent events after a negative temperature shift given by
, where the `effective age' has
an algebraic dependence on , whose exponent contains the temperatures
before and after the shift. The analytical expression is verified by numerical
simulations. Marginal stability suggests that a positive temperature shift could erase the memory of the barrier . The simulations show
that the barrier controls the intermittent dynamics,
whose rate is hence .
Additional `rejuvenation' effects are also identified in the intermittency
data for shifts of both signs.Comment: Revised introduction and discussion. Final version to appear in
Journal of Statistical Mechanics: Theory and Experimen
Monte Carlo calculation of the linear resistance of a three dimensional lattice Superconductor model in the London limit
We have studied the linear resistance of a three dimensional lattice
Superconductor model in the London limit London lattice model by Monte Carlo
simulation of the vortex loop dynamics. We find excellent finite size scaling
at the phase transition. We determine the dynamical exponent for the
isotropic London lattice model.Comment: 4 pages, RevTeX with 3 postscript figures include
Monte Carlo calculation of the current-voltage characteristics of a two dimensional lattice Coulomb gas
We have studied the nonlinear current-voltage characteristic of a two
dimensional lattice Coulomb gas by Monte Carlo simulation. We present three
different determinations of the power-law exponent of the nonlinear
current-voltage characteristic, . The determinations rely on
both equilibrium and non-equilibrium simulations. We find good agreement
between the different determinations, and our results also agree closely with
experimental results for Hg-Xe thin film superconductors and for certain single
crystal thin-film high temperature superconductors.Comment: late
Decoupling and melting in a layered superconductor
We report results for a 3D simulation of a layered superconductor. The
low-temperature phase corresponds to a triangular lattice of vortex
lines. On increasing the temperature order is lost in the a-b plane,
Tab with no detectable thermodynamic signature. On further
increasing the temperature the vortex lines decouple completely at a
first-order phase transition, Tdc. The entropy associated
with the transition is dependent on the anisotropy chosen for the
system and has values of and for the
two anisotropies studied. The width is anisotropy
dependent and is too narrow to be measured for the more anisotropic system
studied, corresponding to sublimation of the vortices as reported
in a recent experiment by Fuchs et al