424 research outputs found
Chaotic dynamics of superconductor vortices in the plastic phase
We present numerical simulation results of driven vortex lattices in presence
of random disorder at zero temperature. We show that the plastic dynamics is
readily understood in the framework of chaos theory. Intermittency "routes to
chaos" have been clearly identified, and positive Lyapunov exponents and
broad-band noise, both characteristic of chaos, are found to coincide with the
differential resistance peak. Furthermore, the fractal dimension of the strange
attractor reveals that the chaotic dynamics of vortices is low-dimensional.Comment: 5 pages, 3 figures Accepted for publication in Physical Review
Letter
Driven flux-line lattices in the presence of weak random columnar disorder: Finite-temperature behavior and dynamical melting of moving Bose glass
We use 3D numerical simulations to explore the phase diagram of driven flux
line lattices in presence of weak random columnar disorder at finite
temperature and high driving force. We show that the moving Bose glass phase
exists in a large range of temperature, up to its melting into a moving vortex
liquid. It is also remarkably stable upon increasing velocity : the dynamical
transition to the correlated moving glass expected at a critical velocity is
not found at any velocity accessible to our simulations. Furthermore, we show
the existence of an effective static tin roof pinning potential in the
direction transverse to motion, which originates from both the transverse
periodicity of the moving lattice and the localization effect due to correlated
disorder. Using a simple model of a single elastic line in such a periodic
potential, we obtain a good description of the transverse field penetration at
surfaces as a function of thickness in the moving Bose glass phase.Comment: 5 pages, 4 figures, New title and minor changes in text and figures.
Accepted for publication in Physical Review
On the nature of Thermal Diffusion in binary Lennard-Jones liquids
The aim of this study is to understand deeper the thermal diffusion transport
process (Ludwig-Soret effect) at the microscopic level. For that purpose, the
recently developed reverse nonequilibrium molecular dynamics method was used to
calculate Soret coefficients of various systems in a systematic fashion. We
studied binary Lennard-Jones (LJ) fluids near the triple point (of one of the
components) in which we separately changed the ratio of one of the LJ
parameters mass, atomic diameter and interaction strength while keeping all
other parameters fixed and identical. We observed that the magnitude of the
Soret coefficient depends on all three ratios. Concerning its sign we found
that heavier species, smaller species and species with higher interaction
strengths tend to accumulate in the cold region whereas the other ones
(lighter, bigger or weaker bound) migrate to the hot region of our simulation
cell. Additionally, the superposition of the influence of the various
parameters was investigated as well as more realistic mixtures. We found that
in the experimentally relevant parameter range the contributions are nearly
additive and that the mass ratio often is the dominating factor.Comment: 27 pages, 9 figures, submitted to J. Chem. Phy
Critical behavior of plastic depinning of vortex lattices in two dimensions: Molecular dynamics simulations
Using molecular dynamics simulations, we report a study of the dynamics of
two-dimensional vortex lattices driven over a disordered medium. In strong
disorder, when topological order is lost, we show that the depinning transition
is analogous to a second order critical transition: the velocity-force response
at the onset of motion is continuous and characterized by critical exponents.
Combining studies at zero and nonzero temperature and using a scaling analysis,
two critical expo- nents are evaluated. We find v\sim (F-F_c)^\beta with
\beta=1.3\pm0.1 at T=0 and F>F_c, and v\sim T^{1/\delta} with
\delta^{-1}=0.75\pm0.1 at F=F_c, where F_c is the critical driving force at
which the lattice goes from a pinned state to a sliding one. Both critical
exponents and the scaling function are found to exhibit universality with
regard to the pinning strength and different disorder realizations.
Furthermore, the dynamics is shown to be chaotic in the whole critical region.Comment: 8 pages, 6 figure
Chaos and plasticity in superconductor vortices: a low-dimensional dynamics
We present new results of numerical simulations for driven vortex lattices in
presence of random disorder at zero temperature. We show that the plastic
dynamics of vortices display dissipative chaos. Intermittency "routes to chaos"
have been clearly identified below the differential resistance peak. The peak
region is characterized by positive Lyapunov exponents characteristic of chaos,
and low frequency broad-band noise. Furthermore we find a low fractal dimension
of the strange attractor, which suggests that only a few dynamical variables
are sufficient to model the complex plastic dynamics of vortices.Comment: 8 pages, 6 figures, accepted for publication in The Physical Review
The elastic depinning transition of vortex lattices in two dimensions
Large scale numerical simulations are used to study the elastic dynamics of
two-dimensional vortex lattices driven on a disordered medium in the case of
weak disorder. We investigate the so-called elastic depinning transition by
decreasing the driving force from the elastic dynamical regime to the state
pinned by the quenched disorder. Similarly to the plastic depinning transition,
we find results compatible with a second order phase transition, although both
depinning transitions are very different from many viewpoints. We evaluate
three critical exponents of the elastic depinning transition. is found for the velocity exponent at zero temperature, and from the
velocity-temperature curves we extract the critical exponent . Furthermore, in contrast with charge-density waves, a
finite-size scaling analysis suggests the existence of a unique diverging
length at the depinning threshold with an exponent , which
controls the critical force distribution, the finite-size crossover force
distribution and the intrinsic correlation length. Finally, a scaling relation
is found between velocity and temperature with the and
critical exponents both independent with regard to pinning strength and
disorder realizations.Comment: 17 pages, 10 figure
Magnetoresistance scaling in the layered cobaltate Ca3Co4O9
We investigate the low temperature magnetic field dependences of both the
resistivity and the magnetization in the misfit cobaltate Ca3Co4O9 from 60 K
down to 2 K. The measured negative magnetoresistance reveals a scaling behavior
with the magnetization which demonstrates a spin dependent diffusion mechanism.
This scaling is also found to be consistent with a shadowed metalliclike
conduction over the whole temperature range. By explaining the observed
transport crossover, this result shed a new light on the nature of the
elementary excitations relevant to the transport
Response to tilted magnetic fields in Bi2Sr2CaCu2O8 with columnar defects: Evidence for transverse Meissner effect
The transverse Meissner effect (TME) in the highly layered superconductor
Bi2Sr2CaCu2O(8+y) with columnar defects is investigated by transport
measurements. We present detailed evidence for the persistence of the
Bose-glass phase when H is tilted at an angle theta < theta_c (T) away from the
column direction: (i) the variable-range vortex hopping process for low
currents crosses over to the half-loops regime for high currents; (ii) in both
regimes near theta_c(T) the energy barriers vanish linearly with tan(theta) ;
(iii) the transition temperature is governed by T_{BG}(0) -T_{BG}(theta) sim
|tan(theta)|^{1/\nu_{\perp}} with \nu_{\perp}=1.0 +/- 0.1. Furthermore, above
the transition as theta->\theta_c+, moving kink chains consistent with a
commensurate-incommensurate transition scenario are observed. These results
thereby clearly show the existence of the TME for theta < theta_c(T).Comment: 4 pages, RevTeX, 5 EPS figure
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