15 research outputs found
Critical scaling and aging near the flux-line-depinning transition
We utilize Langevin molecular dynamics simulations to study dynamical
critical behavior of magnetic flux lines near the depinning transition in
type-II superconductors subject to randomly distributed attractive point
defects. We employ a coarse-grained elastic line Hamiltonian for the mutually
repulsive vortices and purely relaxational kinetics. In order to infer the
stationary-state critical exponents for the continuous non-equilibrium
depinning transition at zero temperature T = 0 and at the critical driving
current density j_c, we explore two-parameter scaling laws for the flux lines'
gyration radius and mean velocity as functions of the two relevant scaling
fields T and j - j_c. We also investigate critical aging scaling for the
two-time height auto-correlation function in the early-time non-equilibrium
relaxation regime to independently measure critical exponents. We provide
numerical exponent values for the distinct universality classes of
non-interacting and repulsive vortices
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Evolutionary dynamics and competition stabilize three-species predator–prey communities
We perform individual-based Monte Carlo simulations in a community consisting
of two predator species competing for a single prey species, with the purpose
of studying biodiversity stabilization in this simple model system. Predators
are characterized with predation efficiency and death rates, to which Darwinian
evolutionary adaptation is introduced. Competition for limited prey abundance
drives the populations' optimization with respect to predation efficiency and
death rates. We study the influence of various ecological elements on the final
state, finding that both indirect competition and evolutionary adaptation are
insufficient to yield a stable ecosystem. However, stable three-species
coexistence is observed when direct interaction between the two predator
species is implemented
Disordered vortex matter out of equilibrium: a Langevin molecular dynamics study
We discuss the use of Langevin molecular dynamics in the investigation of the
non-equilibrium properties of disordered vortex matter. Our special focus is
set on values of system parameters that are realistic for disordered high-
superconductors such as YBCO. Using a discretized elastic line model, we study
different aspects of vortices far from thermal equilibrium. On the one hand we
investigate steady-state properties of driven magnetic flux lines in a
disordered environment, namely the current-voltage characteristics, the
gyration radius, and the pinning time statistics. On the other hand we study
the complex relaxation processes and glassy-like dynamics that emerge in
type-II superconductors due to the intricate competition between the long-range
vortex-vortex repulsion and flux pinning due to randomly placed point defects.
To this end we consider different types of sudden perturbations: temperature,
magnetic field, and external current quenches
Flux line relaxation kinetics following current quenches in disordered type-II superconductors
We investigate the relaxation dynamics of magnetic vortex lines in type-II
superconductors following rapid changes of the external driving current by
means of an elastic line model simulated with Langevin molecular dynamics. A
system of flux vortices in a sample with randomly distributed point-like
defects is subjected to an external current of appropriate strength for a
sufficient period of time so as to be in a moving non-equilibrium steady state.
The current is then instantaneously lowered to a value that pertains to either
the moving or pinned regime. The ensuing relaxation of the flux lines is
studied via one-time observables such as their mean velocity and radius of
gyration. We have in addition measured the two-time flux line height
autocorrelation function to investigate dynamical scaling and aging behavior in
the system, which in particular emerge after quenches into the glassy pinned
state
A field-theoretic approach to the Wiener Sausage
The Wiener Sausage, the volume traced out by a sphere attached to a Brownian
particle, is a classical problem in statistics and mathematical physics.
Initially motivated by a range of field-theoretic, technical questions, we
present a single loop renormalised perturbation theory of a stochastic process
closely related to the Wiener Sausage, which, however, proves to be exact for
the exponents and some amplitudes. The field-theoretic approach is particularly
elegant and very enjoyable to see at work on such a classic problem. While we
recover a number of known, classical results, the field-theoretic techniques
deployed provide a particularly versatile framework, which allows easy
calculation with different boundary conditions even of higher momenta and more
complicated correlation functions. At the same time, we provide a highly
instructive, non-trivial example for some of the technical particularities of
the field-theoretic description of stochastic processes, such as excluded
volume, lack of translational invariance and immobile particles. The aim of the
present work is not to improve upon the well-established results for the Wiener
Sausage, but to provide a field-theoretic approach to it, in order to gain a
better understanding of the field-theoretic obstacles to overcome.Comment: 45 pages, 3 Figures, Springer styl
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Dynamical regimes of vortex flow in type-II superconductors with parallel twin boundaries
We explore the dynamics of driven magnetic flux lines in disordered type-II superconductors in the presence of twin boundaries oriented parallel to the direction of the applied magnetic field, using a three-dimensional elastic line model simulated with Langevin molecular dynamics. The lines are driven perpendicular to the planes to model the effect of an electric current applied parallel to the planes and perpendicular to the magnetic field. A study of the long-time non-equilibrium steady states for varying sample orientation and thickness reveals a rich collection of dynamical regimes spanning the depinning crossover region that separates the pinned and moving-lattice states of vortex matter. We observe the emergence of a preferred direction for the ordering of the Abrikosov lattice in the free-flowing vortex regime due to asymmetric pinning by the planar defects. We have performed novel direct measurements of flux line excitations such as half-loops and double kinks to aid the characterization of the topologically rich flux flow profile
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Flux line relaxation kinetics following current quenches in disordered type-II superconductors
© 2016 IOP Publishing Ltd and SISSA Medialab srl. We investigate the relaxation dynamics of magnetic vortex lines in type-II superconductors following rapid changes of the external driving current by means of an elastic line model simulated with Langevin molecular dynamics. A system of flux vortices in a sample with randomly distributed point-like defects is subjected to an external current of appropriate strength for a sufficient period of time so as to be in a moving non-equilibrium steady state. The current is then instantaneously lowered to a value that pertains to either the moving or pinned regime. The ensuing relaxation of the flux lines is studied via one-time observables such as their mean velocity and radius of gyration. We have in addition measured the two-time flux line height autocorrelation function to investigate dynamical scaling and aging behavior in the system, which in particular emerge after quenches into the glassy pinned state