260 research outputs found
Monte Carlo Dynamics of driven Flux Lines in Disordered Media
We show that the common local Monte Carlo rules used to simulate the motion
of driven flux lines in disordered media cannot capture the interplay between
elasticity and disorder which lies at the heart of these systems. We therefore
discuss a class of generalized Monte Carlo algorithms where an arbitrary number
of line elements may move at the same time. We prove that all these dynamical
rules have the same value of the critical force and possess phase spaces made
up of a single ergodic component. A variant Monte Carlo algorithm allows to
compute the critical force of a sample in a single pass through the system. We
establish dynamical scaling properties and obtain precise values for the
critical force, which is finite even for an unbounded distribution of the
disorder. Extensions to higher dimensions are outlined.Comment: 4 pages, 3 figure
The depinning transition of a driven interface in the random-field Ising model around the upper critical dimension
We investigate the depinning transition for driven interfaces in the
random-field Ising model for various dimensions. We consider the order
parameter as a function of the control parameter (driving field) and examine
the effect of thermal fluctuations. Although thermal fluctuations drive the
system away from criticality the order parameter obeys a certain scaling law
for sufficiently low temperatures and the corresponding exponents are
determined. Our results suggest that the so-called upper critical dimension of
the depinning transition is five and that the systems belongs to the
universality class of the quenched Edward-Wilkinson equation.Comment: accepted for publication in Phys. Rev.
Influence of Hot Band Annealing on Cold-Rolled Microstructure and Recrystallization in AA 6016
The influence of an intermediate heat treatment at the end of hot rolling and before cold rolling on Cube texture formation during the final solution annealing of AA 6016 is investigated. Three hot bands with different initial grain sizes and textures are considered: the first one without annealing before cold rolling, while the other two hot bands are heat treated at 540 °C for 1 hour in air before being cold rolled. One of the heat-treated hot bands was left to cool down in air and the other inside the furnace. Electron-backscatter diffraction (EBSD) maps of the cold-rolled specimens and crystal plasticity simulations show no difference in the amount of Cube remaining in the microstructure at the end of cold rolling for all three specimens. The initial grain size of the hot band has no influence on the Cube texture fraction left in the microstructure at the end of cold rolling for thickness reductions higher than 65 pct. Nevertheless, the grain size of the hot band affects the shape and distribution of the Cube grains left in the microstructure and the kernel average misorientation in the cold-rolled specimens. Moreover, the heat treatment decreases the intensity of the beta fiber components (Brass, Copper, and S) in the hot band and promotes the formation of a cold-rolled microstructure with a low kernel average misorientation. Both these factors lower the probability of preferential Cube nucleation during solution annealing and keep the Cube volume fraction after recrystallization below 10 pct, while it reaches 25 pct without intermediate annealing
Higher correlations, universal distributions and finite size scaling in the field theory of depinning
Recently we constructed a renormalizable field theory up to two loops for the
quasi-static depinning of elastic manifolds in a disordered environment. Here
we explore further properties of the theory. We show how higher correlation
functions of the displacement field can be computed. Drastic simplifications
occur, unveiling much simpler diagrammatic rules than anticipated. This is
applied to the universal scaled width-distribution. The expansion in
d=4-epsilon predicts that the scaled distribution coincides to the lowest
orders with the one for a Gaussian theory with propagator G(q)=1/q^(d+2 \zeta),
zeta being the roughness exponent. The deviations from this Gaussian result are
small and involve higher correlation functions, which are computed here for
different boundary conditions. Other universal quantities are defined and
evaluated: We perform a general analysis of the stability of the fixed point.
We find that the correction-to-scaling exponent is omega=-epsilon and not
-epsilon/3 as used in the analysis of some simulations. A more detailed study
of the upper critical dimension is given, where the roughness of interfaces
grows as a power of a logarithm instead of a pure power.Comment: 15 pages revtex4. See also preceding article cond-mat/030146
Origin of the roughness exponent in elastic strings at the depinning threshold
Within a recently developed framework of dynamical Monte Carlo algorithms, we
compute the roughness exponent of driven elastic strings at the
depinning threshold in 1+1 dimensions for different functional forms of the
(short-range) elastic energy. A purely harmonic elastic energy leads to an
unphysical value for . We include supplementary terms in the elastic
energy of at least quartic order in the local extension. We then find a
roughness exponent of , which coincides with the one
obtained for different cellular automaton models of directed percolation
depinning. The quartic term translates into a nonlinear piece which changes the
roughness exponent in the corresponding continuum equation of motion. We
discuss the implications of our analysis for higher-dimensional elastic
manifolds in disordered media.Comment: 4 pages, 2 figure
Critical behavior of a traffic flow model
The Nagel-Schreckenberg traffic flow model shows a transition from a free
flow regime to a jammed regime for increasing car density. The measurement of
the dynamical structure factor offers the chance to observe the evolution of
jams without the necessity to define a car to be jammed or not. Above the
jamming transition the dynamical structure factor exhibits for a given k-value
two maxima corresponding to the separation of the system into the free flow
phase and jammed phase. We obtain from a finite-size scaling analysis of the
smallest jam mode that approaching the transition long range correlations of
the jams occur.Comment: 5 pages, 7 figures, accepted for publication in Physical Review
Velocity-force characteristics of a driven interface in a disordered medium
Using a dynamic functional renormalization group treatment of driven elastic
interfaces in a disordered medium, we investigate several aspects of the
creep-type motion induced by external forces below the depinning threshold
: i) We show that in the experimentally important regime of forces
slightly below the velocity obeys an Arrhenius-type law
with an effective energy barrier
vanishing linearly when f approaches the threshold . ii) Thermal
fluctuations soften the pinning landscape at high temperatures. Determining the
corresponding velocity-force characteristics at low driving forces for internal
dimensions d=1,2 (strings and interfaces) we find a particular non-Arrhenius
type creep involving the reduced threshold
force alone. For d=3 we obtain a similar v-f characteristic which is,
however, non-universal and depends explicitly on the microscopic cutoff.Comment: 9 pages, RevTeX, 3 postscript figure
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