749 research outputs found
Anisotropic finite-size scaling of an elastic string at the depinning threshold in a random-periodic medium
We numerically study the geometry of a driven elastic string at its
sample-dependent depinning threshold in random-periodic media. We find that the
anisotropic finite-size scaling of the average square width and of
its associated probability distribution are both controlled by the ratio
, where is the
random-manifold depinning roughness exponent, is the longitudinal size of
the string and the transverse periodicity of the random medium. The
rescaled average square width displays a
non-trivial single minimum for a finite value of . We show that the initial
decrease for small reflects the crossover at from the
random-periodic to the random-manifold roughness. The increase for very large
implies that the increasingly rare critical configurations, accompanying
the crossover to Gumbel critical-force statistics, display anomalous roughness
properties: a transverse-periodicity scaling in spite that ,
and subleading corrections to the standard random-manifold longitudinal-size
scaling. Our results are relevant to understanding the dimensional crossover
from interface to particle depinning.Comment: 11 pages, 7 figures, Commentary from the reviewer available in Papers
in Physic
Damage spreading and coupling in Markov chains
In this paper, we relate the coupling of Markov chains, at the basis of
perfect sampling methods, with damage spreading, which captures the chaotic
nature of stochastic dynamics. For two-dimensional spin glasses and hard
spheres we point out that the obstacle to the application of perfect-sampling
schemes is posed by damage spreading rather than by the survey problem of the
entire configuration space. We find dynamical damage-spreading transitions
deeply inside the paramagnetic and liquid phases, and show that critical values
of the transition temperatures and densities depend on the coupling scheme. We
discuss our findings in the light of a classic proof that for arbitrary Monte
Carlo algorithms damage spreading can be avoided through non-Markovian coupling
schemes.Comment: 6 pages, 8 figure
Creep dynamics of elastic manifolds via exact transition pathways
We study the steady state of driven elastic strings in disordered media below
the depinning threshold. In the low-temperature limit, for a fixed sample, the
steady state is dominated by a single configuration, which we determine exactly
from the transition pathways between metastable states. We obtain the dynamical
phase diagram in this limit. At variance with a thermodynamic phase transition,
the depinning transition is not associated with a divergent length scale of the
steady state below threshold, but only of the transient dynamics. We discuss
the distribution of barrier heights, and check the validity of the dynamic
phase diagram at small but finite temperatures using Langevin simulations. The
phase diagram continues to hold for broken statistical tilt symmetry. We point
out the relevance of our results for experiments of creep motion in elastic
interfaces.Comment: 14 pages, 18 figure
A Rapid Dynamical Monte Carlo Algorithm for Glassy Systems
In this paper we present a dynamical Monte Carlo algorithm which is
applicable to systems satisfying a clustering condition: during the dynamical
evolution the system is mostly trapped in deep local minima (as happens in
glasses, pinning problems etc.). We compare the algorithm to the usual Monte
Carlo algorithm, using as an example the Bernasconi model. In this model, a
straightforward implementation of the algorithm gives an improvement of several
orders of magnitude in computational speed with respect to a recent, already
very efficient, implementation of the algorithm of Bortz, Kalos and Lebowitz.Comment: RevTex 7 pages + 4 figures (uuencoded) appended; LPS preprin
Geometry of Reduced Supersymmetric 4D Yang-Mills Integrals
We study numerically the geometric properties of reduced supersymmetric
non-compact SU(N) Yang-Mills integrals in D=4 dimensions, for N = 2,3, ..., 8.
We show that in the range of large eigenvalues of the matrices A^mu, the
original D-dimensional rotational symmetry is spontaneously broken and the
dominating field configurations become one-dimensional, as anticipated by
studies of the underlying surface theory. We also discuss possible implications
of our results for the IKKT model.Comment: 14 pages, Latex + 3 eps fig., a comment added to the conclusion
Genetic and non-genetic determinants of thymic epithelial cell number and function
The thymus is the site of T cell development in vertebrates. In general, the output of T cells is determined by the number of thymic epithelial cells (TECs) and their relative thymopoietic activity. Here, we show that the thymopoietic activity of TECs differs dramatically between individual mouse strains. Moreover, in males of some strains, TECs perform better on a per cell basis than their counterparts in females; in other strains, this situation is reversed. Genetic crosses indicate that TEC numbers and thymopoietic capacity are independently controlled. Long-term analysis of functional parameters of TECs after castration provides evidence that the number of Foxn1-expressing TECs directly correlates with thymopoietic activity. Our study highlights potential complications that can arise when comparing parameters of TEC biology across different genetic backgrounds; these could affect the interpretation of the outcomes of interventions aimed at modulating thymic activity in genetically diverse populations, such as humans
Dynamics below the depinning threshold
We study the steady-state low-temperature dynamics of an elastic line in a
disordered medium below the depinning threshold. Analogously to the equilibrium
dynamics, in the limit T->0, the steady state is dominated by a single
configuration which is occupied with probability one. We develop an exact
algorithm to target this dominant configuration and to analyze its geometrical
properties as a function of the driving force. The roughness exponent of the
line at large scales is identical to the one at depinning. No length scale
diverges in the steady state regime as the depinning threshold is approached
from below. We do find, a divergent length, but it is associated only with the
transient relaxation between metastable states.Comment: 4 pages, 3 figure
The Generic, Incommensurate Transition in the two-dimensional Boson Hubbard Model
The generic transition in the boson Hubbard model, occurring at an
incommensurate chemical potential, is studied in the link-current
representation using the recently developed directed geometrical worm
algorithm. We find clear evidence for a multi-peak structure in the energy
distribution for finite lattices, usually indicative of a first order phase
transition. However, this multi-peak structure is shown to disappear in the
thermodynamic limit revealing that the true phase transition is second order.
These findings cast doubts over the conclusion drawn in a number of previous
works considering the relevance of disorder at this transition.Comment: 13 pages, 10 figure
Vacancy localization in the square dimer model
We study the classical dimer model on a square lattice with a single vacancy
by developing a graph-theoretic classification of the set of all configurations
which extends the spanning tree formulation of close-packed dimers. With this
formalism, we can address the question of the possible motion of the vacancy
induced by dimer slidings. We find a probability 57/4-10Sqrt[2] for the vacancy
to be strictly jammed in an infinite system. More generally, the size
distribution of the domain accessible to the vacancy is characterized by a
power law decay with exponent 9/8. On a finite system, the probability that a
vacancy in the bulk can reach the boundary falls off as a power law of the
system size with exponent 1/4. The resultant weak localization of vacancies
still allows for unbounded diffusion, characterized by a diffusion exponent
that we relate to that of diffusion on spanning trees. We also implement
numerical simulations of the model with both free and periodic boundary
conditions.Comment: 35 pages, 24 figures. Improved version with one added figure (figure
9), a shift s->s+1 in the definition of the tree size, and minor correction
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