419 research outputs found
Thermocycling experiments with the three-dimensional Ising spin glass model
A characteristic feature of the non--equilibrium dynamics of real spin
glasses at low temperatures are strong aging effects. These phenomena can be
manipulated by changing the external parameters in various ways: a
thermo-cycling experiment consists for instance of a short heat pulse during
the waiting time, by which the relaxation might be strongly affected. Results
of numerical experiments of this kind, performed via Monte Carlo simulations of
the three dimensional Ising spin glass model, are presented. The theoretical
implications are discussed and the scenario found is compared with the
experimental situation.Comment: LaTeX 13 pages (+6 figures upon request), THP11-9
Universal properties of shortest paths in isotropically correlated random potentials
We consider the optimal paths in a -dimensional lattice, where the bonds
have isotropically correlated random weights. These paths can be interpreted as
the ground state configuration of a simplified polymer model in a random
potential. We study how the universal scaling exponents, the roughness and the
energy fluctuation exponent, depend on the strength of the disorder
correlations. Our numerical results using Dijkstra's algorithm to determine the
optimal path in directed as well as undirected lattices indicate that the
correlations become relevant if they decay with distance slower than 1/r in d=2
and 3. We show that the exponent relation 2nu-omega=1 holds at least in d=2
even in case of correlations. Both in two and three dimensions, overhangs turn
out to be irrelevant even in the presence of strong disorder correlations.Comment: 8 pages LaTeX, eps figures included, typos added, references added,
content change
Strong Randomness Fixed Point in the Dissipative Random Transverse Field Ising Model
The interplay between disorder, quantum fluctuations and dissipation is
studied in the random transverse Ising chain coupled to a dissipative Ohmic
bath with a real space renormalization group. A typically very large length
scale, L*, is identified above which the physics of frozen clusters dominates.
Below L* a strong disorder fixed point determines scaling at a pseudo-critical
point. In a Griffiths-McCoy region frozen clusters produce already a finite
magnetization resulting in a classical low temperature behavior of the
susceptibility and specific heat. These override the confluent singularities
that are characterized by a continuously varying exponent z and are visible
above a temperature T* ~ L*^{-z}.Comment: 4 pages RevTeX, figures include
Random and aperiodic quantum spin chains: A comparative study
According to the Harris-Luck criterion the relevance of a fluctuating
interaction at the critical point is connected to the value of the fluctuation
exponent omega. Here we consider different types of relevant fluctuations in
the quantum Ising chain and investigate the universality class of the models.
At the critical point the random and aperiodic systems behave similarly, due to
the same type of extreme broad distribution of the energy scales at low
energies. The critical exponents of some averaged quantities are found to be a
universal function of omega, but some others do depend on other parameters of
the distribution of the couplings. In the off-critical region there is an
important difference between the two systems: there are no Griffiths
singularities in aperiodic models.Comment: 4 pages RevTeX, 2 eps-figures include
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