178 research outputs found
Spin-ordering in S = 1 anisotropic Heisenberg models with nondiagonal spin exchange
The properties of S = 1 anisotropic Heisenberg models with nondiagonal
exchange between axial and planar spin components are investigated using Monte
Carlo techniques. The quantum nature is taken into account in a semi-classical
approximation. The ordering of the spins when applying an external field with
axial and planar components is discussed. It is argued that the quantum nature
of the spins and the nondiagonal exchange may explain the peculiar shape of the
magnetic specific heat of FeBr2 as well as the weakly first-order phase
transition observed in the same compound when a tilted field is applied.Comment: 9 pages, 15 figures included, to appear in European Physical Journal
Critical phenomena at perfect and non-perfect surfaces
The effect of imperfections on surface critical properties is studied for
Ising models with nearest-neighbour ferromagnetic couplings on simple cubic
lattices. In particular, results of Monte Carlo simulations for flat, perfect
surfaces are compared to those for flat surfaces with random, 'weak' or
'strong', interactions between neighbouring spins in the surface layer, and for
surfaces with steps of monoatomic height. Surface critical exponents at the
ordinary transition, in particular , are found to be
robust against these perturbations.Comment: 7 pages, 13 figures, submitted to European Physical Journal
Dynamics of the two-dimensional directed Ising model: zero-temperature coarsening
We investigate the laws of coarsening of a two-dimensional system of Ising
spins evolving under single-spin-flip irreversible dynamics at low temperature
from a disordered initial condition. The irreversibility of the dynamics comes
from the directedness, or asymmetry, of the influence of the neighbours on the
flipping spin. We show that the main characteristics of phase ordering at low
temperature, such as self-similarity of the patterns formed by the growing
domains, and the related scaling laws obeyed by the observables of interest,
which hold for reversible dynamics, are still present when the dynamics is
directed and irreversible, but with different scaling behaviour. In particular
the growth of domains, instead of being diffusive as is the case when dynamics
is reversible, becomes ballistic. Likewise, the autocorrelation function and
the persistence probability (the probability that a given spin keeps its sign
up to time ) have still power-law decays but with different exponents.Comment: 29 pages, 36 figure
Phase diagrams of Ising films with competing interactions
The axial next-nearest-neighbour Ising (ANNNI) model of finite thickness is
studied. Using mean-field theory, Monte Carlo simulations, and low-temperature
analyses, phase diagrams are determined, with a distinct phase diagram for each
film thickness. The robustness of the phase diagrams against varying the
couplings in the surface layers is analysed.Comment: 7 pages, 6 figures included, version to appear in Eur. Phys. J.
Low temperature phase diagram and critical behaviour of the four-state chiral clock model
The low temperature behaviour of the four-state chiral clock () model
is reexamined using a systematic low temperature series expansion of the free
energy. Previously obtained results for the low temperature phases are
corrected and the low temperature phase diagram is derived. In addition, the
phase transition from the modulated region to the high temperature paraphase is
shown to belong to the universality class of the 3d-XY model.Comment: 17 pages in ioplppt style, 3 figure
Reply to a Comment
Reply to the Comment by F. Corberi, E. Lipiello and M. Zannetti
(cond-mat/0211609)
Mobility and asymmetry effects in one-dimensional rock-paper-scissors games
As the behavior of a system composed of cyclically competing species is
strongly influenced by the presence of fluctuations, it is of interest to study
cyclic dominance in low dimensions where these effects are the most prominent.
We here discuss rock-paper-scissors games on a one-dimensional lattice where
the interaction rates and the mobility can be species dependent. Allowing only
single site occupation, we realize mobility by exchanging individuals of
different species. When the interaction and swapping rates are symmetric, a
strongly enhanced swapping rate yields an increased mixing of the species,
leading to a mean-field like coexistence even in one-dimensional systems. This
coexistence is transient when the rates are asymmetric, and eventually only one
species will survive. Interestingly, in our spatial games the dominating
species can differ from the species that would dominate in the corresponding
nonspatial model. We identify different regimes in the parameter space and
construct the corresponding dynamical phase diagram.Comment: 6 pages, 5 figures, to appear in Physical Review
Ising cubes with enhanced surface couplings
Using Monte Carlo techniques, Ising cubes with ferromagnetic nearest-neighbor
interactions and enhanced couplings between surface spins are studied. In
particular, at the surface transition, the corner magnetization shows
non-universal, coupling-dependent critical behavior in the thermodynamic limit.
Results on the critical exponent of the corner magnetization are compared to
previous findings on two-dimensional Ising models with three intersecting
defect lines.Comment: 4 pages, 2 figures included, submitted to Phys. Rev.
Ising films with surface defects
The influence of surface defects on the critical properties of magnetic films
is studied for Ising models with nearest-neighbour ferromagnetic couplings. The
defects include one or two adjacent lines of additional atoms and a step on the
surface. For the calculations, both density-matrix renormalization group and
Monte Carlo techniques are used. By changing the local couplings at the defects
and the film thickness, non-universal features as well as interesting crossover
phenomena in the magnetic exponents are observed.Comment: 8 pages, 12 figures included, submitted to European Physical Journal
Corrections to local scale invariance in the non-equilibrium dynamics of critical systems: numerical evidences
Local scale invariance (LSI) has been recently proposed as a possible
extension of the dynamical scaling in systems at the critical point and during
phase ordering. LSI has been applied inter alia to provide predictions for the
scaling properties of the response function of non-equilibrium critical systems
in the aging regime following a quench from the high-temperature phase to the
critical point. These predictions have been confirmed by Monte Carlo
simulations and analytical results for some specific models, but they are in
disagreement with field-theoretical predictions. By means of Monte Carlo
simulations of the critical two- and three-dimensional Ising model with Glauber
dynamics, we study the intermediate integrated response, finding deviations
from the corresponding LSI predictions that are in qualitative agreement with
the field-theoretical computations. This result casts some doubts on the
general applicability of LSI to critical dynamics.Comment: 4 pages, 2 figures, minor changes, version to appear in Phys. Rev. B
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