400 research outputs found
Monte Carlo Study of Mixed-Spin S=(1/2,1) Ising Ferrimagnets
We investigate Ising ferrimagnets on square and simple-cubic lattices with
exchange couplings between spins of values S=1/2 and S=1 on neighbouring sites
and an additional single-site anisotropy term on the S=1 sites. Based mainly on
a careful and comprehensive Monte Carlo study, we conclude that there is no
tricritical point in the two--dimensional case, in contradiction to mean-field
predictions and recent series results. However, evidence for a tricritical
point is found in the three-dimensional case. In addition, a line of
compensation points is found for the simple-cubic, but not for the square
lattice.Comment: 14 pages, 11 figure
Interfacial adsorption phenomena of the three-dimensional three-state Potts model
We study the interfacial adsorption phenomena of the three-state
ferromagnetic Potts model on the simple cubic lattice by the Monte Carlo
method. Finite-size scaling analyses of the net-adsorption yield the evidence
of the phase transition being of first-order and .Comment: 14 page
Determination of the Critical Exponents for the Isotropic-Nematic Phase Transition in a System of Long Rods on Two-dimensional Lattices: Universality of the Transition
Monte Carlo simulations and finite-size scaling analysis have been carried
out to study the critical behavior and universality for the isotropic-nematic
phase transition in a system of long straight rigid rods of length
(-mers) on two-dimensional lattices. The nematic phase, characterized by a
big domain of parallel -mers, is separated from the isotropic state by a
continuous transition occurring at a finite density. The determination of the
critical exponents, along with the behavior of Binder cumulants, indicate that
the transition belongs to the 2D Ising universality class for square lattices
and the three-state Potts universality class for triangular lattices.Comment: 7 pages, 8 figures, uses epl2.cls, to appear in Europhysics Letter
Critical behavior of the pure and random-bond two-dimensional triangular Ising ferromagnet
We investigate the effects of quenched bond randomness on the critical
properties of the two-dimensional ferromagnetic Ising model embedded in a
triangular lattice. The system is studied in both the pure and disordered
versions by the same efficient two-stage Wang-Landau method. In the first part
of our study we present the finite-size scaling behavior of the pure model, for
which we calculate the critical amplitude of the specific heat's logarithmic
expansion. For the disordered system, the numerical data and the relevant
detailed finite-size scaling analysis along the lines of the two well-known
scenarios - logarithmic corrections versus weak universality - strongly support
the field-theoretically predicted scenario of logarithmic corrections. A
particular interest is paid to the sample-to-sample fluctuations of the random
model and their scaling behavior that are used as a successful alternative
approach to criticality.Comment: 10 pages, 8 figures, slightly revised version as accepted for
publication in Phys. Rev.
Anisotropic Lifshitz Point at
We present the critical exponents , and
for an -axial Lifshitz point at second order in an expansion.
We introduced a constraint involving the loop momenta along the -dimensional
subspace in order to perform two- and three-loop integrals. The results are
valid in the range . The case corresponds to the usual
Ising-like critical behavior.Comment: 10 pages, Revte
Interfacial adsorption in Potts models on the square lattice
We study the effect of interfacial phenomena in two-dimensional perfect and
random (or disordered) -state Potts models with continuous phase
transitions, using, mainly, Monte Carlo techniques. In particular, for the
total interfacial adsorption, the critical behavior, including corrections to
scaling, are analyzed. The role of randomness is scrutinized. Results are
discussed applying scaling arguments and invoking findings for bulk critical
properties. In all studied cases, i.e., , , and , the spread
of the interfacial adsorption profiles is observed to increase linearly with
the lattice size at the bulk transition point.Comment: 6 pages, 6 eps figures, 1 table, minor corrections, accepted for
publication in Eur. Phys. J.
Commensurate and modulated magnetic phases in orthorhombic A1C60
Competing magnetically ordered structures in polymerized orthorhombic A1C60
are studied. A mean-field theory for the equilibrium phases is developed using
an Ising model and a classical Heisenberg model to describe the competition
between inter- and intra-chain magnetic order in the solid. In the Ising model,
the limiting commensurate one-dimensional and three-dimensional phases are
separated by a commensurate three-sublattice state and by two sectors
containing higher-order commensurate phases. For the Heisenberg model the
quasi-1D phase is never the equilibrium state; instead the 3D commensurate
phases exhibits a transition to a continuum of coplanar spiral magnetic phases.Comment: 11 pages REVTeX 3.0 plus 4 figures appende
Lifting of Multiphase Degeneracy by Quantum Fluctuations
We study the effect of quantum fluctuations on the multiphase point of the
Heisenberg model with first- and second-neighbor competing interactions and
strong uniaxial spin anisotropy . By studying the structure of perturbation
theory we show that the multiphase degeneracy which exists for
(i.e., for the ANNNI model) is lifted and that the effect of quantum
fluctuations is to stabilize a sequence of phases of wavelength 4,6,8,...~.
This sequence is probably an infinite one. We also show that quantum
fluctuations can mediate an infinite sequence of layering transitions through
which an interface can unbind from a wall.Comment: 55 pages ReVTeX (encoded with uufiles) + 17 uuencoded figure
Step-wise responses in mesoscopic glassy systems: a mean field approach
We study statistical properties of peculiar responses in glassy systems at
mesoscopic scales based on a class of mean-field spin-glass models which
exhibit 1 step replica symmetry breaking. Under variation of a generic external
field, a finite-sized sample of such a system exhibits a series of step wise
responses which can be regarded as a finger print of the sample. We study in
detail the statistical properties of the step structures based on a low
temperature expansion approach and a replica approach. The spacings between the
steps vanish in the thermodynamic limit so that arbitrary small but finite
variation of the field induce infinitely many level crossings in the
thermodynamic limit leading to a static chaos effect which yields a
self-averaging, smooth macroscopic response. We also note that there is a
strong analogy between the problem of step-wise responses in glassy systems at
mesoscopic scales and intermittency in turbulent flows due to shocks.Comment: 50 pages, 18 figures, revised versio
Infinitesimal incommensurate stripe phase in an axial next-nearest-neighbor Ising model in two dimensions
An axial next-nearest-neighbor Ising (ANNNI) model is studied by using the
non-equilibrium relaxation method. We find that the incommensurate stripe phase
between the ordered phase and the paramagnetic phase is negligibly narrow or
may vanish in the thermodynamic limit. The phase transition is the second-order
transition if approached from the ordered phase, and it is of the
Kosterlitz-Thouless type if approached from the paramagnetic phase. Both
transition temperatures coincide with each other within the numerical errors.
The incommensurate phase which has been observed previously is a paramagnetic
phase with a very long correlation length (typically ). We could
resolve this phase by treating very large systems (),
which is first made possible by employing the present method.Comment: 12 pages, 10 figures. To appear in Phys.Rev.
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