371 research outputs found
Heisenberg antiferromagnets with uniaxial exchange and cubic anisotropies in a field
Classical Heisenberg antiferromagnets with uniaxial exchange anisotropy and a
cubic anisotropy term in a field on simple cubic lattices are studied with the
help of ground state considerations and extensive Monte Carlo simulations.
Especially, we analyze the role of non-collinear structures of biconical type
occurring in addition to the well-known antiferromagnetic and spin-flop
structures. Pertinent phase diagrams are determined, and compared to previous
findings.Comment: 14 pages, 8 figure
Wetting and interfacial adsorption in the Blume-Capel model on the square lattice
We study the Blume-Capel model on the square lattice. To allow for wetting
and interfacial adsorption, the spins on opposite boundaries are fixed in two
different states, "+1" and "-1", with reduced couplings at one of the
boundaries. Using mainly Monte Carlo techniques, of Metropolis and Wang-Landau
type, phase diagrams showing bulk and wetting transitions are determined. The
role of the non-boundary state, "0", adsorbed preferably at the interface
between "-1" and "+1" rich regions, is elucidated.Comment: 7 pages, 8 figures, minor corrections to previous versio
Critical Binder cumulant in two-dimensional anisotropic Ising models
The Binder cumulant at the phase transition of Ising models on square
lattices with various ferromagnetic nearest and next-nearest neighbour
couplings is determined using mainly Monte Carlo techniques. We discuss the
possibility to relate the value of the critical cumulant in the isotropic,
nearest neighbour and in the anisotropic cases to each other by means of a
scale transformation in rectangular geometry, to pinpoint universal and
nonuniversal features.Comment: 7 pages, 4 figures, submitted to J. Phys.
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
Critical Binder cumulant for isotropic Ising models on square and triangular lattices
Using Monte Carlo techniques, the critical Binder cumulant U* of isotropic
nearest-neighbour Ising models on square and triangular lattices is studied.
For rectangular shapes, employing periodic boundary conditions, U* is found to
show the same dependence on the aspect ratio for both lattice types. Similarly,
applying free boundary conditions for systems with square as well as circular
shapes for both lattices, the simulational findings are also consistent with
the suggestion that, for isotropic Ising models with short-range interactions,
U* depends on the shape and the boundary condition, but not on the lattice
structure.Comment: 7 pages, 4 figures, submitted to J. Stat. Mec
Quantum phase transitions of the diluted O(3) rotor model
We study the phase diagram and the quantum phase transitions of a
site-diluted two-dimensional O(3) quantum rotor model by means of large-scale
Monte-Carlo simulations. This system has two quantum phase transitions, a
generic one for small dilutions, and a percolation transition across the
lattice percolation threshold. We determine the critical behavior for both
transitions and for the multicritical point that separates them. In contrast to
the exotic scaling scenarios found in other random quantum systems, all these
transitions are characterized by finite-disorder fixed points with power-law
scaling. We relate our findings to a recent classification of phase transitions
with quenched disorder according to the rare region dimensionality, and we
discuss experiments in disordered quantum magnets.Comment: 11 pages, 14 eps figures, final version as publishe
Classical and quantum anisotropic Heisenberg antiferromagnets
We study classical and quantum Heisenberg antiferromagnets with exchange
anisotropy of XXZ-type and crystal field single-ion terms of quadratic and
cubic form in a field. The magnets display a variety of phases, including the
spin-flop (or, in the quantum case, spin-liquid) and biconical (corresponding,
in the quantum lattice gas description, to supersolid) phases. Applying
ground-state considerations, Monte Carlo and density matrix renormalization
group methods, the impact of quantum effects and lattice dimension is analysed.
Interesting critical and multicritical behaviour may occur at quantum and
thermal phase transitions.Comment: 13 pages, 14 figures, conferenc
Relaxation of Surface Profiles by Evaporation Dynamics
We present simulations of the relaxation towards equilibrium of one
dimensional steps and sinusoidal grooves imprinted on a surface below its
roughening transition. We use a generalization of the hypercube stacking model
of Forrest and Tang, that allows for temperature dependent
next-nearest-neighbor interactions. For the step geometry the results at T=0
agree well with the t^(1/4) prediction of continuum theory for the spreading of
the step. In the case of periodic profiles we modify the mobility for the tips
of the profile and find the approximate solution of the resulting free boundary
problem to be in reasonable agreement with the T=0 simulations.Comment: 6 pages, Revtex, 5 Postscript figures, to appear in PRB 15, October
199
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
- …