19 research outputs found
Dynamics of the transverse Ising model with next-nearest-neighbor interactions
We study the effects of next-nearest-neighbor (NNN) interactions on the dynamics of the one-dimensional spin-1/2 transverse Ising model in the high-temperature limit. We use exact diagonalization to obtain the time-dependent transverse correlation function and the corresponding spectral density for a tagged spin. Our results for chains of 13 spins with periodic boundary conditions produce results which are valid in the infinite-size limit. In general we find that the NNN coupling produces slower dynamics accompanied by an enhancement of the central mode behavior. Even in the case of a strong transverse field, if the NNN coupling is sufficiently large, then there is a crossover from collective mode to central mode behavior. We also obtain several recurrants for the continued fraction representation of the relaxation function
Probability Distribution Function of the Order Parameter: Mixing Fields and Universality
We briefly review the use of the order parameter probability distribution
function as a useful tool to obtain the critical properties of statistical
mechanical models using computer Monte Carlo simulations. Some simple discrete
spin magnetic systems on a lattice, such as Ising, general spin- Blume-Capel
and Baxter-Wu, -state Potts, among other models, will be considered as
examples. The importance and the necessity of the role of mixing fields in
asymmetric magnetic models will be discussed in more detail, as well as the
corresponding distributions of the extensive conjugate variables.Comment: 14 pages, 13 figures, accepted for publication (Computer Physics
Communications
Percolation on two- and three-dimensional lattices
In this work we apply a highly efficient Monte Carlo algorithm recently
proposed by Newman and Ziff to treat percolation problems. The site and bond
percolation are studied on a number of lattices in two and three dimensions.
Quite good results for the wrapping probabilities, correlation length critical
exponent and critical concentration are obtained for the square, simple cubic,
HCP and hexagonal lattices by using relatively small systems. We also confirm
the universal aspect of the wrapping probabilities regarding site and bond
dilution.Comment: 15 pages, 6 figures, 3 table
Charging Effects and Quantum Crossover in Granular Superconductors
The effects of the charging energy in the superconducting transition of
granular materials or Josephson junction arrays is investigated using a
pseudospin one model. Within a mean-field renormalization-group approach, we
obtain the phase diagram as a function of temperature and charging energy. In
contrast to early treatments, we find no sign of a reentrant transition in
agreement with more recent studies. A crossover line is identified in the
non-superconducting side of the phase diagram and along which we expect to
observe anomalies in the transport and thermodynamic properties. We also study
a charge ordering phase, which can appear for large nearest neighbor Coulomb
interaction, and show that it leads to first-order transitions at low
temperatures. We argue that, in the presence of charge ordering, a non
monotonic behavior with decreasing temperature is possible with a maximum in
the resistance just before entering the superconducting phase.Comment: 15 pages plus 4 fig. appended, Revtex, INPE/LAS-00
Scaling and universality in the phase diagram of the 2D Blume-Capel model
We review the pertinent features of the phase diagram of the zero-field
Blume-Capel model, focusing on the aspects of transition order, finite-size
scaling and universality. In particular, we employ a range of Monte Carlo
simulation methods to study the 2D spin-1 Blume-Capel model on the square
lattice to investigate the behavior in the vicinity of the first-order and
second-order regimes of the ferromagnet-paramagnet phase boundary,
respectively. To achieve high-precision results, we utilize a combination of
(i) a parallel version of the multicanonical algorithm and (ii) a hybrid
updating scheme combining Metropolis and generalized Wolff cluster moves. These
techniques are combined to study for the first time the correlation length of
the model, using its scaling in the regime of second-order transitions to
illustrate universality through the observed identity of the limiting value of
with the exactly known result for the Ising universality class.Comment: 16 pages, 7 figures, 1 table, submitted to Eur. Phys. J. Special
Topic
Critical behavior of the Ising and Blume-Capel models on directed two-dimensional small-world networks
The critical properties of the two-dimensional Ising and Blume-Capel model on directed small-world lattices with quenched connectivity disorder are investigated. The disordered system is simulated by applying the Monte Carlo method with heat bath update algorithm and histogram re-weighting techniques. The critical temperature, as well as the critical exponents are obtained. For both models the critical parameters have been obtained for several values of the rewiring probability p. It is found that these disorder systems do not belong to the same universality class as two-dimensional ferromagnetic model on regular lattices. In particular, the Blume-Capel model, with zero crystal field interaction, on a directed small-world lattice presents a second-order phase transition for p pc, where pc ≈ 0.25. The critical exponents for p < pc are different from those of the same model on a regular lattice, but are identical to the exponents of the Ising model on directed small-world lattice
A new mean-field-like renormalization group transformation
A new real space renormalization group transformation combining ideas from mean-field and finite-size scaling theories is presented. Application to the two-dimensional site directed percolation problem gives better values for the percolation threshold and critical exponent of longitudinal correlation length than those obtained previously with other real space renormalization group approaches. When applied to one-, two- and three-dimensional) Ising models, the results are comparable to the ones from previous mean-field-like renormalization group transformations. This method can easily be applied to other systems having second-order phase transitions