arXiv subjects

A. Malakis

Publications and source records attributed to A. Malakis.

23 records · Page 2Linked to original sources

arXiv2008

Thermal critical behavior and universality aspects of the three-dimensional random-field Ising model

The three-dimensional bimodal random-field Ising model is investigated using the N-fold version of the Wang-Landau algorithm. The essential energy subspaces are determined by the recently developed critical minimum energy subspace technique, and two implementations of this scheme are utilized. The random fields are obtained from a bimodal discrete $(\pmΔ)$ distribution, and we study the model for various values of the disorder strength $Δ$, $Δ=0.5, 1, 1.5$ and 2, on cubic lattices with linear sizes $L=4-24$. We extract information for the probability distributions of the specific heat peaks over samples of random fields. This permits us to obtain the phase diagram and present the finite-size behavior of the specific heat. The question of saturation of the specific heat is re-examined and it is shown that the open problem of universality for the random-field Ising model is strongly influenced by the lack of self-averaging of the model. This property appears to be substantially depended on the disorder strength.

cond-mat.stat-mech↗

arXiv2008

Phase Diagram of the 3D Bimodal Random-Field Ising Model

The one-parametric Wang-Landau (WL) method is implemented together with an extrapolation scheme to yield approximations of the two-dimensional (exchange-energy, field-energy) density of states (DOS) of the 3D bimodal random-field Ising model (RFIM). The present approach generalizes our earlier WL implementations, by handling the final stage of the WL process as an entropic sampling scheme, appropriate for the recording of the required two-parametric histograms. We test the accuracy of the proposed extrapolation scheme and then apply it to study the size-shift behavior of the phase diagram of the 3D bimodal RFIM. We present a finite-size converging approach and a well-behaved sequence of estimates for the critical disorder strength. Their asymptotic shift-behavior yields the critical disorder strength and the associated correlation length's exponent, in agreement with previous estimates from ground-state studies of the model.

cond-mat.stat-mech↗

arXiv2006

Entropic sampling via Wang-Landau random walks in dominant energy subspaces

Dominant energy subspaces of statistical systems are defined with the help of restrictive conditions on various characteristics of the energy distribution, such as the probability density and the forth order Binder's cumulant. Our analysis generalizes the ideas of the critical minimum energy-subspace (CrMES) technique \cite{malakis04}, applied previously to study the specific heat's finite-size scaling. Here, we illustrate alternatives that are useful for the analysis of further finite-size anomalies and the behavior of the corresponding dominant subspaces is presented for the 2D Baxter-Wu, the 2D and 3D Ising models. In order to show that a CrMES technique is adequate for the study of magnetic anomalies, we study and test simple methods which provide the means for an accurate determination of the energy - order-parameter ($E-M$) histograms via Wang-Landau random walks. The 2D Ising model is used as a test case and it is shown that high-level Wang-Landau sampling schemes yield excellent estimates for all magnetic properties. Our estimates compare very well with those of the traditional Metropolis method. The relevant dominant energy subspaces and dominant magnetization subspaces scale as expected with exponents $α/ν$ and $γ/ν$ respectively. Using the Metropolis method we examine the time evolution of the corresponding dominant magnetization subspaces and we uncover the reasons behind the inadequacy of the Metropolis method to produce a reliable estimation scheme for the tail-regime of the order parameter distribution.

cond-mat.stat-mech↗

arXiv2005

Estimation of critical behavior from the density of states in Classical Statistical Models

We present a simple and efficient approximation scheme which greatly facilitates extension of Wang-Landau sampling (or similar techniques) in large systems for the estimation of critical behavior. The method, presented in an algorithmic approach, is based on a very simple idea, familiar in statistical mechanics from the notion of thermodynamic equivalence of ensembles and the central limit theorem. It is illustrated that, we can predict with high accuracy the critical part of the energy space and by using this restricted part we can extend our simulations to larger systems and improve accuracy of critical parameters. It is proposed that the extensions of the finite size critical part of the energy space, determining the specific heat, satisfy a scaling law involving the thermal critical exponent. The method is applied successfully for the estimation of the scaling behavior of specific heat of both square and simple cubic Ising lattices. The proposed scaling law is verified by estimating the thermal critical exponent from the finite size behavior of the critical part of the energy space. The density of states (DOS) of the zero-field Ising model on these lattices is obtained via a multi-range Wang-Landau sampling.

cond-mat.stat-mech↗

arXiv2005

Monte Carlo studies of the square Ising model with next-nearest-neighbor interactions

We apply a new entropic scheme to study the critical behavior of the square-lattice Ising model with nearest- and next-nearest-neighbor antiferromagnetic interactions. Estimates of the present scheme are compared with those of the Metropolis algorithm. We consider interactions in the range where superantiferromagnetic (SAF) order appears at low temperatures. A recent prediction of a first-order transition along a certain range (0.5-1.2) of the interaction ratio $(R=J_{nnn}/J_{nn})$ is examined by generating accurate data for large lattices at a particular value of the ratio $(R=1)$. Our study does not support a first-order transition and a convincing finite-size scaling analysis of the model is presented, yielding accurate estimates for all critical exponents for R=1 . The magnetic exponents are found to obey ``weak universality'' in accordance with a previous conjecture.

cond-mat.stat-mech↗