Obtaining pressure versus concentration phase diagrams in spin systems from Monte Carlo simulations

TL;DR

This paper introduces an efficient Monte Carlo simulation method combining cluster algorithms and a direct grand canonical potential calculation to accurately determine phase diagrams of spin systems, including first-order transitions.

Contribution

It presents a novel cluster algorithm that eliminates metastability and a technique for calculating thermodynamic limits in first-order phase transitions.

Findings

Accurate phase diagrams for the Andelman model of Langmuir monolayers.

Precise location of first-order transition lines.

Phase diagrams in surface pressure versus concentration plane.

Abstract

We propose an efficient procedure for determining phase diagrams of systems that are described by spin models. It consists of combining cluster algorithms with the method proposed by Sauerwein and de Oliveira where the grand canonical potential is obtained directly from the Monte Carlo simulation, without the necessity of performing numerical integrations. The cluster algorithm presented in this paper eliminates metastability in first order phase transitions allowing us to locate precisely the first-order transitions lines. We also produce a different technique for calculating the thermodynamic limit of quantities such as the magnetization whose infinite volume limit is not straightforward in first order phase transitions. As an application, we study the Andelman model for Langmuir monolayers made of chiral molecules that is equivalent to the Blume-Emery-Griffiths spin-1 model. We have obtained the phase diagrams in the case where the intermolecular forces favor interactions between enantiomers of the same type (homochiral interactions). In particular, we have determined diagrams in the surface pressure versus concentration plane which are more relevant from the experimental point of view and less usual in numerical studies.