Effect of the short-range interaction on critical phenomena in elastic interaction systems

TL;DR

This paper investigates how short-range interactions influence phase transitions in elastic systems, revealing distinct effects on ferromagnetic-like and antiferromagnetic-like transitions, and mapping their phase diagrams.

Contribution

It provides a detailed phase diagram showing the impact of short-range interactions on elastic systems' phase transitions, highlighting different behaviors for FM and AFM phases.

Findings

Short-range interactions enhance FM order cooperatively with elastic long-range interactions.

Elastic interactions have minimal effect on AFM order, maintaining Ising universality.

In the phase diagram, AFM phases dominate at finite temperatures near the FM-AFM boundary.

Abstract

The elastic interaction, induced by the lattice distortion due to the difference of the molecular size, causes an effective long-range interaction. In spin-crossover (SC) compounds, local bistable states, i.e., high-spin and low-spin states have different molecular sizes, and the elastic interaction is important. In bipartite lattices, e.g., the square lattice, the ground state can be two types of phases: ferromagnetic-like (FM) and antiferromagnetic-like (AFM) phases. In systems like SC compounds, the former phase consists of all small or large molecules, and the latter phase has the configuration of alternating small and large molecules. In fact, both cases are observed in SC systems. In this paper, we have studied the effect of the short-range interaction in the elastic system on the properties of those order-disorder phase transitions. We have obtained a phase diagram in the coordinates of the temperature and the strength of the short-range interaction, including the metastable structures. We show that effects of the short-range interaction are essentially different for FM and AFM phase transitions. In the FM transition, the long-range interaction of elasticity is relevant, and the system exhibits a phase transition in the mean-filed universality class. In this case, the long-range interaction strongly enhances the FM order, and it works cooperatively with the short-range interaction. In contrast, in the AFM transition, the elastic interaction slightly enhances the AFM order, but essentially it does not contribute to the ordering, and the system shows a transition in the Ising universality class. We have found that in the border region between FM and AFM phases, the AFM phase has an advantage at finite temperatures. We discuss the critical properties of two-step SC transitions with comparison between the elastic interaction model and conventional SC models (Ising-like models).