2D and 3D quantum rotors in a crystal field: critical points, metastability, and reentrance

TL;DR

This paper reviews models of coupled quantum rotors with dipolar and quadrupolar potentials, focusing on reentrant phase transitions, entropy anomalies, and the effects of crystal fields on phase diagrams in two and three dimensions.

Contribution

It provides a comprehensive analysis of reentrant phase transitions and metastability in quantum rotor models, including the impact of crystal fields on phase behavior.

Findings

Reentrant phase transitions occur in both 2D and 3D rotor models.

Entropy anomalies are present regardless of phase transition occurrence.

Crystal fields can suppress phase transitions and influence metastable states.

Abstract

An overview of results of models of coupled quantum rotors is presented. We focus on rotors with dipolar and quadrupolar potentials in two and three dimensions, potentials which correspond to approximate descriptions of real molecules adsorbed on surfaces and in the solid phase. Particular emphasis is placed on the anomalous reentrant phase transition which occurs in both two and three-dimensional systems. The anomalous behaviour of the entropy, which accompanies the reentrant phase transition, is also analyzed and is shown to be present regardless if a phase transition is present or not. Finally, the effects of the crystal field on the phase diagrams are also investigated. In two-dimensions the crystal field causes the disappearance of the phase transition, and ordering takes place via a continuous increase in the value of the order parameter. This is also true in three dimensions for the dipolar potential. For the quadrupolar potential in three dimensions turning on the crystal field leads to the appearance of critical points where the phase transition ceases, and ordering occurs via a continuous increase in the order parameter. As the crystal field is increased the range of the coupling constant over which metastable states are found decreases.