Extremality of translation-invariant phases for a finite-state SOS-model on the binary tree

TL;DR

This paper classifies translation-invariant phases of a finite-state SOS model on a binary tree, analyzing their extremality and non-extremality, with explicit bounds on phase transitions based on interaction strength.

Contribution

It provides a complete classification of phases and their extremality properties for the SOS model on a Cayley tree, including explicit transition bounds.

Findings

Uniqueness of phases in antiferromagnetic case

Existence of up to seven phases in ferromagnetic case

Identification of extremal and non-extremal phases and their transition points

Abstract

We consider the SOS (solid-on-solid) model, with spin values $0,1,2$, on the Cayley tree of order two (binary tree). We treat both ferromagnetic and antiferromagnetic coupling, with interactions which are proportional to the absolute value of the spin differences. We present a classification of all translation-invariant phases (splitting Gibbs measures) of the model: We show uniqueness in the case of antiferromagnetic interactions, and existence of up to seven phases in the case of ferromagnetic interactions, where the number of phases depends on the interaction strength. Next we investigate whether these states are extremal or non-extremal in the set of all Gibbs measures, when the coupling strength is varied, whenever they exist. We show that two states are always extremal, two states are always non-extremal, while three of the seven states make transitions between extremality and non-extremality. We provide explicit bounds on those transition values, making use of algebraic properties of the models, and an adaptation of the method of Martinelli, Sinclair, Weitz.