Phase Coexistence for the Hard-Core Model on ${\mathbb Z}^2$

TL;DR

This paper proves the existence of multiple Gibbs measures for the hard-core model on the two-dimensional integer lattice when the activity parameter exceeds approximately 5.35, advancing understanding of phase coexistence beyond previous bounds.

Contribution

It provides the first non-trivial lower bound for phase coexistence in the hard-core model on ${f Z}^2$, using novel contour enumeration techniques and boundary condition distinctions.

Findings

Multiple Gibbs measures exist for  > 5.3506

Improved contour bounds via self-avoiding walks

Advances the phase coexistence threshold understanding

Abstract

The hard-core model has attracted much attention across several disciplines, representing lattice gases in statistical physics and independent sets in discrete mathematics and computer science. On finite graphs, we are given a parameter $\lambda$, and an independent set $I$ arises with probability proportional to $\lambda^{|I|}$. On infinite graphs a Gibbs measure is defined as a suitable limit with the correct conditional probabilities, and we are interested in determining when this limit is unique and when there is phase coexistence, i.e., existence of multiple Gibbs measures. It has long been conjectured that on ${\mathbb Z}^2$ this model has a critical value $\lambda_c \approx 3.796$ with the property that if $\lambda < \lambda_c$ then it exhibits uniqueness of phase, while if $\lambda > \lambda_c$ then there is phase coexistence. Much of the work to date on this problem has focused on the regime of uniqueness, with the state of the art being recent work of Sinclair, Srivastava, \v{S}tefankovi\v{c} and Yin showing that there is a unique Gibbs measure for all $\lambda < 2.538$. Here we give the first non-trivial result in the other direction, showing that there are multiple Gibbs measures for all $\lambda > 5.3506$. There is some potential for lowering this bound, but with the methods we are using we cannot hope to replace $5.3506$ with anything below about $4.8771$. Our proof begins along the lines of the standard Peierls argument, but we add two innovations. First, following ideas of Koteck\'y and Randall, we construct an event that distinguishes two boundary conditions and always has long contours associated with it, obviating the need to accurately enumerate short contours. Second, we obtain improved bounds on the number of contours by relating them to a new class of self-avoiding walks on an oriented version of ${\mathbb Z}^2$.