Random-cluster dynamics in $\mathbb Z^2$: rapid mixing with general boundary conditions

TL;DR

This paper proves that the Glauber dynamics for the random-cluster model on $ ext{Z}^2$ mixes rapidly under most boundary conditions, with some exceptions where mixing is slow, advancing understanding of phase transition dynamics.

Contribution

It establishes polynomial mixing times for a broad class of boundary conditions and identifies conditions leading to slow mixing in the random-cluster model.

Findings

Rapid mixing ($O(n^2 ext{ log } n)$) for realizable boundary conditions.

Near-optimal mixing time ($ ilde O(n^2)$) for typical boundary conditions.

Slow (stretched-exponential) mixing for certain non-realizable boundary conditions at low $p$.

Abstract

The random-cluster model with parameters $(p,q)$ is a random graph model that generalizes bond percolation ($q=1$) and the Ising and Potts models ($q\geq 2$). We study its Glauber dynamics on $n\times n$ boxes $\Lambda_{n}$ of the integer lattice graph $\mathbb Z^2$, where the model exhibits a sharp phase transition at $p=p_c(q)$. Unlike traditional spin systems like the Ising and Potts models, the random-cluster model has non-local interactions. Long-range interactions can be imposed as external connections in the boundary of $\Lambda_n$, known as boundary conditions. For select boundary conditions that do not carry long-range information (namely, wired and free), Blanca and Sinclair proved that when $q>1$ and $p\neq p_c(q)$, the Glauber dynamics on $\Lambda_n$ mixes in optimal $O(n^2 \log n)$ time. In this paper, we prove that this mixing time is polynomial in $n$ for every boundary condition that is realizable as a configuration on $\mathbb Z^2 \setminus \Lambda_{n}$. We then use this to prove near-optimal $\tilde O(n^2)$ mixing time for "typical'' boundary conditions. As a complementary result, we construct classes of non-realizable (non-planar) boundary conditions inducing slow (stretched-exponential) mixing at $p\ll p_c(q)$.