Glauber dynamics for Ising models on random regular graphs: cut-off and metastability

Abstract

Consider random d-regular graphs, i.e., random graphs such that there are exactly d edges from each vertex for some d ≥ 3. We study both the configuration model version of this graph, which has occasional multi-edges and self-loops, as well as the simple version of it, which is a d-regular graph chosen uniformly at random from the collection of all d-regular graphs.In this paper, we discuss mixing times of Glauber dynamics for the Ising model with an external magnetic field on a random d-regular graph, both in the quenched as well as the annealed settings. Let β be the inverse temperature, β c be the critical temperature and B be the external magnetic field. Concerning the annealed measure, we show that for β > β c there exists Bc (β) ∈ (0, ∞) such that the model is metastable (i.e., the mixing time is exponential in the graph size n) when β > β c and 0 ≤ B < Bc (β), whereas it exhibits the cut-off phenomenon at c n log n with a window of order n when β < β c or β > β c and B > Bc (β). Interestingly, Bc (β) coincides with the critical external field of the Ising model on the d-ary tree (namely, above which the model has a unique Gibbs measure). Concerning the quenched measure, we show that there exists B c (β) with B c (β) ≤ Bc (β) such that for β > β c , the mixing time is at least exponential along some subsequence (n k ) k≥1 when 0 ≤ B < B c (β), whereas it is less than or equal to Cn log n when B > Bc (β). The quenched results also hold for the model conditioned on simplicity, for the annealed results this is unclear.