Phase transition for the interchange and quantum Heisenberg models on the Hamming graph

TL;DR

This paper investigates phase transitions in permutation models on the Hamming graph, revealing the emergence of macroscopic cycles at critical points and connecting these findings to the quantum Heisenberg ferromagnet's magnetic properties.

Contribution

It introduces a novel analysis of cycle structures in permutation models on the Hamming graph, identifying phase transition thresholds and applying cyclic random walk techniques.

Findings

Existence of a phase transition in cycle sizes depending on transposition count

Precise critical time identified for the interchange process

Phase transition implies spontaneous magnetization in the quantum Heisenberg ferromagnet

Abstract

We study a family of random permutation models on the Hamming graph $H(2,n)$ (i.e., the $2$-fold Cartesian product of complete graphs), containing the interchange process and the cycle-weighted interchange process with parameter $\theta > 0$. This family contains the random walk representation of the quantum Heisenberg ferromagnet. We show that in these models the cycle structure of permutations undergoes a \textit{phase transition} -- when the number of transpositions defining the permutation is $\leq c n^2$, for small enough $c > 0$, all cycles are microscopic, while for more than $\geq C n^2$ transpositions, for large enough $C > 0$, macroscopic cycles emerge with high probability. We provide bounds on values $C,c$ depending on the parameter $\theta$ of the model, in particular for the interchange process we pinpoint exactly the critical time of the phase transition. Our results imply also the existence of a phase transition in the quantum Heisenberg ferromagnet on $H(2,n)$, namely for low enough temperatures spontaneous magnetization occurs, while it is not the case for high temperatures. At the core of our approach is a novel application of the cyclic random walk, which might be of independent interest. By analyzing explorations of the cyclic random walk, we show that sufficiently long cycles of a random permutation are uniformly spread on the graph, which makes it possible to compare our models to the mean-field case, i.e., the interchange process on the complete graph, extending the approach used earlier by Schramm.