Approximating the XY model on a random graph with a $q$-state clock model

TL;DR

This paper demonstrates that the $q$-state clock model rapidly approximates the XY model on random graphs, with exponential convergence of physical observables, enabling efficient simulations of spin glass systems.

Contribution

It provides an analytical and numerical analysis of the convergence rate of the $q$-state clock model to the XY model, showing exponential convergence independent of disorder distribution.

Findings

Exponential convergence of observables with increasing q

Reliable approximation of XY model with small q

Spontaneous replica symmetry breaking at any disorder level

Abstract

Numerical simulations of spin glass models with continuous variables set the problem of a reliable but efficient discretization of such variables. In particular, the main question is how fast physical observables computed in the discretized model converge toward the ones of the continuous model when the number of states of the discretized model increases. We answer this question for the XY model and its discretization, the $q$-state clock model, in the mean-field setting provided by random graphs. It is found that the convergence of physical observables is exponentially fast in the number $q$ of states of the clock model, so allowing a very reliable approximation of the XY model by using a rather small number of states. Furthermore, such an exponential convergence is found to be independent from the disorder distribution used. Only at $T=0$ the convergence is slightly slower (stretched exponential). Thanks to the analytical solution to the $q$-state clock model, we compute accurate phase diagrams in the temperature versus disorder strength plane. We find that, at zero temperature, spontaneous replica symmetry breaking takes place for any amount of disorder, even an infinitesimal one. We also study the one step of replica symmetry breaking (1RSB) solution in the low-temperature spin glass phase.