Infinite volume Gibbs states and metastates of the random field mean-field spherical model

TL;DR

This paper investigates the infinite volume Gibbs states and metastates of the mean-field spherical model with random external fields, revealing three phases and the universal structure of metastates due to Gaussian fluctuations.

Contribution

It provides a comprehensive analysis of the Gibbs states and metastates of the mean-field spherical model under general random external fields, including the construction and characterization of non-trivial metastates.

Findings

Existence of three distinct phases: ferromagnetic, paramagnetic, and spin glass.

Unique Gibbs states in ferromagnetic and paramagnetic phases.

Chaotic size dependence and non-trivial metastates in the spin glass phase.

Abstract

For the discrete random field Curie-Weiss models, the infinite volume Gibbs states and metastates have been investigated and determined for specific instances of random external fields. In general, there are not many examples in the literature of non-trivial limiting metastates for discrete or continuous spin systems. We analyze the infinite volume Gibbs states of the mean-field spherical model, a model of continuous spins, in a general random external field with independent identically distributed components with finite moments of some order larger than four and non-vanishing variances of the second moments. Depending on the parameters of the model, we show that there exist three distinct phases: ordered ferromagnetic, ordered paramagnetic, and spin glass. In the ordered ferromagnetic and ordered paramagnetic phases, we show that there exists a unique infinite volume Gibbs state almost surely. In the spin glass phase, we show the existence of chaotic size dependence, provide a construction of the Aizenman-Wehr metastate, and consider both the convergence in distribution and almost sure convergence of the Newman-Stein metastates. The limiting metastates are non-trivial and their structure is universal due to the presence of Gaussian fluctuations and the spherical constraint.