Multi-state condensation in Berlin-Kac spherical models

TL;DR

This paper analyzes the behavior of the Berlin-Kac spherical model at supercritical densities with multiple global minima, showing how condensation occurs and how the condensate's fluctuations become independent of the normal fluid as the lattice size grows.

Contribution

It extends the understanding of condensation phenomena in the spherical model to cases with multiple minima, demonstrating the independence of condensate fluctuations from the normal fluid in large lattices.

Findings

Condensation occurs at supercritical densities with multiple minima.

The condensate's fluctuations are independent of the normal fluid in large lattices.

Normal fluid converges to the critical Gaussian free field.

Abstract

We consider the Berlin-Kac spherical model for supercritical densities under a periodic lattice energy function which has finitely many non-degenerate global minima. Energy functions arising from nearest neighbour interactions on a rectangular lattice have a unique minimum, and in that case the supercritical fraction of the total mass condenses to the ground state of the energy function. We prove that for any sufficiently large lattice size this also happens in the case of multiple global minima, although the precise distribution of the supercritical mass and the structure of the condensate mass fluctuations may depend on the lattice size. However, in all of these cases, one can identify a bounded number of degrees of freedom forming the condensate in such a way that their fluctuations are independent from the rest of the fluid. More precisely, the original Berlin-Kac measure may be replaced by a measure where the condensate and normal fluid degrees of freedom become independent random variables, and the normal fluid part converges to the critical Gaussian free field. The proof is based on a construction of a suitable coupling between the two measures, proving that their Wasserstein distance is small enough for the error in any finite moments of the field to vanish as the lattice size is increased to infinity.