Protection of Correlation-Induced Phase Instabilities by Exceptional Susceptibilities

TL;DR

This paper reveals that non-Hermitian spectral degeneracies, called exceptional points, in generalized susceptibilities are crucial for stabilizing correlation-induced phase instabilities like phase separation near Mott transitions.

Contribution

It uncovers the inherent non-Hermitian symmetries in susceptibilities of many-electron systems and links exceptional points to thermodynamic instabilities.

Findings

Exceptional points occur in susceptibilities of Fermi-Hubbard models.

EPs are essential for stabilizing phase separation near Mott transitions.

Susceptibilities exhibit non-Hermitian matrix symmetries.

Abstract

At thermal equilibrium, we find that generalized susceptibilities encoding the static physical response properties of Hermitian many-electron systems possess inherent non-Hermitian (NH) matrix symmetries. This leads to the generic occurrence of exceptional points (EPs), i.e., NH spectral degeneracies, in the generalized susceptibilities of prototypical Fermi-Hubbard models, as a function of a single parameter such as chemical potential. We demonstrate that these EPs are necessary to promote correlation-induced thermodynamic instabilities, such as phase-separation occurring in the proximity of a Mott transition, to a topologically stable phenomenon.