Spontaneous Formation of Exceptional Points at the Onset of Magnetism

TL;DR

This paper demonstrates how symmetry-breaking fluctuations at the onset of magnetism can spontaneously create exceptional points in topological semimetals, leading to a novel non-Hermitian magnetic Weyl phase.

Contribution

It introduces a new mechanism for non-Hermitian topology via spontaneous EP formation at magnetic phase transitions in strongly correlated topological insulators.

Findings

Spontaneous formation of exceptional points at magnetic transition.

Emergence of a non-Hermitian Weyl phase on the surface of a topological insulator.

Numerical evidence from a multi-band Hubbard model using dynamical mean-field theory.

Abstract

We reveal how symmetry protected nodal points in topological semimetals may be promoted to pairs of generically stable exceptional points (EPs) by symmetry-breaking fluctuations at the onset of long-range order. This novel route to non-Hermitian (NH) topology is exemplified by a magnetic NH Weyl phase spontaneously emerging at the surface of a strongly correlated three-dimensional topological insulator when entering the ferromagnetic regime from a high temperature paramagnetic phase. Here, electronic excitations with opposite spin acquire significantly different life-times, thus giving rise to an anti-Hermitian structure in spin that is incompatible with the chiral spin texture of the nodal surface states, and hence facilitates the spontaneous formation of EPs. We present numerical evidence of this phenomenon by solving a microscopic multi-band Hubbard model non-perturbatively in the framework of dynamical mean-field theory.