Axion instability and non-linear electromagnetic effect

TL;DR

This paper studies how dynamical axion fields can cause electromagnetic instabilities near topological phase transitions in insulators, revealing conditions that enhance or suppress these effects.

Contribution

It provides a detailed analysis of axion instability bounds and electromagnetic responses near topological phase transitions using two model Hamiltonians.

Findings

Bound on axion amplitude in magnetic insulators suppresses instability

Large axion fluctuations near phase transition lower critical electric field

Increased axion-induced magnetic flux density near transition

Abstract

We investigate the instability due to dynamical axion field near the topological phase transition of insulators. We first point out that the amplitude of dynamical axion field is bounded for magnetic insulators in general, which suppresses the axion instability. Near the topological phase transition, however, the axion field may have a large fluctuation, which decreases the critical electric field for the instability and increases the axion induced magnetic flux density. Using two different model Hamiltonians, we report the electromagnetic response of the axion field in details.