Dynamic axion field in the magnetoelectric antiferromagnet chromia

TL;DR

This paper demonstrates the existence of a dynamic axion field in chromia, a magnetoelectric insulator, by experimentally measuring its non-linear magnetoelectric response and confirming its theoretical behavior.

Contribution

The study provides the first experimental evidence of a dynamic axion field in chromia through temperature-dependent non-linear magnetoelectric measurements.

Findings

The non-linear magnetoelectric response matches theoretical predictions.

Chromia's response couples strongly to quantum and thermal spin fluctuations.

Temperature dependence of the second harmonic signal confirms axion dynamics.

Abstract

Chromia is a magnetoelectric insulator whose bulk magnetoelectric susceptibility contains a non-zero pseudoscalar component, like that present in magnetized topological insulators. We reveal the dynamic axion field of chromia by measuring the temperature dependence of its non-linear magnetoelectric response using a lock-in technique with an AC electric field stimulus. The electric field dependence, magnetic field dependence, and temperature behavior of the second harmonic lock-in signal are all in agreement with theoretical expectations. Our results demonstrate that chromia magnetoelectric response couples strongly to its low-energy quantum and thermal spin fluctuations.