Dual origin of effective axion response

TL;DR

This paper reveals that effective axion responses in condensed matter and photonics originate from two distinct physical mechanisms, leading to dual-symmetric and standard axion electrodynamics with observable differences.

Contribution

It identifies and characterizes two separate physical origins of axion-like electromagnetic responses, introducing a dual-symmetric version and analyzing its unique optical properties.

Findings

Standard and dual-symmetric axion responses have distinguishable optical signatures.

A specific system exemplifies the dual-symmetric axion electrodynamics.

Experimental distinctions between the two axion response types are proposed.

Abstract

Effective axion fields in condensed matter and photonics are manifested as $\mathcal{P}$- and $\mathcal{T}$-odd contributions to the electromagnetic response. Here, we show that the phenomena previously attributed to the effective axion fields have two distinct physical origins. One of them corresponds to the standard axion electrodynamics, while another provides its dual-symmetric version having the same symmetry and featuring similar but distinguishable optical properties. We present an example system described by the dual-symmetric modification of axion electrodynamics, derive the key predictions and pinpoint experimentally observable distinctions between the two versions of axion-type response.