Detecting axion dynamics on the surface of magnetic topological insulators

TL;DR

This paper proposes using magnetic topological insulator surfaces to detect axion dynamics via two-photon decay, showing that surface emissions are significantly stronger and more detectable than bulk signals.

Contribution

It introduces a novel approach to observe axion-like phenomena by exploiting surface states of magnetic topological insulators and calculates the expected photon flux for experimental detection.

Findings

Surface photon flux is in-plane and orders of magnitude larger than bulk flux.

Two-photon decay signatures are detectable with current microwave technology.

Surface axion dynamics are more observable due to the vanishing bulk gap.

Abstract

Axions, initially proposed to solve the strong CP problem, have recently gained attention in condensed matter physics, particularly in topological insulators. However, detecting axion dynamics has proven challenging, with no experimental confirmations to date. In this study, we identify the surface of magnetic topological insulators as an ideal platform for observing axion dynamics. The vanishing bulk gap at the surface allows for order $O(1)$ variations in the axion field, making the detection of axion-like phenomena more feasible. In contrast, these phenomena are strongly suppressed in the bulk due to the small magnetic exchange gap. We investigate two-photon decay as a signature of axion dynamics and calculate the branching ratio using a perturbative approach. Our findings reveal that the photon flux emitted from the surface is in-plane and orders of magnitude larger than that from the bulk, making it detectable with modern microwave technology. We also discuss potential material platforms for detecting axion two-photon decay and strategies to enhance the signal-to-noise ratio.