Quantum decoherence by magnetic fluctuations in a magnetic topological insulator

TL;DR

This paper uses nonlinear optical spectroscopy to study magnetic fluctuations and order in a magnetic topological insulator, revealing a dimensional crossover and domain structures relevant to topological quantum phenomena.

Contribution

It demonstrates the application of magnetic dipole second harmonic generation to probe magnetic fluctuations and order in a candidate axion insulator, revealing a dimensional crossover and symmetry breaking.

Findings

Dimensional crossover from 2D to 3D magnetic correlations at the Néel temperature.

Breaking of rotational symmetry within the antiferromagnetic state.

Mapping of spatial magnetic domain structures.

Abstract

In magnetic topological insulators, spontaneous time-reversal symmetry breaking by intrinsic magnetic order can gap the topological surface spectrum, resulting in exotic properties like axion electrodynamics, the quantum anomalous Hall effect, and other topological magnetoelectric responses. Understanding the magnetic order and its coupling to topological states is essential to harness these properties. Here, we leverage near-resonant magnetic dipole optical second harmonic generation to probe magnetic fluctuations in the candidate axion insulator EuSn$_2$(As,P)$_2$ across its antiferromagnetic phase boundary. We observe a pronounced dimensional crossover in the quantum decoherence induced by magnetic fluctuations, whereby two-dimensional in-plane ferromagnetic correlations at high temperatures give way to three-dimensional long-range order at the N\'eel temperature. We also observe the breaking of rotational symmetry within the long-range-ordered antiferromagnetic state and map out the resulting spatial domain structure. More generally, we demonstrate the unique capabilities of nonlinear optical spectroscopy to study quantum coherence and fluctuations in magnetic quantum materials.