Finite temperature fluctuation-induced order and responses in magnetic topological insulators

TL;DR

This paper develops an effective field theory for magnetic topological insulators, predicting persistent surface gaps and temperature-dependent responses, with quantitative predictions for specific heterostructures using combined theoretical methods.

Contribution

It introduces a novel effective field theory approach that predicts finite temperature effects and surface magnetic gaps in topological insulators, supported by DFT and Monte Carlo simulations.

Findings

Surface magnetic gap persists above bulk ordering temperature.

Up to 15% increase in Néel temperature at MnBi₂Te₄ surface.

Temperature and chemical potential dependence of topological magnetoelectric effects.

Abstract

We derive an effective field theory model for magnetic topological insulators and predict that a magnetic electronic gap persists on the surface for temperatures above the ordering temperature of the bulk. Our analysis also applies to interfaces of heterostructures consisting of a ferromagnetic and a topological insulator. In order to make quantitative predictions for MnBi$_2$Te$_4$, and for EuS-Bi$_2$Se$_3$ heterostructures, we combine the effective field theory method with density functional theory and Monte Carlo simulations. For MnBi$_2$Te$_4$ we predict an upwards N\'eel temperature shift at the surface up to $15 \%$, while the EuS-Bi$_2$Se$_3$ interface exhibits a smaller relative shift. The effective theory also predicts induced Dzyaloshinskii-Moriya interactions and a topological magnetoelectric effect, both of which feature a finite temperature and chemical potential dependence.