Sample-dependent Dirac point gap in MnBi$_2$Te$_4$ and its response to the applied surface charge: a combined photoemission and ab initio study

TL;DR

This study investigates the variability of the Dirac point gap in MnBi$_2$Te$_4$ samples using photoemission spectroscopy and ab initio calculations, revealing its dependence on surface charge and potential for tuning.

Contribution

It provides the first comprehensive analysis of the sample-dependent Dirac gap in MnBi$_2$Te$_4$ and demonstrates how surface charge influences and can modulate this gap.

Findings

Dirac point gaps range from 15 to 65 meV in different samples.

Surface charge can significantly alter or close the Dirac gap.

The gap variability is linked to defects and surface conditions.

Abstract

Recently discovered intrinsic antiferromagnetic topological insulator MnBi$_2$Te$_4$ presents an exciting platform for realization of the quantum anomalous Hall effect and a number of related phenomena at elevated temperatures. An important characteristic making this material attractive for applications is its predicted large magnetic gap at the Dirac point (DP). However, while the early experimental measurements reported on large DP gaps, a number of recent studies claimed to observe a gapless dispersion of the MnBi$_2$Te$_4$ Dirac cone. Here, using micro($\mu$)-laser angle-resolved photoemission spectroscopy, we study the electronic structure of 15 different MnBi$_2$Te$_4$ samples, grown by two different chemists groups. Based on the careful energy distribution curves analysis, the DP gaps between 15 and 65 meV are observed, as measured below the N\'eel temperature at about 10-16 K. At that, roughly half of the studied samples show the DP gap of about 30 meV, while for a quarter of the samples the gaps are in the 50 to 60 meV range. Summarizing the results of both our and other groups, in the currently available MnBi$_2$Te$_4$ samples the DP gap can acquire an arbitrary value between a few and several tens of meV. Further, based on the density functional theory, we discuss a possible factor that might contribute to the reduction of the DP gap size, which is the excess surface charge that can appear due to various defects in surface region. We demonstrate that the DP gap is influenced by the applied surface charge and even can be closed, which can be taken advantage of to tune the MnBi$_2$Te$_4$ DP gap size.