Nature of the Dirac gap modulation and surface magnetic interaction in axion antiferromagnetic topological insulator {hbox {MnBi}}_2 {hbox {Te}}_4

A M Shikin,M M Otrokov,Y Saitoh,Eike F Schwier ,Akio Kimura ,S O Filnov,Yukiharu Takeda ,Shik Shin ,Kenta Kuroda ,И Р Амирасланов ,Nazim Mamedov ,Koji Miyamoto ,И И Климовских ,D A Estyunin,М Б Бабанлы ,Shiv Kumar ,Takashi Okuda ,Koichiro Yaji ,С В Еремеев ,Ziya S Aliev ,Eugene V Chulkov

doi:10.1038/s41598-020-70089-9

Abstract

Modification of the gap at the Dirac point (DP) in axion antiferromagnetic topological insulator {hbox {MnBi}}_2 {hbox {Te}}_4 and its electronic and spin structure have been studied by angle- and spin-resolved photoemission spectroscopy (ARPES) under laser excitation at various temperatures (9–35 K), light polarizations and photon energies. We have distinguished both large (60–70 meV) and reduced (<20~ hbox {meV}) gaps at the DP in the ARPES dispersions, which remain open above the Neél temperature (T_{mathrm{N}} = 24.5~ hbox {K}). We propose that the gap above T_{mathrm{N}} remains open due to a short-range magnetic field generated by chiral spin fluctuations. Spin-resolved ARPES, XMCD and circular dichroism ARPES measurements show a surface ferromagnetic ordering for the “large gap” sample and apparently significantly reduced effective magnetic moment for the “reduced gap” sample. These observations can be explained by a shift of the Dirac cone (DC) state localization towards the second Mn layer due to structural disturbance and surface relaxation effects, where DC state is influenced by compensated opposite magnetic moments. As we have shown by means of ab-initio calculations surface structural modification can result in a significant modulation of the DP gap.

Highlights

Modification of the gap at the Dirac point (DP) in axion antiferromagnetic topological insulator MnBi2Te4 and its electronic and spin structure have been studied by angle- and spin-resolved photoemission spectroscopy (ARPES) under laser excitation at various temperatures (9–35 K), light polarizations and photon energies
We have shown that both a large ( 60 − 70 meV ) and significantly reduced (< 20 meV ) gap at the DP can be clearly distinguished using laser ARPES in the measured dispersion maps for different kinds of samples of the intrinsic AFM topological insulator MnBi2Te4
The first value is close to the predicted from theoretical calculation, while the second one is close to the “gapless”-like dispersion assumed for the samples with significantly rearranged surface structural ordering

Summary

Introduction

Modification of the gap at the Dirac point (DP) in axion antiferromagnetic topological insulator MnBi2Te4 and its electronic and spin structure have been studied by angle- and spin-resolved photoemission spectroscopy (ARPES) under laser excitation at various temperatures (9–35 K), light polarizations and photon energies We have distinguished both large (60–70 meV) and reduced (< 20 meV ) gaps at the DP in the ARPES dispersions, which remain open above the Neél temperature (T N = 24.5 K ). These effects, discovered in magnetic topological insulators (TIs), are very important for both fundamental science and future technological applications, like dissipation-less topological electronics and topological quantum computation They are accompanied by a predicted opening of a gap at the Dirac point (DP) arising as a result of the time reversal symmetry (TRS) breaking due to the induced magnetic ordering. The significant difference in the gap width observed for different samples of MnBi2Te4 and its weak dependence on the long-range magnetic order transition have not been satisfactorily explained so far

Methods

Results

Conclusion