Nature of the Dirac gap modulation and surface magnetic interaction in axion antiferromagnetic topological insulator $${\hbox {MnBi}}_2 {\hbox {Te}}_4$$
A. M. Shikin, D. A. Estyunin, И. И. Климовских, S. O. Filnov, Eike F. Schwier, Shiv Kumar, K. Miyamoto, Taichi Okuda, A. Kimura, Kenta Kuroda, Koichiro Yaji, Sang Hun Shin, Yukiharu Takeda, Y. Saitoh, Ziya S. Aliev, Nazim Mamedov, И. Р. Амирасланов, М. Б. Бабанлы, M. M. Otrokov, С. В. Еремеев, Е. В. Чулков
Abstract
Abstract Modification of the gap at the Dirac point (DP) in axion antiferromagnetic topological insulator $${\hbox {MnBi}}_2 {\hbox {Te}}_4$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mtext>MnBi</mml:mtext> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mtext>Te</mml:mtext> <mml:mn>4</mml:mn> </mml:msub> </mml:mrow> </mml:math> 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 ( $$<20~ \hbox {meV}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo><</mml:mo> <mml:mn>20</mml:mn> <mml:mspace/> <mml:mtext>meV</mml:mtext> </mml:mrow> </mml:math> ) gaps at the DP in the ARPES dispersions, which remain open above the Neél temperature ( $$T_{\mathrm{N}} = 24.5~ \hbox {K}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>N</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>24.5</mml:mn> <mml:mspace/> <mml:mtext>K</mml:mtext> </mml:mrow> </mml:math> ). We propose that the gap above $$T_{\mathrm{N}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>N</mml:mi> </mml:msub> </mml:math> 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.