New insights into the electronic bands of antiferromagnetic topological insulator MnBi₂Te₄

As the first intrinsic magnetic topological insulator, MnBi₂Te₄ has been attracting a wide range of interest in the condensed matter community as it potentially provides a platform to realize high-temperature topological phases, such as axion insulators and quantum anomalous Hall insulators. However, despite an enormous amount of work using angle-resolved photoemission spectroscopy (ARPES), the electronic band structure of MnBi₂Te₄ remains a subject under intense debate. In this talk, we will present an in-depth investigation of the electronic structure of MnBi₂Te₄ utilizing a multi-resolution photoemission spectroscopy (MRPES) setup developed in our lab.^{1, 2} We first estimate an upper bound of 3 meV for the gap size of the topological surface state (TSS) using 6 eV laser-based ARPES. Measurements on the same sample using 21.2 eV photons yield a gap of 150 meV, which suggests a photon-energy-dependent matrix element effect. Furthermore, our circular dichroism measurement reveals an “anti-crossing” behavior between the TSS and other quasi-2D states, leading to a new understanding of band hybridization. A numerical calculation elucidates that the quasi-2D states potentially originate from the surface quantum confinement. Our study represents a solid step forward in reconciling the existing controversies in the electronic structure of MnBi₂Te₄, and provides a conceptual framework to understand the electronic structures of other relevant topological materials MnBi_2nTe_3n+1.³