The unoccupied part of the band structure in the magnetic topological insulator MnBi2Te4 is studied by first-principles calculations. We find a second, unoccupied topological surface state with similar electronic structure to the celebrated occupied topological surface state. This state is energetically located approximate 1.6 eV above the occupied Dirac surface state around Γ point, which permit it to be directly observed by the two-photon angle-resolved photoemission spectroscopy. We propose a unified effective model for the occupied and unoccupied surface states. Due to the direct optical coupling between these two surface states, we further propose two optical effects to detect the unoccupied surface state. One is the polar Kerr effect in odd layer from nonvanishing ac Hall conductance σxy(ω), and the other is higher-order terahertz-sideband generation in even layer, where the non-vanishining Berry curvature of the unoccupied surface state is directly observed from the giant Faraday rotation of optical emission.
@article{arxiv.2108.09055,
title = {Unoccupied Topological Surface State in MnBi$_2$Te$_4$},
author = {Yadong Jiang and Zhaochen Liu and Jing Wang},
journal= {arXiv preprint arXiv:2108.09055},
year = {2022}
}