Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Unoccupied Topological Surface State in MnBi$_2$Te$_4$   The unoccupied part of the band structure in the magnetic topological
insulator MnBi$_2$Te$_4$ 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 $\Gamma$ 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
$\sigma_{xy}(\omega)$, 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.

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The categories of this paper are: q-fin
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The categories of this paper are: stat
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The categories of this paper are: solv-int
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The categories of this paper are: acc-phys
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The categories of this paper are: hep-ex
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The categories of this paper are: comp-gas
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The categories of this paper are: chem-ph
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The categories of this paper are: chao-dyn
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The categories of this paper are: supr-con
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The categories of this paper are: astro-ph
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The categories of this paper are: hep-lat
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The categories of this paper are: nlin
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The categories of this paper are: physics
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The categories of this paper are: mtrl-th
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The categories of this paper are: q-alg
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The categories of this paper are: patt-sol