  Using first-principles simulations combined with many-body calculations, we
show that two-dimensional free-standing quintuple-layer Bi2Se2Te is an
inversion symmetric monolayer expected to achieve spatially indirect exciton
with large exciton radius, small exciton effective mass and long exciton
lifetime. Such system is theoretically predicted to be a promising platform for
realizing excitonic Bose-Einstein condensation and superfluid due to its high
phase transition temperatures of ~257 K and ~64.25 K for the BEC and excitonic
superfluid, respectively. The importance of spin-orbit coupling is revealed,
and the angular momentum selection rules for photon absorption are discussed.
This finding suggests the potential of QL Bi2Se2Te monolayer with exotic
bosonic bound states provides as a tantalizing high-temperature platform to
probe excitonic physics.
