Small exciton effective mass in QL Bi2Se2Te: A material platform towards high-temperature excitonic condensate

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.