High-Temperature Excitonic Bose-Einstein Condensate in Centrosymmetric Two-Dimensional Semiconductors

The realization of high-temperature excitonic Bose-Einstein condensation (BEC) in practical materials poses great challenges, because of strict constraints in symmetry, exciton binding, lifetime, and interaction. Here, using first-principles methods and symmetry analysis, we propose a new route to realize high-temperature excitonic BEC in centrosymmetric 2D materials, exploiting the parity symmetry of band edges and reduced Coulomb screening. We demonstrate it by taking monolayer TiS₃ as an example, whose lowest-energy exciton shows small exciton mass, small Bohr radius, large binding, and long lifetime simultaneously. The phase diagram of electron-hole systems is further constructed, showing that both BEC and superfluidity can be realized at high temperature and in a broad range of exciton density. Importantly, we reveal that the high-temperature character of excitonic BEC is robust against thickness, beneficial for its experimental observation. By application of this general strategy to 2D materials in the database, monolayer AuBr and BiS₂ are identified as promising candidates for high-temperature excitonic BEC.