Valley-Contrasting Linear Dichroism and Excitonic Condensation in LaOBiS2

In conjunction with the first-principles calculations, we confirm the fascinating valley-selective linear dichroism and the possibility of excitonic condensation in multiatomic-layer LaOBiS2 of a tetragonal lattice by the effective tight-binding model and many-body perturbation theory. In LaOBiS2, which indicates a spontaneous valley polarization due to crystalline symmetry reduction rather than conventional time-reversal symmetry breaking, we unravel that the excitons in X and X' valleys exclusively coupled to x- and y-linearly polarized lights, respectively. In sharp contrast to coupled quantum wells and van der Waals architectures, a new type of spatially indirect exciton is identified in single-crystal LaOBiS2, manifesting itself as an engaging platform for investigating the excitonic Bose-Einstein condensation (BEC) and superfluidity. In light of large binding energy, large radius, and small effective mass of the exciton, the transition temperature for BEC and superfluidity achieves record-high values of 325.1 and 81.3 K, respectively.