Valley-ContrastingLinear Dichroism and ExcitonicCondensation 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 <i>x</i>- and <i>y</i>-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.