Electronic band structures and excitonic properties of delafossites: A GW -BSE study

摘要

We report the band structures and excitonic properties of delafossites CuMO_2 (M=Al, Ga, In, Sc, Y, Cr) calculated using the state-of-the-art GW-BSE approach. We evaluate different levels of self-consistency of the GW approximations, namely G_0W_0, GW_0, GW, and QSGW, on the band structures and find that GW_0, in general, predicts the band gaps in better agreement with experiments considering the electron-hole effect. For CuCrO_2, the HSE wave function is used as the starting point for the perturbative GW_0 calculations, since it corrects the band orders wrongly predicted by PBE. The discrepancy about the valence band characters of CuCrO_2 is classified based on both HSE and QSGW calculations. The PBE wave functions, already good, are used for other delafossites. All the delafossites are shown to be indirect band gap semiconductors with large exciton binding energies, varying from 0.24 to 0.44 eV, in consistent with experimental findings. The excitation mechanisms are explained by examining the exciton amplitude projections on the band structures. Discrepancies compared with experiments are also addressed. The lowest and strongest exciton, mainly contributed from either Cu 3d → Cu 3p (Al, Ga, In) or Cu 3d → M 3d (M = Sc, Y, Cr) transitions, is always located at the L point of the rhombohedral Brillouin zone.