Effects of the spin-orbit interaction on the optical properties of ReS2 and ReSe2

ReS(2 )and ReSe2 are less frequently studied transition metal dichalcogenides. They appear in the 1T' phase with a significantly reduced symmetry compared with, for example, MoS2, while inversion symmetry is preserved. Several broad peaks have been observed in previous experimental measurements. The interpretation of their physical origin and properties is a challenging task without the help of theoretical insights. Here, we employ ab initio GW-Bethe-Salpeter equation calculations to investigate the optical properties of neutral and charged monolayers. We present a detailed analysis of the low-energy excitons and trions. In agreement with experiment, we find that the neutral excitons are strongly bound. In contrast, only a tiny trion binding energy of <10 meV is found. By artificially manipulating the spin-orbit coupling (SOC), we demonstrate its importance for the internal structure of the transitions. Interestingly, we find two optically active low-energy excitons. Both stem from transitions between the same valence and conduction bands. In the absence of SOC, only one peak (singlet) can be bright. Therefore, additional peaks are a direct manifestation of SOC on the optical properties.