Anisotropic excitons in bulk 1T'-ReSe2: Origins, binding energy, and interaction with phonons

Polarization-resolved reflectance (R), absorption, and photoreflectance spectroscopy reveal several excitonlike resonances around the band gap of bulk 1T'-ReSe2 at low temperatures. Some show strong linear polarization anisotropy, with oscillator strength maximized for polarizations roughly parallel to the a-axis or b-axis. Electronic structure calculations based on density functional theory, which include electron-hole Coulomb interaction via the GW+Bethe-Salpeter equation, show that bulk 1T'-ReSe2 has a direct band gap at the Z-point of the Brillouin zone, with a single valence and conduction band, each being doubly spin degenerate. We find two prominent sets of excitons that arise around the Z-point. Each set has four excitons that are spin-orbit-couplinginduced mixtures of triplet and singlet states, all of which acquire finite oscillator strength. By comparison with calculated polarization-resolved exciton absorption spectra, we identify origins of five resonances and the band gap. The binding energy Eb of the two dominant lowest energy excitons was estimated to be <^>110 meV and <^>86 meV. However, despite the large Eb, the exciton resonances in R and associated photoluminescence (PL) emission both decay by 200 K (kBT <^> 17.3 meV). The combination of Raman, temperature-dependent R and PL spectroscopy suggests that an unusually strong exciton-optical-phonon interaction leads to this rapid thermal decay of excitons in 1T'-ReSe2.