Location of the valence band maximum in the band structure of anisotropic 1T′−ReSe2

Transition-metal dichalcogenides (TMDCs) are a focus of current research due to their fascinating optical and electronic properties with possible technical applications. ${\mathrm{ReSe}}₂$ is an interesting material of the TMDC family, with unique anisotropic properties originating from its distorted $1T$ structure ($1T$ '). To develop a fundamental understanding of the optical and electric properties, we studied the underlying electronic structure with angle-resolved photoemission (ARPES) as well as band-structure calculations within the density functional theory (DFT)--local density approximation (LDA) and GdW approximations. We identified the $\overline{\mathrm{\ensuremath{\Gamma}}}\phantom{\rule{0.28453pt}{0ex}}{\overline{M}}₁$ direction, which is perpendicular to the $a$ axis, as a distinct direction in k space with the smallest bandwidth of the highest valence band. Using photon-energy-dependent ARPES, two valence band maxima are identified within experimental limits of about 50 meV: one at the high-symmetry point $Z$, and a second one at a non-high-symmetry point in the Brillouin zone. Thus, the position in k space of the global valence band maximum is undecided experimentally. Theoretically, an indirect band gap is predicted on a DFT-LDA level, while quasiparticle corrections lead to a direct band gap at the $Z$ point.