Bulk superconductivity in Pb-substituted BiS2 -based compounds studied by hard x-ray spectroscopy

In this study, we investigate the bulk electronic structure of Pb-substituted LaO0.5F0.5BiS2 single crystals, using two types of hard x-ray spectroscopy. High-energy-resolution fluorescence-detected x-ray absorption spectroscopy revealed a spectral change at low temperatures. Using density functional theory (DFT) simulations, we find that the temperature-induced change originates from a structural phase transition, similar to the pressure-induced transition in LaO0.5F0.5BiS2. This finding suggests that the mechanism of bulk superconductivity induced by Pb substitution is the same as that under high pressure. Furthermore, a low-valence state with a mixture of divalent and trivalent Bi ions is discovered using hard x-ray photoemission spectroscopy with the aid of DFT calculations.