Bulk superconductivity in Pb-substituted BiS$_{\bf 2}$-based compounds studied by hard-x-ray spectroscopy

In this study, we investigate the bulk electronic structure of Pb-substitutedLaO$_{0.5}$F$_{0.5}$BiS$_2$ single crystals, using two types of hard-x-rayspectroscopy. High-energy-resolution fluorescence-detected x-ray absorptionspectroscopy revealed a spectral change at low temperatures. Using densityfunctional theory (DFT) simulations, we find that the temperature-inducedchange originates from a structural phase transition, similar to thepressure-induced transition in LaO$_{0.5}$F$_{0.5}$BiS$_2$. This findingsuggests that the mechanism of bulk superconductivity induced by Pbsubstitution is the same as that under high pressure. Furthermore, a novellow-valence state with a mixture of divalent and trivalent Bi ions isdiscovered using hard x-ray photoemission spectroscopy with the aid of DFTcalculations.