Electronic structure of high-entropy R⁢OBiS2 (R=La, Ce, Pr, Nd, Sm, Gd) superconductors studied by soft x-ray absorption and photoemission spectroscopy

High-entropy alloy (HEA) superconductors are known to exhibit a robust superconducting state with adaptable thermomechanical properties. In this study, we have investigated the electronic structure of HEA-type ROBiS2 (R = rare earth) layered superconductors with mixing entropy using soft x-ray absorption (XAS) and x-ray photoemission spectroscopy (XPS) measurements. The electronic structure evolution differs between equivalent and nonequivalent R mixing as a function of mixing entropy. Ce M4,5-edge XAS and Ce 3d XPS reveal clear evidence of Ce3+/Ce4+ mixed valence in the HEA-type ROBiS2, similar to the pristine CeOBiS2 compound, suggesting a similar charge transfer mechanism responsible for self-doping in these materials. Unlike Ce, all other R atoms exhibit a nominal R3+ valence state across samples with varying mixing entropy. The self-doped charge is consistent with the Luttinger volume estimated by previous angle-resolved photoemission spectroscopy (ARPES) studies. The distinct behaviors observed in equivalent and nonequivalent mixing are discussed in terms of differing potential energy landscapes of the Bi sites manifested in the Bi 4 f XPS spectra, where intrinsic disorder is expected to have different characteristics of the superconductivity.