Correlation between electronic structure and emergence of superconductivity in Bi2-xSbxTe3-ySey (y similar to 1.2) studied by x-ray emission spectroscopy and density functional theory

Chemical composition and pressure dependencies of the electronic structures of Bi2-xSbxTe3-ySey (y similar to 1.2) have been studied by the high-resolution x-ray absorption spectroscopy. We find a shift of the Bi-L-3 absorption edge due to Sb substitution, suggesting a change in the Bi charge state. In the pressure dependence, the electronic structures of Bi2Te2Se and Bi1.5Sb0.5Te2Se start to change below the pressure of the first structural phase transition where the emergence of the superconductivity was observed and then show a large change just around that pressure. This is in contrast to the behavior observed in Bi2Se3-based compounds where the structural phase transition was necessary for the onset of superconductivity. We performed density functional theory calculations using the experimentally determined structures for Bi2Te2Se and Bi1.5Sb0.5Te2Se. We show that both compounds become metallic within the rhombohedral phase below the pressure of the first structural transition and we thus corroborate the experimental observation. The experimental and calculated results show that closing the gap and increasing the density of states in the rhombohedral phase are the triggers to induce superconductivity.