Study of the pressure-induced second superconducting phase of (NH₃)_yCs_0.4FeSe with double-dome superconductivity

The pressure dependence of the crystal and electronic structures of (NH₃)_yCs_0.4FeSe, which has two pressure-induced superconducting domes of the SC1 and SC2 phases, was investigated by x-ray diffraction and emission spectroscopy at low temperatures. We found a pressure-induced change in the crystal structure from tetragonal (T) to collapsed tetragonal (cT) and peculiar pressure dependence of the magnetic moment between the SC1 and SC2 phases at a low temperature. The electronic structure also markedly changed between the two phases. The results suggest that the increase in the electron–electron correlation accompanying the increase in the magnetic moment may relate the high T_c in the SC2 phase. Theoretical calculations using the multi-orbital Hubbard model revealed that the spin susceptibility decreases in the SC1 phase and increases in the SC2 phase with the T → cT phase transition. (author)