Study of the Pressure-Induced Second Superconducting Phase of (NH3)yCs0.4FeSe with Double-Dome Superconductivity

The pressure dependence of the crystal and electronic structures of (NH3)(y)Cs0.4FeSe, 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.