Emergence of double-dome superconductivity in ammoniated metal-doped FeSe

The pressure dependence of the superconducting transition temperature ( T c ) and unit cell metrics of tetragonal (NH 3 ) y Cs 0.4 FeSe were investigated in high pressures up to 41 GPa. The T c decreases with increasing pressure up to 13 GPa, which can be clearly correlated with the pressure dependence of c (or FeSe layer spacing). The T c vs . c plot is compared with those of various (NH 3 ) y M x FeSe (M: metal atoms) materials exhibiting different T c and c , showing that the T c is universally related to c . This behaviour means that a decrease in two-dimensionality lowers the T c . No superconductivity was observed down to 4.3 K in (NH 3 ) y Cs 0.4 FeSe at 11 and 13 GPa. Surprisingly, superconductivity re-appeared rapidly above 13 GPa, with the T c reaching 49 K at 21 GPa. The appearance of a new superconducting phase is not accompanied by a structural transition, as evidenced by pressure-dependent XRD. Furthermore, T c slowly decreased with increasing pressure above 21 GPa and at 41 GPa superconductivity disappeared entirely at temperatures above 4.9 K. The observation of a double-dome superconducting phase may provide a hint for pursuing the superconducting coupling-mechanism of ammoniated/non-ammoniated metal-doped FeSe.