Pressure Dependence of Superconductivity in Alkaline Earth Metal-Doped FeSe: toward Completion of the Phase Diagram of Superconducting Transition Temperature VersusFeSe Layer Distance

The pressure dependence of superconductivity in alkaline earth metal-doped FeSe, (NH3)yAExFeSe (AE: Ca, Sr, and Ba), prepared by the liquid ammonia (NH3) technique was investigated over a wide pressure range, in order to clarify the correlation between superconducting transition temperature Tcand FeSe layer distance. Doping with an alkaline earth metal atom implies the preparation of FeSe doped with a divalent metal atom. The correlation between the Tcand layer distance has been investigated thus far for FeSe doped with a monovalent metal atom, i.e., alkaline metal-doped FeSe ((NH3)yAxFeSe (A: Na, K, Rb, and Cs)). The Tcvalue of (NH3)yAxFeSe increased with an increase in the lattice constant c, which corresponds to twice of the FeSe layer distance. In (NH3)yAExFeSe (AE: Ca, Sr, and Ba), Tcvalues decreased with pressures up to 10 GPa, and superconductivity was not clearly observed above 10 GPa. Pressure-dependent X-ray diffraction of (NH3)yAExFeSe indicated no structural phase transitions up to ∼25 GPa, with a rapid decrease in cwith pressure and saturation above 10 GPa. Tc–cplots of (NH3)yAExFeSe were drawn from Tc–pand c–pplots obtained in this study. Moreover, we analyze Tc–cplots of (NH3)yAExFeSe based on those depicted for both monovalent and divalent atom-doped FeSe, demonstrating a clear linear relationship in the Tc–cplot with similar slopes. Simply put, the different valence between monovalent and divalent atoms does not remarkably affect the cdependence of Tc, and a universal rule that Tcincreases with an increase in cis unambiguously established for both compounds.