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

Abstract

The pressure dependence of superconductivity in alkaline earth metal-doped FeSe, (NH3) y AE x FeSe (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 T c and 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 T c and layer distance has been investigated thus far for FeSe doped with a monovalent metal atom, i.e., alkaline metal-doped FeSe ((NH3) y A x FeSe (A: Na, K, Rb, and Cs)). The T c value of (NH3) y A x FeSe increased with an increase in the lattice constant c, which corresponds to twice of the FeSe layer distance. In (NH3) y AE x FeSe (AE: Ca, Sr, and Ba), T c values decreased with pressures up to 10 GPa, and superconductivity was not clearly observed above 10 GPa. Pressure-dependent X-ray diffraction of (NH3) y AE x FeSe indicated no structural phase transitions up to ∼25 GPa, with a rapid decrease in c with pressure and saturation above 10 GPa. T c–c plots of (NH3) y AE x FeSe were drawn from T c–p and c–p plots obtained in this study. Moreover, we analyze T c–c plots of (NH3) y AE x FeSe based on those depicted for both monovalent and divalent atom-doped FeSe, demonstrating a clear linear relationship in the T c–c plot with similar slopes. Simply put, the different valence between monovalent and divalent atoms does not remarkably affect the c dependence of T c, and a universal rule that T c increases with an increase in c is unambiguously established for both compounds.