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.