Superconductivity in (NH3)(y)Cs0.4FeSe

Alkali-metal-intercalated FeSe materials, (NH3)(y)M0.4FeSe (M: K, Rb, and Cs), have been synthesized using the liquid NH3 technique. (NH3)(y)Cs0.4FeSe shows a superconducting transition temperature (T-c) as high as 31.2 K, which is higher by 3.8 K than the T-c of nonammoniated Cs0.4FeSe. The T(c)s of (NH3)(y)K0.4FeSe and (NH3)(y)Rb0.4FeSe are almost the same as those of nonammoniated K0.4FeSe and Rb0.4FeSe. The T-c of (NH3)(y)Cs0.4FeSe shows a negative pressure dependence. A clear correlation between T-c and lattice constant c is found for ammoniated metal-intercalated FeSe materials, suggesting a correlation between Fermi-surface nesting and superconductivity.