Superconductivity in (NH3)yCs0.4FeSe.

Alkali-metal-intercalated FeSe materials, (NH3)yCs0.4FeSe (M: K, Rb, and Cs), have been synthesized using the liquid NH3 technique. (NH3)yCs0.4FeSe shows a superconducting transition temperature (Tc) as high as 31.2 K, which is higher by 3.8 K than the Tc of nonammoniated Cs0.4FeSe. The Tcs of (NH3)yCs0.4FeSe and (NH3)yCs0.4FeSe are almost the same as those of nonammoniated K0.4FeSe and Rb0.4FeSe. The Tc of (NH3)yCs0.4FeSe shows a negative pressure dependence. A clear correlation between Tc and lattice constant c is found for ammoniated metal-intercalated FeSe materials, suggesting a correlation between Fermi-surface nesting and superconductivity. [ABSTRACT FROM AUTHOR]