Superconductivity of alkali metal intercalated ZrSe2

Alkali metal intercalated ZrSe2 (Asub(x)ZrSe2) has been synthesized by the reaction of ZrSe2 single crystals with n-butyllithium in hexane solution, or alkali metal (Na, K, Rb, Cs) in hexamethylphosphoric triamide solution. Semiconductive ZrSe2 has been converted into a metallic substance by the alkali metal intercalation. Superconductivity has been found for Lisub(x)-, Nasub(x)-, Ksub(x)-, Rbsub(x)- and Cssub(x)ZrSe2 (0 < x <= 1) below 1.7, 2.0, 2.1, 1.7, and 1.4 K, respectively. The angular dependence of the upper critical fields Hsub(c2) for Asub(x)ZrSe2 is interpreted by the effective mass model. The Ginzburg-Landau coherence lengths perpendicular to the layer planes of the crystals at zero temperature are ten to twenty times larger than the layer repeat distances, showing a three dimensional character. Anisotropic superconductivity is discussed on the basis of the Fermi surface of Asub(x)ZrSe2 whose electron number is determined by the charge transfer from alkali metal atoms to ZrSe2. (Auth.)