Potassium Concentration Dependence of the Superconductivity in the Potassium Graphite Intercalation Compounds

Abstract

Magnetic susceptibilities have been measured at temperatures down to 60 mK by AC Hartshorn bridge method for the potassium graphite intercalation compounds with several compositions (C 8.0 K ∼C 21.6 K) prepared by the use of HOPG. T c is nearly constant to be ∼150 mK for C 8.0 K ∼C 14.7 K, while no transition is found down to 60 mK for C 16.7 K and C 21.6 K. As the K concentration is decreased, the superconducting characteristic changes from type II to type I for the magnetic field H parallel to the layers, while it maintains type I for H perpendicular to the layers. The superconducting fluctuation above T c , the effective mass ratio and the curvature of H c2 versus T curve are investigated; all of them lead to the result that 3D character increases with the decrease of the K concentration. The constancy of T c above the critical K concentration is also discussed.