ANISOTROPIC SUPERCONDUCTIVITY IN C4KHG

Superconducting transition temperatures between 0.7 and 1.5 K are reported for the stage-1 potassium-mercury graphite intercalation compound (GIC) C4KHg. A number of experiments have been performed to investigate the differences between the C4KHg samples at either extreme in Tc. The only structural difference appears to be that the phase (characterized by a c-axis lattice constant Ic=10.83 A) is consistently found in the lower-Tc samples, while the higher-Tc samples always contain only the phase (Ic=10.24 A). Adding a minute amount of hydrogen to the Tc=0.8 K samples raises their transition temperature to 1.5 K. Measurements of the angular dependence of the critical-field suggest that the higher-Tc samples have type-I character for a range of applied field orientations near H c^. The temperature dependence of the upper critical field of C4KHg shows extended linearity of Hc2(T) for both types of samples. The critical field data of C4KHg and other GIC's are discussed in light of multiband and anisotropic Fermi-surface models of superconductivity. The thermodynamic parameters obtained from the critical-field experiments are compared to the specific-heat data of Alexander et al. A charge-density-wave hypothesis first proposed by DeLong and Eklund is suggested to explain the hydrogen-induced enhancement of Tc. © 1990 The American Physical Society.