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 T_c. The only structural difference appears to be that the β phase (characterized by a c-axis lattice constant I_c=10.83 Å) is consistently found in the lower-T_c samples, while the higher-T_c samples always contain only the α phase (I_c=10.24 Å). Adding a minute amount of hydrogen to the T_c=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-T_c 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 H_c2(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 T_c.