Anisotropic superconductivity inC4KHg

Superconducting transition temperatures between 0.7 and 1.5 K are reported for the stage-1 potassium-mercury graphite intercalation compound (GIC) ${\mathrm{C}}_4$KHg. A number of experiments have been performed to investigate the differences between the ${\mathrm{C}}_4$KHg 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 ${\mathrm{C}}_4$KHg shows extended linearity of $H_{c2}$(T) for both types of samples. The critical field data of ${\mathrm{C}}_4$KHg 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 $T_c$.