Observation of anomalies in the pressure dependence of the superconducting transition temperature of potassium-based graphite intercalation compounds

Measurements of the pressure ($P$) dependence of the superconducting transition temperature $T_c$ of K${\mathrm{C}}_8$ and KHg${\mathrm{C}}_4$ are reported. K${\mathrm{C}}_8$ undergoes a first-order transition from a $T_c=0.13\mathrm{}$ K to a $T_c=1.7\mathrm{}$ K phase over the range $2<~P<~15$ kbar. A KHg${\mathrm{C}}_4$ sample underwent an anomalously broad superconducting transition at ambient pressure with an onset near 1.4 K, which is nearly double previously reported $T_c$ values. The KHg${\mathrm{C}}_4$ transition sharpened dramatically for $P\gtrsim 1$ kbar and exhibited a strongly negative $\frac{d{T}_c}{\mathrm{dP}}$ at higher pressures. K${\mathrm{C}}_{24}$ was found to be nonsuperconducting to $P=10\mathrm{}$ kbar and $T>~0.07$ K.