Pressure dependence of the superconducting transition temperature in C6Yb and C6Ca

We have studied the evolution, with hydrostatic pressure, of the recently-discovered superconductivity in the graphite intercalation compounds C 6 Yb and C 6 Ca . We present pressure-temperature phase diagrams, for both superconductors, established by electrical transport and magnetization measurements. In the range 0–1.2GPa the superconducting transition temperature increases linearly with pressure in both materials with dT c ∕dP=+0.37±0.01 and +0.50±0.05K∕GPa for C 6 Yb and C 6 Ca , respectively. The transition temperature in C 6 Yb , which has been measured up to 2.3GPa , reaches a peak at around 1.8GPa and then starts to drop. We also discuss how this pressure dependence may be explained within a plasmon pairing mechanism.