Effect of pressure on superconducting Ca-intercalated graphite CaC6

The effect of pressure on the superconducting transition temperature (T-c) of the Ca-intercalated graphite compound CaC6 has been investigated up to similar to 16 kbar. T-c is found to increase under pressure with a large relative ratio Delta T-c/T-c of approximate to 0.4%/kbar. Using first-principles calculations, we show that the positive effect of pressure on T-c can be explained within the scope of electron-phonon theory due to the presence of a soft phonon branch associated with in-plane vibrations of the Ca atoms. The implications of the present findings for the current debate about the superconducting mechanism in graphite intercalation compounds are discussed.