Superconductivity in heavy alkaline-earth intercalated graphites

We report the discovery of superconductivity below 1.65(6) K in Sr-intercalated graphite SrC6, by susceptibility and specific heat (C-p) measurements. In comparison with CaC6, we found that the anisotropy of the upper critical fields for SrC6 is much reduced. The C-p anomaly at T-c is smaller than the BCS prediction, indicating an anisotropic superconducting gap for SrC6 similar to CaC6. The significantly lower T-c of SrC6 as compared to CaC6 can be understood in terms of negative pressure effects, which decreases the electron-phonon coupling for both in-plane intercalant and the out-of-plane C phonon modes. We observed no superconductivity for BaC6 down to 0.3 K.