Microwave losses of bulk CaC6

We report a study of the temperature dependence of the surface resistance R-s in the graphite intercalated compound (GIC) CaC6, where superconductivity at 11.5 K was recently discovered. Experiments are carried out using a copper dielectrically loaded cavity operating at 7 GHz in a 'hot finger' configuration. Bulk CaC6 samples have been synthesized from highly oriented pyrolytic graphite. Microwave data allow to extract unique information on the quasiparticle density and on the nature of pairing in superconductors. The analysis of R-s(T) confirms our recent experimental findings that CaC6 behaves as a weakly-coupled, fully gapped, superconductor. (C) 2007 Elsevier B.V. All rights reserved.