Far-infrared signature of the superconducting gap in intercalated graphite CaC6

Terahertz reflectance spectra of the Ca-intercalated graphite CaC6 reveal a superconducting gap below 11 K. The gap signature lacks a sharp onset to full reflectivity at 2 Delta(0) but rather shows a distribution of gap values consistent with an anisotropic gap. The experimental data were successfully fitted to the gap distribution obtained from density-functional calculations of Sanna [Phys. Rev. B 75, 020511(R) (2007)]. The temperature dependence of the superconducting gap is characteristic for a BCS type superconductor.