Far-infrared signature of a superconducting gap in intercalated graphite CaC$_{6}$.

CaC$_{6}$ is exceptional in the series of intercalated graphite compounds because of its high superconducting transition temperature, $T_{c}$=11.5K. The superconducting gap, 2$\Delta $=25.6 $\pm $ 3.2cm$^{-1}$, measured by scanning tunneling spectroscopy (N. Bergeal et al., PRL \textbf{97}, 077003 (2006)), is consistent with the weak-coupling BCS type superconductivity. The superconducting gap can be directly probed also by far-infrared spectroscopy. We studied the reflectance $R$ of CaC$_{6}$ between 4 and 100cm$^{-1}$ from 3K to 15K. We see the signature of the superconducting gap in the reflectance ratio of superconducting state $R_{s}$ to the normal state $R_{n }$and can follow its temperature dependence. The appearance of the gap signature in $R_{s}/R_{n}$ tells us that CaC$_{6}$ is in the dirty limit. Different models, including an anisotropic gap and a multi-gap scenario, will be discussed to fit the optical data.