Evidence of Strong-Coupled Superconductivity in CaC6 from Tunneling   Spectroscopy
  Point-contact tunneling on CaC$_6$ crystals reproducibly reveals superconducting gaps, $\Delta$, of 2.3$\pm$0.2 meV which are $\sim$~40% larger than earlier reports. That puts CaC$_6$ into the class of very strong-coupled superconductors since 2$\Delta$/kT$_c\sim$~4.6. Thus soft Ca phonons will be primarily involved in the superconductivity, a conclusion that explains the large Ca isotope effect found recently for CaC$_6$. Consistency among superconductor-insulator-normal metal (SIN), SIS and Andreev reflection (SN) junctions reinforces the intrinsic nature of this result. 
C. Kurter, L. Ozyuzer, Daniel Mazur, J.F. Zasadzinski, D. Rosenmann,   H. Claus, D.G. Hinks, K.E. Gray