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$_csim$~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.