Large energy gaps in CaC6 from tunneling spectroscopy:: Possible evidence of strong-coupling superconductivity

Tunneling in CaC6 crystals reproducibly reveals superconducting gaps Delta of 2.3 +/- 0.2 meV that are similar to 40% larger than reported earlier. In an isotropic s-wave scenario, that puts CaC6 into the class of very strongly coupled superconductors, since 2 Delta/kT(c)similar to 4.6, implying that soft Ca phonons are primarily involved in the superconductivity. This conclusion explains the relatively large Ca isotope effect found recently for CaC6, but it could also signal a strong anisotropy in the electron-phonon interaction.