Large energy gaps in Ca C6 from tunneling spectroscopy: Possible evidence of strong-coupling superconductivity

Abstract

Tunneling in Ca C6 crystals reproducibly reveals superconducting gaps Δ of 2.3±0.2 meV that are ∼40% larger than reported earlier. In an isotropic s -wave scenario, that puts Ca C6 into the class of very strongly coupled superconductors, since 2Δ k Tc ∼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 Ca C6, but it could also signal a strong anisotropy in the electron-phonon interaction.