Tunneling in CaC6 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 CaC6 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 CaC6, but it could also signal a strong anisotropy in the electron-phonon interaction.
C. Kurter et al., "Large Energy Gaps in CaC6 from Tunneling Spectroscopy: Possible Evidence of Strong-Coupling Superconductivity," Physical review B: Condensed matter and materials physics, vol. 76, no. 22, pp. 220502-1-220502-4, American Physical Society (APS), Dec 2007.
The definitive version is available at https://doi.org/10.1103/PhysRevB.76.220502
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