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Evidence of Strong-Coupled Superconductivity in CaC6 from Tunneling Spectroscopy

C. Kurter, L. Ozyuzer, Daniel Mazur, J. F. Zasadzinski, D. Rosenmann, H. Claus, D. G. Hinks, K. E. Gray

cond-mat.supr-conarXiv:cond-mat/0612581

Abstract

Point-contact tunneling on CaC6 crystals reproducibly reveals superconducting gaps, Δ, of 2.30.2 meV which are ~40% larger than earlier reports. That puts CaC6 into the class of very strong-coupled superconductors since 2Δ/kTc~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 CaC6. Consistency among superconductor-insulator-normal metal (SIN), SIS and Andreev reflection (SN) junctions reinforces the intrinsic nature of this result.

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