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Berlin 2008 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 36: Superconductivity: Properties, Electronic Structure, Order Parameter

TT 36.2: Talk

Friday, February 29, 2008, 10:30–10:45, H 0104

Anisotropic s-wave superconductivity in graphite intercalation compounds: CaC6 and SrC6 — •Jun Sung Kim1, Lilia Boeri1, Feridoon Razavi2, and Reinhard Kremer11Max-Planck-Institut für Festkörperforschung, Heisenbergstraβe 1, D-70569 Stuttgart, Germany — 2Department of Physics, Brock University, St. Catharines, Ontario, L2S 3A1, Canada

We have investigated the anisotropy of the superconducting properties for newly-discovered superconducting graphite intercalation compounds, CaC6 and SrC6 using specific heat (Cp). The electronic Cp for CaC6 shows an exponential temperature dependence at low temperatures, consistent with a fully gapped s-wave superconducting order parameter. However, the detailed comparison with an isotropic superconducting gap model shows significant deviation between experiment and theory. From the magnetic field dependence of Cp, the anisotropy of upper critical fields (Hc2) for CaC6 is ∼ 5, consistent with that obtained from the magnetic field dependence of Sommerfeld coefficient, but much larger than that of SrC6. In comparison with electronic structure calculations, we found that the isotropic gap model cannot explain observed superconducting properties, suggesting significant anisotropy in the superconducting gap for both CaC6 and SrC6. Recent investigations on a directional point-contact spectroscopy on CaC6 along the c-axis and ab-plane will also be discussed.

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