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 Kremer¹ — ¹Max-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 (C_p). The electronic C_p 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 C_p, the anisotropy of upper critical fields (H_c2) 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.