Anisotropic Superconducting Phase Diagram of C$_{6}$Ca

We present a study of the anisotropic superconducting phase diagram of the new carbon intercalation superconductor C$_{6}$Ca employing magnetization and specific heat measurements. The intercalation of Ca into flakes of natural graphite takes place through vapor transport at temperatures near 500 $^{0}$C. The resulting crystals display an onset of superconductivity at 11.3 K and a transition width of about 1 K as seen in heat capacity and low-field magnetization measurements. A clear step in the heat capacity confirms the bulk nature of the superconducting state. Roughly 1/3 to 1/2 of the sample volume is superconducting as evidenced by the reduced step height of the heat capacity. From measurements of the upper critical field, H$_{c2}$, we determined an in-plane coherence length of $\xi _{ab}$ $\approx $ 36 nm. The angular dependence of H$_{c2}$ is well described within the model of effective mass anisotropy yielding an anisotropy parameter of $\Gamma \quad \approx $ 3.5 to 4.