Anisotropic superconductivity in graphite intercalation compound YbC6

We report anisotropy of the upper critical field (Bc2) of an intercalated graphite superconductor YbC6 (Tc = 6.5 K) determined from angular dependent magnetoresistance measurements. Though the perpendicular coherence length is much longer than interlayer spacing, measured angular dependences of Bc2 are well fitted by the Lawrence–Doniach model or the Tinkham model, which are known to be applicable to quasi two-dimensional materials or thin films, rather than the effective mass model. This observation is similar to the measurements for the other intercalated graphite superconductor, CaC6, by Jobiliong et al. [E. Jobiliong, H.D. Zhou, J.A. Janik, Y.-J. Jo, L. Balicas, J.S. Brooks, C.R. Wiebe, Phys. Rev. B 76 (2007) 052511]. A possible explanation for the unexpected applicability of these models is that our YbC6 samples are synthesized as thin flakes in the host graphite.