  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) 31 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.
