  We have investigated the atomic structure of superconducting Ca-intercalated
bilayer graphene on a SiC(0001) substrate using total-reflection high-energy
positron diffraction. By comparing the experimental rocking-curves with ones
calculated for various structural models using a full-dynamical theory, we have
found that Ca atoms are intercalated in the graphene-buffer interlayer, rather
than between the two graphene layers. From transport measurements, the
superconducting transition was observed to be at Tc_onset = 4K for this
structure. This study is the first to clearly identify the relation between the
atomic arrangement and superconductivity in Ca-intercalated bilayer graphene.
