Superconducting Calcium-Intercalated Bilayer Graphene

We report the direct evidence for superconductivity in Ca-intercalated bilayer graphene C₆CaC₆, which is regarded as the thinnest limit of Ca-intercalated graphite. We performed the electrical transport measurements with the in situ 4-point-probe method in ultrahigh vacuum under zero-or nonzero-magnetic field for pristine bilayer graphene, Li-intercalated bilayer graphene (C₆LiC₆) and C₆CaC₆ fabricated on SiC substrate. We observed that the zero-resistance state occurs in C₆CaC₆ with the onset temperature (T_c^onset) of 4 K, while the T_c^onset is gradually decreased upon applying the magnetic field. This directly proves the superconductivity origin of the zero resistance in C₆CaC₆. On the other hand, both pristine bilayer graphene and C₆LiC₆ exhibit nonsuperconducting behavior, suggesting the importance of intercalated atoms and its species to drive the superconductivity.