Direct Observation of Superconductivity in Calcium-Intercalated Bilayer Graphene by in situ Electrical Transport Measurements

We report the superconductivity in Ca-intercalated bilayer graphene C${}_6$CaC${}_6$, the thinnest limit of Ca graphite intercalation compound. We performed $\mathit{\text{in}}\mathit{\text{situ}}$ electrical transport measurements on pristine bilayer graphene, C${}_6$LiC${}_6$ and C${}_6$CaC${}_6$ fabricated on SiC substrate under zero and non-zero magnetic field. While both bilayer graphene and C${}_6$LiC${}_6$ show non-superconducting behavior, C${}_6$CaC${}_6$ exhibits the superconductivity with transition temperature ($T_{\mathrm{c}}$) of 4.0 K. The observed $T_{\mathrm{c}}$ in C${}_6$CaC${}_6$ and the absence of superconductivity in C${}_6$LiC${}_6$ show a good agreement with the theoretical prediction, suggesting the importance of a free-electron-like metallic band at the Fermi level to drive the superconductivity.