Superconductivity in Ca-intercalated bilayer graphene: C₂CaC₂

The deposition and intercalation of metal atoms can induce superconductivity in monolayer and bilayer graphenes. For example, it has been experimentally proved that Li-deposited graphene is a superconductor with critical temperature T_c of 5.9 K, Ca-intercalated bilayer graphene C₆CaC₆ and K-intercalated epitaxial bilayer graphene C₈KC₈ are superconductors with T_c of 2-4 K and 3.6 K, respectively. However, the T_c of them are relatively low. To obtain higher T_c in graphene-based superconductors, here we predict a new Ca-intercalated bilayer graphene C₂CaC₂, which shows higher Ca concentration than the C₆CaC₆. It is proved to be thermodynamically and dynamically stable. The electronic structure, electron-phonon coupling (EPC) and superconductivity of C₂CaC₂ are investigated based on first-principles calculations. The EPC of C₂CaC₂ mainly comes from the coupling between the electrons of C-p orbital and the high- and low-frequency vibration modes of C atoms. The calculated EPC constant λ of C₂CaC₂ is 0.75, and the superconducting T_c is 18.9 K, which is much higher than other metal-intercalated bilayer graphenes. By further applying -4% biaxial compressive strain to C₂CaC₂, the T_c can be boosted to 26.6 K. Thus, the predicted C₂CaC₂ provides a new platform for realizing superconductivity with the highest T_c in bilayer graphenes.