Strong Coupling Superconductivity in Ca-Intercalated Bilayer Graphene on SiC
Xutao Wang, Ningning Liu, Yanfu Wu, Yueqiao Qu, Wenxuan Zhang, Jinyue Wang, Dandan Guan, Shiyong Wang, Hao Zheng, Yaoyi Li, Canhua Liu, Jinfeng Jia
Abstract
The metal-intercalated bilayer graphene has a flat band with a high density of states near the Fermi energy and thus is anticipated to exhibit an enhanced strong correlation effect and associated fascinating phenomena, including superconductivity. By using a self-developed multifunctional scanning tunneling microscope, we succeeded in observing the superconducting energy gap and diamagnetic response of a Ca-intercalated bilayer graphene below a critical temperature of 8.83 K. The revealed high value of gap ratio, 2Δ/kBTc ≈ 5.0, indicates a strong coupling superconductivity, while the variation of penetration depth with temperature and magnetic field indicates an isotropic s-wave superconductor. These results provide crucial experimental clues for understanding the origin and mechanism of superconductivity in carrier-doped graphene.