Superconductivity in the Graphite Intercalation Compound BaC₆

Among many two-dimensional (2D) high T_C superconductors, graphite intercalation compounds (GICs) are the most famous intercalation family, which are classified as typical electron-phonon mediated superconductors. We show unambiguous experimental facts that BaC₆, the superconductivity of which has been missing for many years so far among various alkaline earth metal (Ca, Sr, and Ba) intercalted GICs, exhibits superconductivity at T_C = 65 mK. By adding this finding as the additional experimental point, a complete figure displaying the relationship between T_C and interlayer distance (d) for GICs is now provided, and their possible superconducting mechanisms raised so far are revisited. The present study settles a long-running debate between theories and experiments on the superconductivity in the first stage GICs.