Superconductivity in the Graphite Intercalation CompoundBaC6

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 ${\mathrm{Ba}\mathrm{C}}_{6}$, 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 ${T}_{C}=65\text{ }\text{ }\mathrm{mK}$. 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.