Phonon-mediated superconductivity in aluminum-deposited graphene AlC 8

It has been theoretically predicted and experimentally confirmed that graphene deposited with atoms of a univalent alkali metal such as Li (${\mathrm{LiC}}_{6}$) or divalent alkaline-earth metal such as Ca (${\mathrm{CaC}}_{6}$) can be a superconductor. For atoms of a trivalent metal such as Al, if deposited on graphene, it was predicted that ${\mathrm{AlC}}_{8}$ can be in a metallic state. Whether this compound is stable and can be made superconducting is an issue which has not been addressed and deserves further investigation. In this work, based on first-principles calculations, it is found that the phonon spectrum of ${\mathrm{AlC}}_{8}$ shows imaginary frequencies for the two lowest branches, indicating the structure is dynamically unstable. By hole doping, the imaginary frequencies basically disappear and the lattice is stabilized. Besides, biaxial tensile strain was applied to study its effect on phonon and electron-phonon coupling. With the increase of tensile strain, the high-energy phonon spectrum associated with the C-C stretching modes softens greatly and the electron-phonon coupling becomes stronger, resulting in the increase of superconducting transition temperature ${T}_{c}$ to a value of more than 22 K for a sample at the experimentally accessible hole doping ($7.5\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$) and tensile strain ($12%$) levels. This is above the liquid hydrogen temperature of 20.3 K. Thus, besides Li and Ca deposited graphene, ${\mathrm{AlC}}_{8}$ provides another platform for realizing superconductivity in graphene.