We present a theoretical investigation on the electron-phonon superconductivity of honeycomb $M{X}_2$ layered structures where $X$ is one element of group IV (C, Si, or Ge) and $M$ is an alkali or an alkaline-earth metal. Among the studied compositions we predict a $T_{\mathrm{C}}$ of 7 K in ${\mathrm{RbGe}}_2$, 9 K in ${\mathrm{RbSi}}_2$, and 11 K in ${\mathrm{SrC}}_2$. All these compounds feature a strongly anisotropic superconducting gap. Our results show that despite the different doping levels and structural properties, the three families of materials fall into a similar description of their superconducting behavior. This allows us to estimate an upper critical temperature of about 20 K for the class of intercalated group-IV structures, including intercalated graphite and doped graphene.

• Received 9 October 2014
• Revised 9 December 2014

DOI:https://doi.org/10.1103/PhysRevB.91.054508