Prediction of phonon-mediated high temperature superconductivity in stoichiometric Li$_2$B$_3$C

The discovery of superconductivity in Magnesium Diborate (MgB$_2$) has stimulated great interest in the search of new superconductors with similar lattice structures. Unlike cuprate or iron-based superconductors, MgB$_2$ is indisputably a phonon-mediated high temperature superconductor. The emergence of high temperature superconductivity in this material results from the strong coupling between the boron $\sigma$-bonding electrons around the Fermi level and the bond-stretching optical phonon modes. Here we show, based on the first-principles calculations, that Li$_2$B$_3$C is such a good candidate of superconductor whose superconducting transition temperature (T$_c$) might be even higher than MgB$_2$. Li$_2$B$_3$C consists of alternating graphene-like boron-carbon layers and boron-boron layers with intercalated lithium atoms between them. Similar to MgB$_2$, Li$_2$B$_3$C is inherently metallic and possesses two $\sigma$- and two $\pi$-electron bands around the Fermi energy. The superconducting pairs are glued predominately by the strong interaction between boron $\sigma$-bonding electrons and various optical phonon modes.