Combining the advantages of superconducting MgB2 and CaC6 in one material: suggestions from first-principles calculations

We show that a recently predicted layered phase of lithium monoboride, Li$_2$B$_2$, combines the key mechanism for strong electron-phonon coupling in MgB$_2$ (i.e., interaction of covalent B $\sigma$ bands with B bond-stretching modes) with the dominant coupling mechanism in CaC$_6$ (i.e., interaction of free-electron-like interlayer states with soft intercalant modes). Yet, surprisingly, the electron-phonon coupling in Li$_{2}$B$_{2}$ is calculated to be weaker than in either MgB$_2$ or CaC$_6$. This is due to the accidental absence of B $\pi$ states at the Fermi level in Li$_{2} $B$_{2}$. In MgB$_2$, the $\pi$ electrons play an indirect but important role in strengthening the coupling of $\sigma$ electrons. We discuss the use of doping to restore $\pi$ electrons at the Fermi level, which would enhance the coupling and the superconducting $T_c$.