Electron-phonon interaction in hole-doped MgB2C2

Based on density functional perturbation theory, we predict that hole-doped MgB2C2, which is isoelectronic and structurally similar to MgB2, has a strong electron-phonon coupling constant. By substituting Mg atoms with alkali metals (Li,Na), pristine insulating MgB2 turns metallic with holes in the sigma bands at the Fermi level. Calculation of the formation enthalphies show that hole-doped LixMg(1-x)B2C2 or NaxMg(1-x)B2C2 for x=0.125-0.25 might be synthesized experimentally under conditions of Mg deficiencies. We find that the contribution of the sigma bands to the electron-phonon coupling constant of Li0.125Mg0.875B2C2 is Lambda(sigma sigma)=0.91, due to the modulation of the sigma bands produced by stretching modes of the borocarbide hexagonal planes. Based on ab initio phonons and electron-phonon coupling constants, we estimate from the McMillan formula a value for superconductive T-c of 67 K (for mu(*)=0.1), higher than that of MgB2 mainly because of larger phonon frequencies (omega(ln)=777 cm(-1)).