Electronic and dynamical properties of the MgB2 surface: Implications for the superconducting properties

We report on a detailed first-principles local-density-functional investigation of the structural, electronic and dynamical properties of a MgB2 free surface in both unreconstructed ideal terminations and in the case of Na coverage. Our study shows that in the case of a B-terminated surface the density of states at the Fermi level is significantly larger than in the bulk, a behavior that might lead to an enhancement of superconducting properties. We find the Mg-terminated surface to be the most stable structure in the whole range admitted by the chemical potentials, in agreement with very recent experimental results. The Na-covered surface is just slightly less stable than the Mg-terminated one in B-rich conditions and cannot be ruled out as a potentially relevant metastable structure. We rationalize the thermodynamical trends and the dynamical properties of the surfaces considered in terms of the B-sigma state filling; in particular, we find that the larger the filling the more stable the surface and the higher the frequency of the vibrational modes localized at the crystal surface.