Out-of-plane Ionicity versus In-plane Covalency Interplay and Electron-phonon Coupling in MgB2 Superconductor

Abstract Using the Density Functional Theory (DFT) within the Generalized Gradient Approximation (GGA) pseudopotential and plane wave basis method along with the frozen-phonon approach that starts from the ab initio evaluation of the total energy Etot of the solid with frozen-in atomic displacements, it is found that a superposition of A2u and the E2gvibrations modes is the key factor in the superconducting mechanism in MgB2 compound. Electron-Phonon coupling to these A2u and E2g phonon modes especially at the zone-boundary A point of the hexagonal Brilliouin zone leads to an interband hole charge transfer (and transfer back) between in-plane σ bond to the out-of-plane π bond along with an interatomic electron charge transfer (and transfer back) between the Magnesium s-states to the Boron out-of-plane pz-state. The direction of the electronic current is opposite to that of hole current so that it reinforces the polarization associated with these currents and may generate a large dynamical charge at a given critical temperature Tc that drives the compound into the superconducting state.