Superconductivity ofMgB2: Covalent Bonds Driven Metallic

A series of calculations on ${\mathrm{MgB}}_2$ and related isoelectronic systems indicates that the layer of ${\mathrm{Mg}}^{2+}$ ions lowers the nonbonding B $\pi$ ( $p_z$) bands relative to the bonding $\sigma$ ( ${\mathrm{sp}}_x{p}_y$) bands compared to graphite, causing $\sigma \to \pi$ charge transfer and $\sigma$ band doping of 0.13 holes/cell. Because of their two dimensionality the $\sigma$ bands contribute strongly to the Fermi level density of states. Calculated deformation potentials of $\Gamma$ point phonons identify the B bond stretching modes as dominating the electron-phonon coupling. Superconductivity driven by $\sigma$ band holes is consistent with the report of destruction of superconductivity by doping with Al.