Superconductivity of MgB2: Covalent bonds driven metallic

A series of calculations on MgB2 and related isoelectronic systems indicate s that the layer of Mg2+ ions lowers the nonbonding B pi (p(z)) bands relat ive to the bonding sigma (sp(x)p(y)) bands compared to graphite, causing si gma --> pi charge transfer and sigma band doping of 0.13 holes/cell. Becaus e of their two dimensionality the a bands contribute strongly to the Fermi level density of states. Calculated deformation potentials of Gamma point p honons identify the B bond stretching modes as dominating the electron-phon on coupling. Superconductivity driven by sigma band holes is consistent wit h the report of destruction of superconductivity by doping with Al.