Superconductivity of MgB₂: Covalent Bonds Driven Metallic

A series of calculations on MgB₂ and related isoelectronic systems indicates that the layer of Mg²⁺ ions lowers the nonbonding B π ( p_z) bands relative to the bonding σ ( sp_xp_y) bands compared to graphite, causing σ→π charge transfer and σ band doping of 0.13 holes/cell. Because of their two dimensionality the σ bands contribute strongly to the Fermi level density of states. Calculated deformation potentials of Γ point phonons identify the B bond stretching modes as dominating the electron-phonon coupling. Superconductivity driven by σ band holes is consistent with the report of destruction of superconductivity by doping with Al.