Superconductivity of MgB2: Covalent Bonds Driven Metallic
Abstract
A series of calculations on MgB2 and related isoelectronic systems indicates that the layer of Mg(2+) ions lowers the non-bonding B "pi" (pz) bands relative to the bonding "sigma" (spxpy) bands compared to graphite, causing sigma --> pi charge transfer and sigma band doping of 0.13 holes/cell. Due to 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.
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