Superconductivity of Metallic Boron inMgB2

Boron in ${\mathrm{MgB}}_2$ forms stacks of honeycomb layers with magnesium as a space filler. Band structure calculations indicate that Mg is substantially ionized, and the bands at the Fermi level derive mainly from B orbitals. Strong bonding with an ionic component and considerable metallic density of states yield a sizable electron-phonon coupling. Together with high phonon frequencies, which we estimate via zone-center frozen phonon calculations to be between 300 and $700{\mathrm{cm}}^{-1\mathrm{}}$, this produces a high critical temperature, consistent with recent experiments. Thus ${\mathrm{MgB}}_2$ can be viewed as an analog of the long sought, but still hypothetical, superconducting metallic hydrogen.