Beyond Eliashberg superconductivity in MgB2: Anharmonicity, two-phonon scattering, and multiple gaps - art. no. 087005

Density-functional calculations of the phonon spectrum and electron-phonon coupling in MgB2 are presented. The E-2g phonons, which involve in-plane B displacements, couple strongly to the p(x,y) electronic bands. The isotropi c electron-phonon coupling constant is calculated to be about 0.8. Allowing for different order parameters in different bands, the superconducting lam bda in the clean limit is calculated to be significantly larger. The E-2g p honons are strongly anharmonic, and the nonlinear contribution to the coupl ing between the E-2g modes and the p(x,y) bands is significant.