Electron-Phonon Coupling And Two-Band Superconductivity Of Al- And C-Doped Mgb2

We have studied the electron-phonon and superconducting properties of the Mg(1-x)AlxB(2) and MgB2(1-y)C2y alloys within the framework of density-functional theory using the self-consistent virtual-crystal approximation. For both alloys, the Eliashberg spectral functions and the electron-phonon coupling constants have been calculated in the two-band model for several concentrations up to x(A1)=0.55 and y(C)=0.175. We solved numerically the two-band Eliashberg gap equations without considering interband scattering. Using a single parameter for the Coulomb pseudopotential, which was determined for the undoped compound, we were able to reproduce the experimental doping dependence of Delta(sigma), Delta(pi), and T-c for both alloys on a quantitative level. In particular, the observed differences in the doping range of superconductivity between A1 and C doping indicate a pronounced influence of the doping site, which can be explained naturally in the present approach without the need to invoke interband scattering, suggesting that this factor plays only a minor role.