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We have studied the electron-phonon and superconducting properties of the Mg 1−x Al x B 2 and MgB 2(1−y) C 2y alloys within the framework of density functional theory using the self-consistent virtual-crystal approximation. For both alloys, the Eliashberg spectral functions and the electronphonon coupling constants have been calculated in the two-band model for several concentrations up to x(Al)= 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 ∆ σ , ∆ π , and T c for both alloys on a quantitative level. In particular, the observed differences in the doping range of superconductivity between Al 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.

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Superconducting thin films of composition Mg 1−x Al x B 2 with 0 Յ x Ͻ 0.5 were prepared in situ by sublimation of Mg combined with B and Al magnetron sputtering. The critical temperature T c decreased linearly with x up to 0.4. For 0.4Ͻ x Ͻ 0.5 the formation of a plateaulike feature at a T c Ϸ 12 K was observed. This effect is supposed to be due to the incipient formation of the superstructure MgAlB 4 with ordered alternating Mg and Al planes separated by B planes. To detailedly study the influence of Al doping on the electron-phonon coupling in the polycrystalline films with a preferred c-axis texture quasiparticle tunneling experiments were performed on planar tunnel junctions with natural thermal oxide or artificial aluminum oxide tunnel barriers. Differential conductance measurements at low-bias voltage and low temperature of superconductor-insulatorsuperconductor tunnel junctions allowed the direct determination of the energy gap of the Fermi surface sheet. The energy gap decreased linearly with decreasing T c of the films in agreement with the model of band filling. Whereas all the tunneling studies published so far mainly revealed features of the two energy gaps, the observation of phonon-induced structures in the differential conductance measurements at high bias voltage, i.e., in the phonon region, in this study enabled the important determination of the energy-dependent Eliashberg function ␣ 2 F of the Fermi surface sheet for various Al doping levels. Compared to the undoped MgB 2 , significant changes in ␣ 2 F could be observed that were confirmed by first-principles calculations.

We have studied the complete phonon dispersion, electron–phonon and superconducting properties of the Nb1−xMox alloy within the framework of density functional perturbation theory using a mixed-basis pseudopotential method and the self-consistent virtual-crystal approximation. Complete phonon dispersions as a function of x were obtained in good agreement with experimental data, independent of the approximation used for the exchange–correlation functional. For the Eliashberg function α2F(ω) we found a shift of weight to higher frequencies as well as an overall reduction with increasing x up to x≈0.7; however, for x = 1 (pure Mo) the spectral weight for α2F(ω) increased again. We used the information of α2F(ω) to calculate and analyze the evolution of the average coupling strength λ(x) and the superconducting temperature Tc(x). The variation of λ(x) closely follows the variation of the electronic density of states at EF. For Tc(x) experimental values were well reproduced provided a proper interpolation scheme for the Coulomb pseudopotential μ*(x) was employed.