  The variation of the superconducting critical temperature Tc as a function of
x in the diboride Mg1-xAlxB2 has been studied in the framework of the two-bands
Eliashberg theory and traditional phonon coupling mechanism. We have solved the
two-bands Eliashberg equations using first-principle calculations or simple
assumptions for the variation of the relevant physical quantities. We have
found that the experimental Tc curve can be explained only if the Coulomb
pseudopotential changes with x by tuning the Fermi level toward the sigma band
edge. In polycrystal samples the x dependence of the sigma and pi-band gap has
been found and is in agreement with experiments.
