Anisotropic, Fermi-Surface-Induced Variation in T_c in MgB₂ Alloys

Using coherent-potential to describe disorder, Gaspari-Gyorffy approach to evaluate electron-phonon coupling, and Allen-Dynes equation to calculate T_c, we show that in Mg_{1-x}M_{x}B_{2} (M = Al,Li or Zn) alloys (i) the way T_c changes depends on the location of the added/modified k-resolved states on the Fermi surface and (ii) the variation of T_c as a function of concentration is dictated by the B p DOS. In addition, using full-potential calculations for MgMB_4, we show that (i) at x=0.5 a superstructure can form in Mg_{1-x}Al_{x}B_2 but not in Mg_{1-x}Li_{x}B_2 or Mg_{1-x}Zn_{x}B_2, and (ii) B layer shifts towards the impurity layer, more for Al than for Li or Zn.