Theoretical study of superconductivity in MgB2 and its alloys

Abstract

Using density-functional-based methods, we have studied the fully-relaxed, full-potential electronic structure of the newly discovered superconductor, MgB2, and BeB2, NaB2 and AlB2. Our results, described in terms of (i) total density of states (DOS) and (ii) the partial DOS around the Fermi energy, EF, clearly show the importance of B p-electrons for superconductivity. For BeB2 and NaB2, our results indicate qualitative similarities but significant quantitative differences in their electronic structure due to differences in the number of valence electrons and the lattice constants a and c. We have also studied Mg1-xMxB2 ((M ≡ Al, Li or Zn) alloys using coherent-potential to describe disorder, Gaspari-Gyorffy approach to evaluate electron-phonon coupling, and Allen-Dynes equation to calculate the superconducting transition temperature, Tc. We find that in Mg1-xMxB2 alloys (i) the way Tc changes depends on the location of the added/modified k-resolved states on the Fermi surface and (ii) the variation of Tc as a function of concentration is dictated by the B p DOS.