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Theoretical study of magnetism and superconductivity in three-dimensional transition-metal–MgB2 alloys

We have studied the electronic structure of three-dimensional transition-metal–MgB2 alloys, Mg0.97TM0.03B2, (TM = Sc, Ti, V, Cr Mn, Fe, Co, Ni, Cu, Zn) using the Korringa–Kohn–Rostoker coherent-potential approximation method in the atomic-sphere approximation. For unpolarized calculations, our results for Mg0.97TM0.03B2 alloys are similar to that of 3d impurities in other s and s–p metals. In particular, the local densities of states (DOS) associated with the 3d impurities are similar to our earlier work on 3d impurities in bulk Al (Singh P P 1991 Phys. Rev. B 43 3975; Singh P P 1991 J. Phys.: Condens. Matter 3 3285). For spin-polarized calculations, we find only the alloys of V, Cr, Mn, Fe and Co with MgB2 to be magnetic of all the 3d elements. We also find that Cr and Mn in MgB2 have a relatively large local magnetic moment of 2.43 and 2.87μB, respectively. We have used the unpolarized, self-consistent potentials of Mg0.97TM0.03B2 alloys, obtained within the coherent-potential approximation, to calculate the electron–phonon coupling constant λ using the Gaspari–Gyorffy formalism and the superconducting transition temperature Tc using the Allen–Dynes equation. We find that the calculated Tc is lowest for Mg0.97Cr0.03B2 and highest for Mg0.97Zn0.03B2, in qualitative agreement with experiment. The calculated trend in variation of Tc from Mn to Zn is also similar to the available experimental data. Our analysis of the variation in Tc, in terms of the DOS and the spectral function along the Γ to A direction, shows the variation to be an interplay between the total DOS at the Fermi energy and the creation/removal of states along the Γ to A direction (Singh P P 2002 Preprint cond-mat/0201093).