Theoretical Study of Magnetism and Superconductivity in 3d Transition-Metal- MgB₂ Alloys

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We have studied the electronic structure of 3d transition-metal- MgB₂ alloys, Mg₀.₉₇TM₀.₀₃B₂, (TM≡ Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) using KKR-CPA 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. 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 μB and 2.87 μB, 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 λusing the Gaspari-Georffy formalism and the superconducting transition temperature Tc using the Allen-Dynes equation. We find that the calculated Tc is the lowest for Mg₀.₉₇Cr₀.₀₃B₂ and the 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 Γto A direction, shows the variation to be an interplay between the total DOS at the Fermi energy and the creation/removal of states along Γto A direction [P. P. Singh, cond-mat/0201093].