The role of stoichiometry in superconducting Nb1-βSnβ: Electronic and vibrational properties from ab initio calculations

Using first principles calculations based on density functional theory, the electronic, vibrational and superconducting properties of Nb3Sn compounds with different stoichiometry ratios in A15 phase have been studied. To this purpose, the λ mass enhancement parameter, that determines the superconducting critical temperature through the Allen-Dynes modification of the McMillan formula, has been explicitly calculated in the context of the density functional perturbation theory that allows the calculation of the matrix elements of the electron-phonon interactions at different compositions related to the Sn content. Our results provide a better understanding of the inho- mogeneous composition of one of the most widely employed low-temperature superconductors, evidencing the electronic properties and the phonon modes that are responsible of the critical temperature degradation as the Sn concentration is varied