Theoretical examination of whether phonon dispersion in Nb3Sn is anomalous

Ab initio psuedopotential calculations have been performed for the electronic structure and lattice dynamics of Nb3Sn. It is found that electronic states near the Fermi level show anomalous dispersion. In agreement with the experimental measurement by Pintschovius [Phys. Rev. Lett. 54, 1260 (1985)] it is found that the dispersion of the longitudinal acoustic branch along [111] does not exhibit a sinusoidal form, and thus can be said to be anomalous. However, our work clearly disproves the presence of a dip in the dispersion curve predicted by Weber in 1984 [Physica B & C 126, 217 (1984)]. Our results further provide a quantitative estimate of strong interaction of electronic states near the Fermi level with the longitudinal acoustic and a group of phonon modes with average frequency of 4.5 THz along the [111] direction. The calculated electron-phonon mass enhancement parameter is 1.51 and the resulting superconducting critical temperature is 17.7 K.