Inelastic-neutron-scattering study of acoustic phonons in Nb3Sn

Transverse-acoustic-phonon frequencies and line shapes have been studied as a function of temperature in Nb3Sn. There is a substantial (~10%) reduction in all of the mode frequencies studied between 300 K and the cubic-tetragonal transformation temperature T_M = 45 K. Even more pronounced elastic softening is observed for [ζζ0]T_1 phonons with q ≈ q_ZB/10. As T → T_M from above, phonons in this latter group acquire an unusual quasielastic "central" component in addition to the phononlike sidebands. The evolution of this central component is adequately described by a phenomenological theory which assumes an additional low-frequency ralaxation mechanism for the acoustic phonons. Finally, abrupt changes in certain phonon lifetimes are deteched near the superconducting transformation temperature T_c = 18.0 K. This behavior is traced to the the inability of phonons with energies less than that of the superconducting gap 2Δ(T) to decay by creation of excited electron-quasiparticle pairs. These measurements give an estimate of 2Δ(0) = (4.4 ± 0.6) k_B T_c and reveal a strong anisotropy in the electron-transverse-phonon interaction.