Inelastic-Neutron-Scattering Study of Acoustic Phonons inNb3Sn

Transverse-acoustic-phonon frequencies and line shapes have been studied as a function of temperature in ${\mathrm{Nb}}_3$Sn. There is a substantial (∼ 10%) reduction in all of the mode frequencies studied between 300 °K and the cubic-tetragonal transformation temperature $T_M=45\mathrm{}$ °K. Even more pronounced elastic softening is observed for $\left[\zeta \zeta 0\right]T_1$ phonons with $\frac{qq_{\mathrm{Z}.\mathrm{B}.\mathrm{}}}{10}$. As $T\to T_M$ 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 $T_c=18.0\mathrm{}$ °K. This behavior is traced to the the inability of phonons with energies less than that of the superconducting gap $2\Delta \mathrm{}\left(T\right)\mathrm{}$ to decay by creation of excited electron-quasiparticle pairs. These measurements given an estimate of $2\Delta \mathrm{}\left(0\right)=(4.4\mathrm{}\pm 0.6)k_B{T}_c$ and reveal a strong anisotropy in the electron-transverse-phonon interaction.