Neutron-scattering profile of Q not equal 0 in BCS superconductors

Phonons in a metal interact with conduction electrons. Ln the normal state, this gives rise to a linewidth gamma(Q) which is small compared with the frequency omega Q. In the superconducting state, the line shape can be altered if (h) over bar omega n(Q) less than or similar to(1+2r)2 Delta where Delta is the superconducting gap, and r is the ratio gamma(Q)/omega(Q), which scales with the strength of the electron-phonon coupling lambda. As long as omega(Q) much less than Q upsilon(F) where upsilon(F) is the Fermi velocity, BCS theory predicts a Line shape which is a universal function of the dimensionless parameters r, omega(Q)/2 Delta, omega/2 Delta and T/T-c where T-c is the superconducting transition temperature, Formulas and curves are given for the full range of these parameters. The BCS predictions correspond well to key features seen In recent experiments on YNi2B2C and LuNi2B2C.