NEUTRON-SCATTERING PROFILE OF Q-NOT-EQUAL-0 IN BCS SUPERCONDUCTORS

Phonons in a metal interact with conduction electrons. Ln the normal s tate, this gives rise to a linewidth gamma(Q) which is small compared with the frequency omega Q. In the superconducting state, the line sha pe 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-pho non coupling lambda. As long as omega(Q) much less than Q upsilon(F) w here upsilon(F) is the Fermi velocity, BCS theory predicts a Line shap e which is a universal function of the dimensionless parameters r, ome ga(Q)/2 Delta, omega/2 Delta and T/T-c where T-c is the superconductin g transition temperature, Formulas and curves are given for the full r ange of these parameters. The BCS predictions correspond well to key f eatures seen In recent experiments on YNi2B2C and LuNi2B2C.