EVIDENCE FOR SOFT-MODE BEHAVIOR OF THE ROTON AT HIGH-DENSITY

Results for the temperature dependence of the inelastic neutron scatte ring at the roton wavevector, q = 2.0 angstrom-1, in liquid He-4 at a constant density of 0.1715 g.cm-3 are presented. The superfluid transi tion temperature, T(lambda) = 1.9202+/-0.0002 K1 (at P = 20.05 bar), i s approached to within 0.0026 K (0.014 K) from below (above). Analysis in terms of the relaxation function nu-1chi''(q,nu) shows that, as T( lambda) is approached from below, there is an increasingly rapid decre ase (increase) in the roton propagation frequency (intrinsic width). T his ''softening'' is complete at T(lambda) where there is a collapse t o the non-propagating behavior (central modes only) characteristic of ordinary fluids.