TEMPERATURE-DEPENDENCE OF THE ROTON IN LIQUID-HE-4

In new neutron scattering measurements on He-4 at saturated vapour pre ssure (SVP), we have probed to within 0.0007 K of T-lambda from below, revealing a dramatic increase in the damping rate of the momentum flu ctuations and a marked softening of the roton frequency, with the chan ges occurring primarily very close to T-lambda and being complete at T -lambda. In contrast to 20 bars, the roton at SVP does not, however, g o completely soft. Above T-lambda, the modes are strongly damped and o verlapping whereas at 20 bars they are overdamped (non-propagating).