Abstract
There is a long standing disagreement between neutron and Raman scattering from rotons in superfluid4He near Tλ. In neutron scattering the linewidth becomes very large and the roton signal seems to disappear at Tλ. A substantially smaller linewidth is observed with Raman scattering and the roton signal is present even at Tλ. We have interpreted this difference (J. Low Temp. Phys. 93, 879 (1993)) as due to a modulation of the roton energy by a fluctuating local superfluid velocity due to proliferation of vorticity as Tλ is approached. This gives rise to an extra contribution to the roton linewidth in neutron scattering but not in Raman scattering in which two rotons with almost opposite momenta are excited. We propose a test of this explanation with evanescent field Raman scattering. It has been suggested (A. Kuklov, A. Bulatov and J.L. Birman, Phys. Rev. Lett. 72, 3855 (1994)) that in such a measurement the scattering by a single roton should be measurable. In this case the presence of a fluctuating local superfluid velocity should show up as in neutron scattering and corroborate our proposal.
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