Two Roton Raman Scattering in Superfluid He⁴

Abstract

The possibility of using Raman scattering as a probe for studying the elementary excitations of superfluid helium was first suggested by Halley¹ in 1968. Raman scattering in helium is a second order process in which two excitations of nearly equal and opposite momenta are created in the liquid. The spectrum of scattered light at an energy loss E is a measure of the density of two excitation states with zero total wavevector and energy E.^1,2 The primary contribution comes from the roton region of the dispersion curve. At low temperatures this consists of a well defined line near E = 2Δ, where Δ is the energy of the roton minimum. I will concentrate on the 2-roton light scattering. A discussion of the spectrum in other energy regions can be found in reference 3.