Neutron scattering from quantum liquids

Recent neutron scattering measurements on the quantum liquids 4He and 3He are described. In the Bose superfluid there is a well-defined excitation for wave vectors less than 3.6 A-1. In the Fermi liquid measurements are much more difficult because of the large absorption cross section, but measurements at the Institute Laue-Langevin have shown that there are no well-defined excitations at 0.630K for wave vectors between 1.0 and 2.6 A-1. The difference between these results is due to the existence of particle-hole excitations in the Fermi liquid into which collective excitations can decay. Because of the simplicity of the excitations in 4He, it has become a testing ground for the effects of the interactions between the excitations. Measurements are described which show that while roton-roton interactions are attractive at small wave vectors they are repulsive at larger wave vectors. The scattering at large momentum transfer in 4He has been measured, but its interpretation is still open to question