Spin fluctuations and zero-sound in normal liquid 3He studied by neutron scattering

High-resolution neutron-inelastic-scattering measurements of liquid 3He at a temperature of 120 mK and pressures between 0 and 20 bars are analyzed. The improved energy resolution provides for the first time information on the line shape of the spin-fluctuation peak. We find that the low-energy enhancement of the spin fluctuations is stronger than predicted by the paramagnon model (m*/m3 = 1), in particular at high pressures. Also, the spin dependent scattering extends to higher energies than can be accounted for in any simple random-phase-approximation (RPA) model that uses the Lindhard function with an effective mass of about 3 m3. This observation is independent of the interaction potential used in the RPA. However, we show that a simple RPA model where the Lindhard function has an effective mass of 1.9 m3 gives a good description of the spin-fluctuation scattering. The zero-sound mode overlaps with both spin fluctuations and multipair excitations, and its energy and line width can only be obtained unambiguously at the smallest wave vectors. For wave vectors larger than 0.6 angstrom -1, the energy and in particular the width of the zero-sound mode depend strongly on the models used for the spin fluctuations and the multipairs. Different damping mechanisms of the zero-sound mode are discussed, and the importance of coherent and incoherent multipairs is illustrated