Excitation Spectrum of a 3He Impurity in 4He
Recent measurements of the effective mass of a 3He impurity on 4He films by Wang and Gasparini1 and Valles et al.2 have provided a challenging problem for many-body theorists. In the case of a very thin 4He film — less than a monolayer — one is studying a homogeneous, quasi-two-dimensional fluid. When the film thickness is increased the impurity begins to float on top of the film and the shape of the surface profile and layered structure of the film become important.3 The effective mass of the impurity depends on the density of surrounding particles and thus can be used as a sensitive test to our understanding of the behaviour of 4He fluid. Recent neutron scattering experiments4 on dilute mixtures of 3He (1% and 5%) in 4He measured the elementary excitation spectrum of 3He for wave vectors between 0.4 and 2.2 °A-1. They tested the effective mass approximation at finite momentum in the three dimensional, homogeneous 4He -3He mixture. Deviations from the quadratic Landau-Pomeranchuck spectrum with increasing momentum were found in agreement with earlier thermodynamical measurements.5
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