Momentum Distributions In Liquid Helium PDF Download

The appearance of the superfluid phase, with its unique macroscopic properties, and the success of the 'two-fluid' model in describing the transport properties are intimately linked to the Bose statistics obeyed by 4He atoms. The most direct signature of the Bose condensate is in the single particle momentum distribution n(p). In the normal liquid the momentum distribution has a broad Gaussian shape, as predicted for classical systems, with a width determined by the quantum zero point motion. In the superfluid phase a new feature appears, the Bose condensate. The condensate appears as a delta-function singularity in n(p) with an intensity proportional to n0, the condensate fraction. There has been extensive experimental work attempting to verify the existence of the condensate. Hohenberg and Platzman originally suggested that inelastic neutron scattering at high momentum transfer Q, where the Impulse Approximation IA can be used to directly relate the observed scattering to n(p), could provide a means of directly observing the condensate. In this review we will concentrate on recent experimental studies that utilize the large flux of high energy neutrons available at spallation neutron sources. These sources, which have become available only recently, have made it possible to make detailed measurements at large enough momentum transfers that the conditions for the Impulse Approximation are approximately satisfied. While deviations from the IA are still present at these higher Q's, they are more amenable to theoretical treatment and detailed predictions are available. Thus, even though no distinct condensate peak is observed, for the first time excellent agreement with the theoretical predictions for n(p) can be obtained. 50 refs., 12 figs.