Light-scattering studies of the dynamical behaviour of liquid4He close to the λ-line

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Abstract
The paper begins with a review of experiments carried out by the present authors and their collaborators to determine by light scattering the dynamic structure factor of liquid 4He at temperatures close to the superfluid transition. This review includes details of the experimental techniques and reports a number of new sets of experimental results ; it concentrates particularly on the low-frequency part of the dynamic structure factor (the second-sound Brillouin doublet and the Rayleigh line), and the results cover both the hydrodynamic and the critical regimes for one particular value of the wave-vector. A comparison is made with the results of other authors, particularly those of Greytak and his collaborators. Following this presentation of the experimental data, an attempt is made to describe the dynamical behaviour of helium near the superfluid transition in terms of simple physical pictures, and then to use these pictures as the basis of simple calculations. The results often seem to agree surprisingly well with experiment, although the agreement may well be fortuitous; this part of the paper is frankly speculative and ought not to be taken too seriously. Comparison is made with the results of rigorous theoretical work, including recent attempts to apply the techniques of the renormalization group ; although these latter attempts have led to a degree of success, some of the results obtained seem to be in serious disagreement with experiment.