Two-Particle Excitations and Pair Correlations in He-II
In this contribution we are concerned with the role of two-particle excitations in liquid He-II. This has become an important aspect to be understood by microscopic theories of the superfluid since Cowley and Woods /1/ reported in their neutron scattering intensities the observation of a broad distribution whose energy width and intensities are strong functions of the momentum transfer k. This peak was seen in addition to the rather sharp peak usually associated with one phonon scattering. The intensity of the broad maximum rises to about 25K and extends to relatively large(about 80K) energy transfers. Such a broad peak was not altogether an unexpected one. Phenomenological arguments of Miller, Pines and Nozieres /2/ indicate the existence of a broad background.
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