Elementary Excitations in Liquid Helium
Recently certain controversies have arisen over the form and direction of the phonon dispersion and the nature of the two-roton excitation in liquid helium. From the standpoint both of theoretical elegance and of satisfactorily interpreting experimental data in a way consistent with sum rules, it is highly desirable to develop a theory which accounts for the two branches of excitations on equal footing, at least in the limit of long wavelengths. We discuss here such an attempt. It is a microscopic theory based on a generalization of Feynman’s treatment of the phonon-roton spectrum.
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