Abstract
The interactions between rotons in superfluid helium are similar to those between dipoles in a polar fluid. Models developed to describe polar fluids may therefore be used to study the influence of roton interactions on various physical quantities in helium II. In this paper we use the Onsager model of a dielectric to calculate the temperature dependence of the energy of the roton minimum in the excitation spectrum, the entropy density, and the normal fluid density, at three different pressures. The results obtained agree well with experimental values for temperatures up to a few tenths of a degree below the $\ensuremath{\lambda}$ temperature; the agreement becomes better with increasing pressure. Our theory contains only a single adjustable parameter, the roton hard-sphere radius, for which we find a value close to half the distance between nearest neighbors in the helium fluid.
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