Some Points of the Theory of Liquid Helium II

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Abstract
Some of the limitations of the hydrodynamic model of liquid He II are being studied in this paper. It is shown that the vortex or roton phase alone, without mutual interaction, undergoes a first-order phase change. The phonon-roton system does not have such a phase change if the zero-point energy or pressure is included explicity in its formalism. In the high temperature or roton region the model has a normal state surface and does not account for the unusual equilibrium properties of liquid He II such as the anomalous expansion coefficient, the entropy increase and decrease in isothermal pressure and volume increases, respectively. The possibility of the abnormal liquid to become normal again at very low temperatures, where the entire thermal excitation is concentrated over the phonons, is briefly discussed.

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