Two-fluid equations for phonons without momentum |
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Authors: | A Thellung |
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Institution: | Institut für Theoretische Physik der Universität Zürich, Schönberggasse 9, 8001 Zürich, Switzerland |
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Abstract: | In the customary microscopic derivation of the two-fluid equations of liquid He II explicit use is made of the assumption that an elementary excitation of wave vector k carries a momentum ?k. In this paper it is shown that phonons in a liquid can be defined as carrying momentum ?k (Eulerian phonons) or zero momentum (Lagrangian phonons). A careful analysis—in particular of the concept “velocity of the bearer fluid”—shows that the two-fluid equations turn out to be the same in either case. |
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