Conjugate variables and a variational principle in superfluid helium

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Abstract
It is shown that the Landau two-fluid equations may be thought of as the Hamilton equations of two pairs of conjugate variables. This result is used to study the vortex dynamics and to obtain a method for generalizing the Landau equations when the superfluid velocity is no longer irrotational. These generalized equations are also derived from a variational principle, using a slightly modified form of Lin's constraint. Lastly we analyze the consequences on the Josephson effect of the existence of two pairs of conjugate variables.