Abstract
It is shown that in superfluid {sup 4}He films, finite-amplitude localized density fluctuations can propagate as pulse-type solitary waves. These excitations are obtained by solving the nonlinear evolution equation for the superfluid order parameter associated with a microscopic pseudospin model of hard-core bosons with nearest-neighbor interactions. For a given film thickness both hot'' and cold'' solitary waves are possible, depending on the initial disturbance. The solitary-wave velocity is always less than that of the linear third-sound mode in the system. The width and amplitude of the solitary wave are determined as functions of its velocity and the microscopic parameters of the model.
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