Superfluid Solitons in Helium Films

It is shown that in monolayer superfluid ${\mathrm{He}}^4$ films, in addition to third-sound modes, small amplitude effects can lead to the existence of gapless solitons made up of superfluid condensate. These nonlinear excitations can be created by localized perturbations in the superfluid density. The conditions are studied under which such initial disturbance evolves into an ordered string of solitons, and the differences to be expected in thicker films are discussed.