Superfluid solitons inHe4films

It is shown that in two-dimensional superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films the one-dimensional solitons do not represent stable states in general. The system, however, admits of nonlinear excitations which are localized two-dimensional "lumps" consisting of superfluid condensate and which share many properties common with solitons. These objects should be experimentally observable in monolayer superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films at very low temperature. For thick films where the finite thickness effect dominates over the surface term in the dispersion and changes its sign and solitons are stable with respect to transverse perturbations and may represent the dominant mode of nonlinear long-wave excitations.