Dynamics of overlapping vortices in complex scalar fields

We investigate dynamics of overlapping vortices in the nonlinear Schr\"{o}dinger equation, the nonlinear heat equation and in the equation with an intermediate Schr\"{o}dinger-diffusion dynamics. Because of formal similarity on a perturbative level we discuss also the nonlinear wave equation (Goldstone model). Special solutions are found like vortex helices, double-helices and braids, breather states and vortex mouths. A pair of vortices in the Goldstone model scatters by the right angle in the head-on collision. It is found that in a dissipative system there is a characteristic lenght scale above which vortices can be entangled but below which the entanglement is unstable.Comment: detailed analysis of the continuation in the complex time plane; latest revision: the paper is made more readable. 9 pages in Late

Quantum Dark Soliton: Non-Perturbative Diffusion of Phase and Position

The dark soliton solution of the Gross-Pitaevskii equation in one dimension has two parameters that do not change the energy of the solution: the global phase of the condensate wave function and the position of the soliton. These degeneracies appear in the Bogoliubov theory as Bogoliubov modes with zero frequencies and zero norms. These ``zero modes'' cannot be quantized as the usual Bogoliubov quasiparticle harmonic oscillators. They must be treated in a non-perturbative way. In this paper I develop non-perturbative theory of zero modes. This theory provides non-perturbative description of quantum phase diffusion and quantum diffusion of soliton position. An initially well localized wave packet for soliton position is predicted to disperse beyond the width of the soliton.Comment: 6 page