Normal and anomalous solitons in the theory of dynamical Cooper pairing

Abstract

We obtain multisoliton solutions of the time-dependent Bogoliubov--de Gennes equations or, equivalently, Gorkov equations that describe the dynamics of a fermionic condensate in the dissipationless regime. There are two kinds of solitons---normal and anomalous. At large times, normal multisolitons asymptote to unstable stationary states of the BCS Hamiltonian with zero order parameter (normal states), while the anomalous ones tend to eigenstates characterized by a nonzero anomalous average. Under certain circumstances, multisoliton solutions break up into sums of single solitons. In the linear analysis near the stationary states, solitons correspond to unstable modes. Generally, they are nonlinear extensions of these modes, so that a stationary state with $k$ unstable modes gives rise to a $k$-soliton solution. We relate parameters of the multisolitons to those of the asymptotic stationary state, which determines the conditions necessary for exciting solitons. We further argue that the dynamics in many physical situations is multisoliton.