Skyrmions in quantum Hall ferromagnets as spin waves bound to unbalanced magnetic-flux quanta

A microscopic description of (baby) skyrmions in quantum Hall ferromagnets is derived from a scattering theory of collective (neutral) spin modes by a bare quasiparticle. We start by mapping the low-lying spectrum of spin wav es in the uniform ferromagnet onto that of freely moving spin excitons, and then we study their scattering by the charge defect. In the presence of th is disturbance, the local spin stiffness Varies in space, and we translate it into an inhomogeneous metric in the Hilbert space supporting the exciton s. An attractive potential is then required to preserve the symmetry under global spin rotations, and it traps the excitons around the charged defect. The quasiparticle now carries a spin texture. Textures containing more tha n one exciton are described within a mean-field theory, the interaction amo ng the excitons being taken into account through a new renormalization of t he metric. The number of excitons actually bound depends on the Zeeman coup ling, which plays the same role as a chemical potential. For small Zeeman e nergies, the defect binds many excitons that condensate. As the bound excit ons have a unit of angular momentum, provided by the quantum of magnetic fl ux left unbalanced by the defect of charge, the resulting texture turns out to be a topological excitation of charge 1. Its energy is that given by th e nonlinear a model for the ground state in this topological sector, i.e., the texture is a skyrmion. [S0163-1829(98)00544-X].