We consider field theory solitons relevant for condensed matter. We start with a field theory arising from a two-dimensional system of spins, the XY model, leading to the “rotor model,” or “O(2) model”. From the bosonic Hubbard model, we show a representation that leads to the same quantum rotor model. In the continuum limit, we obtain a massless scalar that has a global vortex as its solution. The dynamics of these vortices is relevant for the Kosterlitz–Thouless (KT) phase transition, a quantum phase transition appearing for instance in 2+1 dimensional superconductivity. The bosonic Hubbard model leads, in the continuum limit, also to a relativistic Landau–Ginzburg model, that has a kink-like solution.