Physical realization of topological excitations in quantum Heisenberg ferromagnet on lattice

The physical spin configurations corresponding to the topological excitations expected to be present in the XY limit of a purely quantum spin $\frac {1}{2}$ Heisenberg ferromagnet, are investigated on a two dimensional square lattice. Quantum vortices (anti-vortices) are constructed by making use of the coherent spin field components from meronic (anti-meronic) configurations in the limiting case of very strong XY anisotropy. The equations for pseudo-time evolution of coherent spin fields used in this analysis, are obtained by applying variational principle on the quantum Euclidean action corresponding to the Heisenberg ferromagnet on lattice. The important role of the associated topological-like term extracted from the Wess-Zumino-like contribution, is highlighted in our procedure. It is shown that this term can identify a large class of vortices (anti-vortices). In particular the excitations having odd topological charges form this class. Our present formalism is markedly different from the conventional approach for the construction of quantum vortices (anti-vortices). Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2010