Mass and Momentum for Vortices in Two-Dimensional Easy-Plane Magnets

Many quasi-two-dimensional magnetic systems are expected to allow particle-like vortex excitations that are spin configurations with a net 2π twist about a particular point or core. Examples are XY or easy-plane quasi-2d magnets such as BaCoAsO4, and the nearly Heisenberg-like K2CuF4 with a weak easy-plane anisotropy.1 Vortices are also possible in systems such as monolayer magnetic lipid systems.2 Vortices are created or destroyed in particle-antiparticle pairs, carry a conserved circulation charge, exert pair interaction forces on each other, and contribute to thermodynamics and spin-correlation functions.3 There has been much interest in their role in a topological transition due to vortex-antivortex unbinding4 above a characteristic temperature T KT .