Kosterlitz-Thouless vortex-antivortex pairs and phase field dynamics
The XY model consists of planar spins on a 2D lattice. Below the Kosterlitz-Thouless transition temperature T_KT, vortex-antivortex pairs are bound; above it they unbind and proliferate, destroying quasi-long-range order. Vortices (winding number +1) are shown in red, antivortices (−1) in blue.