XY model vortices: click to place +1 or −1 vortices, watch field lines wind around defects
Topological defects in the XY model are vortices characterized by their winding number n = (1/2π)∮∇θ·dl. A +1 vortex has arrows spiraling counterclockwise; a −1 antivortex has them spiraling clockwise. Vortex-antivortex pairs are logarithmically bound (Berezinskii–Kosterlitz–Thouless transition).