The 2D XY model undergoes the Berezinskii-Kosterlitz-Thouless (BKT) transition. Below T_BKT vortex-antivortex pairs are bound; above it they unbind, destroying quasi-long-range order. Vortices (red/blue) carry topological charge ±1 and interact logarithmically.
Click canvas to add a vortex (+) or shift-click to add an antivortex (-)