XY-model spins on a 2D lattice. Vortices (winding number +1) and anti-vortices (−1) are topological defects — their charges are conserved and they interact like 2D Coulomb charges.
The Berezinskii-Kosterlitz-Thouless (BKT) transition occurs at T_BKT≈0.89J: below it, vortex-antivortex pairs are bound and the system has quasi-long-range order; above it, free vortices proliferate and destroy order. A Nobel Prize (2016) was awarded for this topological phase transition.