Topological Defects — 2D XY Model

Vortex/anti-vortex pairs and the BKT transition

Vortex (+1) Anti-vortex (−1) Defects: 0 BKT T_c ≈ 0.89
XY model: H = −J Σ cos(θᵢ − θⱼ). BKT transition: vortex unbinding at T_BKT ≈ 0.89J/k_B