Topological Vortex Defects in 2D Superfluid

In a 2D superfluid, the order parameter ψ = |ψ|e^{iφ} carries a phase field φ. Vortex defects are points where |ψ|→0 and the phase winds by ±2π (winding number ±1). At the BKT transition, vortex-antivortex pairs unbind, destroying superfluid order.