BKT physics: In 2D, the Mermin-Wagner theorem forbids long-range order. But superfluidity survives below T_KT through quasi-long-range order (QLRO): ⟨ψ*(r)ψ(0)⟩ ~ r^{-η(T)}.
Universal jump: At T_KT⁻: ρ_s = 2mT_KT/πħ² (Nelson-Kosterlitz 1977). At T_KT⁺: ρ_s = 0. The jump is universal — independent of microscopic details.
Vortex proliferation: Below T_KT: vortex pairs bound by logarithmic potential V(r) ≈ 2πJ·ln(r/a). Above T_KT: free vortices screen, ρ_s→0. Correlation length: ξ ∝ exp(b/√|T−T_KT|).