BKT Transition: Superfluid Density Universal Jump

Berezinskii-Kosterlitz-Thouless — vortex unbinding and Nelson-Kosterlitz relation
2.00 2.00
1.00 50
BKT transition: In 2D superfluids, vortex-antivortex pairs bind at T < T_BKT and unbind above.
Nelson-Kosterlitz relation: ρ_s(T_BKT⁻) = 2mT_BKT / (πℏ²) — universal superfluid density jump.
K(l) = K₀ − 4π³y²e^(2l) + ... RG flow: y = e^(-E_c/T) fugacity. Jump: Δρ_s = (2/π)T_BKT