Kardar–Parisi–Zhang universality: stochastic interface growth with non-linear slope coupling
KPZ equation: ∂h/∂t = ν∇²h + (λ/2)(∇h)² + η — the nonlinear term breaks up-down symmetry and places interface fluctuations in the KPZ universality class with roughness exponent β=1/3 (1D).