The $\theta$ vacuum reveals itself as the fundamental theory of the   quantum Hall effect II. The Coulomb interaction
  Within the Grassmannian $U(2N) / U(N) \times U(N)$ non-linear $\sigma $ model representation of localization one can study the low energy dynamics of both the free and interacting electron gas. We study the cross-over between these two fundamentally different physical problems. We show how the topological arguments for the exact quantization of the Hall conductance are extended to include the Coulomb interaction problem. We discuss dynamical scaling and make contact with the theory of variable range hopping. 
A.M.M. Pruisken, M.A. Baranov and I.S. Burmistrov