Diagrammatic analysis of the two-state quantum Hall system with chiral   invariance
  The quantum Hall system in the lowest Landau level with Zeeman term is studied by a two-state model, which has a chiral invariance. Using a diagrammatic analysis, we examine this two-state model with random impurity scattering, and find the exact value of the conductivity at the Zeeman energy $E = \Delta$. We further study the conductivity at the another extended state $E = E_1$ ($ E_1 > \Delta$). We find that the values of the conductivities at $E = 0$ and $E = E_1$ do not depend upon the value of the Zeeman energy $\Delta$. We discuss also the case where the Zeeman energy $\Delta$ becomes a random field. 
S. Hikami and K. Minakuchi