Massive and massless phases in self-dual $Z_N$ spin models: some exact   results from the thermodynamic Bethe ansatz
  The generally accepted phase diagrams for the discrete $Z_N$ spin models in two dimensions imply the existence of certain renormalisation group flows, both between conformal field theories and into a massive phase. Integral equations are proposed to describe these flows, and some properties of their solutions are discussed. The infrared behaviour in massless and massive directions is analysed in detail, and the techniques used are applied to a number of other models. 
Patrick Dorey, Roberto Tateo and Kevin E. Thompson