Exact theory of intermediate phases in two dimensions

• Phase separation with appearance of a third phase is studied exactly.
• Interfacial properties are derived from field theory.
• Exact solution of bulk wetting transition is provided.

We show how field theory yields the exact description of intermediate phases in the scaling limit of two-dimensional statistical systems at a first order phase transition point. The ability of a third phase to form an intermediate wetting layer or only isolated bubbles is explicitly related to the spectrum of excitations of the field theory. The order parameter profiles are determined and interface properties such as passage probabilities and internal structure are deduced from them. The theory is illustrated through the application to the qq-state Potts model and the Ashkin–Teller model. The latter is shown to provide the first exact solution of a bulk wetting transition.