THE ANTIFERROMAGNETIC POTTS-MODEL IN 2 DIMENSIONS - BERKER KADANOFF PHASE, ANTIFERROMAGNETIC TRANSITION, AND THE ROLE OF BERAHA NUMBERS

Using methods of integrable systems and conformal field theory, we study the Q-state Potts model on the square lattice with e(K) real. We discover a surprisingly rich phase diagram that involves, besides the usual ferromagnetic critical line, an antiferromagnetic critical line and a Berker-Kadanoff phase (i.e., a massless low-temperature phase with coupling-independent exponents) that has singularities at the Beraha numbers (including Q integer) Q = 4 cos2-pi/n. Critical properties are derived; we show in particular that the Q = 4 cos2-pi/delta-antiferromagnetic critical Potts model is in the "Z-delta-2" universality class with c = 2 - 6/delta. Extensions to other lattices are considered. We discuss the consequences of our results on the coloring problem and the Beraha conjecture. Three appendices deal with the geometrical interpretation of the Temperley-Lieb algebra and U(q)sl(2) symmetry in the Potts and associated loops model, and with the vertex-Potts model correspondence in systems with free boundary conditions.