Phase diagrams of systems with two coupled order parameters

Within the framework of an exactly solvable model, which takes into account only the interaction of fluctuating modes with equal and opposite momenta, we study phase diagrams in systems with coupled scalar order parameters. We show that, in agreement with renormalization group theory, the fluctuation interaction can split the continuous order–disorder transition locus into two phase transitions of first order, replacing a bicritical point by a triple point and two tricritical points. The effect disappears when the fluctuation interaction is reduced.