ISING-MODEL WITH ISOTROPIC COMPETING INTERACTIONS IN THE PRESENCE OF A FIELD - A TRICRITICAL-LIFSHITZ-POINT REALIZATION

We consider a spin system with competing interactions that are isotropic with respect to the axes of a cubic lattice. On the basis of an epsilon expansion, we demonstrate that for small values of the external field H, the paramagnetic-to-modulated-phase transition remains first order. For larger fields, such a transition changes to a continuous one at a tricritical point. As one varies the wave vector q(c) that is related to the modulated phase, one finds a line of such tricritical points. We remark that such a line must end at a Lifshitz tricritical point at q(c) = 0.