Ising model with isotropic competing interactions in the presence of a field : a tricritical-lifshitz-point realization

Abstract

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 e 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 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 qc = 0.