Physical review. B, Condensed matter
Surface energy and free-boson approximation in a competing multispin model
Marcelo D. Grynberg, Horacio Ceva
Abstract
The phase diagram and critical behavior of a two-dimensional system with competing multispin interactions, known as the two-plus-four model, are studied by means of simple approximate analytical methods. Using the M\"uller-Hartmann and Zittartz approximation, the interface free-energy calculation in the ferromagnetic region is reduced to a single-particle problem. We found acceptable agreement with earlier finite-size scaling results. The model is also considered in the Hamiltonian limit, where a linearized Holstein-Primakoff transformation is used to replace spin interactions by a set of independent Bose excitations. We conjecture that the phase transition is first order when the ratio of the four-spin and nearest-neighbor interactions is greater than one-half. Standard perturbative calculations for low temperatures also indicate that the model could display first-order phase transitions.