FOLDING TRANSITIONS OF THE TRIANGULAR LATTICE WITH DEFECTS

A recently introduced model describing the folding of the triangular l attice is generalized, allowing for defects in the lattice, and writte n as an Ising model with nearest-neighbor and plaquette interactions o n the honeycomb lattice. Its phase diagram is determined in the hexago n approximation of the cluster variation method, and the crossover fro m the pure Ising to the pure folding model is investigated, obtaining a quite rich structure with several multicritical points. Our results are in very good agreement with the available exact ones and extend a previous transfer matrix study.