New phase-transition scenario for chiral and algebraic order in a generalized planar model

A generalized planar model on a two-dimensional triangular lattice, with a nearest-neighbor potential which softens the energy barrier to domain-wall formation, is studied by Monte Carlo simulations. A new phase-transition scenario is found, with a low-temperature transition signaled by a sharp nonuniversal jump in the spin-wave stiffness and accompanied by an increase in chiral order, followed by a second transition, in which chiral order and spin-wave stiffness vanish simultaneously, the latter with the universal jump of the standard XY model.