NONUNIVERSALITY IN HELICAL AND CANTED-SPIN SYSTEMS

Vector-spin models with competing interactions have in general nontrivial ground states which completely break rotational invariance. We study the prototypical stacked triangular Heisenberg antiferromagnet by means of a O(3)×O(2)/O(2) nonlinear model in a 2+ expansion. We find a dynamically generated O(4) symmetry. We propose that such systems in three dimensions have a first-order transition or a second-order one with either N=4 or tricritical mean-field exponents. We argue that this view is supported by experimental and numerical data. © 1990 The American Physical Society.