Stiefel models of frustrated antiferromagnets

Lattice nonlinear sigma models, where the Stiefel manifold O(n)/O(n-p) is attached to each lattice site are introduced to analyse phase transitions in frustrated antiferromagnets, with non-collinear spin orderings. A Monte Carlo study of these models in three dimensions indicates either first-order transitions or second-order ones corresponding to new universality classes. There is evidence for a defect mediated transition in some two-dimensional models. None of the results can be reproduced by RG techniques (2+ in and 4- in expansions).