A RENORMALIZATION-GROUP STUDY OF HELIMAGNETS IN D=2+EPSILON DIMENSIONS

The nonlinear sigma model O(N) x O(2)/O(N - 2) x O(2) describing the p hase transition of N-component helimagnets is built and studied up to two-loop order in D = 2 + epsilon dimensions. It is shown that a stabl e fixed point exists as soon as N is greater than 3 (or equal) in the neighborhood of two dimensions. The critical exponents nu and eta are obtained. In the N = 3 case, the symmetry of the system is dynamically enlarged at the fixed point from O(3) x O(2)/O(2) to O(3) x O(3)/O(3) is similar to O(4)/O(3). We show that the order parameter for Heisenb erg helimagnets involves a tensor representation of O(4) and we verify it explicitly at one-loop order on the value of the exponents. We sho w that for large N and in the neighborhood of two dimensions this nonl inear sigma model describes the same critical theory as the Landau-Gin zburg linear theory. As a consequence, the critical behavior evolves s moothly between D = 2 and D = 4 dimensions in this limit. However taki ng into account the old results from the D = 4 - epsilon expansion of the linear theory, we show that most likely the nature of the transiti on must change between D = 2 and D = 4 dimensions for sufficiently sma ll N (including N = 3). The simplest possibility is that there exists a dividing line N(c)(D) in the plane (N, D) separating a first-order r egion containing the Heisenberg point at D = 4 and a second-order regi on containing the whole D = 2 axis. We conclude that the phase transit ion of Heisenberg helimagnets in dimension 3 is either first order or second order with O(4) exponents involving a tensor representation or tricritical with mean-field exponents.