CROSSOVER FROM O(3) TO O(4) BEHAVIOR IN WEAKLY FRUSTRATED ANTIFERROMAGNETS

We consider an anisotropic version of the CP1 model which describes fr ustrated quantum antiferromagnets with incommensurate spin correlation s. We extend the two-component spinon field, describing lattice spins, to the M-component complex vector, and show, in the 1/M expansion, th at for arbitrary small incommensurability longitudinal and transverse stiffnesses lend to the same value as the system approaches the quantu m critical point. For physical spins (M=2), this yields O(4) critical behavior. However, if the spin structure is commensurate, the longitud inal stiffness is identically zero. In this case, the critical behavio r is the same as in the O(3) sigma model. We show how the critical exp onents interpolate between O(3) and O(4) values near the transition. W e also show that the competition between these two fixed points leads to a confinement-deconfinement transition at a finite temperature.