Continuous universality in nonequilibrium relaxational dynamics of O2 symmetric systems

We elucidate a nonconserved relaxational nonequilibrium dynamics of a O(2) symmetric model. We drive the system out of equilibrium by introducing a nonzero noise cross correlation of amplitude D(×) in a stochastic Langevin description of the system, while maintaining the O(2) symmetry of the order parameter space. By performing dynamic renormalization group calculations in a field-theoretic set up, we analyze the ensuing nonequilibrium steady states and evaluate the scaling exponents near the critical point, which now depend explicitly on D(×). Since the latter remains unrenormalized, we obtain universality classes varying continuously with D(×). More interestingly, by changing D(×) continuously from zero, we can make our system move away from its equilibrium behavior (i.e., when D(×)=0) continuously and incrementally.