Fluctuation-induced first order transition in a nonequilibrium steady state

We present the first example of a phase transition in a nonequilibrium steady-state that can be argued analytically to be first order. The system of interest is a two-species reaction-diffusion problem whose control parameter is the total density \rho. Mean-field theory predicts a second-order transition between two stationary states at a critical density \rho=\rho_c. We develop a phenomenological picture that, instead, below the upper critical dimension d_c=4, predicts a first-order transition. This picture is confirmed by hysteresis found in numerical simulations, and by the study of a renormalization-group improved equation of state. The latter approach is inspired by the Weinberg-Coleman mechanism in QED.