High-mobility 2D electron systems in a perpendicular magnetic field exhibit zero resistance states (ZRS) when driven with microwave radiation. We study the nonequilibrium phase transition into this ZRS using phenomenological equations of motion to describe the current and density fluctuations. We focus on two models for the transition into a time-independent steady state. Model-I assumes rotational invariance, density conservation, and symmetry under shifting the density globally by a constant. This model is argued to describe physics on small length scales where the density does not vary appreciably from its mean. The ordered state that arises in this case breaks rotational invariance and consists of a uniform current and transverse Hall field. We discuss some properties of this state, such as stability to fluctuations and the appearance of a Goldstone mode associated with the continuous symmetry breaking. Using dynamical renormalization group techniques, we find that with short-range interactions this model can admit a continuous transition described by mean-field theory, whereas with long-range interactions the transition is driven first-order. Model-II, which assumes only rotational invariance and density conservation and is argued to be appropriate on longer length scales, is shown to predict a first-order transition with either short- or long-range interactions. We discuss implications for experiments, including scaling relations and a possible way to detect the Goldstone mode in the case of a continuous transition into the ZRS, as well as possible signatures of a first-order transition in larger samples. We also point out the connection of our work to the well-studied phenomenon of `flocking.
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