Ordering and Defect Dynamics in Passive and Active Nematopolars

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Original abstract

The coexistence of polar and nematic interactions, observed in a broad range of biological and synthetic active systems, gives rise to a rich phenomenology that continues to challenge our theoretical understanding of non-equilibrium collective behaviour. In this paper, we numerically investigate phase ordering and defect dynamics in a newly introduced minimal single-field model for dry nematopolar systems, where competing polar and nematic contributions enter the free energy, and activity is implemented through a self-advection contribution. At optimal balance between the two alignments, the system develops depolarization strings connecting half-integer defects and separating domains with opposite polarization, together with closed depolarization loops. We first characterize the elementary relaxation mechanisms of defect pairs and loops, showing that the interplay between polar and nematic alignment gives rise to non-monotonic string-mediated interactions, finite equilibrium separations and distinct loop-collapse pathways. Large-scale simulations from disordered states instead show dynamic scaling with a characteristic length growing as $\sim(t/\ln t)^{1/2}$, consistent with coarsening in systems with non-conserved order parameters and point-like defects. Upon introducing self-advection, sufficiently strong activity leads to the coexistence of positive integer and negative half-integer defects, which we term motility-induced charge symmetry breaking, and to saturation of the characteristic length scales, ultimately resulting in arrested coarsening. Overall, our results provide a simple unified framework for understanding the ordering and defect dynamics in biological and synthetic nematopolar systems.

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