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Reciprocity is a hallmark of thermal equilibrium, but ubiquitously broken in far-from-equilibrium systems on various scales. I will give some insights into how nonreciprocal interactions can fundamentally affect the phases and fluctuations of classical many-body systems. Using a two-dimensional XY model, where spins interact only with neighbours within their 'vision cones' inspired by the effective interactions between animals, we show how nonreciprocity can lead to true long-range order and directional propagation of defects [1]. In binary fluids, nonreciprocal coupling between fluid components can cause the emergence of travelling waves through PT symmetry-breaking phase transitions, as observed for mixtures of catalytic particles or membrane protein dynamics. Using a hydrodynamic model, we find that fluctuations not only inflate, as in equilibrium criticality, but also develop an asymptotically increasing time-reversal asymmetry [2-4] and associated surging entropy production. We can trace the formation of dissipative patterns and the emergence of irreversible fluctuations to the same origin, namely a mode-coupling mechanism near critical exceptional points. [1] Loos, Klapp, Martynec, Long-Range Order and Directional Defect Propagation in the Nonreciprocal XY Model with Vision Cone Interactions, Phys. Rev. Lett. 130, 198301 (2023). [2] Suchanek, Kroy, Loos, Irreversible mesoscale fluctuations herald the emergence of dynamical phases, Phys. Rev. Lett. in press (2023). [3] Suchanek, Kroy, Loos, Time-reversal and parity-time symmetry breaking in non-Hermitian field theories, Phys. Rev. E in press (2023). [4] Suchanek, Kroy, Loos, Entropy production in the nonreciprocal Cahn-Hilliard model, Phys. Rev. E in press (2023).

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Sarah Loos/Cambridge (SFB1238/Kolloquium), Cambridge
Seminar Room of the Institute of Physics II
Contact: Carl Philipp Zelle