Abstract

Reciprocity is a hallmark of thermal equilibrium, but ubiquitously broken in far-from-equilibrium systems. I will give some insights into how nonreciprocal interactions can fundamentally affect the phases and fluctuations of many-body systems.  First, in binary fluids, nonreciprocal coupling between fluid components can cause the emergence of travelling waves through PT symmetry-breaking phase transitions. Using a nonreciprocal field model, we show that fluctuations not only inflate, as in equilibrium criticality, but also develop an asymptotically increasing time-reversal asymmetry [1-3]. The formation of dissipative patterns and the emergence of irreversible fluctuations can both be attributed to a mechanism of mode coupling in the vicinity of critical exception points. For a nonreciprocal Cahn-Hilliard model, we show that this manifests itself in actively propelled interfaces whose dynamics can be mapped to the motion of a single microswimmer [3]. Second, introducing nonreciprocal coupling in the two-dimensional XY model, we show how nonreciprocity can lead to the formation of true long-range order [4], as well as dynamical phases and chaos.[1] Suchanek, Kroy \& SL, Irreversible mesoscale fluctuations herald the emergence of dynamical phases, Phys. Rev. Lett. 131, 258302 (2023)[2] Suchanek, Kroy \& SL, Time-reversal and parity-time symmetry breaking in non-Hermitian field theories, Phys. Rev. E 108 , 064123 (2023)[3] Suchanek, Kroy \& SL, Entropy production in the nonreciprocal Cahn-Hilliard model, Phys. Rev. E 108 , 064610 (2023) [4] SL, Klapp \& Martynec, Long-Range Order and Directional Defect Propagation in the Nonreciprocal XY Model with Vision Cone Interactions, Phys. Rev. Lett. 130, 198301 (2023)