About fluctuating systems with nonreciprocal interactions

While the action-reaction principle dictates all fundamental physical interactions, the dynamics we effectively observe in complex nonequilibrium systems ubiquitously breaks reciprocity, giving rise to intriguing new physical phenomena.

We will first show surprising thermodynamic implications for a simple toy model of two nonreciprocally coupled Brownian particles [1]. Then, I will give some insights into how nonreciprocal interactions 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. We show that fluctuations not only inflate, as in equilibrium criticality, but also develop an asymptotically increasing time-reversal asymmetry [2]. Second, we introduce nonreciprocal coupling in the XY model, where nonreciprocity can lead to the formation of true long-range order [3], but can also induce spatiotemporal chaos that can be regarded as a source of self-generated noise.

[1] Loos and Klapp, NJP (2020).
[2] Suchanek, Kroy, and Loos, PRE (2023), PRE (2023), and PRL (2023).
[3] Loos, Klapp, and Martynec, PRL (2023).