Emergent Macroscopic Nonreciprocity from Identical Active Particles via Spontaneous Symmetry Breaking

TL;DR

This paper demonstrates how spontaneous symmetry breaking in a single-species active particle system can lead to emergent macroscopic nonreciprocity, resulting in novel collective behaviors like traveling bands and real-space condensation.

Contribution

It reveals a fundamental mechanism where microscopic single-species nonreciprocity, enhanced by SSB, causes significant macroscopic phenomena in active matter systems.

Findings

Emergent macroscopic nonreciprocity in a single-species active particle system.

Promotion of traveling-band formation due to SSB-enhanced nonreciprocity.

Discovery of a real-space condensation characterized by a traveling line.

Abstract

Nonreciprocity is known to generate a wide range of exotic phenomena in multi-species many-body systems, where different species influence one another through couplings that violate Newton's third law. In contrast, in the absence of explicitly imposed macroscopic nonreciprocal processes, single-species nonreciprocity -- another distinct form of nonreciprocity -- typically plays only a limited role in shaping macroscopic physics. Here, using a single-species Vicsek model with a vision cone and extrinsic noise, we show that spontaneous symmetry breaking (SSB) can dramatically enhance the macroscopic consequences of microscopic single-species nonreciprocity. In the ordered phase, this enhancement gives rise to an emergent macroscopic nonreciprocity that induces the system of identical active particles to admit an effective description with a "two-species" non-Hermitian structure. The resulting SSB-enhanced nonreciprocity substantially promotes traveling-band formation and, more strikingly, drives a novel real-space condensation of identical active particles, characterized by a "traveling line" with vanishing longitudinal width. Our findings uncover a fundamental mechanism by which microscopic single-species nonreciprocity can exert strong macroscopic influences in complex systems.