Reentrance in a Hamiltonian flocking model

TL;DR

This paper demonstrates that a Hamiltonian model of flocking exhibits reentrant phase behavior similar to motility-induced phase separation, revealing a mechanism involving spin-velocity coupling and kinetic frustration.

Contribution

It introduces a Hamiltonian, conservative model that captures reentrant clustering behavior in active matter, bridging equilibrium and non-equilibrium physics.

Findings

Reentrant homogeneous phase observed in the model.

Spin-velocity coupling suppresses transverse diffusion.

Mechanism involves competition between drive amplitude and kinetic frustration.

Abstract

The clustering of self-motile and repulsive particles, so-called motility-induced phase separation (MIPS), is one of the clearest signatures of active physics. Typically, increasing the amplitude of self-motility increases the degree of clustering, however for high enough self-motility the homogeneous phase is reentered. Here, we report that such reentrance naturally emerges in a Hamiltonian (conservative) model known to recapitulate properties of (active) bird flocks, and exhibits clustering behaviour reminiscent of MIPS. We numerically demonstrate the reentrance of the homogeneous phase and identify the underlying mechanism as a competition between the amplitude of a spin-velocity coupled drive and mobility-limited kinetic frustration. Specifically, we reveal that strong spin-velocity coupling suppresses transverse diffusion, thereby leading the system into an arrest that closes the window for phase separation. Overall, our work offers a Hamiltonian, conservative, bridge between reentrant physics across equilibrium and non-equilibrium materials.