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CPP: Fachverband Chemische Physik und Polymerphysik
CPP 11: Active Matter II (joint session DY/BP/CPP)
CPP 11.1: Talk
Monday, March 9, 2026, 15:00–15:15, ZEU/0160
Field-controlled self-organization in an active spin system — Mintu Karmakar1,2,3, •Matthieu Mangeat4, Swarnajit Chatterjee5,4, Heiko Rieger4, and Raja Paul3 — 1WIUCAS, Beijing, China — 2Universitat de Barcelona, Barcelona, Spain — 3IACS, Kolkata, India — 4Saarland University, Saarbrücken, Germany — 5CY Cergy Paris Université, Cergy-Pontoise, France
We investigate the collective response of active Potts particles to an external magnetic field and uncover three striking nonequilibrium phenomena. We first examine how the flocking transition is reshaped for a homogeneous and unidirectional field: the coexistence regime between an apolar gas and a polar liquid is replaced by a phase separation between two polar-ordered phases, a low-density, weakly polarized background and a high-density, strongly polarized band, both moving along the field. Second, when the particles self-organize into a high-density longitudinal lane whose long axis is perpendicular to the field, the lane slowly treadmills against the field direction, driven by the weakly polarized background. Finally, we identify a field-induced interface pinning regime that arises when the domain is divided into two regions with opposite field directions, causing particles to accumulate and perform a back-and-forth motion at the interface. This pinning phenomenon also leads to the emergence of a disordered state in the presence of a random field orientation. A coarse-grained hydrodynamic theory supports and confirms the phenomena observed in our microscopic simulations.
Keywords: active matter; flocking; treadmilling; field-induced pinning