Active magneto gyrator: Memory induced trapped diamagnetism

TL;DR

This paper investigates how active particles in asymmetric potentials and magnetic fields exhibit tunable magnetic phases, including diamagnetism and paramagnetism, with memory effects inducing trapped diamagnetic states.

Contribution

It introduces a model of an active particle with memory effects in a magnetic field, revealing novel magnetic phase transitions and trapped diamagnetic states not seen in equilibrium systems.

Findings

Magnetic phase transitions depend on activity, memory, temperature gradient, and potential asymmetry.

A non-magnetic line in activity-memory space forms a closed loop with diamagnetic and paramagnetic regions.

Trapped diamagnetic phase exists within finite activity-memory regimes and can vanish with parameter tuning.

Abstract

We analytically explore the dynamics of a charged active particle coupled to two thermal baths kept at two different temperatures in two dimensions. The particle is confined to an asymmetric harmonic potential and a magnetic field of constant magnitude is applied perpendicular to the plane of motion of the particle. For such a system, as opposed to Brownian gyrator, the potential asymmetry and temperature gradient are not the key factors for the gyration, as long as finite activity and magnetic field are present. The system shows only a paramagnetic behavior in the absence of either potential asymmetry or temperature gradient. However, by tuning the temperature gradient or potential asymmetry, the system as a function of the duration of activity can exhibit paramagnetic, diamagnetic, or co-existence of both the phases. Interestingly, the magnetic moment vanishes for parameters for which the system possesses a non-equilibrium steady state and hence, a magnetic transition is observed through these non-magnetic points. Further, when the system is suspended in a viscoelastic medium characterized by a finite memory, it exhibits a magnetic transition in the activity-memory parameter space through a non-magnetic line. This non-magnetic line is sensitive to temperature gradient and potential asymmetry. It interestingly forms a closed loop with a diamagnetic phase inside the loop and the entire regime outside as paramagnetic. This results in the emergence of a trapped diamagnetic phase existing only within a finite regime of activity-memory parameter space. This phase eventually disappears as the temperature gradient increases (or decreases) depending on the sign of the potential asymmetry. Moreover, it is observed that by tuning the system parameters, one can obtain zero magnetic moment even for parameter ranges that defy the equilibrium condition of the system.