arXiv subjects

Amir Shee

Publications and source records attributed to Amir Shee.

At least 19 recordsLinked to original sources

arXiv2026

Mobility Anisotropy Reshapes Self-Propelled Motion

We exactly solve the nonequilibrium dynamics of a harmonically trapped self-propelled particle with anisotropic translational mobility in two dimensions, relevant to rodlike microswimmers and wheeled robots. The mean displacement and MSD reveal a quasi-steady plateau with vanishing fluctuations in the high-persistence regime. An exact calculation of steady-state fourth moment yields a negative excess kurtosis that varies non-monotonically with the ratio of mechanical to rotational relaxation timescales. This gives rise to a strictly sub-Gaussian steady-state position distribution, in which the particle with anisotropic mobility, in high persistence regime, is displaced into the high-potential region lying outside the stationary contour set by the activity and harmonic confinement. This is further corroborated by the relaxation of the MSD from the quasi-steady plateau to the steady-state regime.

cond-mat.stat-mech↗

arXiv2026

Passivity-Driven Order-Disorder Transitions in Self-Aligning Active Matter

We study dense mixtures of passive and active self-aligning disks with isotropic or anisotropic mobility. We find that the passive fraction controls an order-disorder transition that is continuous in the isotropic case and discontinuous in the anisotropic one. A mean-field equation derived from the microscopic heading dynamics captures this dichotomy. Near the transition, both ordered regimes can exhibit multiple metastable oscillating or rotating states, depending on the spatial arrangement of passive particles and lattice defects, but with different transient dynamics: Systems with isotropic mobility visit multiple long-lived attractors during each simulation while systems with anisotropic mobility are trapped by a single attractor. Our results reveal the passive fraction as a physically relevant control parameter in active systems, leading to rich self-organizing dynamics.

cond-mat.soft↗

arXiv2026

Characterizing exact dynamics of a trapped active Brownian particle under torque in two and three dimensions

The interplay of chirality, self-propulsion, and spatial confinement generates striking non-equilibrium fluctuations whose higher-order statistics carry information about the dynamics and shape of the position distribution. Here, we present an exact analytical framework, based on a Laplace-transform solution of the Fokker-Planck equation, for the transient dynamics of a chiral active Brownian particle in a harmonic trap, in both two and three dimensions. We obtain closed-form expressions for all time-dependent moments up to fourth order, enabling a complete characterization of the excess kurtosis throughout the transient and steady-state regimes. In two dimensions, the excess kurtosis exhibits a damped oscillatory response with multiple re-entrant crossovers, evolving from negative values that reflect active off-centered ring-like position distributions to positive values characteristic of heavy-tailed fluctuations. This damped oscillatory excess kurtosis appears both for free and harmonic confinement, although increasing the trapping stiffness progressively suppresses it, and the positive excess kurtosis eventually vanishes at sufficiently high stiffness. In contrast, in three dimensions, the excess kurtosis remains negative, indicating a robustly active non-Gaussian state characterized by half-ring-like to band-like position distributions in the two-dimensional plane spanned by the torque axis and its normal radial direction. Our results demonstrate how chirality, propulsion, and confinement, together with dimensionality, shape transient dynamics, while providing experimentally accessible signatures of confined chiral active dynamics.

cond-mat.soft↗

arXiv2026

Controlling inertial active Brownian motion via stochastic resetting

Inertia is intrinsic to many living and synthetic active systems, from animals and robotic agents to colloidal swimmers, and it strongly shapes transport. Many such systems employ intermittent restart protocols to regulate exploration. Stochastic resetting provides a theoretical framework for these strategies and a route to control nonequilibrium steady states, yet the role of inertia in reset-controlled active dynamics remains poorly understood. Here we study an inertial active Brownian particle subject to complete stochastic resetting of position, velocity, and orientation in two dimensions. Using a moment-generating framework together with the Final-Value Theorem, we derive closed-form steady-state moments up to fourth order as functions of inertia, activity, and reset rate. We show that inertia fundamentally modifies reset-controlled transport: at large reset rates the steady-state mean-squared displacement is suppressed much more strongly than in the overdamped limit, yielding enhanced localization near the reset point. At the same time, position excess-kurtosis phase diagrams reveal strongly non-Gaussian steady states characterized by a sharp central peak coexisting with heavy tails in the position distribution, indicating rare long excursions enabled by inertial persistence. The tail weight varies non-monotonically with reset rate, reflecting a competition between inertial momentum relaxation and resetting that selects an optimal regime maximizing rare excursions. Our results provide experimentally testable signatures of inertial effects in reset-controlled active systems.

cond-mat.soft↗

arXiv2025

Chirality, confinement and dimensionality govern re-entrant transitions in active matter

The non-equilibrium dynamics of individual chiral active particles underpin the complex behavior of chiral active matter. Here we present an exact analytical framework, supported by simulations, to characterize the steady states of two-dimensional chiral active Brownian particles and three-dimensional torque-driven counterparts in a harmonic trap. Using a Laplace-transform approach of the Fokker-Planck equation, we derive closed-form expressions for displacement moments and excess kurtosis, providing a precise probe of non-Gaussian statistics. Our analysis reveals three distinct regimes: bimodal active states with off-center peaks, Gaussian-like passive states, and weakly heavy-tailed distributions unique to two dimensions. We show that dimensionality plays a decisive role: in two dimensions, increasing chirality suppresses activity and restores passive behavior, while in three dimensions torque preserves activity along the torque axis, producing anisotropic steady states. These behaviors are captured by simple active length-scale arguments that map the boundaries between passive and active phases. Our results offer concrete experimental signatures - including kurtosis crossovers, off-center peaks, and torque-induced anisotropy - that establish confinement as a powerful tool to probe and control chiral and torque-driven active matter.

cond-mat.soft↗

arXiv2025

Steering chiral active Brownian motion via stochastic position-orientation resetting

Guiding active motion is important for targeted delivery, sensing, and search tasks. Many active systems exhibit circular swimming, ubiquitous in chemical, physical, and biological systems, that biases motion and reduces transport efficiency. We show that stochastic position-orientation resetting can overcome these limitations in two-dimensional chiral active Brownian particles by interrupting circular motion, resulting in tunable dynamics. When resets are infrequent compared to chiral rotation, the steady-state mean-squared displacement varies non-monotonically with rotational diffusion. Steady state excess kurtosis and orientation autocorrelation yields spatiotemporal state diagram comprising three states: an activity-dominated chiral state, and two resetting-dominated states with and without chiral rotation; in contrast, the achiral(or non-chiral) counterpart supports only the resetting-dominated state without chiral rotation. Chirality thus enriches the dynamical landscape, enabling tunable transitions between transport modes absent in achiral systems. A simple reset protocol can therefore transform chiral active dynamics and offer a practical strategy for optimizing search and transport in circle swimmers.

cond-mat.soft↗

arXiv2025

Tuning Steady Shear Rheology through Active Dopants

We numerically investigate the steady shear rheology of mixtures of active and passive Brownian particles, with varying fractions of active components. We find that even a small fraction of active dopants triggers fluidization with comparable efficiency to fully active systems. A combined parameter, active energy, given by dopant fraction multiplied by propulsion speed squared controls the steady shear rheology and glass transition of the active-passive mixtures. These results together provide a quantitative strategy for fine-tuning the mechanical properties of a soft material with small amounts of active dopants.

cond-mat.soft↗

arXiv2024

Dynamical metastability and re-entrant localization of trapped active elements with speed and orientation fluctuations

We explore the dynamics of active elements performing persistent random motion with fluctuating active speed and in the presence of translational noise in a $d$-dimensional harmonic trap, modeling active speed generation through an Ornstein-Uhlenbeck process. Our approach employs an exact analytic method based on the Fokker-Planck equation to compute time-dependent moments of any dynamical variable of interest across arbitrary dimensions. We analyze dynamical crossovers in particle displacement before reaching the steady state, focusing on three key timescales: speed relaxation, persistence, and dynamical relaxation in the trap. Notably, for slow active speed relaxation, we observe an intermediate time metastable saturation in the mean-squared displacement before reaching the final steady state. The steady-state distributions of particle positions exhibit two types of non-Gaussian departures based on control parameters: bimodal distributions with negative excess kurtosis and heavy-tailed unimodal distributions with positive excess kurtosis. We obtain detailed steady-state phase diagrams using the exact calculation of excess kurtosis, identifying Gaussian and non-Gaussian regions and possible re-entrant transitions.

cond-mat.stat-mech↗

arXiv2024

Collective dynamics of densely confined active polar disks with self- and mutual alignment

We study the emerging collective states in a simple mechanical model of a dense group of self-propelled polar disks with off-centered rotation, confined within a circular arena. Each disk presents self-alignment towards the sum of contact forces acting on it, resulting from disk-substrate interactions, while also displaying mutual alignment with neighbors due to having its center of rotation located a distance R behind its centroid so that central contact forces can also introduce torques. The effect of both alignment mechanisms produces a variety of collective states that combine high-frequency localized circular oscillations with low-frequency milling around the center of the arena, in fluid or solid regimes. We consider cases with small/large R values, isotropic/anisotropic disk-substrate damping, smooth/rough arena boundaries, different densities, and multiple systems sizes, showing that the emergent collective states that we identify are robust, generic, and potentially observable in real-world natural or artificial systems.

cond-mat.soft↗

arXiv2024

Active Brownian particle under stochastic position and orientation resetting in a harmonic trap

We present an exact analytical study of an Active Brownian Particle (ABP) subject to both position and orientation stochastic resetting in a two-dimensional harmonic trap. Utilizing a Fokker-Planck-based renewal approach, we derive the system's exact moments, including the mean parallel displacement, mean squared displacement (MSD), and the fourth-order moment of displacement, and compare these with numerical simulations. To capture deviations from Gaussian behavior, we analyze the excess kurtosis, which reveals rich dynamical crossovers over time. These transitions span from Gaussian behavior (zero excess kurtosis) to two distinct non-Gaussian regimes: an activity-dominated regime (negative excess kurtosis) and a resetting-dominated regime (positive excess kurtosis). Furthermore, we quantify the steady-state phase diagrams by varying three key control parameters: activity, resetting rate, and harmonic trap strength, using steady-state excess kurtosis as the primary metric.

cond-mat.stat-mech↗

arXiv2024

UNC-104 transport properties are robust and independent of changes in its cargo binding

Cargo distribution within eukaryotic cells relies on the active transport mechanisms driven by molecular motors. Despite their critical role, the intricate relationship between motor transport properties and cargo binding - and its impact on motor distribution - remains inadequately understood. Additionally, improper regulation of ubiquitination, a pivotal post-translational modification that affects protein degradation, activation, and localization, is associated with several neurodegenerative diseases. Recent data showed that ubiquitination can alter motor-cargo binding of the Kinesin-3 motor UNC-104 / KIF1A that transports synaptic vesicles. To investigate how ubiquitin-like modifications affect motor protein function, particularly cargo binding, transport properties, and distribution, we utilize the PLM neuron of C. elegans as a model system. Using fluorescent microscopy, we assess the distribution of cargo-bound UNC-104 motors along the axon and probe their dynamics using FRAP experiments. We model cargo binding kinetics with a Master equation and motor density dynamics using a Fokker-Planck approach. Our combined experimental and theoretical analysis reveals that ubiquitin-like knockdowns enhance UNC-104's cooperative binding to its cargo. However, these modifications do not affect UNC-104's transport properties, such as processivity and diffusivity. Thus, while ubiquitin-like modifications significantly impact the cargo-binding of UNC-104, they do not alter its transport dynamics, keeping the homeostatic distribution of UNC-104 unchanged.

physics.bio-ph↗

arXiv2024

Emergent Mesoscale Correlations in Active Solids with Noisy Chiral Dynamics

We present the linear response theory for an elastic solid composed of active Brownian particles with intrinsic individual chirality, deriving both a normal mode formulation and a continuum elastic formulation. Using this theory, we compute analytically the velocity correlations and energy spectra under different conditions, showing an excellent agreement with simulations. We generate the corresponding phase diagram, identifying chiral and achiral disordered regimes (for high chirality or noise levels), as well as chiral and achiral states with mesoscopic-range order (for low chirality and noise). The chiral ordered states display mesoscopic spatial correlations and oscillating time correlations, but no wave propagation. In the high chirality regime, we find a peak in the elastic energy spectrum that leads to a non-monotonic behavior with increasing noise strength that is consistent with the emergence of the `hammering' state recently identified in chiral glasses. Finally, we show numerically that our theory, despite its linear response nature, can be applied beyond the idealized homogeneous solid assumed in our derivations. Indeed, by increasing the level of activity, we show that it remains a good approximation of the system dynamics until just below the melting transition. In addition, we show that there is still an excellent agreement between our analytical results and simulations when we extend our results to heterogeneous solids composed of mixtures of active particles with different intrinsic chirality and noise levels. Our work provides a thorough analytical and numerical description of the emergent states in a densely packed system of chiral self-propelled Brownian disks, thus allowing a detailed understanding of the phases and dynamics identified in a minimal chiral active systems.

cond-mat.soft↗

arXiv2024

Impact of chirality on active Brownian particle: Exact moments in two and three dimensions

In this work, we investigate the effects of chirality, accounting for translational diffusion, on active Brownian particles in two and three dimensions. Despite the inherent complexity in solving the Fokker-Planck equation, we demonstrate a Laplace transform method for precisely calculating the temporal evolution of various dynamic moments. Our analysis yields explicit expressions for multiple moments, such as the second and fourth moments of displacement, revealing the impact of persistence and chirality. These moments exhibit oscillatory behaviour, and excess kurtosis indicates deviations from the Gaussian distribution during intermediate time intervals.

cond-mat.stat-mech↗

arXiv2024

Self-Aligning Polar Active Matter

Self-alignment describes the property of a polar active unit to align or anti-align its orientation towards its velocity. In contrast to mutual alignment, where the headings of multiple active units tend to directly align to each other -- as in the celebrated Vicsek model --, self-alignment impacts the dynamics at the individual level by coupling the rotation and displacements of each active unit. This enriches the dynamics even without interactions or external forces, and allows, for example, a single self-propelled particle to orbit in a harmonic potential. At the collective level, self-alignment modifies the nature of the transition to collective motion already in the mean field description, and it can also lead to other forms of self-organization such as collective actuation in dense or solid elastic assemblies of active units. This has significant implications for the study of dense biological systems, metamaterials, and swarm robotics. Here, we review a number of models that were introduced independently to describe the previously overlooked property of self-alignment and identify some of its experimental realizations. Our aim is three-fold: (i)~underline the importance of self-alignment in active systems, especially in the context of dense populations of active units and active solids; (ii)~provide a unified mathematical and conceptual framework for the description of self-aligning systems; (iii)~discuss the common features and specific differences of the existing models of self-alignment. We conclude by discussing promising research avenues in which the concept of self-alignment could play a significant role.

cond-mat.soft↗

arXiv2023

Noise-induced quenched disorder in dense active systems

We report and characterize the emergence of a noise-induced state of quenched disorder in a generic model describing a dense sheet of active polar disks with non-isotropic rotational and translational dynamics. In this state, randomly oriented self-propelled disks become jammed, only displaying small fluctuations about their mean positions and headings. The quenched disorder phase appears at intermediate noise levels, between the two states that typically define the flocking transition (a standard disordered state that displays continuously changing headings due to rotational diffusion and a polar order state of collective motion). We find that the angular fluctuations in this dense system follow an Ornstein-Uhlenbeck process leading to retrograde forces that oppose self-propulsion, and determine its properties. Using this result, we explain the mechanism behind the emergence of the quenched state and compute analytically its critical noise, showing that it matches our numerical simulations. We argue that this novel type of state could be observed in a broad range of natural and artificial dense active systems with repulsive interactions.

cond-mat.soft↗

arXiv2021

Self-propulsion with speed and orientation fluctuation: exact computation of moments and dynamical bistabilities in displacement

We consider the influence of active speed fluctuations on the dynamics of a $d$-dimensional active Brownian particle performing a persistent stochastic motion. We use the Laplace transform of the Fokker-Planck equation to obtain exact expressions for time-dependent dynamical moments. Our results agree with direct numerical simulations and show several dynamical crossovers determined by the activity, persistence, and speed fluctuation.The persistence in the motion leads to anisotropy, with the parallel component of displacement fluctuation showing sub-diffusive behavior and non-monotonic variation. The kurtosis remains positive at short times determined by the speed fluctuation, crossing over to a negative minimum at intermediate times governed by the persistence before vanishing asymptotically. The probability distribution of particle displacement obtained from numerical simulations in two-dimension shows two crossovers between contracted and expanded trajectories via two bimodal distributions at intervening times. While the speed fluctuation dominates the first crossover, the second crossover is controlled by persistence like in the worm-like chain model of semiflexible polymers.

cond-mat.stat-mech↗

arXiv2021

Active Brownian motion with speed fluctuations in arbitrary dimensions: exact calculation of moments and dynamical crossovers

We consider the motion of an active Brownian particle with speed fluctuations in d-dimensions in the presence of both translational and orientational diffusion. We use an Ornstein-Uhlenbeck process for active speed generation. Using a Laplace transform approach, we describe and use a Fokker-Planck equation-based method to evaluate the exact time dependence of all relevant dynamical moments. We present explicit calculations of such moments and compare our analytical predictions against numerical simulations to demonstrate and analyze several dynamical crossovers. The kurtosis of displacement shows positive or negative deviations from a Gaussian behavior at intermediate times depending on the dominance of speed or orientational fluctuations.

cond-mat.stat-mech↗

arXiv2020

Filament-motor protein system under loading: instability and limit cycle oscillations

We consider the dynamics of a rigid filament in a motor protein assay under external loading. The motor proteins are modeled as active harmonic linkers with tail ends immobilized on a substrate. Their heads attach to the filament stochastically to extend along it, resulting in a force on the filament, before detaching. The rate of extension and detachment are load dependent. Here we formulate and characterize the governing dynamics in the mean field approximation using linear stability analysis, and direct numerical simulations of the motor proteins and filament. Under constant loading, the system shows transition from a stable configuration to instability towards detachment of the filament from motor proteins. Under elastic loading, we find emergence of stable limit cycle oscillations via a supercritical Hopf bifurcation with change in activity and the number of motor proteins. Numerical simulations of the system for large number of motor proteins show good agreement with the mean field predictions.

physics.bio-ph↗