arXiv subjects

Urna Basu

Publications and source records attributed to Urna Basu.

At least 19 recordsLinked to original sources

arXiv2026

Non-monotonic diffusion from nonequilibrium driving

The stochastic dynamics of interacting particles far from equilibrium remains a fundamental challenge in statistical physics. While reciprocal interactions often permit effective one-body descriptions, such reductions generally fail for nonreciprocal interactions, which are ubiquitous in driven and active systems. We develop a unified theoretical framework for interacting particles with reciprocal and nonreciprocal couplings, applicable to the principal classes of active matter, including run-and-tumble, active Brownian, and active Ornstein-Uhlenbeck particles. As a minimal example, we study a passive particle driven by an active particle. On a periodic ring, we show analytically that the driven particle is always diffusive at long times, independent of the microscopic driving mechanism. Analytical predictions and numerical simulations reveal a nonmonotonic dependence of the effective diffusivity on the driving activity, giving rise to both enhanced and suppressed transport. Remarkably, the same behavior occurs in an equilibrium system driven out of equilibrium by coupling the driving particle to a higher local temperature. Our framework quantitatively captures both systems, identifies the common mechanism underlying the nonmonotonic transport, and establishes a unified description of transport under active and passive nonequilibrium driving.

cond-mat.stat-mech↗

arXiv2026

Collective Ring Formation in Active Matter

We study the formation of ring-like structures in interacting active particle systems in two dimensions. The emergent structure shows signatures of both spatial and orientational organization. The spatial organization is characterized by the radial distance of a tagged particle from the centroid of the assembly, while orientational organization is characterized by the radial alignment of its self-propulsion direction. We derive exact analytical expressions for the radial and polarization distributions for systems of active Brownian particles and run-and-tumble particles. While both models exhibit annular steady states, we show that their spatial and orientational organization differ qualitatively in the strongly active regime. A direct comparison of the two models reveals how the nature of the propulsion mechanism leads to the distinction in both the structure of the annulus and the statistics of particle orientations. Our results provide a unified analytical framework for characterizing emergent annular states in active matter and identify robust signatures that distinguish persistent active dynamics with continuous and discrete reorientation.

cond-mat.soft↗

arXiv2026

Universal Features in Atmospheric Particulate Matter Dynamics

We study statistical properties of atmospheric particulate matter fluctuations using six years of daily PM2.5 concentration data from fifty-four Indian cities. Despite diverse urban settings and heterogeneous climatic conditions, we find that the fluctuations show strikingly universal behaviour in both the distributional properties and temporal dynamics. After removing slow trends and seasonal components, the rescaled probability density functions of the residual fluctuations collapse onto a single curve and are well described by an exponentially modified Gaussian distribution. The rescaled residual time-series for all the cities further exhibit certain robust dynamical features, with similar decay of auto-correlation functions, and power spectral densities displaying a similar 1/f decay at the tails. Finally, we propose a minimal stochastic model for the residual dynamics, which explains the observed universal features -- the stationary distribution, temporal correlation, and spectral scaling.

physics.soc-ph↗

arXiv2026

Propulsion dispersion mediated ordering transition in active particles

We show that dispersion in propulsion strength qualitatively alters collective behavior of active multi-particle systems interacting via short-range attractive potential, giving rise to novel ordered phases that combine spatial and orientational ordering. Considering a binary mixture of active Brownian particles with two distinct self-propulsion strengths, we find that, the interplay between interaction range, self-propulsion strengths and the relative numbers of the particles with different propulsion strengths can lead to three different phases, namely, a disordered one, and two ordered ones with partial and complete spatial and orientational ordering. The partially ordered phase is characterized by formation of a ring-like assembly of the slower particles while the faster particles diffuse randomly. Two concentric rings, comprising faster and slower particles, form in the fully ordered phase. Using the example of a truncated harmonic potential, we analytically characterize the phase boundaries and identify the associated order parameters. Our results demonstrate that propulsion dispersion provides a robust and novel route to collective ordering in attractive active matter.

cond-mat.stat-mech↗

arXiv2025

Ergodicity Breaking in Active Run-and-Tumble Particles in a Double-Well Potential

We investigate the dynamics of a run-and-tumble particle in a double-well potential and demonstrate that, in stark contrast to Brownian particles, active dynamics can lead to strong ergodicity breaking. When the barrier height exceeds a critical threshold, the long-time position distribution depends crucially on the initial condition: if the particle starts within the basin of attraction of one well, it remains trapped there, while if it begins between the two basins, it can reach either well with a finite probability, which we compute exactly via hitting probabilities. Below the critical barrier height, ergodicity is restored and the system converges to a unique stationary distribution, which we derive analytically. Using this result, we also estimate the characteristic barrier crossing time and show that it violates Kramer's-Arrhenius law, and displays a divergence near the critical height following a Vogel-Fulcher-Tammann-like form with an anomalous exponent $1/2$.

cond-mat.stat-mech↗

arXiv2025

Classifying Urban Regions by Aggregated Pollutant Weather Correlation Strength: A Spatiotemporal Study

Understanding pollutant meteorology interactions is essential for environmental risk assessment. This study develops an entropy-based statistical framework to analyze static and temporal dependencies between urban air pollutants and meteorological variables across multiple Indian cities. Dependence is quantified using complementary linear and nonlinear measures, including Pearson correlation, mutual information, and relative conditional entropy. A key methodological contribution is a PCA based composite indexing framework that integrates these heterogeneous metrics into a unified and interpretable correlation score. For each pollutant meteorological pair within a city, PCA is used to extract a joint variability index, while spatial variability is assessed by aggregating correlations across cities. These indices are further combined to derive a comprehensive city-level correlation score that represents overall pollutant meteorology coupling strength and enables classification of cities into distinct interaction regimes. Sensitivity analysis, performed by systematically excluding individual variable pairs, demonstrates the robustness of the framework, with no single pair exerting disproportionate influence. Temporal dependencies are examined using transfer entropy and time-delayed mutual information. Results indicate that relative humidity generally leads changes in pollutant concentrations, whereas ambient temperature tends to lag, highlighting contrasting causal influences. Mutual information peaks at zero lag and decays rapidly, indicating strong short term interactions with limited persistence. Overall, the proposed framework provides a unified and interpretable approach for assessing complex pollutant meteorology interactions across diverse locations and time.

physics.soc-ph↗

arXiv2025

Stochastic Two-temperature Nonequilibrium Ising model

We investigate the nonequilibrium stationary state (NESS) of the two-dimensional Ising model under a stochastic dichotomous modulation of temperature, which alternates between $T_c \pm \delta$ around the critical temperature $T_c$ at a rate $\gamma$. Both magnetization and energy exhibit non-monotonic dependence on $\gamma$, explained by a renewal approach in the slow-switching limit, while for small $\delta$ dynamical response theory quantitatively captures the $\gamma$-dependence of the observables. In the fast-switching regime, the NESS appears Boltzmann-like with a $\gamma$-dependent effective temperature. However, a finite energy current flowing through the system from hot to cold reservoir confirms the intrinsic nonequilibrium nature of the dynamics.

cond-mat.stat-mech↗

arXiv2025

Entropy-Based Analysis of Urban Pollutant-Weather Correlations

We employ statistical physics and information-theoretic methods to quantify the dependencies between key atmospheric pollutants and meteorological variables across multiple Indian cities. To capture both linear and nonlinear relationships, we introduce a Composite Correlation Index (CCI) that combines the Pearson correlation coefficient with entropy-based measures, including mutual information and conditional entropy. Based on the CCI values, cities are clustered into distinct groups, uncovering regional similarities in pollutant-meteorology interactions that may reflect shared climatic or environmental conditions. To explore temporal structure and causal dynamics, we analyze the relationship between particulate matter (PM2.5) and relative humidity (RH) using transfer entropy, which reveals a bidirectional flow of information in most locations. Further time-domain analysis via time-delayed mutual information shows that, in many cities, the dependence between PM2.5 and RH peaks at zero lag and decays exponentially thereafter, indicating predominantly contemporaneous interactions with limited memory. This integrative framework provides a robust approach to characterizing atmospheric interaction regimes, bridging statistical physics with environmental complexity and revealing new insights into the pollutant-meteorology dynamics.

physics.soc-ph↗

arXiv2025

Universal winding properties of chiral active motion

We propose the area swept $A(t)$ and the winding angle $\Omega(t)$ as the key observables to characterize chiral active motion. We find that the distributions of the scaled area and the scaled winding angle are described by universal scaling functions across all well-known models of active particles, parametrized by the chirality $\omega$, along with a self-propulsion speed $v_0$, and the persistence time $\tau$. In particular, we show that, at late times, the average winding angle grows logarithmically with time $\la\Omega \ra\sim(\omega\tau/2)\,\ln t$, while the average area swept has a linear temporal growth $\la A(t)\ra\simeq(\omega\tau D_{\text{eff}})\,t$, where $D_{\text{eff}}=v_0^2 \tau /[2(1+ \omega^2 \tau^2)]$ is the effective diffusion coefficient. Moreover, we find that the distribution of the scaled area $z=[A-\la A\ra]/(2D_{\text{eff}}t)$ is described by the universal scaling function $F_{\text{ch}}(z)=\text{sech}(\pi z)$. From extensive numerical evidence, we conjecture the emergence of a new universal scaling function $G_{\text{ch}}(z)=\mathcal {N}/[e^{\alpha z} + e^{-\beta z}]$ for the distribution of the scaled winding angle $z=\Omega/[\ln t]$, where the parameters $\alpha$ and $\beta$ are model-dependent and $\mathcal{N}$ is the normalization constant. In the absence of chirality, i.e., $\omega=0$, the scaling function becomes $G_{\text{ch}}(z)=(\alpha/\pi)\,\mathrm{sech}(\alpha z)$.

cond-mat.stat-mech↗

arXiv2025

From Attraction to Repulsion: Emergent Interactions in Harmonically Coupled Active Binary System

We investigate the emergent interactions between two active Brownian particles coupled by an attractive harmonic potential and in contact with a thermal reservoir. By analyzing the stationary distribution of their separation, we demonstrate that the effective interaction can be either attractive or repulsive, depending on the interplay between activity, coupling strength, and temperature. Notably, we find that an effective short-range repulsion emerges in the strong and moderate-coupling regimes, when the temperature is below some threshold value, which we characterize analytically. In the strong-coupling regime, the repulsion emerges solely due to the difference in the self-propulsion speeds of the particles. We also compute the short-time position distribution of the centroid of the coupled particles, which shows strongly non-Gaussian fluctuations at low temperatures.

cond-mat.stat-mech↗

arXiv2025

An entropy based comparative study of regional and seasonal distributions of particulate matter in Indian cities

Particulate matter (PM), especially $\text{PM}_{2.5}$, is a critical air pollutant posing significant risks to human health and the environment in India. This study, using six years (2018-2024) of daily $\text{PM}_{2.5}$ data, investigates the seasonal characteristics of the distributions of $\text{PM}_{2.5}$ concentrations across eleven Indian cities, selected from different regions of the country. We find that, while each city has its own unique seasonal patterns, all of them show a universal exponential decay in the tail of the $\text{PM}_{2.5}$ distribution for all the seasons. However, the decay rates of this tail vary across cities, highlighting regional and seasonal disparities in pollution levels. To quantitatively characterize the {\it randomness} of the seasonal $\text{PM}_{2.5}$ concentration distributions, we compute Shannon entropy, a key information theoretic measure. This allows for classifying cities into different groups, according to the level of randomness observed in their seasonal distributions. To further explore the inter-city relationships, we employ Jensen-Shannon divergence (JSD), a symmetric measure of relative entropy, to quantitatively assess the degree of similarity in the $\text{PM}_{2.5}$ distributions among different cities. Remarkably, we find that several cities show very similar distributions in the winter months, which helps us to categories them into several groups. The groups obtained from these entropy based measures, namely, individual Shannon entropy and the JSD estimate, are consistent with each other, providing a robust framework for efficient air quality management and policy-making in India.

physics.soc-ph↗

arXiv2024

Emergent short-range repulsion for attractively coupled active particles

We show that heterogeneity in self-propulsion speed can lead to the emergence of a robust effective short-range repulsion among active particles interacting via long-range attractive potentials. Using the example of harmonically coupled active Brownian particles, we analytically derive the stationary distribution of the pairwise distances and reveal that the heterogeneity in propulsion speeds induces a characteristic scale of repulsion between particles. This length scale algebraically increases with the difference in their self-propulsion speeds. In contrast to the conventional view that activity in active matter systems typically leads to effective attraction, our results demonstrate that activity can give rise to an emergent repulsive interaction. This phenomenon is universal, independent of the specific dynamics of the particles or the presence of thermal fluctuations. We also discuss possible experimental realization of this counter-intuitive phenomenon.

cond-mat.stat-mech↗

arXiv2024

Inertial Dynamics of Run-and-Tumble Particle

We study the dynamics of a single inertial run-and-tumble particle on a straight line. The motion of this particle is characterized by two intrinsic time-scales, namely, an inertial and an active time-scale. We show that interplay of these two time-scales leads to the emergence of four distinct regimes, characterized by different dynamical behaviour of mean-squared displacement and survival probability. We analytically compute the position distributions in these regimes when the two time-scales are well separated. We show that in the large-time limit, the distribution has a large deviation form and compute the corresponding large deviation function analytically. We also find the persistence exponents in the different regimes theoretically. All our results are supported with numerical simulations.

cond-mat.stat-mech↗

arXiv2024

Universal Dynamics of a Passive Particle Driven by Brownian Motion

We investigate the overdamped dynamics of a `passive' particle driven by nonreciprocal interaction with a `driver' Brownian particle. When the interaction between them is short-ranged, the long-time behavior of the driven particle is remarkably universal -- the mean-squared displacement (MSD) and the typical position of the driven particle exhibits the same qualitative behaviors independent of the specific form of the potential. In particular, the MSD grows as $t^{1/2}$ in one dimension and $\log t$ in two spatial dimensions. We compute the exact scaling functions for the position distribution in $d=1$ and $d=2$. These functions are universal when the interaction is short-ranged. For long-ranged interactions, the MSD of the driven particle grows as $t^{\phi}$ with exponent $\phi$ depending on the tail of the potential.

cond-mat.stat-mech↗

arXiv2024

Harmonically trapped inertial run-and-tumble particle in one dimension

We study the nonequilibrium stationary state of a one-dimensional inertial run-and-tumble particle (IRTP) trapped in a harmonic potential. We find that the presence of inertia leads to two distinct dynamical scenarios, namely, overdamped and underdamped, characterized by the relative strength of the viscous and the trap time-scales. We also find that inertial nature of the active dynamics leads to the particle being confined in specific regions of the phase plane in the overdamped and underdamped cases, which we compute analytically. Moreover, the interplay of the inertial and active time-scales gives rise to several sub-regimes, which are characterized by very different behaviour of position and velocity fluctuations of the IRTP. In particular, in the underdamped regime, both the position and velocity undergoes transitions from a novel multi-peaked structure in the strongly active limit to a single peaked Gaussian-like distribution in the passive limit. On the other hand, in the overdamped scenario, the position distribution shows a transition from a U-shape to a dome-shape, as activity is decreased. Interestingly, the velocity distribution in the overdamped scenario shows two transitions -- from a single-peaked shape with an algebraic divergence at the origin in the strongly active regime to a double peaked one in the moderately active regime to a dome-shaped one in the passive regime.

cond-mat.stat-mech↗

arXiv2024

Harmonic chain driven by active Rubin bath: transport properties and steady-state correlations

Characterizing the properties of an extended system driven by active reservoirs is a question of increasing importance. Here we address this question in two steps. We start by investigating the dynamics of a probe particle connected to an `active Rubin bath' -- a linear chain of overdamped run-and-tumble particles. We derive exact analytical expressions for the effective noise and dissipation kernels, acting on the probe, and show that the active nature of the bath leads to a modified fluctuation-dissipation relation. In the next step, we study the properties of an activity-driven system, modeled by a chain of harmonic oscillators connected to two such active reservoirs at the two ends. We show that the system reaches a nonequilibrium stationary state (NESS), remarkably different from that generated due to a thermal gradient. We characterize this NESS by computing the kinetic temperature profile, spatial and temporal velocity correlations of the oscillators, and the average energy current flowing through the system. It turns out that, the activity drive leads to the emergence of two characteristic length scales, proportional to the activities of the reservoirs. Strong signatures of activity are also manifest in the anomalous short-time decay of the velocity autocorrelations. Finally, we find that the energy current shows a non-monotonic dependence on the activity drive and reversal in direction, corroborating previous findings.

cond-mat.stat-mech↗

arXiv2024

Tagged particle behavior in a harmonic chain of direction reversing active Brownian particles

We study the tagged particle dynamics in a harmonic chain of direction reversing active Brownian particles, with spring constant $k$, rotation diffusion coefficient $D_{\text{r}}$, and directional reversal rate $\gamma$. We exactly compute the tagged particle position variance for quenched and annealed initial orientations of the particles. For well-separated time scales, $k^{-1}$, $D_{\text{r}}^{-1}$ and $\gamma^{-1}$, the strength of spring constant $k$ relative to $D_{\text{r}}$ and $\gamma$ gives rise to different coupling limits and for each coupling limit there are short, intermediate, and long time regimes. In the thermodynamic limit, we show that, to the leading order, the tagged particle variance exhibits an algebraic growth $t^{\nu}$, where the value of the exponent $\nu$ depends on the specific regime. For a quenched initial orientation, the exponent $\nu$ crosses over from $3$ to $1/2$, via intermediate values $5/2$ or $1$, depending on the specific coupling limits. On the other hand, for the annealed initial orientation, $\nu$ crosses over from $2$ to $1/2$ via an intermediate value $3/2$ or $1$ for strong coupling limit and weak coupling limit respectively. An additional time scale $t_N=N^2/k$ emerges for a system with a finite number of oscillators $N$. We show that the behavior of the tagged particle variance across $t_N$ can be expressed in terms of a crossover scaling function, which we find exactly. Finally, we characterize the stationary state behavior of the separation between two consecutive particles by calculating the corresponding spatio-temporal correlation function.

cond-mat.stat-mech↗

arXiv2023

Target search by active particles

Active particles, which are self-propelled nonequilibrium systems, are modelled by overdamped Langevin equations with colored noise, emulating the self-propulsion. In this chapter, we present a review of the theoretical results for the target search problem of these particles. We focus on three most well-known models, namely, run-and-tumble particles, active Brownian particles, and direction reversing active Brownian particles, which differ in their self-propulsion dynamics. For each of these models, we discuss the first-passage and survival probabilities in the presence of an absorbing target. We also discuss how resetting helps the active particles find targets in a finite time.

cond-mat.stat-mech↗