arXiv subjects

R. K. Singh

Publications and source records attributed to R. K. Singh.

At least 19 recordsLinked to original sources

arXiv2026

Harvesting Reshapes Dynamical Populations

Harvesting -- the periodic removal of individuals above or below a threshold trait value -- reshapes heterogeneous populations without altering their underlying stochastic dynamics. We study how repeated harvesting events steer the evolution of probability densities for classes of stochastic processes exhibiting both normal and anomalous dynamics, as well as a prototypical predator-prey model. Removal of the upper portion of the density drives the system to a quasi-steady state when viewed at the ``harvesting clock''. This state depends only on the harvesting threshold and frequency but not on the initial conditions. Removal of the lower portion of the density fixes its shape while generating a constant effective drift that exceeds that of the unharvested mean. Our results suggest the possibility of manipulating the dynamics of stochastic populations through external selection interventions.

cond-mat.stat-mech↗

arXiv2026

Network exploration by random walks: A large deviation perspective

We study exploration properties of a random walk on a network. For a fully connected network we find that the problem can be mapped to the well known coupon collector problem, thus allowing us to estimate form of $P(S,t)$: the distribution of number of distinct nodes $S$ visited by the random walk upto time $t$. From a practical point of view, however, both the fully connected network and hops taking place after fixed intervals are an idealization. We solve this problem by introducing the formalism of continuous time random walks wherein the random walk spends a random amount of time a node before hopping to its neighboring node. The formalism allows us to study the large deviation limit of $P(S,t)$ under very mild conditions that the distribution of waiting times $ψ(τ)$ exhibits analyticity at small times. Furthermore, we find that at small times, the properties of $P(S,t)$ are largely independent of the network topology, and are governed solely by the waiting time characteristics.

physics.soc-ph↗

arXiv2025

Deep Learning-Enhanced Analysis for Delineating Anticoagulant Essay Efficacy Using Phase Microscopy

The coagulation of blood after it is drawn from the body poses a significant challenge for hematological analysis, potentially leading to inaccurate test results and altered cellular characteristics, compromising diagnostic reliability. This paper presents a deep learning-enhanced framework for delineating anticoagulant efficacy ex vivo using Digital Holographic Microscopy (DHM). We demonstrate a label-free, non-invasive approach for analyzing human blood samples, capable of accurate cell counting and morphological estimation. A DHM with an automated image processing and deep learning pipeline is built for morphological analysis of the blood cells under two different anti-coagulation agents, e.g. conventional EDTA and novel potassium ferric oxalate nanoparticles (KFeOx-NPs). This enables automated high-throughput screening of cells and estimation of blood coagulation rates when samples are treated with different anticoagulants. Results indicated that KFeOx-NPs prevented human blood coagulation without altering the cellular morphology of red blood cells (RBCs), whereas EDTA incubation caused notable changes within 6 hours of incubation. The system allows for quantitative analysis of coagulation dynamics by assessing parameters like cell clustering and morphology over time in these prepared samples, offering insights into the comparative efficacy and effects of anticoagulants outside the body.

physics.optics↗

arXiv2025

Steady state and relaxation dynamics of run and tumble particles in contact with a heat bath

We study the relaxation dynamics of a run and tumble particle in a one-dimensional piecewise linear potential $U(x)=b|x|$, from delta-function initial conditions at $x=0$ to steady state. In addition to experiencing active telegraphic noise, the particle is in contact with a heat bath at temperature $T$ that applies white thermal noise. We find that the position distribution of the RTP is described by a sum of two distributions ("modes"), each of which of the form $P(x,t\to\infty)\sim e^{-λ_i|x|}$ ($i=1,2$) at steady state. The two modes are dynamically coupled: At very short times ($t\to 0$), each mode stores half of the probability, and exhibits thermal diffusive spreading with a Gaussian profile. With progressing time and evolution toward steady state, the partition of probability between the modes becomes increasingly uneven and, depending on the model parameters, the mode with the smaller value of $λ_i$ may carry an overwhelming majority of the probability. Moreover, we identify that the characteristic relaxation time of each mode is $τ_i=(λ_i^2T)^{-1}$, which implies that the minority mode also relaxes much faster than the dominant one. A more detailed analysis reveals that $τ_i$ is characteristic of the mode relaxation only close to the origin at the core of the distribution, while further away it increases linearly with $|x|$ as if a relaxation front is propagating at constant speed $v_i^*=2\sqrt{T/τ_i}$ in the system. The rate of non-equilibrium entropy production can be related to the two-mode splitting of the probability distribution and be expressed in terms of their correlation-lengths $λ_i$ and their contributions to the steady state distribution.

cond-mat.stat-mech↗

arXiv2024

The Emergence of Laplace Universality in Correlated Processes

In transport processes across materials like glasses, living cells, and porous media, the probability density function of displacements exhibits exponential decay rather than Gaussian behavior. We show that this universal behavior of rare events, termed Laplace tails, emerges even when correlations and memory affect the dynamics. Using a renormalization-based approach, we demonstrate that correlations and memory do not suppress these tails but rather enhance their visibility, even at short timescales. The developed analytical framework refines the concept of correlations for rare events and enables the computation of effective parameters that govern Laplace tails in correlated processes. These findings suggest that correlations can serve as a tunable parameter to control the behavior of rare events in transport.

cond-mat.stat-mech↗

arXiv2024

Restarts delay escape over a potential barrier

In the barrier escape problem, a random searcher starting at the energy minima tries to escape the barrier under the effect of thermal fluctuations. If the random searcher is subject to successive restarts at the bottom of the well, then its escape over the barrier top is delayed compared to the time it would take in absence of restarts. When restarting at an intermediate location, the time required by the random searcher to go from the bottom of the well to the restart location should be considered. Taking into account this time overhead, we find that restarts delay escape, independent of the specific nature of the distribution of restart times, or the location of restart, or the specific details of the random searcher. For the special case of Poisson restarts, we study the escape problem for a Brownian particle with a position-dependent restart rate $r(x)θ(x^p_0-x)$, with $x^p_0$ being the location of restart. We find that position-dependent restarts delay the escape as compared to Kramers escape time. We also study ways of modifying time overheads which help expedite escape under restarts.

cond-mat.stat-mech↗

arXiv2024

Competing random searchers under restarts

We study independent searchers competing for a target under restarts and find that introduction of restarts tends to enhance the search efficiency of an already efficient searcher. As a result, the difference between the search probabilities of the individual searchers increases when the system is subject to restarts. This result holds true independent of the identity of individual searchers or the specific details of the distribution of restart times. However, when only one of a pair of searchers is subject to restarts while the other evolves in an unperturbed manner, a concept termed as subsystem restarts, we find that the search probability exhibits a nonmonotonic dependence on the restart rate. We also study the mean search time for a pair of run and tumble and Brownian searchers when only the run and tumble particle is subject to restarts. We find that, analogous to restarting the whole system, the mean search time exhibits a nonmonotonic dependence on restart rates.

cond-mat.stat-mech↗

arXiv2024

Synchronous and Asynchronous Updates of Active Ising Spins in One Dimension

How do update rules affect the dynamical and steady state properties of a flock? In this study, we have explored the active Ising spins (s = +-1) in one dimension, where spin updates its orientation according to the Metropolis algorithm (based on the neighbors) via two different update rules. (i) Parallel, and (ii) Random-sequential. We explore the effect of Parallel and Random-sequential updates on the dynamical properties of flocks in one dimension. Due to the inherent asynchronous nature of the Random-sequential update, the directional switching of the flock is increased compared to the Parallel one. The nature of phase transition is affected by the difference in the updating mechanism: discontinuous for Parallel and continuous for Random-sequential updates.

cond-mat.soft↗

arXiv2023

Escape of a lamb to safe haven in pursuit by a lion under restarts

We study the escape behavior of a lamb to safe haven pursued by a hungry lion. Identifying the system with a pair of vicious Brownian walkers we evaluate the probability density function for the vicious pair and from there we estimate the distribution of first passage times. The process ends in two ways: either the lamb makes it to the safe haven (success) or is captured by the lion (failure). We find that the conditional distribution for both success and failure possesses a finite mean, but no higher moments exist. This makes it interesting to study these first passage properties of this Bernoulli process under restarts, which we do via Poissonian and sharp restart protocols. We find that under both restart protocols the probability of success exhibits a monotonic dependence on the restart parameters, however, their approach to the case without restarts is completely different. The distribution of first passage times exhibits an exponential decay for the two restart protocols. In addition, the distribution under sharp resetting also exhibits a periodic behavior, following the periodicity of the sharp restart protocol itself.

cond-mat.stat-mech↗

arXiv2023

Universal to Non-Universal Transition of the statistics of Rare Events During the Spread of Random Walks

Particle hopping is a common feature in heterogeneous media. We explore such motion by using the widely applicable formalism of the continuous time random walk and focus on the statistics of rare events. Numerous experiments have shown that the decay of the positional probability density function P (X, t), describing the statistics of rare events, exhibits universal exponential decay. We show that such universality ceases to exist once the threshold of exponential distribution of particle hops is crossed. While the mean hop is not diverging and can attain a finite value; the transition itself is critical. The exponential universality of rare events arises due to the contribution of all the different states occupied during the process. Once the reported threshold is crossed, a single large event determines the statistics. In this realm, the big jump principle replaces the large deviation principle, and the spatial part of the decay is unaffected by the temporal properties of rare events.

cond-mat.stat-mech↗

arXiv2022

Capture of a diffusing lamb by a diffusing lion when both return home

A diffusing lion pursues a diffusing lamb when both of them are allowed to get back to their homes intermittently. Identifying the system with a pair of vicious random walkers, we study their dynamics under Poissonian and sharp resetting. In absence of any resets, the location of intersection of the two walkers follows a Cauchy distribution. In presence of resetting, the distribution of the location of annihilation is composed of two parts: one in which the trajectories cross without being reset (center) and the other where trajectories are reset at least once before they cross each other (tails). We find that the tail part decays exponentially for both the resetting protocols. The central part of the distribution, on the other hand, depends on the nature of the restart protocol, with Cauchy for Poisson resetting and Gaussian for sharp resetting. We find good agreement of the analytical results with numerical calculations.

cond-mat.stat-mech↗

arXiv2022

Inclusive $α$ Production for $^{6}$Li+$^{51}$V System

Measurement of angular distributions and energy spectra of $α$ and deuterons through breakup, transfer and incomplete fusion processes to dis-entangle their relative contributions and to investigate relative importance of breakup-fusion compared to transfer. Inclusive $α$ production cross-sections have been measured for $^6$Li + $^{51}$V system near Coulomb barrier energies. Theoretical calculations for estimation of various reaction channels contributing to $α$ production have been performed with finite range coupled reaction method using \textsc{FRESCO} code. The cross-sections from non-capture breakup (NCBU) ($α$ + \textit{d}) and 1\textit{n}, 1\textit{p}, and 1\textit{d} transfer channels, compound nuclear decay channel and incomplete fusion (ICF) leading to $α$ production were estimated to get the cumulative production cross-sections. Contributions from breakup, transfer and incomplete fusion channels could reproduce the integral direct $α$ production cross-sections and their angular distributions quite well. The direct $α$ production cross-sections are in agreement with other targets. The $α$ production cross-sections are higher compared to the deuteron production. Kinematic analysis of the energy spectra of $α$ particles and deuterons suggest that $α$ particle spectra is dominated by breakp-fusion and deuteron spectra have contribution of breakup and transfer reactions. A systematic study of direct $α$ production with various targets follow a universal behavior on average but noticeable differences are observed for different targets. A ratio of $α$ and deuteron yields for a wide mass range of targets shows a saturation above barrier and an increasing production of $α$ particles relative to deuteron around Coulomb barrier.

nucl-ex↗

arXiv2022

General approach to stochastic resetting

We address the effect of stochastic resetting on diffusion and subdiffusion process. For diffusion we find that MSD relaxes to a constant only when the distribution of reset times possess finite mean and variance. In this case, the leading order contribution to the PDF of a Gaussian propagator under resetting exhibits a cusp independent of the specific details of the reset time distribution. For subdiffusion we derive the PDF in Laplace space for arbitrary resetting protocol. Resetting at constant rate allows evaluation of the PDF in terms of H-function. We analyze the steady state and derive the rate function governing the relaxation behavior. For a subdiffusive process the steady state could exist even if the distribution of reset times possesses only finite mean.

cond-mat.stat-mech↗

arXiv2021

A 3D Magnetohydrodynamic simulation for the propagation of plasma plume transverse to applied magnetic field

We have carried out a 3D ideal-MHD (Magnetohydrodynamic) simulation to study the evolution of laser generated plasma plume in a moderate external magnetic field (0.13 T) oriented perpendicular to the flow direction of the plasma plume. The simulation shows that the plasma plume pushes the external magnetic field lines outward in the direction of the expansion. This leads to compression and bending of the magnetic field lines.The force resulting from the change in shape and the density of magnetic field lines opposes the expansion of the plume. An elliptic layer of shocked plasma is formed at the plasma/external field interface leaving a cavity in the plume core due to the outward expansion and the inertia of the plume. As the plasma pressure drops due to expansion, the imbalance between the magnetic energy and the internal energy results in the collapse of the cavity. These observations have striking similarities with the observations of the experiments [Phys. Plasmas 24, 033511 (2017)] performed recently to study the plasma plume expansion in the presence of an external transverse magnetic field. This similarity indicates that the physical mechanisms dominantly governing the plasma plume expansion in the moderate magnetic field are aptly described in the ideal MHD regime. The studies thus show that the laser generated plasma plume can be utilized to carry out interesting experiments on MHD phenomena in a simple laboratory set up.

physics.plasm-ph↗

arXiv2021

Cavitation and charge separation in laser-produced copper and carbon plasma in transverse magnetic field

In the present work, we report the dynamics and geometrical features of the plasma plume formed by the laser ablation of copper and graphite (carbon) targets in the presence of different transverse magnetic field. This work emphasizes on the effect of atomic mass of the plume species on the diamagnetic behaviour and geometrical aspect of the expanding plasma plume in the magnetic field. The time-resolved analysis of the simultaneously captured two directional images in orthogonal to the expansion axis is carried out for the comparative study of projected three-dimensional structure of copper and carbon plasma plume. In the presence of magnetic field, sharp differences are observed between the copper and carbon plasma plumes in terms of formation of diamagnetic cavity and structure formation. An elliptical cavity-like structure is observed in case of copper plasma plume which attains the sharp conical shape with increasing the time delay or magnetic field strength. On the other hand, splitted carbon plasma plume appears as a Y-shape structure in the presence of magnetic field where the cavity-like structure is not observed for the considered time and magnetic field. Based on the modified energy balance relation for the elliptic cylindrical geometry, we have also simulated the dynamics of the plume which is in close agreement with observed plasma expansion in diamagnetic and non-diamagnetic regions.

physics.plasm-ph↗

arXiv2021

Backbone diffusion and first-passage dynamics in a comb structure with confining branches under stochastic resetting

We study the diffusive motion of a test particle in a two-dimensional comb structure consisting of a main backbone channel with continuously distributed side branches, in the presence of stochastic Markovian resetting to the initial position of the particle. We assume that the motion along the infinitely long branches is biased by a confining potential. The crossover to the steady state is quantified in terms of a large deviation function, which is derived for the first time for comb structures in present paper. We show that the relaxation region is demarcated by a nonlinear "light-cone" beyond which the system is evolving in time. We also investigate the first-passage times along the backbone and calculate the mean first-passage time and optimal resetting rate.

cond-mat.stat-mech↗

arXiv2020

Numerical study of the effect of mass of the background gas on the lateral interactions of two plasma plumes at high pressure

The characteristic of the lateral interaction of two plasma plumes in $Ar$ background gas at high pressures was reported in recent publication [Yadav {\it et. al.}, J. Phys. D: Appl. Phys. {\bf 50}, 053421 (2017)]. Further we have investigated the interaction characteristics of plumes in $He$, $Ne$, $Ar$ and $Xe$ background gases to see the effect of mass on the interaction. The present work illustrate the applicability of the present model for theoretical understanding of dynamics, structure, density variation, shock wave formations and their interactions of two propagating plasma plumes in a wide range of ambient conditions. The formation of interaction region, geometrical shape and strength of the shock fronts and subsequent regular and Mach reflections in accordance with the nature and pressure of ambient gas are successfully captured in the simulations. The observed results are supported by the reported experimental observations under identical conditions.

physics.plasm-ph↗

arXiv2020

Effect of magnetic field on the laterally colliding plasma plumes

An experimental investigation of laser produced colliding plasma of aluminium target in the presence of external magnetic field in vacuum is done. Characteristic parameters and line emission of plasma plume in the presence of magnetic field are compared with those for field free case. Axial expansion of the plasma is slowed down in the presence of magnetic field as compared to the field free case. Contrary to the field free case no sharp interaction zone is observed. Higher electron temperature and increased ionic line emission from singly as well as doubly ionized aluminium can be attributed to the Joule heating phenomenon.

physics.plasm-ph↗