arXiv subjects

Pawan Kumar

Publications and source records attributed to Pawan Kumar.

At least 55 records · Page 3Linked to original sources

arXiv2025

The role of magnetic and rotation axis alignment in driving fast radio burst phenomenology

We propose a scenario that can describe a broad range of FRB phenomenology, from non-repeating bursts to highly prolific repeaters. Coherent radio waves in these bursts are produced in the polar cap region of a magnetar, where magnetic field lines are open. The angle between the rotation and magnetic axes, relative to the angular size of the polar cap region, partially determines the repetition rate and polarization properties of FRBs. We discuss how many of the properties of repeating FRBs-such as their lack of periodicity, energetics, small PA swing, spectro-temporal correlation and inferred low source density are explained by this scenario. The systematic PA swing and the periodic modulation of long duration bursts from non-repeaters are also natural outcomes. We derive a lower limit of about 400 on the Lorentz factor of FRB sources applying this scenario to bursts with a linear polarization degree greater than 95\%.

astro-ph.HE↗

arXiv2025

Emergence of Order in Chemically Active Droplets: Temporal Dynamics and Collective Behavior

Collective behaviors such as swarming, chemical signaling, and clustering are fundamental to biological microorganisms, enabling hierarchical colony formation, coordinated motion, and enhanced nutrient accessibility crucial for their survival. Over the past few decades, extensive research has been dedicated to unraveling the mechanisms underlying these diverse collective patterns through experimental model systems. Among these, active droplets have emerged as valuable synthetic analogs, effectively replicating key biological attributes and serving as ideal platforms for investigating collective phenomena. This research explores the collective behavior of 4-Cyano-4-pentyl-biphenyl (5CB) oil droplets across varying Péclet ($Pe$) numbers. At high $Pe$, droplets exhibit a pusher mode of propulsion and form dynamic chain-like patterns. Decreasing $Pe$ enhances repulsive interactions among droplets, resulting in the inhibition of clustering. In the low $Pe$ regime, their repulsive interactions predominated by chemical field lead to the emergence of an ordered structure. Furthermore, we illustrate how active droplets efficiently navigate within a soft structured environment. These findings contribute to our comprehension of self-organized phenomena in active matter systems and provide insights for designing strategies for controlled locomotion in intricate fluidic environments.

cond-mat.soft↗

arXiv2025

Chemical interactions in active droplets

Interactions among biologically active agents is facilitated by their self-generated chemical and hydrodynamic fields. In order to elucidate the pair-wise interactions between such micro-organisms, we employ active droplets as a model system, capable of self-generating chemical and hydrodynamic fields. We demonstrate that the solute Péclet number ($Pe$), characterizing the relative strength of its convective to diffusive transport, plays a crucial role in determining how the chemical and hydrodynamic fields impact their interactions. Our findings reveal that at low $Pe$, the interaction is predominantly governed by chemo-repulsive effects, leading to droplets avoiding physical contact. Conversely, at elevated $Pe$, hydrodynamic interactions become more influential, leading to physical engagement. However, irrespective of $Pe$, the interaction of a droplet with the chemical trail of another droplet is always governed by chemo-repulsive effects. Furthermore, our results establish that the chemo-repulsive deflection/rebounding of droplets is influenced by the droplets' inherent chemical polarity, as determined by its $Pe$, independent of their approach orientation. Our findings offer a methodology for tuning the outcomes of binary interactions among chemically active droplets, laying the groundwork for potential studies on their collective dynamics.

cond-mat.soft↗

arXiv2025

Thermal spin wave noise as a probe for the Dzyaloshinkii-Moriya interaction

Interfacial Dzyaloshinkii-Moriya interaction (DMI) is a key ingredient in the stabilization of chiral magnetic states in thin films. Its sign and strength often determine crucial properties of magnetic objects, like their topology or how they can be manipulated with currents. A few experimental techniques are currently available to measure DMI quantitatively, based on the study of domain walls, spin waves, or spin-orbit torques. In this work, we propose a qualitative variant of spin wave methods. We rely on magnetic noise from confined thermal spin waves in domain walls and skyrmions in perpendicularly magnetized thin films, which we probe with scanning NV center relaxometry. We show both numerically and experimentally that the sign of the DMI can be inferred from the amplitude of the detected noise, which is affected by the non-reciprocity in the spin wave dispersion. Furthermore, we also demonstrate that the noise distribution around the contour of magnetic skyrmions reveals their Néel/Bloch nature, giving therefore also insight into the strength of DMI involved in their stabilization.

cond-mat.mes-hall↗

arXiv2025

Data-Constrained Magnetohydrodynamics Simulation of a Confined X-class Flare in NOAA Active Region 11166

In this paper, we present a magnetohydrodynamics simulation of NOAA active region 11166 to understand the origin of a confined X-class flare that peaked at 23:23 UT on 2011 March 9. The simulation is initiated with a magnetic field extrapolated from the corresponding photospheric magnetogram using a non-force-free-field extrapolation technique. Importantly, the initial magnetic configuration identifies three-dimensional (3D) magnetic nulls and quasi-separatrix layers (QSLs), which nearly agree with the bright structures appeared in multi-wavelength observations. The Lorentz force associated with the extrapolated field self-consistently generates the dynamics that leads to the magnetic reconnections at the 3D nulls and the QSLs. These reconnections are found to contribute to the pre-flare activities and, ultimately, lead to the development of the flare ribbons. Notably, the anchored spine of the 3D null and the complete absence of flux rope in the flaring region are congruent with the confined nature of the flare. Furthermore, the simulation also suggests the role of reconnections at the 3D null with an open spine in the onset of a jet away from the flaring site.

astro-ph.SR↗

arXiv2025

Analyses of features of magnetic cycles at different amounts of dynamo supercriticality: Solar dynamo is about two times critical

The growth of a large-scale magnetic field in the Sun and stars is usually possible when the dynamo number (D) is above a critical value Dc. As the star ages, its rotation rate and thus D decrease. Hence, the question is how far the solar dynamo is from the critical dynamo transition. To answer this question, we have performed a set of simulations using Babcock-Leighton type dynamo models at different values of dynamo supercriticality and analyzed various features of magnetic cycle. By comparing the recovery rates of the dynamo from the Maunder minimum and statistics (numbers and durations) of the grand minima and maxima with that of observations and we show that the solar dynamo is only about two times critical and thus not highly supercritical. The observed correlation between the polar field proxy and the following cycle amplitudes and Gnevyshev-Ohl rule are also compatible with this conclusion.

astro-ph.SR↗

arXiv2024

Dispersion Measures of Fast Radio Bursts through the Epoch of Reionization

Dispersion measures (DM) of fast radio bursts (FRBs) probe the density of electrons in the intergalactic medium (IGM) along their lines-of-sight, including the average density versus distance to the source and its variations in direction. While previous study focused on low-redshift, FRBs are potentially detectable out to high redshift, where their DMs can, in principle, probe the epoch of reionization (EOR) and its patchiness. We present the first predictions from large-scale, radiation-hydrodynamical simulation of fully-coupled galaxy formation and reionization, using Cosmic Dawn (``CoDa")~II to model the density and ionization fields of the universe down to redshifts through the end of the EOR at $z_{re}\approx6.1$. Combining this with an N-body simulation CoDa~II--Dark Matter of the fully-ionized epoch from the EOR to the present, we calculate the mean and standard deviation of FRB DMs as functions of their source redshift. The mean and standard deviation of DM increase with redshift, reaching a plateau by $z(x_{HII}\lesssim0.25)\gtrsim8$, i.e. well above $z_{re}$. The mean-DM asymptote $\mathcal{DM}_{max} \approx 5900~\mathrm{pc\, cm^{-3}}$ reflects the end of the EOR and its duration. The standard deviation there is $σ_{DM, max}\approx497 ~\mathrm{pc\, cm^{-3}}$, reflecting inhomogeneities of both patchy reionization and density. Inhomogeneities in ionization during the EOR contribute $\mathcal{O}(1$ per cent) of this value of $σ_{DM,max}$ from FRBs at redshifts $z\gtrsim 8$. Current estimates of FRB rates suggest this may be detectable within a few years of observation.

astro-ph.CO↗

arXiv2024

Cost-Effective Cyber-Physical System Prototype for Precision Agriculture with a Focus on Crop Growth

In precision agriculture, integrating advanced technologies is crucial for optimizing plant growth and health monitoring. Cyber-physical system (CPS) platforms tailored to specific agricultural environments have emerged, but the diversity of these environments poses challenges in developing adaptive CPS platforms. This paper explores rapid prototyping methods to address these challenges, focusing on non-destructive techniques for estimating plant growth. We present a CPS prototype that combines sensors, microcontrollers, digital image processing, and predictive modeling to measure leaf area and biomass accumulation in hydroponic environments. Our results show that the prototype effectively monitors and predicts plant growth, highlighting the potential of rapid CPS prototyping in promoting sustainability and improving crop yields at a moderate cost of hardware.

eess.SY↗

arXiv2024

The Origins of Narrow Spectra of Fast Radio Bursts

Observations find that some fast radio bursts (FRBs) have extremely narrow-band spectra, i.e., $Δν/ν_0 \ll 1$. We show that when the angular size of the emission region is larger than the Doppler beaming angle, the observed spectral width ($Δν/ν_0$) exceeds 0.58 due to the high latitude effects for a source outside the magnetosphere, even when the spectrum in the source's comoving frame is monochromatic. The angular size of the source for magnetospheric models of FRBs can be smaller than the Doppler beaming angle, in which case this geometric effect does not influence the observed bandwidth. We discuss various propagation effects to determine if any could transform a broad-spectrum radio pulse into a narrow-spectrum signal at the observer's location. We find that plasma lensing and scintillation can result in a narrow bandwidth in the observed spectrum. However, the likelihood of these phenomena being responsible for the narrow observed spectra with $Δν/ν_0 < 0.58$ in the fairly large observed sample of FRBs is exceedingly small.

astro-ph.HE↗

arXiv2024

Nuclear forces and nuclear excited states

In this work, the role of the central, spin-force and tensor forces of two-nucleon interaction in building the first 2$^+$ and 4$^+$ states of 20 sd-shell nuclei is studied. Calculations are performed within the framework of the nuclear shell model. It is shown that the central force predominantly contributes to the excitation energy. While the spin-orbit and tensor forces contribute relatively less, their roles are noted crucial in a few cases, such as in $^{22}$O. Further, it is demonstrated that the Hamiltonian projection approach should be preferred for rightly assessing the contributions of each force to the excitation energy.

nucl-th↗

arXiv2024

Fabrication of low-loss lithium niobate on insulator waveguides on the wafer scale

We report on the wafer scale fabrication of single mode low-loss lithium niobate on insulator waveguides utilizing a chemically amplified resist and an optimized dry etching method. The fabricated single mode waveguides are free of residuals and re-deposition, with measured losses for straight waveguides around 2 dB/m (0.02 dB/cm). We present on a method offering advantages for large-scale production due to its cost-effectiveness, faster writing time, and simplified processes. This work holds promise for advancing integrated photonics and optical communication technologies.

physics.optics↗

arXiv2024

> 2π Phase Modulation using Exciton-Polaritons in a Two-Dimensional Superlattice

Active metamaterials promise to enable arbitrary, temporal control over the propagation of wavefronts of light for applications such as beam steering, optical communication modulators, and holograms. This has been done in the past using patterned silicon photonics to locally control the phase of light such that the metasurface acts as a large number of wavelets. Although phase modulation only requires refractive index modulation when the interaction length is on the order of the wavelength, this is not enough to significantly modulate the phase of light in flatland. Instead, phase modulation is achieved using a resonant mode such as a plasmon or high-Q cavity mode that enable light to accumulate a large amount of phase over a short distance and coupling it to an active material that modulates the light-matter interactions. Here, we report that electrostatic doping can modulate the light-matter interaction strength of a two-dimensional WS2 based multi quantum well (MQW) structure going from strongly-coupled, phase-accumulating exciton-polaritons to weakly-coupled exciton-trion-polaritons. As a result of this transition, 2.02π radians of phase modulation is observed using spectroscopic ellipsometry. This result demonstrates the potential of the MQW structure as a compact, lightweight electro-optical modulators for LiDAR and optical communications in the red region of visible spectrum.

physics.optics↗

arXiv2024

Overlay Space-Air-Ground Integrated Networks with SWIPT-Empowered Aerial Communications

In this article, we consider overlay space-air-ground integrated networks (OSAGINs) where a low earth orbit (LEO) satellite communicates with ground users (GUs) with the assistance of an energy-constrained coexisting air-to-air (A2A) network. Particularly, a non-linear energy harvester with a hybrid SWIPT utilizing both power-splitting and time-switching energy harvesting (EH) techniques is employed at the aerial transmitter. Specifically, we take the random locations of the satellite, ground and aerial receivers to investigate the outage performance of both the satellite-to-ground and aerial networks leveraging the stochastic tools. By taking into account the Shadowed-Rician fading for satellite link, the Nakagami-\emph{m} for ground link, and the Rician fading for aerial link, we derive analytical expressions for the outage probability of these networks. For a comprehensive analysis of aerial network, we consider both the perfect and imperfect successive interference cancellation (SIC) scenarios. Through our analysis, we illustrate that, unlike linear EH, the implementation of non-linear EH provides accurate figures for any target rate, underscoring the significance of using non-linear EH models. Additionally, the influence of key parameters is emphasized, providing guidelines for the practical design of an energy-efficient as well as spectrum-efficient future non-terrestrial networks. Monte Carlo simulations validate the accuracy of our theoretical developments.

cs.IT↗

arXiv2024

Magnetospheric origin of a fast radio burst constrained using scintillation

Fast radio bursts (FRBs) are micro-to-millisecond duration radio transients that originate mostly from extragalactic distances. The emission mechanism responsible for these high luminosity, short duration transients remains debated. The models are broadly grouped into two classes: physical processes that occur within close proximity to a central engine; and central engines that release energy which moves to large radial distances and subsequently interacts with surrounding media producing radio waves. The expected emission region sizes are notably different between these two types of models. FRB emission size constraints can therefore be used to distinguish between these competing models and inform on the physics responsible. Here we present the measurement of two mutually coherent scintillation scales in the frequency spectrum of FRB 20221022A: one originating from a scattering screen located within the Milky Way, and the second originating from a scattering screen located within its host galaxy or local environment. We use the scattering media as an astrophysical lens to constrain the size of the lateral emission region, $R_{\star\mathrm{obs}} \lesssim 3\times10^{4}$ km. We find that this is inconsistent with the expected emission sizes for the large radial distance models, and is more naturally explained with an emission process that operates within or just beyond the magnetosphere of a central compact object. Recently, FRB 20221022A was found to exhibit an S-shaped polarisation angle swing, supporting a magnetospheric emission process. The scintillation results presented in this work independently support this conclusion, while highlighting scintillation as a useful tool in our understanding of FRB emission physics and progenitors.

astro-ph.HE↗

arXiv2024

On the spectral redundancy of pineapple graphs

In this article, we explore the concept of spectral redundancy within the class of pineapple graphs, denoted as $\mathcal{P}(α,β)$. These graphs are constructed by attaching $β$ pendent edges to a single vertex of a complete graph $K_α$. A connected graph $G$ earns the title of being spectrally non-redundant if the spectral radii of its connected induced subgraphs remain distinct. Spectral redundancy, on the other hand, arises when there is a repetition of spectral radii among the connected induced subgraphs within $G$. Specifically, we analyze the adjacency spectrum of $\mathcal{P}(α,β)$, revealing distinct eigenvalues including $0$, $-1$, and additional eigenvalues, some negative and others positive. Our investigation focuses on determining the spectral redundancy within this class of graphs, shedding light on their unique structural properties and implications for graph theory.

math.CO↗

arXiv2024

Explainable machine learning to enable high-throughput electrical conductivity optimization and discovery of doped conjugated polymers

The combination of high-throughput experimentation techniques and machine learning (ML) has recently ushered in a new era of accelerated material discovery, enabling the identification of materials with cutting-edge properties. However, the measurement of certain physical quantities remains challenging to automate. Specifically, meticulous process control, experimentation and laborious measurements are required to achieve optimal electrical conductivity in doped polymer materials. We propose a ML approach, which relies on readily measured absorbance spectra, to accelerate the workflow associated with measuring electrical conductivity. The classification model accurately classifies samples with a conductivity > 25 to 100 S/cm, achieving a maximum of 100 % accuracy rate. For the subset of highly conductive samples, we employed a regression model to predict their conductivities, yielding an impressive test R2 value of 0.984. We tested the models with samples of the two highest conductivities (498 and 506 S/cm) and showed that they were able to correctly classify and predict the two extrapolative conductivities at satisfactory levels of errors. The proposed ML-assisted workflow results in an improvement in the efficiency of the conductivity measurements by 89 % of the maximum achievable using our experimental techniques. Furthermore, our approach addressed the common challenge of the lack of explainability in ML models by exploiting bespoke mathematical properties of the descriptors and ML model, allowing us to gain corroborated insights into the spectral influences on conductivity. Through this study, we offer an accelerated pathway for optimizing the properties of doped polymer materials while showcasing the valuable insights that can be derived from purposeful utilization of ML in experimental science.

physics.app-ph↗

arXiv2024

Fast current-induced skyrmion motion in synthetic antiferromagnets

Magnetic skyrmions are topological magnetic textures that hold great promise as nanoscale bits of information in memory and logic devices. Although room-temperature ferromagnetic skyrmions and their current-induced manipulation have been demonstrated, their velocity has been limited to about 100 meters per second. In addition, their dynamics are perturbed by the skyrmion Hall effect, a motion transverse to the current direction caused by the skyrmion topological charge. Here, we show that skyrmions in compensated synthetic antiferromagnets can be moved by current along the current direction at velocities of up to 900 meters per second. This can be explained by the cancellation of the net topological charge leading to a vanishing skyrmion Hall effect. Our results open an important path toward the realization of logic and memory devices based on the fast manipulation of skyrmions in tracks.

cond-mat.mes-hall↗

arXiv2024

Variabilities in the polar field and solar cycle due to irregular properties of Bipolar Magnetic Regions

Decay and dispersal of the tilted Bipolar Magnetic Regions (BMRs) on the solar surface are observed to produce large-scale poloidal field, which acts as the seed for the toroidal field and, thus, the next sunspot cycle. However, various properties of BMR, namely, the tilt, time delay between successive emergences, location, and flux, all have irregular variations. Previous studies show that these variations can lead to changes in the polar field. In this study, we first demonstrate that our 3D kinematic dynamo model, STABLE, reproduces the robust feature of the surface flux transport (SFT) model, namely the variation of the generated dipole moment with the latitude of the BMR position. Using STABLE in both SFT and dynamo modes, we perform simulations by varying the individual properties of BMR and keeping their distributions the same in all the cycles as inspired by the observations. We find that randomness due to the distribution in either the time delay or the BMR latitude produces negligible variation in the polar field and the solar cycle. However, randomness due to BMR flux distribution produces substantial effects, while the scatter in the tilt around Joy law produces the largest variation. Our comparative analyses suggest that the scatter of BMR tilt around Joy law is the major cause of variation in the solar cycle. Furthermore, our simulations also show that the magnetic field-dependent time delay of BMR emergence produces more realistic features of the magnetic cycle, consistent with observation.

astro-ph.SR↗