arXiv subjects

Ankit Kumar

Publications and source records attributed to Ankit Kumar.

At least 55 records · Page 3Linked to original sources

arXiv2025

Universal relation involving fundamental modes in two-fluid dark matter admixed neutron stars

We systematically investigate the fundamental oscillation frequencies of dark matter admixed neutron stars, focusing on models with self-interacting fermionic dark matter that couples to normal matter solely through gravity. The analysis is carried out within a two-fluid formalism under the relativistic Cowling approximation, where the perturbation equations follow from the linearized energy-momentum conservation laws of both components. We find that the mass-scaled fundamental frequencies of the nuclear (dark) fluid in dark core (halo) configurations exhibit a remarkably tight correlation with the total stellar compactness. This universality persists across the dark matter parameter space explored in this study and is largely insensitive to the choice of nuclear equation of state. In contrast, we also find the breakdown of such universality with the tidal deformability, i.e, the same frequencies show substantial deviations from universality when expressed in terms of the tidal deformability. These contrasting behaviors highlight possible observational imprints of dark matter in neutron star interiors.

astro-ph.HE↗

arXiv2025

Bar Evolution in Edge-on Galaxies: A Demographic Study of Boxy/Peanut Bulges

Boxy/peanut and X-shaped (BP/X) bulges are prominent features in edge-on disk galaxies and are believed to be vertically thickened bars. Despite their relevance in bar evolution, a statistically robust census of these structures in large surveys has been lacking. We aim to provide the largest catalog of BP/X structures in edge-on galaxies to date, and to investigate their properties and role in shaping galaxy scaling relations. We selected a sample of 6684 edge-on galaxies from SDSS DR8 using Galaxy Zoo classifications, requiring a high edge-on probability ($> 0.9$) and a minimum of 10 independent votes. Two-dimensional image decomposition is performed using GALFIT to obtain structural parameters. Residual images are visually inspected to classify BP/X features into four categories: strong both-sided, both-sided, one-sided, and control (no BP/X). We also estimated stellar mass, distance, and physical size for each galaxy. Out of 6653 classified galaxies, we identified 1673 ($\sim$25%) with both-sided BP/X features-504 ($\sim$8%) strong and 1169 ($\sim$17%) weak-as well as 1112 ($\sim$17%) one-sided structures, making up a total of 2785 BP/X-hosting galaxies ($\sim$42%). One-sided structures, likely signatures of ongoing buckling, are more frequent than strong both-sided bulges across all stellar masses. The fraction of BP/X bulges increases with stellar surface mass density, indicating a connection with bar formation in dense disks. We also find that galaxies with strong BP/X bulges contribute to increased scatter in the stellar mass-size and stellar mass-surface density relations, particularly at higher masses.

astro-ph.GA↗

arXiv2025

ODIN: Using multiplicity of Lyman-Alpha Emitters to assess star formation activity in dark matter halos

We investigate if systems of multiple Lyman-alpha emitters (LAEs) can serve as a proxy for dark matter halo mass, assess how their radiative properties relate to the underlying halo conditions, and explore the physics of star formation activity in LAEs and its relation to possible physically related companions. We use data from the One-hundred-deg$^2$ DECam Imaging in Narrowbands (ODIN) survey, which targets LAEs in three narrow redshift slices. We identify physically associated LAE multiples in the COSMOS field at $z = 2.4$, $z = 3.1$, and $z=4.5$, and use a mock catalog from the IllustrisTNG100 simulation to assess the completeness and contamination affecting the resulting sample of LAE multiples. We then study their statistical and radiative properties as a function of multiplicity, where we adopt the term multiplicity to refer to the number of physically associated LAEs. We find a strong correlation between LAE multiplicity and host halo mass in the mocks, with higher multiplicity systems preferentially occupying more massive halos. In both ODIN and the mock sample, we find indications that the mean Ly$α$ luminosity and UV magnitude of LAEs in multiples increase with multiplicity. The halo-wide LAE surface brightness densities in Ly$α$ and UV increase with multiplicity, reflecting more compact and actively star-forming environments. The close agreement between the model and ODIN observations supports the validity of the Ly$α$ emission model in capturing key physical processes in LAE environments. Finally, a subhalo-based perturbation induced star formation model reproduces the minimum subhalo mass distribution in simulations at $z=2.4$, suggesting that local perturbations, rather than the presence of LAE companions, drive star formation in these systems. For the higher redshifts, neighbor perturbations do not seem to be the main driver that triggers star formation.

astro-ph.GA↗

arXiv2025

Revisiting the Nandakumar-Ramana Rao Conjecture

We reprove the generalized Nandakumar-Ramana Rao conjecture for the prime case using representation ring-graded Bredon cohomology. Our approach relies solely on the $RO(C_p)$-graded cohomology of configuration spaces, viewed as a module over the $RO(C_p)$-graded Bredon cohomology of a point.

math.AT↗

arXiv2025

Room temperature Planar Hall effect in nanostructures of trigonal-PtBi2

Trigonal-PtBi2 has recently garnered significant interest as it exhibits unique superconducting topological surface states due to electron pairing on Fermi arcs connecting bulk Weyl nodes. Furthermore, topological nodal lines have been predicted in trigonal-PtBi2, and their signature was measured in magnetotransport as a dissipationless, i.e. odd under a magnetic field reversal, anomalous planar Hall effect. Understanding the topological superconducting surface state in trigonal-PtBi2 requires unravelling the intrinsic geometric properties of the normal state electronic wavefunctions and further studies of their hallmarks in charge transport characteristics are needed. In this work, we reveal the presence of a strong dissipative, i.e. even under a magnetic field reversal, planar Hall effect in PtBi2 at low magnetic fields and up to room temperature. This robust response can be attributed to the presence of Weyl nodes close to the Fermi energy. While this effect generally follows the theoretical prediction for a planar Hall effect in a Weyl semimetal, we show that it deviates from theoretical expectations at both low fields and high temperatures. We also discuss the origin of the PHE in our material, and the contributions of both the topological features in PtBi2 and its possible trivial origin. Our results strengthen the topological nature of PtBi2 and the strong influence of quantum geometric effects on the electronic transport properties of the low energy normal state.

cond-mat.mes-hall↗

arXiv2025

Incommensurate Moiré Stacking and Landau Quantization Without External Magnetic Field in Turbostratic Graphene

Turbostratic multilayer graphene, composed of randomly twisted and stacked graphene sheets, offers a naturally disordered yet tunable platform for exploring moiré physics beyond tedious artificial stacking. Using scanning tunneling microscopy/spectroscopy (STM/STS) and Raman analysis, we uncover a wide distribution of twist angles and stacking configurations spontaneously formed across large-area turbostratic films. In several regions, we identify overlapping incommensurate moiré patterns consistent with locally chiral trilayer stacking. We observe van Hove singularities and reconstructed Dirac-like spectra whose angle dependence supports strong interlayer electronic coherence. In the highly strained trilayered regions, we observe peaks in the local density-of states with characteristic scaling of the quantized Landau levels strikingly even in the absence of a magnetic field. They arise from the strain-induced pseudo-magnetic fields (~ 26 T), making turbostratic graphene a single natural platform to explore the physics of moiré structures as well as of the pseudo-electromagnetic fields.

cond-mat.mes-hall↗

arXiv2025

Set-Valued Fractal Approximation for Countable Data Sets

Fractal geometry deals mainly with irregularity and captures the complexity of a structure or phenomenon. In this article, we focus on the approximation of set-valued functions using modern machinery on the subject of fractal geometry. We first provide a construction of fractal functions for countable data sets and use these functions in the approximation and study of set-valued mappings. We also show the existence and uniqueness of an invariant Borel measure supported on the graph of a set-valued fractal function. In addition, we obtain some effective bounds on the dimensions of the constructed set-valued fractal functions.

math.FA↗

arXiv2025

Stability analysis of two-fluid neutron stars featuring twin star and ultradense configurations

We perform a detailed analysis of radial oscillations to discuss dynamical stability in two-fluid neutron stars composed of ordinary nuclear matter and a gravitationally coupled dark matter component. Using a fully relativistic two-fluid formalism, we solve the eigenvalue problem for a coupled system of equations with small-amplitude radial perturbations and derive the critical line corresponding to stability boundaries. We also compare these stability boundary lines obtained from the radial perturbations with those obtained from a generalized turning-point criterion based on extremization of mass and particle numbers, and find that the two methods agree to within better than $1\%$ across the parameter space explored. We consider both mirror dark matter and self-interacting fermionic dark matter models, and examine how microphysical properties$-$such as nuclear equations of state, dark matter mass, and vector coupling strength$-$reshape the topology of the stability boundary and gravitational mass contours. Our results reveal the emergence of ultra-dense and compact stars, with nuclear central densities exceeding single-fluid instability thresholds by factors of two or more, and the appearance of twin-star configurations with identical masses but distinct radii and internal fluid compositions. These findings have direct implications for the interpretation of neutron star observables and motivate future studies involving phase transitions, density discontinuities, or additional interactions in multi-component stellar systems. In particular, the emergence of exotic stable configurations beyond conventional stability limits underscores the need to reassess standard criteria in light of multi-fluid dynamics, with significant consequences for multimessenger probes of dense matter$-$including gravitational wave signals, mass-radius constraints, and post-merger remnants.

astro-ph.HE↗

arXiv2025

Global well-posedness and small time asymptotics of stochastic Ladyzhenskaya-Smagorinsky equations with damping on unbounded domains

The Ladyzhenskaya-Smagorinsky equations model turbulence phenomena, and are given by $$\frac{\partial \boldsymbol{u}}{\partial t}-μ\mathrm{div}\left(\left(1+|\nabla\boldsymbol{u}|^2\right)^{\frac{p-2}{2}}\nabla\boldsymbol{u}\right)+(\boldsymbol{u}\cdot\nabla)\boldsymbol{u}+\nabla p=\boldsymbol{f}, \ \nabla\cdot\boldsymbol{u}=0,$$ for $p\geq 2.$ In this work, we consider the stochastic Ladyzhenskaya-Smagorinsky equations with the damping $α\boldsymbol{u}+β|\boldsymbol{u}|^{r-2}\boldsymbol{u},$ for $r\geq 2$ ($α,β\geq 0$), subjected to multiplicative Gaussian noise in a Poincaré domain (which may be bounded or unbounded) $\mathcal{O}\subset\mathbb{R}^d$ ($2\leq d\leq 4$). We show the local monotonicity ($p\geq \frac{d}{2}+1,\ r\geq 2$) as well as global monotonicity ($p\geq 2,\ r\geq 4$) properties of the linear and nonlinear operators, which along with an application of a stochastic version of the Minty-Browder technique imply the existence of a unique pathwise strong solution satisfying the energy equality (Itô formula), which is proved with the help of the methodology developed in [Krylov, \emph{Probab. Theory Related Fields}, {\bf 147} (2010), 583--605.] Then, we discuss the small time asymptotics by studying the effect of small, highly nonlinear, unbounded drifts (small time large deviation principle) for the stochastic Ladyzhenskaya-Smagorinsky equations with damping.

math.PR↗

arXiv2025

Stabilization of sawteeth instability by short gas pulse injection in ADITYA-U tokamak

Experiments on ADITYA-U tokamak show a marked enhancement in the sawtooth period by application of short gas puffs of fuel that cause a modification of the radial density profile. A consequent suppression of the trapped electron modes (TEMs) then leads to an increase in the core electron temperature. This slows down the heat propagation following a sawtooth crash, causing a delay in achieving the critical temperature gradient inside the q = 1 surface required for the next sawtooth crash to happen. The overall scenario has strong similarities with the behavior of sawtooth under electron cyclotron resonance heating (ECRH). Our findings suggest an alternate, simpler technique for sawtooth control that may be usefully employed in small/medium-sized tokamaks that do not have an ECRH or any other auxiliary heating facility.

physics.plasm-ph↗

arXiv2025

Favorable modifications of Scrape-Off Layer (SOL) heat flux width through pulsed fuelling in ADITYA-U Tokamak

Enhancement of the scrape-off layer (SOL) heat flux width has been observed in the ADITYA-U Tokamak following the injection of short fuel gas pulses. A notable reduction in parallel heat flux near the last closed flux surface (LCFS) is observed after each pulse. Comparative analysis indicates that pulsed fuelling is more effective in mitigating heat flux with improved core confinement than continuous gas feeding via real-time density control. Analytical and simulation works are also carried out for validation of experimental results. The analytical model shows that SOL width modification cannot be attributed solely to the decrease of temperature due to gas pulse injection; cross-field plasma diffusion also needs to increase. Simulations with the UEDGE code suggest that an increase in both the cross-field diffusion coefficient and inward pinch velocity is necessary to replicate the experimentally observed broadening of the heat flux SOL width. These findings provide insights into efficient SOL heat flux control strategies for future fusion devices.

physics.plasm-ph↗

arXiv2025

Electric, thermal and thermoelectric response of a hot pion gas in a time dependent background magnetic field

The prime focus of the work is to determine the electric, thermal and thermoelectric transport coefficients of a hot pion gas in the presence of time-dependent background magnetic fields. The thermoelectric effect is analyzed by examining the magneto-Seebeck and Nernst coefficients in the hot pionic medium under such conditions. Furthermore, the phenomenologically relevant elliptic flow coefficient, linked to the Knudsen number, is examined. The analysis reveals the significant impact of both the strength and time dependence of the magnetic field on the transport coefficients of the pionic medium. The results are analyzed in contrast to those obtained under a constant magnetic field.

hep-ph↗

arXiv2025

Open quantum dynamics of Josephson charge pumps

We investigate the macroscopic dynamics of Josephson charge pumps in the light of Alicki et al.'s theoretical description of the Josephson junction as an open quantum system described by a Markovian master equation. Once the electrostatic interaction between the terminals is taken into account via nonlinear capacitive terms in the Hamiltonian, we find that the resulting description of pumping is physically reasonable and in good qualitative agreement with experimental observations. We comment on how this approach relates to other theoretical treatments of quantum pumps based on time-dependent potentials or scattering amplitudes. We also highlight the significance of our results in the broader context of the dynamics of charge pumping by active systems.

quant-ph↗

arXiv2025

A Formalization of the Correctness of the Floodsub Protocol

Floodsub is a simple, robust and popular peer-to-peer publish/subscribe (pubsub) protocol, where nodes can arbitrarily leave or join the network, subscribe to or unsubscribe from topics and forward newly received messages to all of their neighbors, except the sender or the originating peer. To show the correctness of Floodsub, we propose its specification: Broadcastsub, in which implementation details like network connections and neighbor subscriptions are elided. To show that Floodsub does really implement Broadcastsub, one would have to show that the two systems have related infinite computations. We prove this by reasoning locally about states and their successors using Well-Founded Simulation (WFS). In this paper, we focus on the mechanization of a proof which shows that Floodsub is a simulation refinement of Broadcastsub using WFS. To the best of our knowledge, ours is the first mechanized refinement-based verification of a real world pubsub protocol.

cs.LO↗

arXiv2025

Simplified approach to estimate Lorenz number using experimental Seebeck coefficient for non parabolic band

Reduction of lattice thermal conductivity ($κ_L$) is one of the most effective ways of improving thermoelectric properties. However extraction of $κ_L$ from the total measured thermal conductivity can be misleading if Lorenz ($L$) number is not estimated correctly. The $κ_L$ is obtained using Wiedemann-Franz law which estimates electronic part of thermal conductivity $κ_e$ = $L$$σ$T where, $σ$ and T are electrical conductivity and temperature. The $κ_L$ is then estimated as $κ_L$ = $κ_T$ - $L$$σ$T. For the metallic system the Lorenz number has universal value of 2.44 $\times$ 10$^{-8}$ W$Ω$K$^{-2}$ (degenerate limit), but for no-degenerate semiconductors, the value can deviate significantly for acoustic phonon scattering, the most common scattering mechanism for thermoelectric above room temperatures. Up till now, $L$ is estimated by solving a series of equation derived form Boltzmann transport equations. For the single parabolic band (SPB) an equation was proposed to estimate $L$ directly from the experimental Seebeck coefficient. However using SPB model will lead to overestimation of $L$ in case of low band gap semiconductors which result in underestimation of $κ_L$ sometimes even negative $κ_L$. In this letter we propose a simpler equation to estimate $L$ for a non parabolic band. Experimental Seebeck coefficient, band gap($E_g$), and Temperature ($T$) are the main inputs in the equation which nearly eliminates the need of solving multiple Fermi integrals besides giving accurate values of $L$.

cond-mat.mtrl-sci↗

arXiv2025

Identification and Characterization of a New Disruption Regime in ADITYA-U Tokamak

Disruptions continue to pose a significant challenge to the stable operation and future design of tokamak reactors. A comprehensive statistical investigation carried out on the ADITYA-U tokamak has led to the observation and characterization of a novel disruption regime. In contrast to the conventional Locked Mode Disruption (LMD), the newly identified disruption exhibits a distinctive two-phase evolution: an initial phase characterized by a steady rise in mode frequency with a nonlinearly saturated amplitude, followed by a sudden frequency collapse accompanied by a pronounced increase in amplitude. This behaviour signifies the onset of the precursor phase on a significantly shorter timescale. Clear empirical thresholds have been identified to distinguish this disruption type from conventional LMD events, including edge safety factor, current decay coefficient, current quench (CQ) time, and CQ rate. The newly identified disruption regime is predominantly governed by the (m/n = 2/1) drift-tearing mode (DTM), which, in contrast to typical disruptions in the ADITYA-U tokamak that involve both m/n = 2/1 and 3/1 modes, consistently manifests as the sole dominant instability. Initiated by core temperature hollowing, the growth of this mode is significantly enhanced by a synergistic interplay between a strongly localized pressure gradient and the pronounced steepening of the current density profile in the vicinity of the mode rational surface.

physics.plasm-ph↗

arXiv2025

ODIN: Clustering Analysis of 14,000 Lyα Emitting Galaxies at z=2.4, 3.1, and 4.5

Lyman Alpha Emitters (LAEs) are star-forming galaxies that efficiently probe the spatial distribution of galaxies in the high redshift universe. The spatial clustering of LAEs reflects the properties of their individual host dark matter halos, allowing us to study the evolution of the galaxy-halo connection. We analyze the clustering of 5233, 5220, and 3706 LAEs at $z$ = 2.4, 3.1, and 4.5, respectively, in the 9 deg$^2$ COSMOS field from the One-hundred-deg$^2$ DECam Imaging in Narrowbands (ODIN) survey. After correcting for redshift space distortions, LAE contamination rates, and the integral constraint, the observed angular correlation functions imply linear galaxy bias factors of $b$ = $1.72^{+0.26}_{-0.27}, 2.01^{+0.26}_{-0.29},$ and $2.95^{+0.40}_{-0.46}$, for $z$ = 2.4, 3.1, and 4.5, respectively. The median dark matter halo masses inferred from these measurements are $\log(M_{h}/M_{\odot})$ = $11.44^{+0.30}_{-0.28}, 11.13^{+0.26}_{-0.26}$, and $10.85^{+0.24}_{-0.24}$ for the three samples, respectively. The analysis also reveals that LAEs occupy roughly 3-7% of the halos whose clustering strength matches that of the LAEs.

astro-ph.GA↗

arXiv2025

np spin correlations in the deuteron ground state

The deuteron is the simplest atomic nucleus made of two particles - a proton and a neutron. In this work, we study how their spins are quantum entangled with each other. We study two cases: when the deuteron is in a fixed projection of total angular momentum, and when it exists in a superposition of all projections. Our findings show that the spins are most entangled when the total projection is zero, and that strong entanglement still exists even when all spin states are superposed.

nucl-th↗