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Researcher profile

Ajay Kumar

Ajay Kumar contributes to research discovery and scholarly infrastructure.

ResearcherAffiliation not importedOpen to collaborate
cond-mat.mtrl-scimath.ACastro-ph.HEcond-mat.str-elastro-ph.COastro-ph.IMastro-ph.SRComputational ComplexityComputer Visioncond-mat.mes-hallDistributed, Parallel, and Cluster Computingeess.IVhep-exhep-phMachine Learningmath.COmath.CVphysics.app-phphysics.ins-detphysics.plasm-phquant-ph

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Published work

18 published item(s)

preprint2026arXiv

Effect of hole pitch reduction on electron transport and diffusion: A comparative simulation study of Triple GEM detectors

Advances in fabrication techniques and high-performance electronics have facilitated the development of fine-pitch Gas Electron Multipliers (GEMs). Earlier experimental and simulation findings suggest that these reduced-pitch GEMs can outperform the standard configuration in terms of effective gain, collection efficiency, and position resolution. However, a noticeable fraction of avalanche electrons is lost within the GEM systems, resulting in a degradation of charge collection efficiency. Therefore, a comprehensive simulation-based study is essential to provide deeper insights into the extent of degradation and its contributing factors. In this context, we employ ANSYS and Garfield++ to model the Triple GEM detectors with reduced pitch sizes of 90 and 60 $μ$m, and perform a comparative performance analysis with the standard configuration (pitch size: 140 $μ$m). At first, the simulation framework is validated by comparing the results of the standard configuration with available experimental data and previously reported simulation outcomes. Despite the characteristic gain offset, the framework remains physically consistent and reliable in capturing microscopic avalanche dynamics, reproducing the experimental trend. Following validation, we investigate electron losses at the metal electrodes and within the Kapton holes, electron transmission through the transfer and induction regions, electron diffusion on the induction electrode, and the overall collection efficiency. These parameters are analyzed as functions of GEM potential, outer hole diameter, inner hole diameter, Kapton thickness, metal thickness, and gas composition, thereby offering insights for designing efficient GEM detectors.

0 citations0 reviewsNo code linkedOpen access
preprint2026arXiv

Evidence of energy conversion in weakly collisional plasma during an interplanetary coronal mass ejection

Intervals of enhanced turbulent fluctuations are typically less frequent within the magnetic cloud region of an interplanetary coronal mass ejection (ICME). We investigate two such intervals inside an ICME observed by the \textit{Wind} spacecraft on 8--9 June 2000 and characterize their associated wave populations. We focus on spectral analysis and plasma instability analysis, using ion-scale normalized magnetic helicity and polarization properties with respect to the background magnetic field $B_0$. In the first interval, the ion-scale normalized magnetic helicity shows a left-handed circularly polarized signature. In the second interval, the left-handed signature persists and an additional high-frequency right-handed population appears. The propagation is approximately parallel to $B_0$. The left-handed fluctuations are compatible with Alfvén ion-cyclotron (AIC) waves, while the right-handed fluctuations are consistent with fast magnetosonic/whistler (FM/W) waves. The ICME plasma accesses resonance conditions that support multiple ion-scale wave modes. Evolving anisotropies in the plasma and the approach to marginal stability allow the coexistence of AIC-like and fast-magnetosonic/whistler-like fluctuations, with enhanced electron heating favoring the growth of the FM/W contribution and strengthening the density--magnetic-field magnitude correlation.

0 citations0 reviewsNo code linkedOpen access
preprint2026arXiv

SURGE: SuperBatch Unified Resource-efficient GPU Encoding for Heterogeneous Partitioned Data

We present SURGE, a streaming GPU encoding system deployed in production to generate embeddings for over 800 million texts across 40,000 logical partitions. Production embedding pipelines face a tension between logical data partitioning and efficient GPU utilization: processing each partition independently incurs $P$ inter-process communication (IPC) calls whose overhead limits throughput for compute-light models. Our contributions are analytical: (i) a cost model (Theorem 1) predicting throughput within 2% across three encoders spanning a 15$\times$ parameter range; (ii) a memory-safety bound (Lemma 3) enabling a streaming two-threshold policy with peak memory $O(B_{\min} + n_{\max})$ rather than $O(N)$; and (iii) a $φ$/CV decision framework characterizing when the pattern applies beyond our workload. The naive fix of batching at fixed size requires $O(N)$ peak memory (32.7 GB at 10M texts; infeasible beyond ~60M on 192 GB nodes), produces no output until all encoding completes, and offers no fault tolerance. SURGE achieves the same throughput with $O(B_{\min} + n_{\max})$ bounded memory (2.6 GB), 68$\times$ faster time-to-first-output, and crash recovery at SuperBatch granularity. On 10M texts with 4 NVIDIA L4 GPUs, SURGE delivers 26,413 texts/s -- matching fixed-batch throughput while using 12.6$\times$ less memory. We validate on bge-base (109M, $d$=768, error 1.3%) and across log-normal $σ$ in {1.0, 1.72, 2.5} (speedup invariant within $\pm$3%), and compare against a partition-batched baseline (PB-PBP-LB), against which SURGE retains a 7% throughput edge and 2.5$\times$ faster TTFO. Complementary engineering -- zero-copy Arrow serialization (22-25$\times$ speedup) and async I/O pipelining (up to 93% benefit) -- realizes the design but is not the contribution.

0 citations0 reviewsNo code linkedOpen access
preprint2026arXiv

The Second CHIME/FRB Catalog of Fast Radio Bursts

We present a catalog of 4539 fast radio bursts (FRBs) observed with the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope between 25 July 2018 and 15 September 2023. These bursts originate from 3641 unique sources, including 981 bursts from 83 known repeating sources. For each FRB, the catalog provides a $O(10')$ estimate of sky location along with corresponding measurements of cumulative exposure time and survey sensitivity over the observing period. It includes a total-intensity dynamic spectrum between 400 and 800 MHz at 0.983 ms resolution. From this spectrum, we constrain a model of the burst morphology and measure key parameters such as arrival time, intrinsic temporal width, dispersion measure, scattering time, and flux density. This second catalog includes all FRBs from the first catalog, with every event reprocessed using a uniform and improved analysis framework. We show that previously published inferences remain valid under the updated measurements. We assess consistency of the detection rate across observational parameters, present initial distributions of burst properties, and outline ongoing and future studies that will use this catalog to investigate the nature of FRBs and their utility as astrophysical and cosmological probes.

0 citations0 reviewsNo code linkedOpen access
preprint2025arXiv

Long-term monitoring of repeating FRB 20220912A with the uGMRT at low radio frequencies

Some repeating FRBs exhibit occasional extreme repetition rates, but very few show a sustained high activity level. One such hyperactive repeater is FRB 20220912A, which was discovered by CHIME/FRB Collaboration on 2022 September 12. Here, we present results from a long-term monitoring campaign of FRB 20220912A using the upgraded Giant Metrewave Radio Telescope (uGMRT) in the frequency range from 300 to 750 MHz. Over the course of nearly two years, we detected a total of 643 bursts in this frequency range. The source exhibited extreme activity for a few months after its discovery and sustained its active phase for over 500 days, with unsystematic modulations in the activity during this phase. The cumulative energy distributions in both bands show a break, consistent with other active repeaters like FRB 20121102A, FRB 202011124A, etc., suggesting common underlying emission mechanisms. Moreover, we show that the energy distribution shape for FRB 20220912A remains broadly same across a large range of frequencies and over time. Overall, the extended high activity, estimated total energy output, persistent power-law tails in the energy distributions, and the lack of detectable short timescale periodicity favor progenitor models invoking young dynamic magnetars, potentially emitting pulses across large rotation phase ranges.

0 citations0 reviewsNo code linkedOpen access
preprint2025arXiv

Non-trivial critical behavior at the magnetic transitions: A case study of Sm$_7$Pd$_3$

We present a comprehensive analysis of the critical behavior of Sm$_7$Pd$_3$ in the vicinity of its second-order magnetoelastic transition at $T_ {\rm c} = 173$ K. The critical exponents (CEs) $β$ and $γ$, determined using both the standard convergence procedure and the average normalized slope (ANS) method, diverge at $T_{\rm c}$: a characteristic typically associated with first-order transitions. Notably, none of the established universality classes satisfactorily describe the critical behavior of Sm$_7$Pd$_3$, and we discuss the possible origins of this deviation in the context of the strong spin-lattice coupling intrinsic to the sample. We emphasize the importance of accurately selecting the critical temperature and magnetic field ranges to ensure robust critical behavior analysis and propose a quantitative approach to assess the reliability of the extracted CEs. Additionally, we demonstrate that in the ANS method, the critical exponents $β$ and $γ$ should be calculated separately using data for $T \leqslant T_{\rm c}$ and $T \geqslant T_{\rm c}$, respectively. Our findings underscore the need for a revised theoretical framework to accurately describe second-order magnetoelastic transitions.

0 citations0 reviewsNo code linkedOpen access
preprint2025arXiv

Study of Heavy Hadron Production in Au + Au Collisions at a Center-of-Mass Energy of $\sqrt{s_{NN}}=200$ GeV

Using the Monte Carlo HYDJET++ model, the transverse momentum ($p_{T}$) spectra of heavy hadrons ($D^{0}$, $\overline{D}^{0}$, $D^{+}$, $D^{-}$ and $Λ_{c}$), as well as the nuclear modification factors of $D^{0}$ and $D^{\pm}$, produced in Au + Au collisions at $\sqrt{s_{NN}} = 200$ GeV RHIC energy across various centrality bins, are presented. This study is motivated by the need to understand the centrality dependence of the charm enhancement factor ($γ_{c}$) and the roles of different hadronization mechanisms such as coalescence and fragmentation, in charm hadron production. To achieve the best description of heavy hadron production, several input parameters in both the soft and hard components of the model are tuned. The study finds a decreasing trend of $γ_{c}$ from central to peripheral collisions and a mass dependence across charm hadrons. Moreover, the model effectively reproduces experimental data of $p_{T}$ spectra at low and intermediate $p_{T}$, capturing key features of charm hadron production in the quark-gluon plasma medium. However, it overpredicts the data at high $p_{T}$, indicating the need for improvements in modeling heavy quark energy loss mechanisms. Further, the nuclear modification factors ($R_{AA}$ and $R_{CP}$) for $D^{0}$ mesons exhibit significant suppression in central collisions, which matches with experimental observations. This highlights the roles of collisional and radiative energy loss due to collective effects, such as coalescence and radial flow. The antiparticle-to-particle and mixed particle ratios are also presented, showing good agreement with experimental data and revealing limitations in baryon production due to the absence of heavy quark coalescence in HYDJET++.

0 citations0 reviewsNo code linkedOpen access
preprint2024arXiv

Multiple magnetic interactions and large inverse magnetocaloric effect in TbSi and TbSi$_{0.6}$Ge$_{0.4}$

We present a comprehensive investigation of the electronic structure, magnetization, specific heat, and crystallography of TbSi (FeB structure type) and TbSi$_{0.6}$Ge$_{0.4}$ (CrB structure type) compounds. Both TbSi and TbSi$_{0.6}$Ge$_{0.4}$ exhibit two antiferromagnetic (AFM) transitions at T$_{\rm N1}\approx$ 58~K and 57~K, and T$_{\rm N2}\approx$ 36~K and 44~K, respectively, along with an onset of weak metamagnetic-like transition around 6~T between T$_{\rm N1}$ and T$_{\rm N2}$. High-resolution specific heat (C$_{\rm P}$) measurements show the second- and first-order nature of the magnetic transition at T$_{\rm N1}$ and T$_{\rm N2}$, respectively, for both samples. However, in the case of TbSi, the low-temperature (LT) AFM to high-temperature (HT) AFM transition takes place via an additional AFM phase at the intermediate temperature (IT), where both LT to IT AFM and IT to HT AFM phase transitions exhibit a first-order nature. Both TbSi and TbSi$_{0.6}$Ge$_{0.4}$ manifest significant magnetic entropy changes ($ΔS_{\rm M}$) of 9.6 and 11.6~J/kg-K, respectively, for $Δμ_0H$=7~T, at T$_{\rm N2}$. The HT AFM phase of TbSi$_{0.6}$Ge$_{0.4}$ is found to be more susceptible to the external magnetic field, causing a significant broadening in the peaks of $ΔS_{\rm M}$ curves at higher magnetic fields. Temperature and field-dependent specific heat data have been utilized to construct the complex H-T phase diagram of these compounds. Furthermore, temperature-dependent x-ray diffraction measurements demonstrate substantial magnetostriction and anisotropic thermal expansion of the unit cell in both samples.

0 citations0 reviewsNo code linkedOpen access
preprint2023arXiv

Powers of facet ideals of simplicial trees

In this article, we study the linearity of the minimal free resolution of powers of facets ideals of simplicial trees. We give a complete characterization of simplicial trees for which (some) power of its facet ideal has a linear resolution. We calculate the regularity of the $t$-path ideal of a perfect rooted tree. We also obtain an upper bound for the regularity of the $t$-path ideal of a rooted tree. We give a procedure to calculate the regularity of powers of facet ideals of simplicial trees. As a consequence of this result, we study the regularity of powers of $t$-path ideals of rooted trees. We pose a regularity upper bound conjecture for facet ideals of simplicial trees, which is as follows: if $Δ$ is a $d$-dimensional simplicial tree, then $\reg(I(Δ)^s) \leq (d+1)(s-1)+\reg(I(Δ))$ for all $s \geq 1$. We prove this conjecture for some special classes of simplicial trees.

0 citations0 reviewsNo code linkedOpen access
preprint2022arXiv

High temperature dielectric and impedance spectroscopy study of LaCo$_{0.7}$Nb$_{0.3}$O$_3$

We report the high temperature dielectric and {\it ac} impedance spectroscopy investigation of Nb substituted LaCo$_{0.7}$Nb$_{0.3}$O$_3$ polycrystalline sample. The maximum dielectric constant value was observed $\approx$1400 at around 400~K where the peak value shows a decreasing trend at higher temperatures and frequency. Similar variation was reflected in the dielectric loss (tan$δ$) behavior with temperature, which shows the thermal activation of the charge carriers in the material. The analysis of high temperature impedance spectroscopy data shows the grain and grain boundary contributions by fitting the Nyquist plots to the equivalent circuit. From the analysis of the impedance and modulus spectra, it was possible to discern between the effects of overlapping grains, grain boundaries, and electrode interfaces. The relaxation time decreases with an increase in the temperature and the activation energy changes from 0.44~eV to 0.56~eV at around 400~K, which is due to involvement of thermal activation in the conduction of charge carriers. The conductivity is found to be increased with temperature for a given frequency, which again shows the semiconducting behavior. Whereas the conductivity increases with increase in frequency at lower temperatures. Also, the conductivity almost saturates with frequency at high temperatures.

0 citations0 reviewsNo code linkedOpen access
preprint2021arXiv

Herzog, Hibi and Ohsugi conjecture for trees

Let $S=\mathbb{K}[x_1,\dots, x_n]$ be a polynomial ring, where $\mathbb{K}$ is a field, and $G$ be a simple graph on $n$ vertices. Let $J(G)\subset S$ be the vertex cover ideal of $G$. Herzog, Hibi and Ohsugi have conjectured that all powers of vertex cover ideals of chordal graph are componentwise linear. Here we establish the conjecture for the special case of trees. We also show that if $G$ is a unicyclic vertex decomposable graph that does not contain $C_3$ or $C_5$, then symbolic powers of $J(G)$ are componentwise linear.

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preprint2021arXiv

Regularity, Rees algebra and Betti numbers of certain cover ideals

Let $S={\sf k}[X_1,\dots, X_n]$ be a polynomial ring, where ${\sf k}$ is a field. This article deals with the defining ideal of the Rees algebra of squarefree monomial ideal generated in degree $n-2$. As a consequence, we prove that Betti numbers of powers of the cover ideal of the complement graph of a tree do not depend on the choice of tree. Further, we study the regularity and Betti numbers of powers of cover ideals associated to certain graphs.

0 citations0 reviewsNo code linkedOpen access
preprint2020arXiv

Quadratic Sieve Factorization Quantum Algorithm and its Simulation

Quantum computing is a winsome field that concerns with the behaviour and nature of energy at the quantum level to improve the efficiency of computations. In recent years, quantum computation is receiving much attention for its capability to solve difficult problems efficiently in contrast to classical computers. Specifically, some well-known public-key cryptosystems depend on the difficulty of factoring large numbers, which takes a very long time. It is expected that the emergence of a quantum computer has the potential to break such cryptosystems by 2020 due to the discovery of powerful quantum algorithms (Shor's factoring, Grover's search algorithm and many more). In this paper, we have designed a quantum variant of the second fastest classical factorization algorithm named "Quadratic Sieve". We have constructed the simulation framework of quantized quadratic sieve algorithm using high-level programming language Mathematica. Further, the simulation results are performed on a classical computer to get a feel of the quantum system and proved that it is more efficient than its classical variants from computational complexity point of view.

0 citations0 reviewsNo code linkedOpen access
preprint2020arXiv

Structural and transport properties of La$_{1-x}$Sr$_x$Co$_{1-y}$Nb$_y$O$_3$ thin films

We present the structural and transport properties of La$_{1-x}$Sr$_x$Co$_{1-y}$Nb$_y$O$_3$ ($y=$ 0.1 and $x=$ 0; $y=$ 0.15 and $x=$ 0.3) thin films grown on (001) orientated single crystalline ceramic substrates to investigate the effect of lattice induced compressive and tensile strain. The high resolution x-ray diffraction measurements, including $θ$-2$θ$ scan, $Φ$-scan, and reciprocal space mapping, affirm single phase; four-fold symmetry; good quality of deposited thin films. The atomic force micrographs confirm that these films have small root mean square roughness in the range of $\sim$0.5--7~nm. We observed additional Raman active modes in the films owing to the lowered crystal symmetry as compared to the bulk. More interestingly, the temperature dependent dc-resistivity measurements reveal that films become insulating due to induced lattice strain in comparison to bulk, however for the larger compressive strained films conductivity increase significantly owing to the higher degree of $p-d$ hybridization and reduction in bandwidth near the Fermi level.

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preprint2020arXiv

Structural, transport, optical and electronic properties of Sr$_2$CoNbO$_6$ thin films

We study the effect of substrate induced strain on the structural, transport, optical and electronic properties of Sr$_2$CoNbO$_6$ double perovskite thin films. The reciprocal space mapping, $ϕ$-scan and high-resolution $θ$-2$θ$ scans of x-ray diffraction patterns suggest the epitaxial nature and high-quality of the films deposited on various single crystal ceramic substrates. A systematic enhancement in the dc electronic conductivity is observed with increase in the compressive strain, while a sharp reduction in case of tensile strain, which are further supported by change in the activation energy and density of states near the Fermi level. The optical band gap extracted from two distinct absorption bands, observed in the visible-near infrared spectroscopy show a non-monotonic behavior in case of compressive strain while significant enhancement with tensile strain. Unlike the bulk Sr$_2$CoNbO$_6$ (Co$^{3+}$ and Nb$^{5+}$), we observe different valence states of Co namely 2+, 3+ and 4+, and tetravalent Nb (4$d^1$) in the x-ray photoemission spectroscopy measurements. Moreover, a reduction in the average oxygen valency with the compressive strain due to enhancement in the covalent character of Co/Nb--O bond is evident. Interestingly, we observe sharp Raman active modes in these thin films, which indicates a significant enhancement in structural ordering as compared to the bulk.

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preprint2020arXiv

Unraveling the magnetic interactions and spin state in insulating Sr$_{2-x}$La$_x$CoNbO$_6$

We investigate the structural, magnetic and spin-state transitions, and magnetocaloric properties of Sr$_{2-x}$La$_x$CoNbO$_6$ ($x=$ 0--1) double perovskites. The structural transition from tetragonal to monoclinic phase at $x$ $\geqslant$ 0.6, and an evolution of (101)/(103) superlattice reflections and Raman active modes indicate the enhancement in the B-site ordering with $x$. The magnetic susceptibility data reveal the transition from weak ferromagnetic (FM) to antiferromagnetic (AFM) ordering for $x$ $\geqslant$ 0.6 with T$_{\rm N}$$\approx$9--15~K. Interestingly, the La substitution drives towards more insulating state due to increase in high-spin Co$^{2+}$, whereas a spin-state crossover is observed in Co$^{3+}$ from high-spin to intermediate/low-spin states with $x$. We discuss the correlation between complex magnetic interactions and the presence of various Co spin-states in the system. Moreover, the emergence of metamagnetic nature due to the competition between FM and AFM interactions as well as crossover from conventional to inverse magnetocaloric effect have been demonstrated by detailed analysis of temperature and field dependent change in magnetic entropy.

0 citations0 reviewsNo code linkedOpen access
preprint2019arXiv

Segmentation-Aware and Adaptive Iris Recognition

Iris recognition has emerged as one of the most accurate and convenient biometric for the human identification and has been increasingly employed in a wide range of e-security applications. The quality of iris images acquired at-a-distance or under less constrained imaging environments is known to degrade the iris matching accuracy. The periocular information is inherently embedded in such iris images and can be exploited to assist in the iris recognition under such non-ideal scenarios. Our analysis of such iris templates also indicates significant degradation and reduction in the region of interest, where the iris recognition can benefit from a similarity distance that can consider importance of different binary bits, instead of the direct use of Hamming distance in the literature. Periocular information can be dynamically reinforced, by incorporating the differences in the effective area of available iris regions, for more accurate iris recognition. This paper presents such a segmentation-assisted adaptive framework for more accurate less-constrained iris recognition. The effectiveness of this framework is evaluated on three publicly available iris databases using within-dataset and cross-dataset performance evaluation and validates the merit of the proposed iris recognition framework.

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