arXiv subjects

Ajay Kumar

Publications and source records attributed to Ajay Kumar.

At least 73 records · Page 4Linked to original sources

arXiv2024

A Scalable Tool For Analyzing Genomic Variants Of Humans Using Knowledge Graphs and Machine Learning

The integration of knowledge graphs and graph machine learning (GML) in genomic data analysis offers several opportunities for understanding complex genetic relationships, especially at the RNA level. We present a comprehensive approach for leveraging these technologies to analyze genomic variants, specifically in the context of RNA sequencing (RNA-seq) data from COVID-19 patient samples. The proposed method involves extracting variant-level genetic information, annotating the data with additional metadata using SnpEff, and converting the enriched Variant Call Format (VCF) files into Resource Description Framework (RDF) triples. The resulting knowledge graph is further enhanced with patient metadata and stored in a graph database, facilitating efficient querying and indexing. We utilize the Deep Graph Library (DGL) to perform graph machine learning tasks, including node classification with GraphSAGE and Graph Convolutional Networks (GCNs). Our approach demonstrates significant utility using our proposed tool, VariantKG, in three key scenarios: enriching graphs with new VCF data, creating subgraphs based on user-defined features, and conducting graph machine learning for node classification.

cs.AI↗

arXiv2024

Predicting edge-localized monovacancy defects in zigzag graphene nanoribbons from Floquet quasienergy spectrum

In this work, we prescribe a theoretical framework aiming at predicting the position of monovacancy defects at the edges of zigzag graphene nanoribbons (ZGNRs) using Floquet-Bloch formalism, which can be experimentally observed through time- and angle-resolved photoemission spectroscopy (tr-ARPES). Our methodology involves an in-depth investigation of the Floquet quasienergy band spectrum influenced by light with varying polarization across a range of frequencies. Particularly under the influence of circularly polarized light with a frequency comparable to the bandwidth of the system, our findings suggest a promising approach for locating monovacancy defects at either edge, a challenge that proves intricate to predict from the ARPES spectrum of ZGNRs with monovacancy defects. This has been achieved by analyzing the orientation of the Floquet edge state and the appearance of new Dirac points in the vicinity of the Fermi level. The real-world applications of these captivating characteristics underscore the importance and pertinence of our theoretical framework, paving the way for additional exploration and practical use. Our approach, employing the Floquet formalism, is not limited to monovacancy-type defects; rather, it can be expanded to encompass various types of vacancy defects.

cond-mat.mes-hall↗

arXiv2024

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.

cond-mat.mtrl-sci↗

arXiv2024

Antiferromagnetic ordering and glassy nature in NASICON type NaFe$_2$PO$_4$(SO$_4$)$_2$

We investigate crystal structure and magnetic properties including spin relaxation and magnetocaloric effect in NASICON type NaFe$_2$PO$_4$(SO$_4$)$_2$ sample. The Rietveld refinement of x-ray and neutron diffraction patterns show a rhombohedral crystal structure with the R$\bar{3}$c space group. The core-level spectra confirm the desired oxidation state of constituent elements. The {\it dc}--magnetic susceptibility ($χ$) behavior in zero field-cooled (ZFC) and field-cooled (FC) modes show the ordering temperature $\approx$50~K. Interestingly, the analysis of temperature dependent neutron diffraction patterns reveal an A-type antiferromagnetic (AFM) structure with the ordered moment of 3.8 $μ_{B}$/Fe$^{3+}$ at 5~K, and a magnetostriction below $T_{\rm N}=$ 50~K. Further, the peak position in the {\it ac}--$χ$ is found to be invariant with the excitation frequency supporting the notion of dominating AFM transition. Also, the unsaturated isothermal magnetization curve supports the AFM ordering of the moments; however, the observed coercivity suggests the presence of weak ferromagnetic (FM) correlations at 5~K. On the other hand, a clear bifurcation between ZFC and FC curves of {\it dc}--$χ$ and the observed decrease in peak height of {\it ac}--$χ$ with frequency suggest for the complex magnetic interactions. The spin relaxation behavior in thermo-remanent magnetization and aging measurements indicate the glassy states at 5~K. Moreover, the Arrott plots and magnetocaloric analysis reveal the AFM--FM interactions in the sample at lower temperatures.

cond-mat.str-el↗

arXiv2024

Radio-loud fraction of z>6 quasars

Quasars at redshifts $z>6$ are an excellent probe of the formation and evolution of supermassive black holes in the early Universe. The population of radio-luminous quasars is of particular interest, as such quasars could potentially be used to study the neutral intergalactic medium during cosmic reionisation via H$\,$I 21$\,$cm absorption studies. However, the lack of deep radio observations of $z>6$ quasars leaves the population poorly constrained, and suitable candidates for an H$\,$I 21$\,$cm absorption study have yet to be found. In this work, we present Jansky Very Large Array (VLA) 1$-$2 GHz radio continuum observations of 138 quasars at redshifts $6.0 \leq z<7.6$. We detect the radio continuum emission of the $z=6.1$ quasar J1034-1425, with a 1.6 GHz flux density of $170\pm 36\,μ$Jy. This quasar is radio-quiet with radio-loudness, $R \equiv f_{5\text{~GHz}}/f_{ν,\text{4400 A}} = 2.4\pm0.5$. In addition, we detect 7 other quasars at z>6, which have previously been characterised in the literature at these frequencies. Using the full sample, we estimate the radio-loud fraction to be $3.8^{+6.2}_{-2.4}\%$, where the uncertainties are 95% confidence intervals. This is lower than recent estimates of the radio-loud fraction in the literature, but is still marginally consistent with no redshift evolution of the radio-loud fraction. We explore the undetected quasar population by stacking their continuum images at their optical positions and obtain a median stacked flux density of $13.8\pm 3.9~μ$Jy and luminosity of $\log{L_{5~\mathrm{GHz}}/(\mathrm{W~Hz}^{-1})}=24.2\pm0.1$.

astro-ph.GA↗

arXiv2023

Theoretical insights on structural, electronic and thermoelectric properties of inorganic biphenylene: non-benzenoid Boron nitride

The first-principles calculations predict a stable biphenylene carbon network (BPN) like the Boron-nitride structure named inorganic biphenylene network (I-BPN). A comparison has been done between BPN and I-BPN to examine the stability of the I-BPN monolayer. We calculate the formation energy, phonon dispersion and mechanical parameters: young modulus and Poisson ratio for mechanical stability. It has been found that the stability of I-BPN is comparable with the BPN. The lattice transport properties reveal that the phonon thermal conductivity of I-BPN is 10th order low than the BPN. The electronic band structure reveals that I-BPN is a semiconductor with an indirect bandgap of 1.88 eV with valence band maximum (VBM) at Y and conduction band maximum (CBM) at the X high symmetry point. In addition, the thermoelectric parameters, such as the seebeck coefficient, show the highest peak value of 0.00292 V/K at 324K. Electronic transport properties reveal that I-BPN is highly anisotropic along the x and y-axes. Furthermore, the thermoelectric power factor as a function of chemical potential shows a peak value of 0.0056 W/mK2 (900K) along the x-axis in the p-type doping region. An electronic figure of merit shows an amplified peak approach to 1. The total figure of merit (including lattice transport parameters) shows peak values of 0.378 (0.21) for p-type and 0.24 (0.198) n-type regions along the x(y) direction. It is notice that the obtain ZT peaks values are higher than any B-N compositions.

cond-mat.mtrl-sci↗

arXiv2023

RayDF: Neural Ray-surface Distance Fields with Multi-view Consistency

In this paper, we study the problem of continuous 3D shape representations. The majority of existing successful methods are coordinate-based implicit neural representations. However, they are inefficient to render novel views or recover explicit surface points. A few works start to formulate 3D shapes as ray-based neural functions, but the learned structures are inferior due to the lack of multi-view geometry consistency. To tackle these challenges, we propose a new framework called RayDF. It consists of three major components: 1) the simple ray-surface distance field, 2) the novel dual-ray visibility classifier, and 3) a multi-view consistency optimization module to drive the learned ray-surface distances to be multi-view geometry consistent. We extensively evaluate our method on three public datasets, demonstrating remarkable performance in 3D surface point reconstruction on both synthetic and challenging real-world 3D scenes, clearly surpassing existing coordinate-based and ray-based baselines. Most notably, our method achieves a 1000x faster speed than coordinate-based methods to render an 800x800 depth image, showing the superiority of our method for 3D shape representation. Our code and data are available at https://github.com/vLAR-group/RayDF

cs.CV↗

arXiv2023

Dielectric behavior and impedance spectroscopy of Niobium substituted Lanthanum based orthovanadates at high temperatures

We present the detailed study of the temperature dependent dielectric properties, impedance spectroscopy and electrical conductivity of LaV$_{1-x}$Nb$_x$O$_4$ ($x=$ 0-1) samples prepared by the solid-state reaction method. The dielectric constant ($ε_r'$) increases (decreases) with increase in the temperature (frequency); while, the magnitude of $ε_r'$ remains almost invariant (order of 10$^4$ at 100~Hz and 600\degree C) with $x$. The single phase $x=$ 0 (monoclinic-monazite, $P2_1/n$) and $x =$ 1 (monoclinic fergusonite, $I2/a$) samples show the lower values of loss factor [tan$δ=$ (4-8)] as compared to the $x=$ 0-0.8 samples [tan$δ=$(12-18)] having mixed tetragonal and monoclinic phases, which indicates the strong correlation between the crystal structure and the dielectric properties. The real part of impedance decreases with both temperature and frequency, and the observed weak relaxation remains almost unaltered with $x$. The imaginary part of impedance shows strong relaxation peaks shifting towards higher temperatures with frequency, which is attributed to the effect of grains, grain boundaries, and electrodes in the samples. The activation energy of the relaxation process is estimated to be 0.8-1.0~eV for the $x=$ 0-0.6 samples, and $\approx$1.4~eV for the $x=$ 0.8; whereas the $x=$ 1 sample shows two values ($\approx$0.5~eV and $\approx$1.0~eV) in the higher and lower temperature range, respectively. Further, the change in total conductivity with the angular frequency, which found to be in the range of 10$^{-3}$ to 10$^{-5}$~S/m, is fitted using the Jonsher power law. The analysis suggests the OLPT model for all the samples, whereas the $x$ = 1 sample exhibit a transition near 480\degree C and at higher temperatures it shows NSPT and QMT. This transition is corroborated by the tangent loss curves and may be associated with the change in structure.

cond-mat.mtrl-sci↗

arXiv2023

Correlation between the exchange bias effect and antisite disorders in Sr$_{2-x}$La$_x$CoNbO$_6$

We unravel the effect of La substitution and hence antisite disorders on the exchange bias (EB) mechanism in Sr$_{2-x}$La$_x$CoNbO$_6$ ($x=$ 0, 0.2) double perovskite samples using the detailed analysis of the field cooled magnetization isotherms (M--H) and training effect. The field dependence of the freezing temperature deviates from both Gabay-Toulouse (GT) and de Almeida-Thouless (AT) lines and the analysis suggests that the $x=$ 0 sample follows a different universality class with moderate anisotropy in the frozen spins. Interestingly, we find that the EB effect is significantly suppressed in the $x=$ 0.2 sample due to increase [decrease] in the size of ferromagnetic (FM) [cluster glass (CG)] domain, which reduces the effective disordered interface responsible for the EB. The changes in fraction of FM, AFM, and CG like interactions with the La substitution and applied magnetic field are found to be crucial in governing the EB effect in these samples. Further, the training effect measurements show the unequal shift in the left and right branches of the M--H loops and their different evolution with the field cycles ($n$). The analysis reveals that the rotatable spins relax approximately one order of magnitude faster than the frozen spins at the disordered interface. We find a possible correlation between the observed EB effect and the antisite disorders in these samples.

cond-mat.str-el↗

arXiv2023

Study of neutron multiplicity in $^{232}$Th (n,f) reaction using TALYS-1.96

The nuclear scientific community views $^{232}$Th as an option for fuel in the future nuclear energy program. Numerous experimental studies have been conducted to determine the cross-section; however, very few have been performed to calculate the total neutron multiplicity above 10 MeV energy. In this work, we have compared the experimental data of average neutron multiplicity at different incident energies from EXFOR with the evaluated data from ENDF/B-VI, JENDL-4.0, and the calculated data from TALYS-1.96. The experimental data are in good agreement with the evaluated data from both the ENDF/B-VI and JENDL-4.0 libraries and at high incident energy (7 MeV), the TALYS-1.96 data are also in agreement with the experimental data.

nucl-ex↗

arXiv2023

Effect of the growth orientation on the physical properties of Sr$_2$CoNbO$_6$ thin films

We study the effect of the growth orientation on the structural, electronic, and hence transport properties of Sr$_2$CoNbO$_6$ thin films grown on the orthorhombic NGO(100) and cubic MgO(100) substrates. The x-ray diffraction patterns show the growth of the thin film along $a$-axis resulting in the asymmetric ($b\neq c$) in-plane compressive strain in case of NGO(100), whereas along $c$-axis with tensile strain in case of MgO(100) substrate. The temperature dependent resistivity measurements indicate the lower electronic conductivity for the film grown on the NGO(100) substrate, which is found to be correlated with the higher degree of the oxygen deficiencies and hence larger concentration of the insulating Co$^{2+}$ in this sample. Further, the x-ray photoemission spectroscopy measurements show that Sr and Nb are present in the 2+ and 4+ valence state, whereas Co exist in the 2+, 3+ as well as 4+ states, fraction of which was found to vary with the growth orientation. Moreover, the analysis of leakage current using the sum exponent model indicate the presence of two different relaxation mechanisms in these samples.

cond-mat.str-el↗

arXiv2023

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.

math.AC↗

arXiv2023

A Novel Deep Learning based Model for Erythrocytes Classification and Quantification in Sickle Cell Disease

The shape of erythrocytes or red blood cells is altered in several pathological conditions. Therefore, identifying and quantifying different erythrocyte shapes can help diagnose various diseases and assist in designing a treatment strategy. Machine Learning (ML) can be efficiently used to identify and quantify distorted erythrocyte morphologies. In this paper, we proposed a customized deep convolutional neural network (CNN) model to classify and quantify the distorted and normal morphology of erythrocytes from the images taken from the blood samples of patients suffering from Sickle cell disease ( SCD). We chose SCD as a model disease condition due to the presence of diverse erythrocyte morphologies in the blood samples of SCD patients. For the analysis, we used 428 raw microscopic images of SCD blood samples and generated the dataset consisting of 10, 377 single-cell images. We focused on three well-defined erythrocyte shapes, including discocytes, oval, and sickle. We used 18 layered deep CNN architecture to identify and quantify these shapes with 81% accuracy, outperforming other models. We also used SHAP and LIME for further interpretability. The proposed model can be helpful for the quick and accurate analysis of SCD blood samples by the clinicians and help them make the right decision for better management of SCD.

q-bio.QM↗

arXiv2023

Powers of vertex cover ideals of Simplicial Trees

In $2011$, Herzog, Hibi, and Ohsugi conjectured that if $J$ is the cover ideal of a chordal graph, then $J^s$ is componentwise linear for all $s \ge 1.$ In 2022, Hà and Tuyl considered objects more general than chordal graphs and posed the following problem: Let $J(Δ)$ be the cover ideal of a simplicial tree $Δ.$ Is it true that $J(Δ)^s$ is componentwise linear for all $s \geq 1$? In this article, we give an affirmative answer to this problem.

math.AC↗

arXiv2023

Vertex cover ideals of simplicial complexes

Given a simplicial complex $Δ$, we investigate how to construct a new simplicial complex $\barΔ$ such that the corresponding monomial ideals satisfy nice algebraic properties. We give a procedure to check the vertex decomposability of an arbitrary hypergraph. As a consequence, we prove that attaching non-pure skeletons at all vertices of a cycle cover of a simplicial complex $Δ$ results in a simplicial complex $\barΔ$ such that the associated hypergraph $\mathcal{H}(\barΔ)$ is vertex decomposable. Also, we prove that all symbolic powers of the cover ideal of $\barΔ$ are componentwise linear. Our work generalizes the earlier known result where non-pure complete graphs were added to all vertices of a cycle cover of a graph.

math.AC↗

arXiv2022

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.

cond-mat.mtrl-sci↗

arXiv2022

Probing the electronic and local structure of Sr$_{2-x}$La$_x$CoNbO$_6$ using near-edge and extended x-ray absorption fine structures

We report the electronic and local structural investigation of double pervoskites Sr$_{2-x}$La$_x$CoNbO$_6$ ($x=$ 0--1) using x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structures (EXAFS) at the Nb, Co, and Sr $K$-edges. The $ab$ $initio$ simulations and detailed analysis of the Nb and Co $K$-edge XANES spectra demonstrate that the observed pre-edge features arise from the transition of 1$s$ electrons to the mixed $p-d$ hybridized states. We reveal a $z$-out Jahn-Teller (JT) distortion in the CoO$_6$ octahedra, which decreases monotonically due to an enhancement in the JT inactive Co$^{2+}$ ions with $x$ . On the other hand, the $z$-in distortion in NbO$_6$ octahedra remains unaltered up to $x=$ 0.4 and then decreases with further increase in $x$. This sudden change in the local coordination around Nb atoms is found to be responsible for the evolution of the antiferromagnetic interactions in $x \geqslant$ 0.6 samples. Also, we establish a correlation between the degree of octahedral distortion and intensity of the white line feature in the XANES spectra and possible reason for this are discussed. More interestingly, we observe the signature of KN$_1$ double electron excitation in the Sr $K$-edge EXAFS spectra for all the samples, which is found to be in good agreement with the $Z$+1 approximation. Further, the Co L$_{2,3}$ edge shows the reduction in the crystal field strength and hence an increase in the charge transfer energy ($Δ_{ct}$) with the La substitution.

cond-mat.str-el↗

arXiv2022

Observation of anisotropic thermal expansion and Jahn-Teller effect in double perovskites Sr$_{2-x}$La$_x$CoNbO$_6$ using neutron diffraction

We use temperature dependent neutron powder diffraction (NPD) to investigate the structural changes and magnetic interactions in double perovskite Sr$_{2-x}$La$_x$CoNbO$_6$ ($x=$ 0.4 and 0.6). A structural phase transition from tetragonal ($I4/m$) to monoclinic ($P2_1/n$) is observed between $x=$ 0.4 and 0.6 samples. Interestingly, temperature evolution of the unit cell parameters follow the Grüneisen approximation, and the analysis suggest the isotropic thermal expansion in case of the $x=$ 0.4 sample, whereas the $x=$ 0.6 sample shows the anisotropy where the thermal expansion along the $c$-axis significantly deviates from the Grüneisen function. We observe the $z$-out Jahn-Teller distortion in the CoO$_6$ and consequently $z$-in distortion in the adjacent NbO$_6$ octahedra. With increase in the La substitution, a decrease in the degree of octahedral distortion is evident from the significant reduction in the local distortion parameter $Δ$ around the B-site atoms.

cond-mat.str-el↗