arXiv subjects

Suman Sarkar

Publications and source records attributed to Suman Sarkar.

At least 19 recordsLinked to original sources

arXiv2026

Transforming Discarded Thermoelectrics into High-Performance HER Catalysts

With the increase in the complexity of the materials used in various sophisticated electronic devices, recycling of E-waste is becoming challenging. In the present study, we have converted thermoelectric (TE) waste into functional HER electrocatalyst by considering circular-economy and low-carbon approach. The as received TE waste was processed through ball milling (TE waste-BM) and melting casting (TE waste-M) routes. Morphological and structural evaluations revealed that the formation of BiSbTe3/ZnTe heterostructure in TE-waste-M promote HE efficiency when compared to the presence of Bi2Te3/BiSbTe3 heterostructure (TE-waste-BM). TE waste-M exhibited lower overpotential (641 mV at 10 mA/sq.cm), smaller Tafel slope (233 mV/dec) and stable operation for 5.5 h with negligible current decay than that of TE waste-BM, attributed to the accelerated charge transfer, fast water dissociation steps and rapid hydrogen adsorption in TE waste-M, originated from the presence of BiSbTe3/ZnTe heterostructure, defect enriched interfaces. Density functional theory calculations supported the experimental findings, revealing that heterostructures strengthens the bonding states near the Fermi level, thereby enhancing the HER activity of BiSbTe3/ZnTe heterostructure. This work simultaneously integrates waste management with green hydrogen production by offering an economically viable, scalable and low-carbon approach for HER catalysts.

cond-mat.mtrl-sci↗

arXiv2026

Galaxy evolution in the cosmic web: the relative impact of nodes and filaments in the EAGLE simulation

Galaxies evolve within the intricate geometry of the cosmic web, yet the distinct roles of its primary components - nodes and filaments remain incompletely understood. Using the EAGLE cosmological hydrodynamical simulation, we investigate how distances to filament spines and cluster-scale nodes jointly and independently influence galaxy evolution. Galaxies are classified into red, green, and blue populations through a fully data-driven entropic thresholding technique, and the nodes and filaments are identified using DisPerSE. We find that red galaxies preferentially reside near filament cores and nodes, whereas blue and green galaxies dominate the outskirts. This spatial segregation reveals two characteristic transition scales: a node-related crossover at $\sim 2.5~\mathrm{Mpc}$ and a filament-related crossover at $\sim 0.75~\mathrm{Mpc}$. To further quantify environmental influence, we adopt an information-theoretic approach and measure the normalised mutual information between dominant mass component and galaxy colour across the $d_{\mathrm{f}}$-$d_{\mathrm{n}}$ plane. The mass-colour coupling increases with distance from nodes at fixed filament proximity, indicating a weakening of this relation in cluster-scale environments and a stronger coupling within filamentary regions. This behaviour is strongly mass dependent, with low-mass galaxies exhibiting a more pronounced environmental modulation than high-mass systems. These results support a scale-dependent view of galaxy evolution across the cosmic web, highlighting the distinct and complementary influence of nodes and filaments.

astro-ph.GA↗

arXiv2026

Negentropy as Diagnostic of Cosmic Density Fields and Dynamical Dark Energy Models

We employ negentropy ($J$), defined as the difference between the information content of a non-Gaussian probability distribution and a Gaussian with identical variance, as an information-theoretic probe of non-Gaussianity in the cosmic density field. We quantify its sensitivity to dynamical dark energy by studying the evolution of $J(a)$ and its derivatives $\Gamma_1(a)$ and $\Gamma_2(a)$ across three parameterisation schemes: CPL, JBP, and BA. We determine the characteristic redshift $z_{NG}$, marking the epoch of maximal non-Gaussian structure formation, and the turnaround redshift $z_{TA}$, when information production transitions due to dark-energy domination, finding $z_{NG}\sim0.81$ and $z_{TA}\sim0.18$ for $\Lambda$CDM. Our diagnostics clearly discriminate between thawing and freezing quintessence models and phantom dark energy at low redshifts. Thawing models show small departures from $\Lambda$CDM, freezing models display higher $z_{TA}$, while phantom models exhibit lower $z_{TA}$, reflecting late-time evolution. We provide a practical prescription for measuring negentropy from discrete galaxy distributions, establishing a framework that can be applied to simulations and observations. This information-theoretic approach offers a robust and complementary tool for probing dark energy dynamics, enabling sensitive discrimination between evolving and cosmological-constant scenarios.

astro-ph.CO↗

arXiv2025

Collective Fluorescence of Graphene Quantum Dots on a surface

This study explores the organization of graphene quantum dots on the surface of monocrystalline halide perovskite. We show that graphene quantum dots tends to aggregate on the surface of perovskite unlike in solution or on other substrates, even at very low concentration of the initial solution that should yield single-molecule samples. Spectral analysis on small clusters shows a back-and-forth dynamical transition between an uncoupled, monomer-like state, and an excimer state. Following this "dance" between states, a drastic one-way increase in fluorescence intensity combined with a shortening of the excited state lifetime has been observed on some clusters. This behavior is related to the emission of a collective state that may be a consequence of the dynamical organization of graphene quantum dots under illumination on the surface of the perovksite.

cond-mat.mes-hall↗

arXiv2025

Identifying lopsidedness in spiral galaxies using a Deep Convolutional Neural Network

About 30\% of disk galaxies show lopsidedness in their stellar disk. Although such a large-scale asymmetry in the disk can be primarily looked upon as a long-lived mode ($m=1$), the physical origin of the lopsidedness in the disk continues to be a puzzle. In this work, we employ a transfer-learning approach for the automated identification of lopsided galaxies using SDSS DR18 imaging by fine-tuning a Zoobot model, a deep convolutional neural network package pre-trained on the Galaxy Zoo dataset. We obtain 7,042 well-resolved, nearly face-on spiral galaxies from SDSS DR18 over the redshift range 0.01 $\leq z \leq 0.1$, with extinction-corrected g-band model magnitude < 16 and Petrosian radius (enclosing 90 \% of the flux) $\geq$ 3 arcsec. Out of these, we visually identify 490 lopsided and 444 symmetric galaxy samples suitable for training. The trained model achieves a testing accuracy of $(87 \pm 0.02)$ \%, averaged over 10 independent trials. Using the best-performing model, we identify 3,679 lopsided and 2,429 symmetric galaxies from the remaining sample. Of these, 2,658 lopsided and 1,455 symmetric galaxies are predicted with are predicted with high prediction probability $P_{pred} \geq 0.85$. Lopsided galaxies in our predicted samples are relatively high star-forming, bluer, low-concentration (late-type), low-mass galaxies compared to the symmetric galaxies. Our study produces an usable catalogue of lopsided and symmetric galaxies, which will offer new insights into the formation of lopsidedness in disk galaxies. The dataset and the best-performing model are made publicly available through GitHub at https://github.com/bijusaha-astro/CNN_lopsided

astro-ph.GA↗

arXiv2025

Exceptionally High Nonlinear Optical Response in Two-dimensional Type II Dirac Semimetal Nickel di-Telluride (NiTe2)

Nickel ditelluride (NiTe2) is a newly identified Type-II Dirac semimetal, showing novel characteristics in electronic transport and optical experiments. In this study, we explored the nonlinear optical properties of two-dimensional NiTe2 using experimental and computational techniques (density functional theory-based approach). Few layered two-dimensional NiTe2 (2D-NiTe2) are synthesized using liquid phase exfoliation (LPE), which is characterized using X-ray diffraction technique, transmission electron, and atomic force microscopy. The nonlinear refractive index and third-order nonlinear susceptibility of the prepared 2D-NiTe2 are determined from the self-induced diffraction pattern generated using different wavelengths ( 405, 532, and 650 nm) in the far field. In addition, the diffraction pattern generated by spatial self-phase modulation (SSPM) is further verified by varying concentration (2D-NiTe2 in the IPA solvent), wavelength (of incoming laser beams), and cuvette width (active path length). The high value of third-order nonlinear susceptibility (in order of 10-9 e.s.u.) determined using SSPM in the 2D-NiTe2 can be attributed to the laser-induced hole coherence effect. Lastly, utilizing the reverse saturable absorption property of 2D-hBN, asymmetric light propagation is also demonstrated in the 2D-NiTe2/2D-hBN heterostructure.

cond-mat.mes-hall↗

arXiv2025

Emamectin benzoate sensing using vivianenes (2D vivianites)

The excessive application of pesticides, particularly the overreliance on insecticides for the protection of desirable crops from pests, has posed a significant threat to both ecological systems and human health due to environmental pollution. This research outlines a comprehensive approach to recognizing and quantifying the presence of insecticides through the application of spectroscopic and electrochemical sensing methods. The detection of Emamectin benzoate (EB), a commonly used insecticide, was performed utilizing vivianenes, a 2D phosphate that has been mechanically exfoliated from the naturally occurring vivianite minerals. This investigation examined the structural and compositional characteristics of vivianenes, utilizing a range of characterization methods. The spectroscopic analyses reveal the molecular interactions and structural modifications that take place during the interaction of EB with the 2D template. Electrochemical investigations employing cyclic voltammetry were performed for different concentrations of EB to enable real-time monitoring of the pesticide. The modified sensing electrode using vivianene demonstrated a linear range of from 50 mg/L to 10 micro g/L, effectively detecting EB molecules at levels significantly below the hazardous threshold. Fully atomistic molecular dynamics simulations were also carried out to obtain further insights into the interaction mechanisms of the EB with the vivianites, and the results corroborate the adsorption mechanism. Our results highlight the potential application of 2D phosphate minerals as advanced sensors to enhance agricultural monitoring and promote sustainable development.

physics.chem-ph↗

arXiv2025

Laser patterning of the room temperature van der Waals ferromagnet 1$T$-CrTe$_2$

Lamellar crystalline materials, whose layers are bond by van der Waals forces, can be stacked to form ultrathin artificial heterostructures, and in particular vertical magnetic junctions when some of the stacked materials are (ferro)magnetic. Here, using the room temperature van der Waals ferromagnet 1$T$-CrTe$_2$, we report a method for patterning lateral magnetic junctions. Exploiting the heat-induced phase transformation of the material into Cr$_x$Te$_y$ compounds ($x/y>1/2$), we use local laser heating to imprint patterns at the micron-scale. Optimizing laser heat dissipation, we further demonstrate the crucial role of the substrate to control the phase transformation. If plain, unstructured poorly heat-conducting substrates allow for direct writing of magnetic patterns, structured $h$-BN layers can serve as heat stencils to draw potentially thinner patterns. Besides, $h$-BN encapsulation turns out to be heat-protective (in addition from protecting against oxidation as it is generally used for), allowing the demonstration of room temperature ferromagnetism in $<$7~nm-thick 1$T$-CrTe$_2$.

cond-mat.mtrl-sci↗

arXiv2024

Unveiling galaxy pair alignment in cosmic filaments: A 3D exploration using EAGLE simulation

We investigate how galaxy pairs are oriented in three dimensions within cosmic filaments using data from the EAGLE simulation. We identify filament spines using DisPerSE and isolate galaxies residing in filamentary environments. Employing a FoF algorithm, we delineate individual filaments and determine their axes by diagonalizing the moment of inertia tensor. The orientations of galaxy pairs relative to the axis of their host filament are analyzed. Our study covers diverse subsets of filaments identified through varying linking lengths, examining how galaxy pairs align with the filament axis across different spatial parameters such as pair separation and distance from the filament spine. We observe a nearly uniform probability distribution for the cosine of the orientation angle, which is nearly identical in each case. We also investigate the effects of redshift space distortions and confirm that the probability distributions remain uniform in both real space and redshift space. To validate our approach, we conduct Monte Carlo simulations using various theoretical probability distributions. Our analysis does not reveal any evidence of preferential alignment of galaxy pairs within cosmic filaments in hydrodynamical simulations.

astro-ph.CO↗

arXiv2024

Impact of Large-Scale Structure Formation on the Global Information Budget

This paper offers an original theoretical framework to quantify the information content associated with cosmological structure formation. By taking into account the growth in perturbations for the matter density field, we are able to calculate the relative reduction in the information entropy from the projection for a fully homogeneous setting. An analytical study is carried out to track back the evolution of global information content up to $z=20$, where a log-normal density distribution with redshift-dependent variance, skewness, and kurtosis is used to mimic the observable Universe. We report a $0.004\%$ drop in the information entropy of the Universe, that is caused by the formation of large-scale structures in the present universe. The accelerated expansion of the Universe is suspected as a countermeasure by nature to override the exponential growth in this entropic drop. We also analyse how the scale factor, expansion rate and growth rate of structure formation are connected with the entropic drop, taking the $ΛCDM$ model into account. The formalism is further developed and employed to study the spectrum of information underlying the galaxy distribution in the observable Universe. Using data from SDSS DR18 we also quantify the information sharing across different parts of the studied volume. An attempt to validate the assumption of cosmic homogeneity is made, which rules out the presence of a Universal scale of uniformity below 130 $h^{-1}$ Mpc.

astro-ph.CO↗

arXiv2024

Harvesting magneto-acoustic waves using magnetic two-dimensional chromium telluride (CrTe3)

A vast majority of electrical devices have integrated magnetic units, which generate constant magnetic fields with noticeable vibrations. The majority of existing nanogenerators acquire energy through friction/mechanical forces and most of these instances overlook acoustic vibrations and magnetic fields. Magnetic two-dimensional (2D) tellurides present a wide range of possibilities for devising a potential flexible energy harvester. We have synthesized two-dimensional chromium telluride (2D CrTe3) which exhibits ferromagnetic (FM) nature with a Tc of 224 K. The structure exhibits stable high remnant magnetization, making 2D CrTe3 flakes a potential material for harvesting of magneto-acoustic waves at room temperature. A magneto-acoustic nanogenerator (MANG) was fabricated composing of 2D CrTe3 dispersed in a polymer matrix. Basic mechanical stability and sensitivity of the device with change in load conditions were tested. A high surface charge density of 2.919 mC m-2 was obtained for the device. The thermal strain created in the lattice structure was examined using in-situ Raman spectroscopic measurements. The magnetic anisotropy energy (MAE) responsible for long-range FM ordering was calculated with the help of theoretical modelling. The theoretical calculations also showed opening of electronic bandgap which enhances the flexoelectric effects. The MANG can be a potential energy harvester to synergistically tap into the magneto-acoustic vibrations generated from the frequency changes of a vibrating device such as loudspeakers.

physics.app-ph↗

arXiv2023

Thickness dependent mechanical properties of soft ferromagnetic two-dimensional CoTe2

Two dimensional (2D) layered transition-metal-based tellurides (chalcogens) are known to harness their surface atoms characteristics to enhance topographical activities for energy conversion, storage, and magnetic applications. High surface energy due to unsaturated dangling bonds and larger lateral size than the thickness (volume) makes them a potential candidate for emerging electronics. Nevertheless, the gradual stacking of each sheet alters the surface atoms' subtle features, such as lattice expansion, leading to several phenomena and rendering tunable properties. In the present work, we have monitored thickness-dependent properties of the 2D CoTe2 sheets from nanoscale mechanics, tribology, surface potential distributions, interfacial interaction and magnetism using atomically resolved spectroscopy and different surface probe techniques, in conjunction with theoretical investigations: density functional theory (DFT) and molecular dynamics (MD). The variation in properties observed in theoretical investigation unleashes the crucial role of crystal planes of the CoTe2. The presented results are beneficial in expanding the use of 2D telluride family in flexible electronics, piezo sensors, tribo-generator, and next-generation memory devices.

cond-mat.mtrl-sci↗

arXiv2023

Galaxy interactions in filaments and sheets: insights from EAGLE simulations

We study the colour and star formation rates of paired galaxies in filaments and sheets using the EAGLE simulations. We find that the major pairs with pair separation $<50$ kpc are bluer and more star forming in filamentary environments compared to those hosted in sheet-like environments. This trend reverses beyond a pair separation of $\sim 50$ kpc. The interacting pairs with larger separations ($>50$ kpc) in filaments are on average redder and low-star forming compared to those embedded in sheets. The galaxies in filaments and sheets may have different stellar mass and cold gas mass distributions. Using a KS test, we find that for paired galaxies with pair separation $<50$ kpc, there are no significant differences in these properties in sheets and filaments. The filaments transport gas towards the cluster of galaxies. Some earlier studies find preferential alignment of galaxy pairs with filament axis. Such alignment of galaxy pairs may lead to different gas accretion efficiency in galaxies residing in filaments and sheets. We propose that the enhancement of star formation rate at smaller pair separation in filaments is caused by the alignment of galaxy pairs. A recent study with the SDSS data (Das et al., 2023) reports the same findings. The confirmation of these results by the EAGLE simulations suggests that the hydrodynamical simulations are powerful theoretical tools for studying the galaxy formation and evolution in the cosmic web.

astro-ph.GA↗

arXiv2023

Do minor interactions trigger star formation in galaxy pairs?

We analyze the galaxy pairs in a set of volume limited samples from the SDSS to study the effects of minor interactions on the star formation rate (SFR) and colour of galaxies. We carefully design control samples of the isolated galaxies by matching the stellar mass and redshift of the minor pairs. The SFR distributions and colour distributions in the minor pairs differ from their controls at $>99\%$ significance level. We also simultaneously match the control galaxies in stellar mass, redshift and local density to assess the role of the environment. The null hypothesis can be rejected at $>99\%$ confidence level even after matching the environment. Our analysis shows a quenching in the minor pairs where the degree of quenching decreases with the increasing pair separation and plateaus beyond 50 kpc. We also prepare a sample of minor pairs with $H_α$ line information. We calculate the SFR of these galaxies using the $H_α$ line and repeat our analysis. We observe a quenching in the $H_α$ sample too. We find that the majority of the minor pairs are quiescent systems that could be quenched due to minor interactions. Combining data from the Galaxy Zoo and Galaxy Zoo2, we find that only $\sim 1\%$ galaxies have a dominant bulge, $4\%-7\%$ galaxies host a bar, and $5\%-10\%$ galaxies show the AGN activity in minor pairs. This indicates that the presence of bulge, bar and AGN activity plays an insignificant role in quenching the galaxies in minor pairs. The more massive companion satisfies the criteria for mass quenching in most of the minor pairs. We propose that the stripping and starvation likely caused the quenching in the less massive companion at a later stage of evolution.

astro-ph.GA↗

arXiv2023

Analyzing the cosmic architecture in the vicinity of Grand-design and Flocculent spiral galaxies with Local Geometric Index

We explore the environment of 351 grand-design and 541 flocculent spiral galaxies recently identified employing convolutional neural networks from the $17^{th}$ data release of Sloan Digital Sky Survey. We introduce a novel estimator called the Local Geometric Index to quantify the morphology of the local environment of these 892 spirals. Based on the local geometric index of the galaxies, we identify their local environments to be voids, sheets, filaments or clusters. We find that grand-designs are mostly located in dense environments like clusters and filaments ($\sim 76\%$), whereas a reasonable fraction of the flocculents lie in sparse environments like voids and sheets ($\sim 45\%$). A $p$-value $<$ $10 ^{-10}$ from a Kolmogorov-Smirnov test indicates that our results are statistically significant at $99.9\%$ confidence level. Further, we note that dense environments with large tidal flows are dominated by the grand-designs. Metal-poor environments, such as sheets and voids, with a high abundance of gas clouds, on the other hand, are mostly populated by the flocculents.

astro-ph.GA↗

arXiv2023

Analyzing the cosmic web environment in the vicinity of grand-design and flocculent spirals with local geometric index

We explore the environment of a combined set of $367$ grand-design and $619$ flocculent spiral galaxies. We introduce a novel estimator called the \textit{local geometric index} to quantify the morphology of the local environment of these $986$ spirals. The local geometric index allows us to classify the environment of galaxies into voids, sheets, filaments, and clusters. We find that grand-designs are mostly located in dense environments like clusters and filaments ($\sim 78\%$), whereas the fraction of the flocculents lying in sparse environments like voids and sheets is significantly higher ($ > 10\%$) than that of the grand-designs. A $p$-value $<$ $10 ^{-3}$ from a Kolmogorov-Smirnov test indicates that our results are statistically significant at $99.9\%$ confidence level. Further, we note that dense environments with large tidal flows are dominated by the grand-designs. On the other hand, low-density environments such as sheets and voids favor the growth of flocculents.

astro-ph.GA↗

arXiv2023

Stability of the In-Plane Room Temperature van der Waals Ferromagnet Chromium Ditelluride and Its Conversion to Chromium-Interleaved CrTe$_2$ Compounds

Van der Waals magnetic materials are building blocks for novel kinds of spintronic devices and playgrounds for exploring collective magnetic phenomena down to the two-dimensional limit. Chromium-tellurium compounds are relevant in this perspective. In particular, the 1$T$ phase of CrTe$_2$ has been argued to have a Curie temperature above 300~K, a rare and desirable property in the class of lamellar materials, making it a candidate for practical applications. However, recent literature reveals a strong variability in the reported properties, including magnetic ones. Using electron microscopy, diffraction and spectroscopy techniques, together with local and macroscopic magnetometry approaches, our work sheds new light on the structural, chemical and magnetic properties of bulk 1$T$-CrTe$_2$ exfoliated in the form of flakes having a thickness ranging from few to several tens of nanometers. We unambiguously establish that 1$T$-CrTe$_2$ flakes are ferromagnetic above room temperature, have an in-plane easy axis of magnetization, low coercivity, and we confirm that their Raman spectroscopy signatures are two modes, $E_{2\text{g}}$ (103.5~cm$^{-1}$) and $A_{1\text{g}}$ (136.5~cm$^{-1}$). We also prove that thermal annealing causes a phase transformation to monoclinic Cr$_5$Te$_8$ and, to a lesser extent, to trigonal Cr$_5$Te$_8$. In sharp contrast with 1$T$-CrTe$_2$, none of these compounds have a Curie temperature above room temperature, and they both have perpendicular magnetic anisotropy. Our findings reconcile the apparently conflicting reports in the literature and open opportunities for phase-engineered magnetic properties.

cond-mat.mtrl-sci↗

arXiv2022

Galaxy interactions in filaments and sheets: effects of the large-scale structures versus the local density

The major interactions are known to trigger star formation in galaxies and alter their colour. We study the major interactions in filaments and sheets using the SDSS data to understand the influence of large-scale environments on the galaxy interactions. We identify the galaxies in filaments and sheets using the local dimension and also find the major pairs residing in these environments. The star formation rate and colour of the interacting galaxies as a function of pair separation are separately analyzed in filaments and sheets. The analysis is repeated for three volume limited samples covering different magnitude ranges. The major pairs residing in the filaments show a significantly higher star formation rate (SFR) and bluer colour than those residing in the sheets up to the projected pair separation of $\sim 50$ kpc. We observe a complete reversal of this behaviour for both the SFR and colour of the galaxy pairs having a projected separation larger than 50 kpc. Some earlier studies report that the galaxy pairs align with the filament axis. Such alignment inside filaments indicates anisotropic accretion that may cause these differences. We do not observe these trends in the brighter galaxy samples. The pairs in filaments and sheets from the brighter galaxy samples trace relatively denser regions in these environments. The absence of these trends in the brighter samples may be explained by the dominant effect of the local density over the effects of the large-scale environment.

astro-ph.GA↗