arXiv subjects

Debashish Das

Publications and source records attributed to Debashish Das.

13 recordsLinked to original sources

arXiv2026

Contact Formation and Viscoelastic Detachment in Non-Circular Soft Adhesive Contacts

Adhesive contact measurements on soft polymers are commonly interpreted using Johnson-Kendall-Roberts (JKR) theory, which is formulated for circular contacts. Here, we examine contact formation and detachment in non-circular soft adhesive contacts using PDMS crossed-cylinder experiments. The crossing angle was varied from 30 to 90 degrees, producing contacts from highly elongated ellipses to nearly circular geometries while keeping the material pair fixed. During loading, the contact aspect ratio b/a rapidly approached an angle-dependent plateau, indicating approximately self-similar growth. This motivates use of the area-equivalent radius c=sqrt(a*b) and geometric-mean curvature radius Reff=sqrt(R1*R2). The loading branches follow a JKR-type linearization and yield a nearly angle- and preload-independent work of adhesion, W_load=24 mJ/m^2. Johnson-Greenwood elliptical-contact fits give comparable values. In contrast, unloading and pull-off are strongly history dependent. The unloading branches require a substantially larger effective separation energy, W_unload,eff, which increases with preload and decreasing crossing angle. A reduced viscoelastic model based on the same area-equivalent description captures the principal unloading response over 50-80 degrees using a single shared parameter set across angles and preloads. These results show that contact formation is governed primarily by area-equivalent scaling, whereas detachment is governed by geometry- and history-dependent dissipative separation.

cond-mat.soft↗

arXiv2026

Convective Preheating Enhances Front Propagation in DCPD Frontal Polymerization

Frontal polymerization (FP) enables rapid curing of thermosets via a self-sustaining thermal wave, but its propagation mechanism can shift dramatically depending on processing conditions. In this study, we investigate the effect of trigger direction and monomer viscosity - controlled via hold time - on the front velocity in frontal ring-opening metathesis polymerization (FROMP) of dicyclopentadiene (DCPD). Our experiments reveal that at low viscosities, bottom-triggered FP fronts propagate significantly faster, ~50% faster front speed compared to top-triggered ones, driven by buoyancy-enhanced convection that preheats the unreacted monomer ahead of the front, that can have important implications for manufacturing applications. However, with increasing hold time, the monomer viscosity rises steeply, suppressing convection and causing the front velocity for top and bottom triggering to converge. This behavior reflects a convection-to-conduction (thermal-diffusion) transition in heat transport during FP. Complementary simulations incorporating buoyancy-driven advection reproduce the observed trends and highlight the importance of fluid flow in front dynamics. These results provide new insight into the coupled thermo-fluid-chemical mechanisms in FP offer strategies to tailor front behavior through viscosity and initiation geometry.

cond-mat.soft↗

arXiv2025

Twist-Free Enhancement of Strength and Modulus in Electrospun Yarns via Liquid-Assisted Capillary Densification

Electrospun yarns often fall short of the strength and stiffness of their constituent nanofibers because of loose packing and inter-fiber slip. We report a simple, twist-free route to close this gap by liquid-assisted rolling: yarns are briefly wetted (water or ethanol) and subjected to gentle rolling action (mechanical strokes perpendicular and parallel to the yarn axis), then dried under controlled conditions so that meniscus forces compact the assembly into tightly bound bundles. The treatment yields large gains in tensile strength and modulus, and as yarn diameter decreases the properties of liquid-treated yarns approach single-fiber limits, indicating more efficient load transfer. Dry-rolling controls produce negligible changes compared to as-spun yarns, confirming that capillarity-driven consolidation, rather than mechanical pressing, dominates the improvement. Water consistently outperforms ethanol, reflecting its larger elastocapillary driving term gamma*(1 + cos theta) on PAN and thus stronger capillary compaction; a short post-treatment anneal near Tg further increases stiffness with a corresponding reduction in ductility. To rationalize these trends, we quantify microstructure via SEM-derived alignment and packing density and show that these complementary descriptors jointly explain variability in mechanical response. A compact constitutive framework, grounded in distributed fiber recruitment and adhesion/frictional contact, captures the observed strengthening-ductility trade-off across processing routes. The results establish capillarity-driven consolidation as a scalable pathway to engineer processing-structure-property relationships in hierarchical polymer fiber assemblies and provide practical guidance for upgrading electrospun yarns, alone or as precursors to twisted and composite architectures.

cond-mat.soft↗

arXiv2025

A hinge effect that anomalously decreases the stiffness of slender fiber-reinforced composite structures

We present experimental evidence for an anomalous decrease in stiffness in a fiber-reinforced polymer composite because of the embedded fiber. A shell with carbon fiber showed about 20% less stiffness and 100% more strength under compressive loading. We ruled out the role of debonding of fiber due to imperfect impregnation by using a fiber-pullout test, which revealed that the fiber-matrix interface is strong in the direction of the fiber. Therefore, we hypothesize that a fiber allows the matrix material to rotate around it as in a hinge. We corroborate this phenomenon, which we call the hinge effect, with analytical modelling and experimental data for small and large deformations of a fiber embedded in slender composite beams. We also demonstrate the design of foldable and deployable sheets with hill and valley folds enabled by the embedded fibers. Moreover, the hinge effect warrants further research into physics of how fibers in slender composite structures give rise to the anomalous flexibility. This effect can be gainfully used in designing novel origami structures and compliant mechanisms should be flexible and strong.

cond-mat.soft↗

arXiv2025

MSAD-Net: Multiscale and Spatial Attention-based Dense Network for Lung Cancer Classification

Lung cancer, a severe form of malignant tumor that originates in the tissues of the lungs, can be fatal if not detected in its early stages. It ranks among the top causes of cancer-related mortality worldwide. Detecting lung cancer manually using chest X-Ray image or Computational Tomography (CT) scans image poses significant challenges for radiologists. Hence, there is a need for automatic diagnosis system of lung cancers from radiology images. With the recent emergence of deep learning, particularly through Convolutional Neural Networks (CNNs), the automated detection of lung cancer has become a much simpler task. Nevertheless, numerous researchers have addressed that the performance of conventional CNNs may be hindered due to class imbalance issue, which is prevalent in medical images. In this research work, we have proposed a novel CNN architecture ``Multi-Scale Dense Network (MSD-Net)'' (trained-from-scratch). The novelties we bring in the proposed model are (I) We introduce novel dense modules in the 4th block and 5th block of the CNN model. We have leveraged 3 depthwise separable convolutional (DWSC) layers, and one 1x1 convolutional layer in each dense module, in order to reduce complexity of the model considerably. (II) Additionally, we have incorporated one skip connection from 3rd block to 5th block and one parallel branch connection from 4th block to Global Average Pooling (GAP) layer. We have utilized dilated convolutional layer (with dilation rate=2) in the last parallel branch in order to extract multi-scale features. Extensive experiments reveal that our proposed model has outperformed latest CNN model ConvNext-Tiny, recent trend Vision Transformer (ViT), Pooling-based ViT (PiT), and other existing models by significant margins.

cs.CV↗

arXiv2025

Fast Interlayer Energy Transfer from the Lower Bandgap MoS2 to the Higher Bandgap WS2

Energy transfer (ET) is a dipole-dipole interaction, mediated by the virtual photon. Traditionally, ET happens from the higher (donor) to lower bandgap (acceptor) material. However, in some rare instances, ET can happen from the lower-to-higher bandgap material, depending on the strong overlap between the acceptor photoluminescence (PL) and the donor absorption spectra. In this work, we report an ET process from the lower bandgap MoS2 to the higher bandgap WS2, due to a near 'resonant' overlap between the MoS2 B and WS2 A excitonic levels. Changing the MoS2 bandgap from direct-to-indirect by increasing the layer number results in a reduced ET rate, evidenced by the quenching of the WS2 PL emission. Our work shows at 300 K, the ET timescale of ~33 fs is faster than the reported thermalization of the MoS2 excitonic intervalley scattering (K to K') time and competing with the ultrafast charge transfer timescale. Thus, allowing us to open a new direction in understanding the competing inter/intralayer processes.

cond-mat.mtrl-sci↗

arXiv2025

Texture- and Stress-Dependent Electromechanical Response in Ferroelectric PZT: Insights from a Micromechanical Model

The electromechanical response of PbZr0.52Ti0.48O3 (PZT) near the morphotropic phase boundary (MPB) is strongly influenced by crystallographic texture and residual stress, both of which affect domain switching behavior. While these effects are critical for optimizing sensors, actuators, and MEMS devices, their combined influence remains poorly understood. We present a computational micromechanical model that captures texture- and stress-dependent polarization switching in MPB PZT. The framework incorporates both tetragonal and rhombohedral domain switching, along with interphase transformations, enabling accurate simulation of nonlinear electromechanical behavior. The model reproduces key experimental trends, including enhanced piezoelectric response in (001)-textured ceramics, and degradation under high in-plane stress. The implementation, provided as open-source MATLAB code, offers an accessible platform for experimentalists and materials designers to explore and interpret electromechanical behavior. By linking microstructural orientation and stress state to macroscopic response, this work provides a practical tool for understanding and designing next-generation piezoelectric materials.

cond-mat.mtrl-sci↗

arXiv2023

Ligand hole driven metal-insulator transition in a prototypical transition metal double perovskite oxide Ca$_2$FeMnO$_6$

Ca$_2$FeMnO$_6$ (CFMO) double perovskite was studied using first principles density functional theory and tight-binding (TB) Hamiltonian modeling using extended Hubbard model. We have shown by electronic structure analysis that charge- and magnetic-ordering are driven by charge disproportionation at low temperature caused by partial localization of O-$2p$ ligand holes at alternate Fe sites that creates Jahn-Teller distortion, which leads to metal to insulator transition (MIT) in CFMO. Our results suggests MIT was triggered by negative charge-transfer energy of self-hole doping, responsible for symmetry lowering transitions. Notably, the band-gap was found to fundamentally controlled by the strength of the charge-transfer energy, and not by the Mott-Hubbard interactions, which can be modeled by composition, pressure or stoichiometry modulations. The fundamental insights presented in this work will help understand similar physics and mechanisms in other class of perovskites and correlated metals.

cond-mat.mtrl-sci↗

arXiv2020

Exotic multiferroic properties of spinel structured $AB_2O_4$ compounds: A Monte Carlo Study

Spinel structured compounds, $AB_2O_4$, are special because of their exotic multiferroic properties. In $ACr_2O_4$ ($A$=$Co$, $Mn$,$Fe$), a switchable polarization has been observed experimentally due to a non-collinear magnetic spin order. In this article, we demonstrated the microscopic origin behind such magnetic spin order, hysteresis, polarisation and the so-called magnetic compensation effect in $ACr_2O_4$ ($A$=$Co$, $Mn$,$Fe$, $Ni$) using Monte Carlo simulation. With a careful choice of the exchange interaction, we were able to explain various experimental findings such as magnetization vs. temperature (T) behavior, conical stability, unique magnetic ordering and polarization in a representative compound $CoCr_2O_4$ which is the best known multiferroic compound in the $AB_2O_4$ spinel family. We have also studied the effect of $Fe$-substitution in $CoCr_2O_4$, with an onset of few exotic phenomena such as magnetic compensation and sign reversible exchange bias effect. These effects are investigated using an effective interactions mimicking the effect of substitution. Two other compounds in this family, $CoMn_2O_4$ and $CoFe_2O_4$, are also studied where no conical magnetic order and polarisation was observed, as hence provide a distinct contrast. Here all calculations are done using the polarisation calculated by the spin-current model. This model has certain limitation and it works quite good for low temperature and low magnetic field. But the model despite its limitation it can reproduce sign reversible exchange bias and magnetic compensation like phenomena quite well.

cond-mat.mtrl-sci↗

arXiv2017

Site occupancies and their effects on the physical properties of spinel $Co\left(Cr_{1-x}Fe_{x} \right)_{2}O_{4}$: an {\it ab initio} study

Recent experimental studies on Fe substituted spinel CoCr$_{2}$O$_{4}$ have discovered multiple functional properties in the system such as temperature and composition dependent magnetic compensation, tunable exchange bias and magnetostriction. These properties are attributed to the renormalisation of the inter-atomic magnetic exchange interactions arising due to the non-regular site occupancies of the magnetic cations in the system. In this work, we perform {\it ab initio} electronic structure calculations by DFT+U method and combine with a generalised thermodynamic model to compute the site occupancy patterns of the magnetic cations, the structural properties and the magnetic exchange interactions of Co$\left(Cr_{1-x}Fe_{x} \right)_{2}$O$_{4}$ for the entire composition range $0<x<1$. We find that the substituting Fe atoms prefer to occupy the tetrahedral sites of the spinel structure for the entire range of $x$, in agreement with the experimental inferences. Our results on the variations of the structural parameters with compositions agree very well with the experiments. By computing the variations of the various inter-atomic magnetic exchange interactions, we provide a microscopic picture of the evolution of a collinear structure from a non-collinear one due to substitution of Fe in CoCr$_{2}$O$_{4}$. The computed results are analysed in terms of the elements of the crystal field theory, and the features in the atoms and orbital-projected densities of states. The results and analysis presented in this work is the first comprehensive study on this system which would help understanding the complexities associated with the site occupancies, the electronic structures and the magnetic interactions in this multi-functional material.

cond-mat.mtrl-sci↗

arXiv2016

Systematic analysis of structural and magnetic properties of spinel $CoB_2O_4$(B=Cr,Mn and Fe)compounds from their electronic structures

The structural and magnetic properties of spinel compounds $CoB_2O_4$ (B=Cr,Mn and Fe) are studied using the DFT+U method and generalized gradient approximation (GGA). We concentrate on understanding the trends in the properties of these materials as the B cation changes, in terms of relative strengths of crystal fields and exchange fields through an analysis of their electronic densities of states. We find that the electron-electron correlation plays a significant role in obtaining the correct structural and electronic ground states. Significant structural distortion in CoMn$_{2}$O$_{4}$ and "inverted" sublattice occupancy in CoFe$_{2}$O$_{4}$ affects the magnetic exchange interactions substantially. The trends in the magnetic exchange interactions are analysed in terms of the structural parameters and the features in their electronic structures. We find that the Fe states in CoFe$_{2}$O$_{4}$ are extremely localised, irrespective of the symmetry of the site, which makes it very different from the features of the states of the B cations in other two compounds. These results provide useful insights into the trends in the properties of CoB$_{2}$O$_{4}$ compounds with variation of B cation which would help in understanding the results of recent experiments on doping of Mn and Cr in multiferroic CoCr$_{2}$O$_{4}$.

cond-mat.str-el↗

arXiv2016

First-principles investigations into the thermodynamics of cation disorder and it's impact on electronic structure and magnetic properties of spinel $Co\left(Cr_{1-x}Mn_{x} \right)_{2}O_{4}$

Recent experiments on Mn doped multiferroic $CoCr_{2}O_{4}$ indicate that a possible distribution of Mn atoms among tetrahedrally and octahedrally coordinated sites in the spinel lattice give rise to different variations in the structural parameters and saturation magnetisations in different concentration regimes of Mn atoms substituting the Cr. A composition dependent magnetic compensation behaviour points to the role conversions of the magnetic constituents. In this work, we have investigated the thermodynamics of cation disorder in $Co\left(Cr_{1-x}Mn_{x}\right)_{2}O_{4}$ system and it's consequences on the structural, electronic and magnetic properties, using results from first-principles electronic structure calculations. We have computed the variations in the cation-disorder as a function of Mn concentration and the temperature and found that at the annealing temperature of the experiment many of the systems exhibit cation disorder. Our results support the interpretations of the experimental results regarding the qualitative variations in the sub-lattice occupancies and the associated magnetisation behaviour, with composition. We have analysed the variations in structural, magnetic and electronic properties of this system with variations in the compositions and the degree of cation disorder from the variations in their electronic structures and by using the ideas from crystal field theory. Our study provides a complete microscopic picture of the effects that are responsible for composition dependent behavioural differences of the properties of this system. This work lays down a general framework, based upon results from first-principles calculations, to understand and analyse the substitutional magnetic spinel oxides $A\left(B_{1-x}C_{x} \right)_{2}O_{4}$ in presence of cation disorder.

cond-mat.mtrl-sci↗

arXiv2016

Structural, electronic and magnetic properties of Fe doped CoCr$_{2}$O$_{4}$: insights from ab initio calculations

CoCr$_2$O$_4$ has attracted significant attention recently due to several interesting properties such as magnetostriction, magnetoelectricity etc.. More recent experiments on Fe substituted CoCr$_2$O$_4$ observed a variety of novel phenomena such as the magnetic compensation accompanied by the occurrence of exchange bias, which reverses its sign. Understanding of such phenomena may lead to control the properties of these material in an efficient way to enhance its potential for multifunctional applications. In this paper, we study the microscopic understanding of Fe doping in modifying the structural and magnetic properties of CoCr$_{2}$O$_{4}$ with varying composition and substitution of Fe at different sublattices by first-principles density functional calculations. We have analysed in detail the effect of Fe substitution on crystal field and exchange splittings, magnetic moments and interatomic exchange parameters. It is also observed that with increasing concentration of Fe impurity, the system has a tendency towards forming an "Inverse Spinel" structure as observed in experiments. Such tendencies are crucial to understand this system as it would lead to modifications in the magnetic exchange interactions associated with sites with different symmetry finally affecting the magnetic structure and the multiferrocity in turn.

cond-mat.mtrl-sci↗