arXiv subjects

Jagadish Kumar

Publications and source records attributed to Jagadish Kumar.

12 recordsLinked to original sources

arXiv2025

Unraveling the electronic, vibrational, thermodynamic, optical and piezoelectric properties of LiNbO$_3$, LiTaO$_3$ and Li$_2$NbTaO$_6$ from first-principles calculations

We have investigated the electronic, vibrational, optical, thermal and piezoelectric properties of LiNbO$_3$, LiTaO$_3$ and Li$_2$NbTaO$_6$ using the first-principles calculation based on the density functional theory. It also shows structural phase transition below $T_c$ due to ionic displacement that may alter the properties of material. We have checked the structural stability by calculating the tolerance factor and formation energy before proceeding to the further calculations. The ground state electronic band structures and corresponding density of states show its semiconducting nature with a band gap range of 3.5-3.7 eV. Optical properties such as dielectric function, absorption coefficient, optical conductivity, refractive index, absorbance and reflectance are calculated using time-dependent density functional theory. Furthermore, the piezoelectric properties and Born effective charges were analyzed to find the correlation between them. In these materials, the distortion induced by the small ionic radius of Li$^{+}$ coupled with strong covalent interaction between transition metal and oxygen leads to high spontaneous polarization which can enhance both piezoelectric and optical properties.

cond-mat.mtrl-sci↗

arXiv2025

Structural, electronic, vibrational, optical, piezoelectric, thermal and thermoelectric properties of BCZT from first-principles calculations

Perovskite material such as BCZT (Ba$_{0.875}$Ca$_{0.125}$(Zr$_{0.125}$Ti$_{0.875}$)O$_{3}$) is well known for its high value of piezoceramic properties and Curie temperature which has potential applications in sensors, actuators, optoelectronic and thermoelectric devices. Based on its composition and physical parameters such as pressure and temperature, experimentally, BCZT shows different crystal structures (rhombohedral, tetragonal and orthorhombic) with multiferroic properties. Here, we have designed these materials by comparing experimental stoichiometry and evaluated their stability by calculating the tolerance factor, formation energy, and cohesive energy. The structural, electronic and vibrational properties of BCZT are explored using generalized gradient approximation (GGA) within the framework of density functional theory. We have also shown variation of piezoelectric and optical properties through multiple phases using time dependent density functional theory. The electronic band gap, optical response in the visible light range, as well as piezoelectric, electrical, thermal, and thermoelectric properties, demonstrate excellent characteristics, making this material a promising lead-free ferroelectric candidate for various energy harvesting applications. Boltzmann transport theory is used for the calculation of Seebeck coefficient, electron thermal conductivity, and electrical conductivity to estimate the power factor and figure of merit which represents the efficiency of the material. The high values at the Fermi level suggest that these materials are well-suited for future device applications.

cond-mat.mtrl-sci↗

arXiv2022

Interface properties of CsPbBr$_3$ /CsPbI$_3$ perovskite heterostructure for solar cell

We explore the interface properties of perovskite heterostructure CsPbBr$_3$/CsPbI$_3$ through first-principles calculations. The structural interface is formed by the bonding of Cs-Br and Cs-I with bond length of $\sim$4.106 and 3.922 \AA. The upshift of Goldsmith tolerance factor in the range $0.8<t<1$ from $t<0.8$ is revealed for the bi-layer interface, from bulk, reflecting the structural rearrangement from anisotropy to isotropy in confinement. The band gap arises mainly due to the energy difference of I-5p orbital than that of Br-4p at the valence band and Pb-6p at the conduction band. Heavier halide shows the red shift in the absorption spectra, for the pristine monolayer component. For the bilayer geometry, iodine contribution is more observed than that of bromine and the underlying interface properties may be useful for solar cell devices application.

cond-mat.mtrl-sci↗

arXiv2020

A reductive analysis of a compartmental model for COVID-19: data assimilation and forecasting for the United Kingdom

We introduce a deterministic model that partitions the total population into the susceptible, infected, quarantined, and those traced after exposure, the recovered and the deceased. We hypothesize 'accessible population for transmission of the disease' to be a small fraction of the total population, for instance when interventions are in force. This hypothesis, together with the structure of the set of coupled nonlinear ordinary differential equations for the populations, allows us to decouple the equations into just two equations. This further reduces to a logistic type of equation for the total infected population. The equation can be solved analytically and therefore allows for a clear interpretation of the growth and inhibiting factors in terms of the parameters in the full model. The validity of the 'accessible population' hypothesis and the efficacy of the reduced logistic model is demonstrated by the ease of fitting the United Kingdom data for the cumulative infected and daily new infected cases. The model can also be used to forecast further progression of the disease. In an effort to find optimized parameter values compatible with the United Kingdom coronavirus data, we first determine the relative importance of the various transition rates participating in the original model. Using this we show that the original model equations provide a very good fit with the United Kingdom data for the cumulative number of infections and the daily new cases. The fact that the model calculated daily new cases exhibits a turning point, suggests the beginning of a slow-down in the spread of infections. However, since the rate of slowing down beyond the turning point is small, the cumulative number of infections is likely to saturate to about $3.52 \times 10^5$ around late July, provided the lock-down conditions continue to prevail.

q-bio.PE↗

arXiv2020

Epidemiological study of novel coronavirus (COVID-19)

We report a statistical analysis of some highly infected countries by the novel coronavirus (COVID-19). The cumulative infected data were fitted with various growth models (e.g. Logistic equation, Weibull equation and Hill equation) and obtained the power index of top ten highly infected countries. The newly infected data were fitted with Gaussian distribution with the peak at ~40 days for the countries whose infection curves are seem to be saturated. The similarity in growth kinetics of infected people of different countries provides first-hand guidelines to take proper precautions to minimize human damage.

q-bio.PE↗

arXiv2017

General framework for acoustic emission during plastic deformation

Despite the long history, so far there is no general theoretical framework for calculating the acoustic emission spectrum accompanying any plastic deformation. We set up a discrete wave equation with plastic strain rate as a source term and include the Rayleigh-dissipation function to represent dissipation accompanying acoustic emission. We devise a method of bridging the widely separated time scales of plastic deformation and elastic degrees of freedom. The efficacy of the framework is illustrated by considering three distinct cases of plastic deformation. The first one is the acoustic emission during a typical continuous yield exhibiting a smooth stress-strain curve. We first construct an appropriate set of evolution equations for two types of dislocation densities and then show that the shape of the model stress-strain curve and accompanying acoustic emission spectrum match very well with experimental results. The second and the third are the more complex cases of the Portevin-Le Chatelier bands and the L\"uders band. These two cases are dealt with in the context of the Ananthakrishna model since the model predicts the three types of the Portevin-Le Chatelier bands and also L\"uders-like bands. Our results show that for the type-C bands where the serration amplitude is large, the acoustic emission spectrum consists of well-separated bursts of acoustic emission. At higher strain rates of hopping type-B bands, the burst-type acoustic emission spectrum tends to overlap, forming a nearly continuous background with some sharp acoustic emission bursts. The latter can be identified with the nucleation of new bands. The acoustic emission spectrum associated with the continuously propagating type-A band is continuous. These predictions are consistent with experimental results. The acoustic emission spectrum of the L\"uders-like band matches with recent experiments as well.

cond-mat.mtrl-sci↗

arXiv2017

Influence of Visco-elastic Nature on the Intermittent Peel Front Dynamics of the Adhesive Tape

We investigate the influence of viscoelastic nature of the adhesive on the intermittent peel front dynamics by extending a recently introduced model for peeling of an adhesive tape. As time and rate-dependent deformation of the adhesives are measured in stationary conditions, a crucial step in incorporating the viscoelastic effects applicable to unstable intermittent peel dynamics is the introduction of a dynamization scheme that eliminates the explicit time dependence in terms of dynamical variables. We find contrasting influences of viscoelastic contribution in different regions of tape mass, roller inertia, and pull velocity. As the model acoustic energy dissipated depends on the nature of the peel front and its dynamical evolution, the combined effect of the roller inertia and pull velocity makes the acoustic energy noisier for small tape mass and low-pull velocity while it is burstlike for low-tape mass, intermediate values of the roller inertia and high-pull velocity. The changes are quantified by calculating the largest Lyapunov exponent and analyzing the statistical distributions of the amplitudes and durations of the model acoustic energy signals. Both single and two stage power-law distributions are observed. Scaling relations between the exponents are derived which show that the exponents corresponding to large values of event sizes and durations are completely determined by those for small values. The scaling relations are found to be satisfied in all cases studied. Interestingly, we find only five types of model acoustic emission signals among multitude of possibilities of the peel front configurations.

cond-mat.soft↗

arXiv2017

Modeling the complexity of acoustic emission spectra during intermittent plastic deformation: Power laws and multifractal spectra

Scale invariant power law distributions for acoustic emission signals are ubiquitous to several plastically deforming materials. However, power law distributions for the acoustic emission energies are reported in distinctly different plastically deforming situations such as in hcp and, fcc single and polycrystalline samples exhibiting smooth stress-strain curves, and in dilute metallic alloys exhibiting discontinuous flow. This is surprising since the underlying dislocation mechanisms in these two types of deformations are very different. So far, there has been no models that predict the power law statistics for the discontinuous flow. Furthermore, the statistics of the acoustic emission signals in jerky flow is even more complex requiring multifractal measures for a proper characterization. There has been no model that explains the complex statistics either. Here, we address the problem of statistical characterization of the acoustic emission signals associated with the three types of Portevin-Le Chatelier bands. We set-up a wave equation for elastic degrees of freedom with plastic strain rate as a source term. Using the plastic strain rate obtained from the Ananthakrishna model for the Portevin-Le Chatelier effect, we compute the acoustic emission spectrum corresponding to the three Portevin-Le Chatelier bands, which is used for further statistical characterization. Our results show that the model predicts power law statistics for all the three types of Portevin-Le Chatelier bands with the exponent values increasing with increasing strain rate. The calculated multifractal spectra corresponding to the acoustic emission signals associated with these three band types has a maximum spread for the type C decreasing with type B and A. We further show that the acoustic emission signals associated with L\"uders like band also exhibits power law distribution and multifractality.

cond-mat.mtrl-sci↗

arXiv2011

Multi-scale Modeling Approach to Acoustic Emission during Plastic Deformation

We address the long standing problem of the origin of acoustic emission commonly observed during plastic deformation. We propose a frame-work to deal with the widely separated time scales of collective dislocation dynamics and elastic degrees of freedom to explain the nature of acoustic emission observed during the Portevin-Le Chatelier effect. The Ananthakrishna model is used as it explains most generic features of the phenomenon. Our results show that while acoustic emission bursts correlated with stress drops are well separated for the type C serrations, these bursts merge to form nearly continuous acoustic signals with overriding bursts for the propagating type A bands.

cond-mat.mtrl-sci↗

arXiv2010

Correlation between stick-slip frictional sliding and charge transfer

A decade ago, Budakian and Putterman (Phys. Rev. Lett., {\bf 85}, 1000 (2000)) ascribed friction to the formation of bonds arising from contact charging when a gold tip of a surface force apparatus was dragged on polymethylmethacrylate surface. We propose a stick-slip model that captures the observed correlation between stick-slip events and charge transfer, and the lack of dependence of the scale factor connecting the force jumps and charge transfer on normal load. Here, stick-slip dynamics arises as a competition between the visco-elastic and plastic deformation time scales and that due to the pull speed with contact charging playing a minor role. Our model provides an alternate basis for explaining most experimental results without ascribing friction to contact charging.

cond-mat.mtrl-sci↗

arXiv2008

Intermittent Peel Front Dynamics and the Crackling Noise in an Adhesive Tape

We report a comprehensive investigation of a model for peeling of an adhesive tape along with a nonlinear time series analysis of experimental acoustic emission signals in an effort to understand the origin of intermittent peeling of an adhesive tape and its connection to acoustic emission. The model represents the acoustic energy dissipated in terms of Rayleigh dissipation functional that depends on the local strain rate. We show that the nature of the peel front exhibits rich spatiotemporal patterns ranging from smooth, rugged and stuck-peeled configurations that depend on three parameters, namely, the ratio of inertial time scale of the tape mass to that of the roller, the dissipation coefficient and the pull velocity. The stuck-peeled configurations are reminiscent of fibrillar peel front patterns observed in experiments. We show that while the intermittent peeling is controlled by the peel force function, the model acoustic energy dissipated depends on the nature of the peel front and its dynamical evolution. Even though the acoustic energy is a fully dynamical quantity, it can be quite noisy for a certain set of parameter values suggesting the deterministic origin of acoustic emission in experiments. To verify this suggestion, we have carried out a dynamical analysis of experimental acoustic emission time series for a wide range of traction velocities. Our analysis shows an unambiguous presence of chaotic dynamics within a subinterval of pull speeds within the intermittent regime. Time series analysis of the model acoustic energy signals is also found to be chaotic within a subinterval of pull speeds.

nlin.CD↗

arXiv2008

Hidden Order in Crackling Noise during Peeling of an Adhesive Tape

We address the long standing problem of recovering dynamical information from noisy acoustic emission signals arising from peeling of an adhesive tape subject to constant traction velocity. Using phase space reconstruction procedure we demonstrate the deterministic chaotic dynamics by establishing the existence of correlation dimension as also a positive Lyapunov exponent in a mid range of traction velocities. The results are explained on the basis of the model that also emphasizes the deterministic origin of acoustic emission by clarifying its connection to sticks-slip dynamics.

cond-mat.stat-mech↗