Showing 1–13 of 13 results for author: K, V

Effect of Vacancies on Hydrogen Mobility and Trapping in Elemental Fe and Cr: A DFT and kMC Study

Authors: Vallinathan K, Gurpreet Kaur, Sharat Chandra

Abstract: Hydrogen-vacancy interactions play an important role in governing hydrogen transport and embrittlement in body-centered cubic (BCC) metals. In this study, a multiscale approach combining density functional theory (DFT) and kinetic Monte Carlo (kMC) simulations is employed to investigate hydrogen behavior in BCC Fe and Cr. The DFT-calculated binding energies and Bader charge analysis indicate stron… ▽ More Hydrogen-vacancy interactions play an important role in governing hydrogen transport and embrittlement in body-centered cubic (BCC) metals. In this study, a multiscale approach combining density functional theory (DFT) and kinetic Monte Carlo (kMC) simulations is employed to investigate hydrogen behavior in BCC Fe and Cr. The DFT-calculated binding energies and Bader charge analysis indicate stronger hydrogen trapping in Cr than in Fe. Migration and detrapping energy barriers are determined using the climbing-image nudged elastic band method, showing that the detrapping energy generally decreases with increasing hydrogen occupancy. However, the sixth hydrogen atom in Fe exhibits a finite barrier, contrary to some previous reports. kMC simulations are then used to evaluate hydrogen diffusion over extended time and length scales. The results demonstrate that vacancy defects significantly reduce hydrogen mobility and increase the effective activation energy, with a more pronounced effect observed in Cr due to stronger trapping. The combined DFT-kMC framework provides detailed insight into the mechanisms of hydrogen trapping, detrapping, and diffusion in BCC metals, offering important implications for understanding hydrogen embrittlement in structural materials. △ Less

Submitted 27 May, 2026; originally announced May 2026.

Robust quantum metrology using disordered probes

Authors: Vishnupriya K., Harikrishnan K. J., Amit Kumar Pal

Abstract: Disorder is ubiquitous in quantum devices including quantum probes designed and fabricated for quantum parameter estimation and sensing. We investigate the robustness of a quantum probe against the presence of glassy disorder. We define a disorder marker quantifying the effect of the disorder by expanding the quantum Fisher information in terms of different orders of the standardized central momen… ▽ More Disorder is ubiquitous in quantum devices including quantum probes designed and fabricated for quantum parameter estimation and sensing. We investigate the robustness of a quantum probe against the presence of glassy disorder. We define a disorder marker quantifying the effect of the disorder by expanding the quantum Fisher information in terms of different orders of the standardized central moments of the disorder-distributions. We classify the quantum probes in terms of the possible values of the disorder marker, and analytically show, for a disorder-sensitive probe with identical and weak disorder on all or a subset of the parameters of the probe-Hamiltonian, that the absolute value of the disorder marker exhibits a quadratic dependence on the disorder strength. We derive a robustness scale intrinsic to the probe that competes with the disorder, and provide a prescription for estimating the maximum disorder strength that the probe can withstand from the disorder-free probe-Hamiltonian for a given initial state of the probe, which can be computed without the disorder averaging. We demonstrate our results in the case of a single-qubit probe under disordered magnetic field, and a multi-qubit probe described by a disordered one-dimensional Kitaev model with nearest-neighbor interactions. △ Less

Submitted 13 April, 2026; originally announced April 2026.

Comments: 16 pages, 3 figures

Ising Model with Power Law Resetting

Authors: Anagha V K, Apoorva Nagar

Abstract: We investigate the nonequilibrium dynamics of the nearest-neighbour Ising model subjected to stochastic resetting, where the system is intermittently returned to an initial configuration with magnetisation $m_0$, with the inter-reset times drawn from the power law distribution $ατ_0^α/ τ^{α+1}$. The heavy-tailed resets generate magnetisation distributions that differ significantly from both equili… ▽ More We investigate the nonequilibrium dynamics of the nearest-neighbour Ising model subjected to stochastic resetting, where the system is intermittently returned to an initial configuration with magnetisation $m_0$, with the inter-reset times drawn from the power law distribution $ατ_0^α/ τ^{α+1}$. The heavy-tailed resets generate magnetisation distributions that differ significantly from both equilibrium dynamics and the previously studied Ising model with exponentially distributed reset times. In two dimensions, for $T > T_C$, we find a quasi-ferro state for all $α$, marked by a double-peaked distribution that diverges at $m=0$ and $m=m_0$; no steady state exists for $α< 1$, while a stationary state emerges for $α> 1$. For $T < T_C$, power law resetting produces two distinct regimes separated by a crossover exponent $α^* = 1-c$: a single-peak ferromagnetic phase localised at $m_{eq}$ for $α< α^*$, and a dual-peak ferromagnetic phase with divergences at $m_{eq}$ and $m_0$ for $α> α^*$. Analytic results in one and two dimensions, supported by simulations, yield a rich phase diagram in the $(T,α)$ plane and reveal how heavy-tailed resetting generates nonequilibrium phases very different from those seen in the case of exponential resetting. △ Less

Submitted 17 February, 2026; originally announced February 2026.

Journal ref: Phys. Rev. Research 8, 023197, 2026

Strain-induced Ettingshausen effect in spin-orbit coupled noncentrosymmetric metals

Authors: Gautham Varma K, Azaz Ahmad, Gargee Sharma

Abstract: Elastic deformations couple with electronic degrees of freedom in materials to generate gauge fields that lead to interesting transport properties. Recently, it has been well studied that strain-induced chiral magnetic fields in Weyl semimetals lead to interesting magnetotransport induced by the chiral anomaly (CA). Recent studies have revealed that CA is not necessarily only a Weyl-node property,… ▽ More Elastic deformations couple with electronic degrees of freedom in materials to generate gauge fields that lead to interesting transport properties. Recently, it has been well studied that strain-induced chiral magnetic fields in Weyl semimetals lead to interesting magnetotransport induced by the chiral anomaly (CA). Recent studies have revealed that CA is not necessarily only a Weyl-node property, but is rather a Fermi surface property, and is also present in a more general class of materials, for example, in spin orbit-coupled noncentrosymmetric metals (SOC-NCMs). The interplay of strain, CA, and charge and thermomagnetic transport in SOC-NCMs, however, remains unexplored. Here we resolve this gap. Using a tight-binding model for SOC-NCMs, we first demonstrate that strain in SOC-NCMs induces anisotropy in the spin-orbit coupling and generates an axial electric field. Then, using the quasi-classical Boltzmann transport formalism with momentum-dependent intraband and interband scattering processes, we show that strain in the presence of external magnetic field can generate temperature gradients via the Nernst-Ettingshausen effect, whose direction and behavior depends the on interplay of multiple factors: the angle between the applied strain and magnetic field, the presence of the chiral anomaly, the Lorentz force, and the strength of interband scattering. We further reveal that time-reversal symmetry breaking in the presence of an external magnetic field generates the Berry-curvature-driven anomalous Ettingshausen effect, which is qualitatively distinct from the conventional Lorentz-force-driven counterpart. In light of recent and forthcoming theoretical and experimental advances in the field of SOC-NCMs, we find our study to be particularly timely and relevant. △ Less

Submitted 29 August, 2025; v1 submitted 18 August, 2025; originally announced August 2025.

Comments: 13 pages, 8 figures. Figure 2 slightly modified in this version

Chiral anomaly-induced nonlinear Hall effect in spin-orbit coupled noncentrosymmetric metals

Authors: Gautham Varma K, Mohd. Hashim Raza, Azaz Ahmad

Abstract: Recent studies have shown that chiral anomaly is not limited to Weyl semimetals (WSMs), but are also shown by a larger class of materials called spin orbit coupled noncentrosymmetric metals (SOC-NCMs),which has shed more insight into the origin of chiral anomaly as a Fermi surface property rather than a nodal property. In this study, we explore nonlinear transport responses in SOC-NCMswithin the f… ▽ More Recent studies have shown that chiral anomaly is not limited to Weyl semimetals (WSMs), but are also shown by a larger class of materials called spin orbit coupled noncentrosymmetric metals (SOC-NCMs),which has shed more insight into the origin of chiral anomaly as a Fermi surface property rather than a nodal property. In this study, we explore nonlinear transport responses in SOC-NCMswithin the framework of semiclassical dynamics, employing the Maxwell-Boltzmann transport theory augmented by charge conservation and momentum-dependent scattering processes. We take into account both non-magnetic and magnetic impurity scattering mechanisms. We demonstrate that the chiral-anomaly-induced nonlinear Hall (CNLH) response exhibits a characteristic quadratic dependence on the applied magnetic field and remains negative for both types of impurities. We find that magnetic scatterers leading to enhanced/suppressed interband scattering modifies the magnitude of the signal, but does not affect its qualitative behavior. In contrast, the presence of tilt in the band dispersion induces a pronounced anisotropic response, including a magnetic-field-direction dependent sign reversal that can be categorized into weak and strong regimes. Furthermore, the CNLH response shows substantial directional anisotropy governed by the relative orientation of the external magnetic field and the tilt vector. Our findings will be helpful in designing the experimental setup to get direction-dependent conductivity, which can be tuned externally with the help of magnetic impurity sites. △ Less

Submitted 6 January, 2026; v1 submitted 1 August, 2025; originally announced August 2025.

Comments: Updated version

Journal ref: Phys. Rev. B 113, 035112(2026)

Nonlinear anomalous Hall effect in three-dimensional chiral fermions

Authors: Azaz Ahmad, Gautham Varma K., Gargee Sharma

Abstract: Chiral fermionic quasiparticles emerge in certain quantum condensed matter systems such as Weyl semimetals, topological insulators, and spin-orbit coupled noncentrosymmetric metals. Here, a comprehensive theory of the chiral anomaly-induced nonlinear anomalous Hall effect (CNLAHE) is developed for three-dimensional chiral quasiparticles, advancing previous models by rigorously including momentum-d… ▽ More Chiral fermionic quasiparticles emerge in certain quantum condensed matter systems such as Weyl semimetals, topological insulators, and spin-orbit coupled noncentrosymmetric metals. Here, a comprehensive theory of the chiral anomaly-induced nonlinear anomalous Hall effect (CNLAHE) is developed for three-dimensional chiral quasiparticles, advancing previous models by rigorously including momentum-dependent chirality-preserving and chirality-breaking scattering processes and global charge conservation. Focusing on two specific systems-Weyl semimetals (WSMs) and spinorbit coupled non-centrosymmetric metals (SOC-NCMs), we uncover that the nonlinear anomalous Hall conductivity in WSMs shows nonmonotonic behavior with the Weyl cone tilt and experiences a "strong-sign-reversal" with increasing internode scattering, diverging from earlier predictions. For SOC-NCMs, where nonlinear anomalous Hall conductivity has been less explored, we reveal that unlike WSM, the orbital magnetic moment alone can drive a large CNLAHE with distinctive features: the CNLAH conductivity remains consistently negative regardless of interband scattering intensity and exhibits a quadratic dependence on the magnetic field, contrasting the linear dependence in WSMs. Furthermore, we discover that in SOC-NCMs the Zeeman coupling of the magnetic field acts like an effective tilt term which can further enhance the CNLAH current. These findings offer fresh insights into the nonlinear transport dynamics of chiral quasiparticles and can be verified in upcoming experiments on such materials. △ Less

Submitted 4 September, 2024; originally announced September 2024.

Geometry, anomaly, topology, and transport in Weyl fermions

Authors: Azaz Ahmad, Gautham Varma K., Gargee Sharma

Abstract: Weyl fermions are one of the simplest objects that link ideas in geometry and topology to highenergy physics and condensed matter physics. Although the existence of Weyl fermions as elementary particles remains dubious, there is mounting evidence of their existence as quasiparticles in certain condensed matter systems. Such systems are termed Weyl semimetals (WSMs). Needless to say, WSMs have emer… ▽ More Weyl fermions are one of the simplest objects that link ideas in geometry and topology to highenergy physics and condensed matter physics. Although the existence of Weyl fermions as elementary particles remains dubious, there is mounting evidence of their existence as quasiparticles in certain condensed matter systems. Such systems are termed Weyl semimetals (WSMs). Needless to say, WSMs have emerged as a fascinating class of materials with unique electronic properties, offering a rich playground for both fundamental research and potential technological applications. This review examines recent advancements in understanding electron transport in Weyl semimetals (WSMs). We begin with a pedagogical introduction to the geometric and topological concepts critical to understanding quantum transport in Weyl fermions. We then explore chiral anomaly (CA), a defining feature of WSMs, and its impact on transport phenomena such as longitudinal magnetoconductance (LMC) and the planar Hall effect (PHE). The Maxwell-Boltzmann transport theory extended beyond the standard relaxation-time approximation is then discussed in the context of Weyl fermions, which is used to evaluate various transport properties. Attention is also given to the effects of strain-induced gauge fields and external magnetic fields in both time-reversal broken and inversion asymmetric inhomogeneous WSMs. The review synthesizes theoretical insights, experimental observations, and numerical simulations to provide a comprehensive understanding of the complex transport behaviors in WSMs, aiming to bridge the gap between theoretical predictions and experimental verification. △ Less

Submitted 3 June, 2024; originally announced June 2024.

Comments: Invited review. Comments welcome

Critical crack-length during fracture

Authors: Viswakannan R. K., Subhadeep Roy

Abstract: Through controlled numerical simulations in a one dimensional fiber bundle model with local stress concentration, we established an inverse correlation between the strength of the material and the cracks which grow inside it - both the maximum crack and the one that set in instability within the system, defined to be the critical crack. Through Pearson correlation function as well as probabilistic… ▽ More Through controlled numerical simulations in a one dimensional fiber bundle model with local stress concentration, we established an inverse correlation between the strength of the material and the cracks which grow inside it - both the maximum crack and the one that set in instability within the system, defined to be the critical crack. Through Pearson correlation function as well as probabilistic study of individual configurations, we found that the maximum and the critical crack often differ from each other unless the disorder strength is extremely low. A phase diagram on the plane of disorder vs system size demarcates between the regions where the largest crack is the most vulnerable one and where they differ from each other but still shows moderate correlation. △ Less

Submitted 19 February, 2024; originally announced February 2024.

Comments: 7 pages, 7 figures

Autonomous Multi-Rotor UAVs: A Holistic Approach to Design, Optimization, and Fabrication

Authors: Aniruth A, Chirag Satpathy, Jothika K, Nitteesh M, Gokulraj M, Venkatram K, Harshith G, Shristi S, Anushka Vani, Jonathan Spurgeon

Abstract: Unmanned Aerial Vehicles (UAVs) have become pivotal in domains spanning military, agriculture, surveillance, and logistics, revolutionizing data collection and environmental interaction. With the advancement in drone technology, there is a compelling need to develop a holistic methodology for designing UAVs. This research focuses on establishing a procedure encompassing conceptual design, use of c… ▽ More Unmanned Aerial Vehicles (UAVs) have become pivotal in domains spanning military, agriculture, surveillance, and logistics, revolutionizing data collection and environmental interaction. With the advancement in drone technology, there is a compelling need to develop a holistic methodology for designing UAVs. This research focuses on establishing a procedure encompassing conceptual design, use of composite materials, weight optimization, stability analysis, avionics integration, advanced manufacturing, and incorporation of autonomous payload delivery through object detection models tailored to satisfy specific applications while maintaining cost efficiency. The study conducts a comparative assessment of potential composite materials and various quadcopter frame configurations. The novel features include a payload-dropping mechanism, a unibody arm fixture, and the utilization of carbon-fibre-balsa composites. A quadcopter is designed and analyzed using the proposed methodology, followed by its fabrication using additive manufacturing and vacuum bagging techniques. A computer vision-based deep learning model enables precise delivery of payloads by autonomously detecting targets. △ Less

Submitted 4 January, 2024; originally announced January 2024.

Magnetotransport in spin-orbit coupled noncentrosymmetric and Weyl metals

Authors: Gautham Varma K, Azaz Ahmad, Sumanta Tewari, G. Sharma

Abstract: Recently, chiral anomaly (CA) has been proposed to occur in spin-orbit coupled noncentrosymmetric metals (SOC-NCMs), motivating CA to be a Fermi surface property rather than a Weyl node property. Although the nature of the anomaly is similar in both SOC-NCMs and Weyl systems, here we point out significant fundamental differences between the two. We show that the different nature of the orbital mag… ▽ More Recently, chiral anomaly (CA) has been proposed to occur in spin-orbit coupled noncentrosymmetric metals (SOC-NCMs), motivating CA to be a Fermi surface property rather than a Weyl node property. Although the nature of the anomaly is similar in both SOC-NCMs and Weyl systems, here we point out significant fundamental differences between the two. We show that the different nature of the orbital magnetic moment (OMM) in the two systems leads to non-trivial consequences -- particularly the sign of the longitudinal magnetoconductance always remains positive in a SOC non-centrosymmetric metal, unlike a Weyl metal that displays either sign. Furthermore,we investigate the planar Hall effect and the geometrical contribution to the Hall effect in the two systems and point out significant differences in the two systems. We conduct our analysis for magnetic and non-magnetic impurities, making our study important in light of current and upcoming experiments in both SOC-NCMs and Weyl metals. △ Less

Submitted 30 October, 2023; originally announced October 2023.

Current redistribution model of anomalous resistance behaviour in superconductor-topological insulator heterostructures

Authors: Abhirami S., Edward Prabu Amaladass, Prashant Sharma, Vinod K., Thanikaiarasu A. V., Awadhesh Mani

Abstract: Anomalous resistance upturn and downturn have been observed on the topological insulator (TI) surface in superconductor-TI (NbN-Bi1.95Sb0.05Se3) heterostructures at ~ mm length scales away from the interface. Magnetotransport measurements were performed to verify that the anomaly is caused due to the superconducting transition of the NbN layer. The possibility of long range superconducting proximi… ▽ More Anomalous resistance upturn and downturn have been observed on the topological insulator (TI) surface in superconductor-TI (NbN-Bi1.95Sb0.05Se3) heterostructures at ~ mm length scales away from the interface. Magnetotransport measurements were performed to verify that the anomaly is caused due to the superconducting transition of the NbN layer. The possibility of long range superconducting proximity effect due to the spin-polarized TI surface state was ruled out due to the observation of similar anomaly in NbN-Au and NbN-Al heterostructures. It was discovered that the unusual resistance jumps were caused due to current redistribution at the superconductor-TI interface on account of the geometry effects. Results obtained from finite element analysis using COMSOL package has validated the proposed current redistribution (CRD) model of long range resistance anomalies in superconductor-TI and superconductor-metal heterostructures. △ Less

Submitted 9 December, 2022; originally announced December 2022.

Comments: 37 pages (including references), 17 figures

Shape and motion of drops in the inertial regime

Authors: Vijaya Senthil Kumar K., Baburaj A. Puthenveettil

Abstract: In this paper, we report experimental results on the shape and motion of a mercury droplet, placed in a horizontally rotating cylinder in the rpm range 8-93, so that the Reynolds number of the drop 2500<Re<26000 and its capillary number 0.0002<Ca<0.0023. When contact angle variations can be neglected at low speeds (Re<8150), the velocity of the drop is much lower than that predicted by the Ho. You… ▽ More In this paper, we report experimental results on the shape and motion of a mercury droplet, placed in a horizontally rotating cylinder in the rpm range 8-93, so that the Reynolds number of the drop 2500<Re<26000 and its capillary number 0.0002<Ca<0.0023. When contact angle variations can be neglected at low speeds (Re<8150), the velocity of the drop is much lower than that predicted by the Ho. Young Kim's [6] relation. This observed discrepancy is overcome by modifying Kim's relation by substituting the dissipation estimated from a boundary layer near the solid surface instead of bulk dissipation. Based on the changes at the rear side of the mercury droplet, there are three distinct regimes identified with varying speeds of rotation (i) oval or rounded regime (ii) corner regime and (iii) cusping regime. The oval to corner transition happens at a finite receding contact angle of 950. The ratio of critical contact angle (θc) at which the transition occurs to the static receding contact angle (θs) was found to be 0.657. The de Gennes model [4], extended to high contact angle by substituting the dissipation for wedge flow, predicts a critical contact angle ratio (θc/θs) that is in close agreement with the experimental value. At higher Re, the dynamic contact angle variation with velocity was compared with Cox-Voinov model [3]. Though the trend of the variation of data is approximately represented by the model, the fit coefficient according to experimental data is very high when compared to theoretical value. △ Less

Submitted 12 May, 2011; v1 submitted 8 June, 2010; originally announced June 2010.

Comments: Published in Proceedings of Asian Conference on Fluid Mechanics (ACFM) December 2010 Dhaka Bangladesh

Noncommutative deformation and a topological nature of singularity Koiter

Authors: Trinh V. K

Abstract: In this paper we constructed the model of noncommutative plastic deformation and give the proof of hypothesis Koiter. We showed, that occurrence of singularity Koiter has the topological reasons and number of singularities Koiter - it is topological number Pontriagin. In this paper we constructed the model of noncommutative plastic deformation and give the proof of hypothesis Koiter. We showed, that occurrence of singularity Koiter has the topological reasons and number of singularities Koiter - it is topological number Pontriagin. △ Less

Submitted 10 April, 2003; v1 submitted 8 April, 2003; originally announced April 2003.

Comments: LaTeX, 10 pages, no figures

Report number: Moscow State University, thesis 2003g