arXiv subjects

Vivek Pandey

Publications and source records attributed to Vivek Pandey.

At least 37 records · Page 2Linked to original sources

arXiv2025

Disorder induced dynamical interband response in Dirac nodal line semimetals

To obtain the total response of the system, the effect of disorder cannot be neglected, as it introduces a new contribution (i.e. extrinsic) in the total response of the system. In the study of dynamical (AC) effects, the interband response exhibits an exotic resonance peak due to interband transitions. Here, the dynamical interband response of Dirac nodal line semimetal is investigated by using the quantum kinetic approach. The scattering driven effect is analyzed under the first-order Born approximation (i.e., in the weak disorder limit) and reveals a resonance peak at $2\tildeμ$. In contrast, the field driven intrinsic response peak depends on both the mass ($\tilde{M}$) and chemical potential ($\tildeμ$). The results indicate that the total interband response of the 3D nodal line semimetals, is mainly dominated by the disorder induced contributions.

cond-mat.mes-hall↗

arXiv2025

Reshaping the anomalous Hall response in tilted 3D system with disorder correction

The anomalous Hall conductivity in the nodal line semimetals (NLSMs) due to the presence of a symmetry-protected nodal ring adds complexity in the investigation of their transport properties. By employing quantum kinetic theory and considering the weak disorder limit, we analyze the intraband and interband parts of anomalous Hall conductivity in the tilted 3D Dirac NLSMs. Our findings reveal that the net anomalous response is mainly contributed by the interband part. Further, the latter part gives non zero results by breaking inversion symmetry via tilt. We observe that the competition between the tilt and the chemical potential emerges kinks at distinct characteristic frequencies in the intrinsic interband part of the anomalous conductivity. On the other hand, the disorder driven interband component of the conductivity exhibits a prominent peak at low chemical potential, followed by a sign change. Notably, the disorder or extrinsic contribution to the response dominates over the intrinsic interband contribution, making it a crucial factor for the study of the overall response of a three-dimensional system.

cond-mat.mes-hall↗

arXiv2025

C-BerryANC: A first-principle C++ code to calculate Berry Curvature dependent anomalous Nernst conductivity in any material

The anomalous Nernst conductivity (ANC) is a key transport property in magnetic and topological materials, arising from the Berry curvature ($\boldsymbolΩ$) of electronic bands. It offers deep insight into the underlying topology and thermoelectric behavior. While Wannier interpolation have become popular for calculating ANC due to their computational efficiency, their accuracy critically depends on the quality of the Wannierization, which can be challenging for entangled bands or materials with complex band crossings. These limitations highlight the need for a direct first-principles approach to reliably compute ANC from ab-initio electronic structures. Here, we present a C++ based code named C-BerryANC that calculates $\boldsymbolΩ$-dependent ANC by directly using the eigenvalues and momentum-matrices obtained from DFT calculations. Presently, the code is interfaced with WIEN2k package which uses all-electron approach and full-potential linearized augmented plane wave (FP-LAPW) based method. For efficiently handling dense k-mesh, calculation of $\boldsymbolΩ$ is made parallel over k-points using the OpenMP method. Additionally, the code stores band-resolved components of $\boldsymbolΩ$ in binary files thereby reducing the memory occupancy and providing fast post-process option to compute ANC for any range of chemical potential and temperature values. Also, as compilation of C++ modules produce executable files which are in machine level language, computational speed of C-BerryANC is very fast. The code is benchmarked over some well-known materials exhibiting ANC. These includes- Pd, Fe$_3$Al & Co$_2$FeAl. The obtained values of ANC is found to in good agreement with the previously reported data. This highlights the accuracy, efficieny and reliability of the C-BerryANC code.

cond-mat.mtrl-sci↗

arXiv2025

$\textit{PY-BerryAHC}$: An $\textit{ab-initio}$ python 3 code to calculate Berry Curvature dependent Anomalous Hall Conductivity in any material

The anomalous Hall conductivity (AHC) in materials has long been a topic of debate. Studies reveal that AHC originates from the Berry curvature ($\boldsymbolΩ$) of Bloch states. Accurate computation of AHC is crucial for predicting material properties and guiding experimental studies in topological and spintronic applications. Traditional approaches often rely on wannier interpolation, which can introduce inaccuracies and computational overhead. Also, reliability of the wannierization technique becomes questionable when the bands are highly entangled and dispersive. This demands the calculation of AHC using the $\textit{first-principle}$ approach. Here, we present $\textit{PY-BerryAHC}$, a Python 3 based code that directly computes $\boldsymbolΩ$ and AHC using WIEN2k output. Since, WIEN2k employs an all-electron full-potential linearized augmented plane wave method, $\textit{PY-BerryAHC}$ provides highly accurate AHC results. The code efficiently handles large $\textbf{k}$-grids by parallelizing $\boldsymbolΩ$ computations over $\textbf{k}$-points. Also, it stores band-resolved $\boldsymbolΩ$ in a binary file, thereby greatly reducing the required storage memory and allowing fast post-processing to compute AHC. $\textit{PY-BerryAHC}$ has been validated on well-known materials exhibiting AHC. These include- Fe, Fe$_3$Ge & Co$_2$FeAl. At 300 K, the calculated magnitude of $σ_{xy}$ for Fe & Fe$_3$Ge is found to be 744 $S/cm$ & 311 $S/cm$, respectively. For Co$_2$FeAl, the magnitude of $σ_{xy}$ is obtained to be $\sim$56 $S/cm$ and is found to be constant with the change in temperature from 0-300 K. These results are in good agreement with previously reported theoretical and experimental data. This ensures the accuracy, reliability and efficiency of the code. The code is also provided with a post-processing tool to visualize $\boldsymbolΩ$.

cond-mat.str-el↗

arXiv2025

Tunable optical bistability of two-dimensional tilted Dirac system

We study the phenomenon of controlling the light by light known as the optical bistability for the two-dimensional tilted Dirac system. Using the Boltzmann approach under relaxation time approximation, we find that the optical bistability can be controlled by the nonlinear response of the system. For the prototype, we consider an inversion symmetry broken system. We find that the optical bistability associated with the nonlinear response is tunable with the strength of the tilt, gap and chemical potential. This suggests the inputs for the development of future-generation optical devices.

physics.optics↗

arXiv2025

Beyond Uncertainty: Risk-Aware Active View Acquisition for Safe Robot Navigation and 3D Scene Understanding with FisherRF

The active view acquisition problem has been extensively studied in the context of robot navigation using NeRF and 3D Gaussian Splatting. To enhance scene reconstruction efficiency and ensure robot safety, we propose the Risk-aware Environment Masking (RaEM) framework. RaEM leverages coherent risk measures to dynamically prioritize safety-critical regions of the unknown environment, guiding active view acquisition algorithms toward identifying the next-best-view (NBV). Integrated with FisherRF, which selects the NBV by maximizing expected information gain, our framework achieves a dual objective: improving robot safety and increasing efficiency in risk-aware 3D scene reconstruction and understanding. Extensive high-fidelity experiments validate the effectiveness of our approach, demonstrating its ability to establish a robust and safety-focused framework for active robot exploration and 3D scene understanding.

cs.RO↗

arXiv2024

Conditional entropy and information of quantum processes

What would be a reasonable definition of the conditional entropy of bipartite quantum processes, and what novel insight would it provide? We develop this notion using four information-theoretic axioms and define the corresponding quantitative formulas. Our definitions of the conditional entropies of channels are based on the generalized state and channel divergences, for instance, quantum relative entropy. We find that the conditional entropy of quantum channels has potential to reveal insights for quantum processes that aren't already captured by the existing entropic functions, entropy or conditional entropy, of the states and channels. The von Neumann conditional entropy $S[A|B]_{\mathcal{N}}$ of the channel $\mathcal{N}_{A'B'\to AB}$ is based on the quantum relative entropy, with system pairs $A',A$ and $B',B$ being nonconditioning and conditioning systems, respectively. We identify a connection between the underlying causal structure of a bipartite channel and its conditional entropy. In particular, we provide a necessary and sufficient condition for a bipartite quantum channel $\mathcal{N}_{A'B'\to AB}$ in terms of its von Neumann conditional entropy $S[A|B]_{\mathcal{N}}$, to have no causal influence from $A'$ to $B$. As a consequence, if $S[A|B]_{\mathcal{N}}< -\log|A|$ then the channel necessarily has causal influence (signaling) from $A'$ to $B$. Our definition of the conditional entropy establishes the strong subadditivity of the entropy for quantum channels. We also study the total amount of correlations possible due to quantum processes by defining the multipartite mutual information of quantum channels.

quant-ph↗

arXiv2024

Longitudinal DC Conductivity in Dirac Nodal Line Semimetals: Intrinsic and Extrinsic Contributions

Nodal line semimetals, a class of topological quantum materials, exhibit a variety of novel phenomena due to their properties, such as bands touching on a one-dimensional line or a ring in the Brillouin zone and drumhead-like surface states. In addition, these semimetals are protected by the combined space-inversion and time-reversal ($\mathcal{PT}$) symmetry. In this study, we investigate the longitudinal DC conductivity of the Dirac nodal line semimetals for the broken $\mathcal{PT}$-symmetric system by the mass term. Here, using the quantum kinetic technique, we find the intrinsic (field-driven) and extrinsic (scattering-driven) contributions to the total DC conductivity due to interband effects. Interestingly, the resulting intrinsic conductivity is the Fermi sea contribution, while the extrinsic stems from the Fermi surface contribution. We show that at low chemical potential, the extrinsic part contributes more and dominates over the traditional Drude intraband term, while at the high chemical potential, the intrinsic conductivity contributes. Furthermore, the total DC response due to interband effects saturates at high chemical potential and its strength decreases with increasing mass value. Our findings suggest that the extrinsic contributions are rich enough to understand the overall feature of the response for the three-dimensional system.

cond-mat.str-el↗

arXiv2024

Effect of measurements on quantum speed limit

Given the initial and final states of a quantum system, the speed of transportation of state vector in the projective Hilbert space governs the quantum speed limit. Here, we ask the question what happens to the quantum speed limit under continuous measurement process. We model the continuous measurement process by a non-Hermitian Hamiltonian which keeps the evolution of the system Schr{ö}dinger-like even under the process of measurement. Using this specific measurement model, we prove that under continuous measurement, the speed of transportation of a quantum system tends to zero. Interestingly, we also find that for small time scale, there is an enhancement of quantum speed even if the measurement strength is finite. Our findings can have applications in quantum computing and quantum control where dynamics is governed by both unitary and measurement processes.

quant-ph↗

arXiv2024

An ab-initio study of nodal-arcs, axial strain's effect on nodal-lines & Weyl nodes and Weyl-contributed Seebeck coefficient in TaAs class of Weyl semimetals

This work verifies the existence of dispersive \textit{nodal-arcs} and their evolution into Weyl nodes under the effect of spin-orbit coupling (SOC) in NbAs & NbP. The obtained features mimic the observations as reported for TaAs & TaP in our previous work. In addition, this work reports that the number of nodes in TaAs class of Weyl semimetals (WSMs) can be altered via creating strain along $a$ or $c$ direction of the crystal. For instance, the number of nodes in NbAs under SOC-effect along with 2% (3%) tensile-strain in $a$ direction is found to be 40 (56) in its full Brillouin zone (BZ). Besides the nodes, such strain are found to have considerable impact on the nodal-lines of these WSMs when effect of SOC is ignored. A 3\% tensile (compressive) strain along the $a$ ($c$) direction leads to the partially merging of nodal-lines (without SOC) in the extended BZ of NbAs \& NbP, which is not observed in TaAs & TaP within the range of -3% to 3% strain. Apart from this, the work discusses the role of Weyl physics in affecting the Seebeck coefficient ($S$) of any WSM. In this direction, it is discussed that how a symmetric Weyl cone, even if tilted, will have no contribution to the $S$ of WSMs. Furthermore, the work highlights the conditions under which a Weyl cone can contribute to the $S$ of a given WSM. Lastly, the discussion of Weyl contribution to $S$ is validated over TaAs class of WSMs via investigating the features of their Weyl cones and calculating the contributions of such cones to the $S$ of these semimetals. The value of $S$ contributed from Weyl cone is found to be as large as $\sim$65 $μ$\textit{V}/\textit{K} below 25 K in case of TaAs. The findings of this work present a possibility of engineering the topological properties of TaAs class of WSMs via creating strain in their crystal. It also makes the picture of Weyl physics impact on the $S$ of WSMs a more clear.

cond-mat.str-el↗

arXiv2024

Scalable Networked Feature Selection with Randomized Algorithm for Robot Navigation

We address the problem of sparse selection of visual features for localizing a team of robots navigating an unknown environment, where robots can exchange relative position measurements with neighbors. We select a set of the most informative features by anticipating their importance in robots localization by simulating trajectories of robots over a prediction horizon. Through theoretical proofs, we establish a crucial connection between graph Laplacian and the importance of features. We show that strong network connectivity translates to uniformity in feature importance, which enables uniform random sampling of features and reduces the overall computational complexity. We leverage a scalable randomized algorithm for sparse sums of positive semidefinite matrices to efficiently select the set of the most informative features and significantly improve the probabilistic performance bounds. Finally, we support our findings with extensive simulations.

cs.RO↗

arXiv2023

Data-Driven Distributionally Robust Mitigation of Risk of Cascading Failures

We introduce a novel data-driven method to mitigate the risk of cascading failures in delayed discrete-time Linear Time-Invariant (LTI) systems. Our approach involves formulating a distributionally robust finite-horizon optimal control problem, where the objective is to minimize a given performance function while satisfying a set of distributionally chances constraints on cascading failures, which accounts for the impact of a known sequence of failures that can be characterized using nested sets. The optimal control problem becomes challenging as the risk of cascading failures and input time-delay poses limitations on the set of feasible control inputs. However, by solving the convex formulation of the distributionally robust model predictive control (DRMPC) problem, the proposed approach is able to keep the system from cascading failures while maintaining the system's performance with delayed control input, which has important implications for designing and operating complex engineering systems, where cascading failures can severely affect system performance, safety, and reliability.

math.OC↗

arXiv2023

Quantification of Distributionally Robust Risk of Cascade of Failures in Platoon of Vehicles

Achieving safety is a critical aspect of attaining autonomy in a platoon of autonomous vehicles. In this paper, we propose a distributionally robust risk framework to investigate cascading failures in platoons. To examine the impact of network connectivity and system dynamics on the emergence of cascading failures, we consider a time-delayed network model of the platoon of vehicles as a benchmark. To study the cascading effects among pairs of vehicles in the platoon, we use the measure of conditional distributionally robust functional. We extend the risk framework to quantify cascading failures by utilizing a bi-variate normal distribution. Our work establishes closed-form risk formulas that illustrate the effects of time-delay, noise statistics, underlying communication graph, and sets of soft failures. The insights gained from our research can be applied to design safe platoons that are robust to the risk of cascading failures. We validate our results through extensive simulations.

eess.SY↗

arXiv2023

Investigating the effect of electronic correlation on transport properties and phononic states of Vanadium

In the present work, we have tried to investigate the importance of electronic correlation on transport properties and phononic states of Vanadium (V). The temperature-dependent electrical resistivity ($ρ$) and electronic part of thermal conductivity ($κ_e$) due to electron-electron interactions (EEIs) and electron-phonon interactions (EPIs) are computed. The values of $ρ$ due to EEIs are found to be extremely small in comparison to $ρ$ due to EPIs. For instance, at 300 K, the calculated value of $ρ$ due to EEIs (EPIs) is $\sim$ 0.859$\times10^{-3}$ ($\sim$ 0.20) $μΩ$m. The magnitudes of $κ_e$ due to EPIs are found to be in good agreement with the experimental results. These observations indicate the negligible importance of EEIs to these quantities for V. However, at 300 K, the value of Seebeck coefficient ($S$) at DFT+DMFT level ($\sim$ -0.547 $μ$VK$^{-1}$) is found to be entirely different than at DFT level ($\sim 7.401$ $μ$VK$^{-1}$). Also, the DFT+DMFT value of $S$ at 300 K is in good match with the available experimental data (-1.06 $μ$VK$^{-1}$, 1.0 $μ$VK$^{-1}$). Apart from this, the study of phononic states at DFT and DFT+DMFT level is performed. The obtained phononic band structure and phonon DOS at DFT+DMFT differ to a good extent from that at DFT. The maximum energy of phononic state obtained at DFT (DFT+DMFT) is $\sim$ 33.83 ($\sim$ 35.15) meV, where the result of DFT+DMFT is obtained more closer to the experimental data (35.15, 36.98 & 41.77 meV). These results highlight the importance of electronic correlation on $S$ & phononic states of simple correlated V metal.

cond-mat.str-el↗

arXiv2023

Existence of nodal-arc and its evolution into Weyl-nodes in the presence of spin-orbit coupling in TaAs & TaP

In this work, we report the existence of nodal-arc, which acts as the building block of all the nodal-rings in TaAs & TaP. This nodal-arc is found to be capable of generating all the nodal-rings in these materials upon the application of space-group symmetry operations including time-reversal symmetry. The arcs are obtained to be dispersive with the energy spread of $\sim$109 ($\sim$204) meV in TaAs (TaP). Also, the orbitals leading to bands-inversion and thus the formation of nodal-arcs are found to be Ta-5d & As-4p (P-3p) in TaAs (TaP). The area of nodal-rings is found to be highly sensitive to the change in hybridization-strength, where the increase in hybridization-strength leads to the decrease in the area of nodal-rings. In the presence of spin-orbit coupling (SOC), all the points on these arcs get gaped-up and two pairs of Weyl-nodes are found to evolve from them. Out of the two pair, one is found to be situated close to the joining point of the two arcs forming a ring. This causes the evolution of each nodal-ring into three pairs of Weyl-nodes. The coordinates of these Weyl-nodes are found to be robust to the increase in SOC-strength from $\sim$ 0.7-3.5 eV. All the results are obtained at the first-principle level. This work provides a clear picture of the existence of nodal-arc due to accidental degeneracy and its evolution into Weyl-nodes under the effect of SOC.

cond-mat.mes-hall↗

arXiv2023

Speed limits on correlations in bipartite quantum systems

Quantum speed limit is bound on the minimum time a quantum system requires to evolve from an initial state to final state under a given dynamical process. It sheds light on how fast a desired state transformation can take place which is pertinent for design and control of quantum technologies. In this paper, we derive speed limits on correlations such as entanglement, Bell-CHSH correlation, and quantum mutual information of quantum systems evolving under dynamical processes. Our main result is speed limit on an entanglement monotone called negativity which holds for arbitrary dimensional bipartite quantum systems and processes. Another entanglement monotone which we consider is the concurrence. To illustrate efficacy of our speed limits, we analytically and numerically compute the speed limits on the negativity, concurrence, and Bell-CHSH correlation for various quantum processes of practical interest. We are able to show that for practical examples we have considered, some of the speed limits we derived are actually attainable and hence these bounds can be considered to be tight.

quant-ph↗

arXiv2023

Cascading Waves of Fluctuation in Time-delay Multi-agent Rendezvous

We develop a framework to assess the risk of cascading failures when a team of agents aims to rendezvous in time in the presence of exogenous noise and communication time-delay. The notion of value-at-risk (VaR) measure is used to evaluate the risk of cascading failures (i.e., waves of large fluctuations) when agents have failed to rendezvous. Furthermore, an efficient explicit formula is obtained to calculate the risk of higher-order cascading failures recursively. Finally, from a risk-aware design perspective, we report an evaluation of the most vulnerable sequence of agents in various communication graphs.

eess.SY↗

arXiv2022

PY-Nodes: An ab-initio python code for searching nodes in a material using Nelder-Mead's simplex approach

With the discovery of topological semimetals, it has been found that the band touching points near the Fermi level are of great importance. They give rise to many exciting phenomena in these materials. Moreover, these points, commonly known as nodes, are related to several properties of these semimetals. Thus, the proper estimation of their coordinates is extremely needed for better understanding of the properties of these materials. We have designed a Python 3 based code named PY-Nodes for efficiently finding the nodes present in a given material using first-principle approach. The present version of the code is interfaced with the WIEN2k package. For benchmarking the code, it has been tested on some famous materials which possess characteristic nodes. These include - TaAs, a well-known Weyl semimetal, Na$_3$Bi, which is categorized as Dirac semimetal, CaAgAs, classified as a nodal-line semimetal and YAuPb, which is claimed to be non-trivial topological semimetal. In case of TaAs, 24 nodes are obtained from our calculations. On computing their chiralities, it is found that 12 pairs of nodes having equal and opposite chirality are obtained. Furthermore, for Na$_3$Bi, a pair of nodes are obtained on the either side of $Γ$-point in the $\boldsymbol{k_3}$ direction. In case of CaAgAs, several nodes are obtained in the $k_z$=0 plane. These nodes, when plotted in the $k_x$-$k_y$ plane, form a closed loop which is generally referred to as nodal-line. Finally, in the case of YAuPb, large number of nodes are obtained in the vicinity of $Γ$-point. The results obtained for these materials are in good match with the previous works carried out by different research groups. This assures the reliability and the efficiency of the PY-Nodes code for estimating the nodes present in a given material.

cond-mat.mtrl-sci↗