arXiv subjects

Amit Gupta

Publications and source records attributed to Amit Gupta.

31 records · Page 2Linked to original sources

arXiv2019

Na-ion diffusion and electrochemical performance of NaVO$_3$ anode in Li/Na batteries

We study Na ion diffusion and electrochemical performance of NaVO$_3$ (NVO) as anode material in Li/Na--ion batteries with the specific capacity of $\approx$350 mAhg$^{-1}$ at the current density 11~mAg$^{-1}$ after 300 cycles. Remarkably, the capacity retains $\ge$200~mAhg$^{-1}$ even after 400~cycles at 44~mAg$^{-1}$ with Coulombic efficiency $>$99\%. The deduced diffusion coefficient from galvanostatic intermittent titration technique (GITT), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements for NVO as anode in Li--ion battery is in the range of 10$^{-10}$--10$^{-12}$~cm$^2$s$^{-1}$. In case of Na-ion batteries, the NVO electrode exhibits initial capacity of 385~mAhg$^{-1}$ at 7~mAg$^{-1}$ current rate, but the capacity degradation is relatively faster in subsequent cycles. We find the diffusion coefficient of NVO--Na cells similar to that of NVO--Li. On the other hand, our charge discharge measurements suggest that the overall performance of NVO anode is better in Li--ion battery than Na-ion. Moreover, we use the density functional theory to simulate the energetics of Na vacancy formation in the bulk of the NVO structure, which is found to be 0.88~eV higher than that of the most stable (100) surface. Thus, the Na ion incorporation at the surface of the electrode material is more facile compared to the bulk.

physics.app-ph↗

arXiv2019

Novel electric field effects on Landau levels in multi- Weyl semimetals

Multi-Weyl semimetals(WSMs) have an anisotropic non-linear dispersion along a 2-D plane and a linear dispersion in an orthogonal direction. They have topological charge $\lq n $' and are created when two or multiple Weyl points or nodes with nonzero net monopole charge are brought together onto a high-symmetry point. We study the perturbation corrections up to second order of such multi-WSMs in crossed electric($\mathcal{E}$) and magnetic (B) fields in the low electric field approximation. As a result, the first order correction lifts only n-fold degeneracy of the lowest Landau levels(LLs) while the higher Landau levels are modified due to the second order perturbation. We study the signatures of these corrections due to electric fields on the density of states (DOS) subjected to a magnetic field.

cond-mat.str-el↗

arXiv2019

Electrochemical properties of Na$_{0.66}$V$_4$O$_{10}$ nanostructures as cathode material in rechargeable batteries for energy storage applications

We report the electrochemical performance of nanostructures of Na$_{0.66}$V$_4$O$_{10}$ as cathode material for rechargeable batteries. The Rietveld refinement of room temperature x-ray diffraction pattern shows the monoclinic phase with C2/m space group. The cyclic voltammetry curves of prepared half-cells exhibit redox peaks at ~3.1 and 2.6~V, which are due to two-phase transition reaction between V$^{5+/4+}$ and can be assigned to the single step deintercalation/intercalation of Na-ion. We observe a good cycling stability with specific discharge capacity (measured vs. Na$^+$/Na) between 80 ($\pm$2) and 30 ($\pm$2) mAh g$^{-1}$ at a current density 3 and 50~mA g$^{-1}$, respectively. The electrochemical performance of Na$_{0.66}$V$_4$O$_{10}$ electrode was also tested with Li anode, which showed higher capacity, but decay faster than Na. Using density functional theory, we calculate the Na vacancy formation energies; 3.37~eV in the bulk of the material and 2.52~eV on the (100) surface, which underlines the importance of nanostructures.

cond-mat.mtrl-sci↗

arXiv2018

Physical properties and electrochemical performance of Zn-substituted Na$_{0.44}$Mn$_{1-x}$Zn$_x$O$_2$ nanostructures as cathode in Na-ion batteries

We report the synthesis, physical properties and electrochemical performance of Zn substituted Na$_{0.44}$Mn$_{1-x}$Zn$_x$O$_2$ ($x=$ 0 -- 0.02) nanostructures as cathode in Na-ion batteries for energy storage applications. These samples stabilize in the orthorhombic structure and the morphology is found to be slab like with 100 -- 200~nm width and few micrometer of length. The resistivity measurements show highly insulating nature for all the samples, where the activation energy decreases with increasing Zn concentration indicating more defect levels in the band gap. The cyclic voltammogram (CV) shows reversible oxidation and reduction peaks, which clearly shift towards higher/lower potentials with increasing Zn concentration up to 2\%. We observed the specific capacity of about 100~mAh/g at current density of 4 mA/g and improved cycle life for Zn substituted $x=$ 0.005 sample. However, with further increasing the Zn concentration ($x=$ 0.02), the specific capacity decreases, which can be a manifestation of the decreased number of reduction peaks in the CV data.

cond-mat.mtrl-sci↗

arXiv2018

Synthesis and physical properties of Na$_x$TO$_2$ (T=Mn, Co) nanostructures for cathode materials in Na--ion batteries

We prepared Na$_x$TO$_2$ (T = Mn, Co) nanostructures to use as cathode material in Na-ion batteries. The Rietveld refinement of x-ray diffraction data confirms the hexagonal symmetry. Scanning and transmission electron microscopy measurements show hexagonal and rod-shape morphology for T = Co and Mn, respectively. The magnetic measurements indicate the presence of variable oxidation states of Mn/Co ions, but no long range ordering till 5~K. We find that the Na$_{0.6}$MnO$_2$ is highly insulating whereas Na$_{0.7}$CoO$_2$ is semiconducting in nature. The conduction in Na$_{0.7}$CoO$_2$ takes place through both variable range hopping (VRH) as well as activation mechanisms in different temperature ranges. However, for Na$_{0.6}$MnO$_2$ the VRH prevails at elevated temperatures. Furthermore, the coin cells have been fabricated and tested the electrochemical behavior using cyclic voltammetry, which confirms the reversibility of Na--ions during intercalation/de-intercalation.

cond-mat.mtrl-sci↗

arXiv2017

Taxonomy Induction using Hypernym Subsequences

We propose a novel, semi-supervised approach towards domain taxonomy induction from an input vocabulary of seed terms. Unlike all previous approaches, which typically extract direct hypernym edges for terms, our approach utilizes a novel probabilistic framework to extract hypernym subsequences. Taxonomy induction from extracted subsequences is cast as an instance of the minimumcost flow problem on a carefully designed directed graph. Through experiments, we demonstrate that our approach outperforms stateof- the-art taxonomy induction approaches across four languages. Importantly, we also show that our approach is robust to the presence of noise in the input vocabulary. To the best of our knowledge, no previous approaches have been empirically proven to manifest noise-robustness in the input vocabulary.

cs.AI↗

arXiv2017

280 Birds with One Stone: Inducing Multilingual Taxonomies from Wikipedia using Character-level Classification

We propose a simple, yet effective, approach towards inducing multilingual taxonomies from Wikipedia. Given an English taxonomy, our approach leverages the interlanguage links of Wikipedia followed by character-level classifiers to induce high-precision, high-coverage taxonomies in other languages. Through experiments, we demonstrate that our approach significantly outperforms the state-of-the-art, heuristics-heavy approaches for six languages. As a consequence of our work, we release presumably the largest and the most accurate multilingual taxonomic resource spanning over 280 languages.

cs.CL↗

arXiv2017

Floquet dynamics in multi-Weyl semimetals

Multi-Weyl semimetals(m-WSMS) are characterized by anisotropic non-linear energy dispersion along 2-D plane and a linear dispersion in orthogonal direction. They have topological charge J and are realized when two or multiple Weyl nodes with nonzero net monopole charge bring together onto a high-symmetry point. The model Hamiltonians of such systems show various phases such as trivial normal insulators (NIs), WSMs and quantum anomalous Hall(QAH) states depending on the model parameters. We study a periodic driving of such Hamiltonians in trivial NI phase by circularly polarized light and shows this phase can be tunable to Weyl semimetals phase. This transition can be accomplished with large anomalous Hall conductivity which could be observed in transport measurements or pump-probe angle resolved photoemission spectroscopy experiments.

cond-mat.mes-hall↗

arXiv2015

Thermoelectric transport in the topological phase due to the coexistence of superconductivity and spin-density-wave

We study the thermoelectric transport in two dimensional topological system which has coexistence of superconductivity(SC) and spin-density wave(SDW). The SC is presumed to be of $d_{x^2-y^2}+(p_x + i p_y) $ type whereas the SDW order parameter is of $BCS$ symmetry. The Hamiltonian describing such a coexistence phase is shown to have topological phase in addition to the conventional one. The transport properties in such topological system have two distinct contributions: (i) the surface/edge and (ii) the bulk. The competition between the surface/edge versus the bulk transport is analyzed in different parameter regimes and the possibility of enhancing the figure of merit is discussed.

cond-mat.supr-con↗

arXiv2015

Topological phase in a $d_{x^2-y^2}+(p+ip)$ superconductor in presence of spin-density-wave

We consider a mean-field Hamiltonian for a $d_{x^2-y^2}+(p+ip)$ superconductor(SC) in presence of spin-density-wave(SDW) order. This is due to the fact that the non-commutativity of any two orders produces the third one. The energy spectrum of such a Hamiltonian is shown to be gapped and it yields a topological phase in addition to the conventional one. A phase diagram characterizing different topological phases is construted. The Chern numbers and hence the nature of the topological phases are determined. The edge state spectrum and the possibility of whether the vortex state harbouring the zero modes are discussed.

cond-mat.supr-con↗

arXiv2014

Three Experiments to Analyze the Nature of the Heat Spreader

In this paper, we describe ongoing work to investigate the properties of the heat spreader, and its implication on architecture research. In specific, we conduct two experiments to quantify the heat distribution across the surface of a spreader during normal operation. The first experiment uses T-type thermocouples, to find the temperature difference across different points on the spreader. We observe about a 6 degree celsius temperature difference on average. In the second experiment, we try to capture the temperature gradients using an infrared camera. However, this experiment was inconclusive because of some practical constraints such as the low emissivity of the spreader. We conclude that to properly model the spreader, it is necessary to conduct detailed finite element simulations. We describe a method to accurately measure the thermal conductivity of the heat spreader such that it can be used to compute the steady state temperature distribution across the spreader.

cs.OH↗

arXiv2014

Topological phases due to the coexistence of superconductivity and spin-density wave: Application to high Tc superconductors in the underdoped regime

We consider the coexistence of superconductivity(SC) and spin-density wave(SDW). The SC is presumed to be of $d_{x^2-y^2}+id_{xy} (d_1 + i d_2) $ type whereas the SDW order parameter is of $BCS$/$d_{x y} $ symmetry. The Hamiltonian having such a structure is shown to have topological coexistence phases in addition to the conventional one. It is shown that the amplitudes of both the order parameters determine the nature of topological phases. A phase diagram characterizing different topological phases with their Chern numbers are obtained. The experimental realization of such topological phases with reference to high temperature superconductors in the extreme underdoped regime are discussed.

cond-mat.supr-con↗

arXiv2010

New Frontiers of Network Security: The Threat Within

Nearly 70% of information security threats originate from inside an organization. Opportunities for insider threats have been increasing at an alarming rate with the latest trends of mobility (portable devices like Laptop, smart phones etc.), ubiquitous connectivity (wireless or through 3G connectivity) and this trend increases as more and more web-based applications are made available over the Internet. Insider threats are generally caused by current or ex-employees, contractors or partners, who have authorized access to the organization's network and servers. Theft of confidential information is often for either material gain or for willful damage. Easy availability of hacking tools on the Internet, USB devices and wireless connectivity provide for easy break-ins. The net result is losses worth millions of dollars in terms of IP theft, leakage of customer / individual information, etc. This paper presents an understanding of Insider threats, attackers and their motives and suggests mitigation techniques at the organization level

cs.CR↗