arXiv subjects

Rakesh Kumar

Publications and source records attributed to Rakesh Kumar.

At least 73 records · Page 4Linked to original sources

arXiv2019

First principles investigation of topological phase in XMR material TmSb under hydrostatic pressure

In this article, we report emergence of topological phase in XMR material TmSb under hydrostatic pressure using first principles calculations. We find that TmSb, a topologically trivial semimetal, undergoes a topological phase transition with band inversion at X point without breaking any symmetry under a hydrostatic pressure of 12 GPa. At 15 GPa, it again becomes topologically trivial with band inversion at $Γ$ as well as X point. We find that the pressures corresponding to the topological phase transitions are far below the pressure corresponding to structural phase transition at 25.5 GPa. The reentrant behaviour of topological quantum phase with hydrostatic pressure would help in finding a correlation between topology and XMR effect through experiments.

cond-mat.mtrl-sci↗

arXiv2019

On the geometry of lightlike submanifolds of indefinite statistical manifolds

We study lightlike submanifolds of indefinite statistical manifolds. Contrary to the classical theory of submanifolds of statistical manifolds, lightlike submanifolds of indefinite statistical manifolds need not to be statistical submanifold. Therefore we obtain some conditions for a lightlike submanifold of indefinite statistical manifolds to be a lightlike statistical submanifold. We derive the expression of statistical sectional curvature and finally obtain some conditions for the induced statistical Ricci tensor on a lightlike submanifold of indefinite statistical manifolds to be symmetric.

math.DG↗

arXiv2018

Hybrid BSQI-WENO Based Numerical Scheme for Hyperbolic Conservation Laws

In this paper, we intend to use a B-spline quasi-interpolation (BSQI) technique to develop higher order hybrid schemes for conservation laws. As a first step, we develop cubic and quintic B-spline quasi-interpolation based numerical methods for hyperbolic conservation laws in 1 space dimension, and show that they achieve the rate of convergence 4 and 6, respectively. Although the BSQI schemes that we develop are shown to be stable, they produce spurious oscillations in the vicinity of shocks, as expected. In order to suppress the oscillations, we conjugate the BSQI schemes with the fifth order weighted essentially non-oscillatory (WENO5) scheme. We use a weak local truncation based estimate to detect the high gradient regions of the numerical solution. We use this information to capture shocks using WENO scheme, whereas the BSQI based scheme is used in the smooth regions. For the time discretization, we consider a strong stability preserving (SSP) Runge-Kutta method of order three. At the end, we demonstrate the accuracy and the efficiency of the proposed schemes over the WENO5 scheme through numerical experiments.

math.NA↗

arXiv2018

Network Coding for Critical Infrastructure Networks

The applications in the critical infrastructure systems pose simultaneous resilience and performance requirements to the underlying computer network. To meet such requirements, the networks that use the store-and-forward paradigm poses stringent conditions on the redundancy in the network topology and results in problems that becoming computationally challenging to solve at scale. However, with the advent of programmable data-planes, it is now possible to use linear network coding (NC) at the intermediate network nodes (i.e. hardware and software switches) to meet resilience requirements of the applications. To that end, we propose an architecture that realizes linear NC in programmable networks by decomposing the linear NC functions into the atomic coding primitives. We designed and implemented the primitives using the features offered by the P4 ecosystem. Using an empirical evaluation of an open-source prototype, we show that the theoretical gains promised by linear network coding can be realized with a per-packet processing cost.

cs.NI↗

arXiv2018

Simple smoothness indicator and multi-level adaptive order WENO scheme for hyperbolic conservation laws

In the present work, we propose two new variants of fifth order finite difference WENO schemes of adaptive order. We compare our proposed schemes with other variants of WENO schemes with special emphasize on WENO-AO(5,3) scheme [Balsara, Garain, and Shu, {\it J. Comput. Phys.}, 326 (2016), pp 780-804]. The first algorithm (WENO-AON(5,3)), involves the construction of a new simple smoothness indicator which reduces the computational cost of WENO-AO(5,3) scheme. Numerical experiments show that accuracy of WENO-AON(5,3) scheme is comparable to that of WENO-AO(5,3) scheme and resolution of solutions involving shock or other discontinuities is comparable to that of WENO-AO(5,3) scheme. The second algorithm denoted as WENO-AO(5,4,3), involves the inclusion of an extra cubic polynomial reconstruction in the base WENO- AO(5,3) scheme, which leads to a more accurate scheme. Extensive numerical experiments in 1D and 2D are performed, which shows that WENO-AO(5,4,3) scheme has better resolution near shocks or discontinuities among the considered WENO schemes with negligible increase in computational cost.

math.NA↗

arXiv2018

Generalized least square homotopy perturbations for system of fractional partial differential equations

In this paper, generalized aspects of least square homotopy perturbations are explored to treat the system of non-linear fractional partial differential equations and the method is called as generalized least square homotopy perturbations (GLSHP). The concept of partial fractional Wronskian is introduced to detect the linear independence of functions depending on more than one variable through Caputo fractional calculus. General theorem related to Wronskian is also proved. It is found that solutions converge more rapidly through GLSHP in comparison to classical fractional homotopy perturbations. Results of this generalization are validated by taking examples from nonlinear fractional wave equations.

math.NA↗

arXiv2018

Band Gap tunability in One-dimensional system

The ability to tune the gaps of direct bandgap materials has tremendous potential for applications in the fields of LEDs and solar cells. However, lack of reproducibility of bandgaps due to quantum confinement observed in experiments on reduced dimensional materials, severely affects tunability of their bandgaps. In this letter, we report broad theoretical investigations of direct bandgap one-dimensional functionalized isomeric system using their periodic potential profile, where bandgap tunability is demonstrated simply by modifying the potential profile by changing the position of the functional group in a periodic supercell. It is verified for known synthetic, as well as natural polymers (biological and organic), and also for other one-dimensional direct bandgap systems. This insight would greatly help experimentalists in designing new isomeric systems of various bandgap values for polymers and one-dimensional inorganic systems for LEDs applications, and for effectively harvesting energy in solar cells.

cond-mat.mtrl-sci↗

arXiv2018

Effect of edge defects on band structure of zigzag graphene nanoribbons

In this article, we report band structure studies of zigzag graphene nanoribbons (ZGNRs) on introducing defects (sp_3 hybridized carbon atoms) in different concentrations at edges by varying the ratio of sp_3 to sp_2 hybridized carbon atoms. On the basis of theoretical analyses, band gap values of ZGNRs are found to be strongly dependent on relative arrangement of sp3 to sp2 hybridized carbon atoms at the edges for a defect concentration; so the findings would greatly help in understanding band gap of nanoribbons for their electronic applications.

cond-mat.mes-hall↗

arXiv2017

Triplon mean-field analysis of an antiferromagnet with degenerate Shastry-Sutherland ground states

We look into the quantum phase diagram of a spin-$\frac{1}{2}$ antiferromagnet on the square lattice with degenerate Shastry-Sutherland ground states, for which only a schematic phase diagram is known so far. Many exotic phases were proposed in the schematic phase diagram by the use of exact diagonalization on very small system sizes. In our present work, an important extension of this antiferromagnet is introduced and investigated in the thermodynamic limit using triplon mean-field theory. Remarkably, this antiferromagnet shows a stable plaquette spin-gapped phase like the original Shastry-Sutherland antiferromagnet, although both of these antiferromagnets differ in the Hamiltonian construction and ground state degeneracy. We propose a sublattice columnar dimer phase which is stabilized by the second and third neighbor antiferromagnetic Heisenberg exchange interactions. There are also some commensurate and incommensurate magnetically ordered phases, and other spin-gapped phases which find their places in the quantum phase diagram. Mean-field results suggest that there is always a level-crossing phase transition between two spin gapped phases, whereas in other situations, either a level-crossing or a continuous phase transition happens.

cond-mat.str-el↗

arXiv2017

End-to-End Network Delay Guarantees for Real-Time Systems using SDN

We propose a novel framework that reduces the management and integration overheads for real-time network flows by leveraging the capabilities (especially global visibility and management) of software-defined networking (SDN) architectures. Given the specifications of flows that must meet hard real-time requirements, our framework synthesizes paths through the network and associated switch configurations - to guarantee that these flows meet their end-to-end timing requirements. In doing so, our framework makes SDN architectures "delay-aware" - remember that SDN is otherwise not able to reason about delays. Hence, it is easier to use such architectures in safety-critical and other latency-sensitive applications. We demonstrate our principles as well as the feasibility of our approach using both - exhaustive simulations as well as experiments using real hardware switches.

cs.NI↗

arXiv2017

Adsorption and dynamics of Si atoms at the monolayer Pb/Si(111) surface

In this work, we studied the adsorption behavior of deposited Si atoms along with their diffusion and other dynamic processes on a Pb monolayer-covered Si(111) surface from 125-230 K using a variable-temperature scanning tunneling microscope (STM). The Pb-covered Si(111) surface form a low-symmetry row-like structure in this temperature range and the Si atoms bind favorably to two specific on-top sites (T1A and T1B) on the trimer row after deposition at the sample temperature of 125 K. The Si atoms were immobile at low temperatures and started to switch between the two neighboring T1A and T1B sites within the same trimer when the temperature was raised to 150 K. When the temperature was raised above 160 K, the adsorbed Si atoms could hop to other trimers along the same trimer row. Below 170 K, short hops to adjacent trimers dominated, but long hops dominated at temperatures above 170 K. The activation energy and prefactor for the Si atoms diffusion were derived through analysis of continuous-time imaging at temperatures from 160-174 K. In addition, irreversible aggregation of single Si atoms into Si clusters started to occur at the phase boundaries or defective sites at temperatures above 170 K. This study provides crucial information for understanding the very initial stage of nucleation and growth behavior of epitaxial Si layers on a Pb-covered Si(111) surface. In addition, our study provides strong evidence for breaking in the mirror symmetry in the Pb-covered structure, which has implication for the atomic model of this controversial structure.

cond-mat.mes-hall↗

arXiv2016

Origin of multiple band gap values in single width nanoribbons

Deterministic band gap in quasi-one-dimensional nanoribbons is prerequisite for their integrated functionalities in high-performance molecular-electronics based devices. However, multiple band gap values commonly observed in the same width of graphene nanoribbons fabricated in same slot of the experiments remains unresolved, and raise a critical concern over scalable production of pristine and/or hetero-structure nanoribbons with deterministic properties and functionalities for plethora of applications. Here, we show that a modification in the depth of potential wells in the periodic direction of a supercell on relative shifting of passivating atoms at the edges is the origin of multiple band gap values for the same width of nanoribbons in a crystallographic orientation, although they carry practically the same ground state energy. The results are similar when calculations are extended from planar graphene to buckled silicene nanoribbons. Thus, the findings facilitate tuning of the electronic properties of quasi-one-dimensional materials such as bio-molecular chains, organic and inorganic nanoribbons by performing edge engineering.

cond-mat.mtrl-sci↗

arXiv2015

Nonlinear convection stagnation point heat transfer and MHD fluid flow in porous medium towards a permeable shrinking sheet

This investigation deals with the analysis of stagnation point heat transfer and corresponding flow features of hydromagnetic viscous incompressible fluid over a vertical shrinking sheet. The considered sheet is assumed to be permeable and subject to addition of stagnation point to control the generated vorticity in the boundary layer. The sheet is placed on the right side of the fluid saturated porous medium which is having permeability of specified form. Nonlinear convection waves in the flow field are realized due to the envisaged nonlinear relation between density and temperature. The equations governing the nonlinear convection boundary layer flow are modeled and simplified using similarity transformations. The economized equations are solved for numerical solutions by employing the implicit finite difference scheme also known as Keller-box method. The influence of the associated parameters of the problem on velocity and temperature distributions, skin friction and rate of heat transfer are presented through graphs and tables, and qualitatively discussed. The study reveals that interaction among magnetic field, porous medium permeability and nonlinear convection parameters substantially enhance the solution range and thus endorse their control to sustain the boundary layer flow.

physics.flu-dyn↗

arXiv2014

Edge Configurational Effect on Band Gaps in Graphene Nanoribbons

In this Letter, we put forward a resolution to the prolonged ambiguity in energy band gaps between theory and experiments of fabricated graphene nanoribbons (GNRs). Band structure calculations using density functional theory are performed on oxygen passivated GNRs supercells of customized edge configurations without disturbing the inherent sp2 hybridization of carbon atoms. Direct band gaps are observed for both zigzag and armchair GNRs, consistent with the experimental reports. In addition, band gap values of GNRs scattered about an average value curve for a given crystallographic orientation are correlated with their width on basis of the edge configurations elucidates the band gaps in fabricated GNRs. We conclude that edge configurations of GNRs significantly contribute to band gap formation in addition to its width for a given crystallographic orientation, and would play a crucial role in band gap engineering of GNRs for future research works on fabrication of nanoelectronic devices.

cond-mat.mes-hall↗

arXiv2014

Controlled Smooth Edge Formation of Graphene Nanoribbons

We report energy estimated to dissociate a C-C bond of a graphene sheet to form nanoribbons of armchair and zigzag configurations using first principles calculations. For the ground state energy calculations, the configurations considered are with spin, and without spin polarization. It is observed that the energy required to dissociate a C-C bond of a graphene sheet to form zigzag configuration is higher than that of armchair configuration for both spin polarized state, as well as non-spin polarized state. Therefore, formation of smooth edged graphene nanoribbons along the crystallographic directions might be engineered by a control over energy.

cond-mat.mtrl-sci↗

arXiv2013

Energy Controlled Edge Formation for Graphene Nano Ribbons

On the basis of first principles calculations, we report energy estimated to cut a graphene sheet into nanoribbons of armchair and zigzag configurations. Our calculations show that the energy required to cut a graphene sheet into zigzag configuration is higher than that of armchair configuration by an order of 0.174 eV. Thus, a control over the threshold energy might be helpful in designing an experiment for cutting a graphene sheet into smooth edged armchair or zigzag configurations.

cond-mat.mes-hall↗

arXiv2011

A Comparative Study of Various Routing Protocols in VANET

Vehicular Ad Hoc Networks (VANET) is a subclass of Mobile ad hoc networks which provides a distinguished approach for Intelligent Transport System (ITS). The survey of routing protocols in VANET is important and necessary for smart ITS. This paper discusses the advantages / disadvantages and the applications of various routing protocols for vehicular ad hoc networks. It explores the motivation behind the designed, and traces the evolution of these routing protocols. F inally the paper concludes by a tabular comparison of the various routing protocols for VANET.

cs.NI↗

arXiv2011

Mobile Agent as an Approach to Improve QoS in Vehicular Ad Hoc Network

Vehicular traffic is a foremost problem in modern cities. Huge amount of time and resources are wasted while traveling due to traffic congestion. With the introduction of sophisticated traffic management systems, such as those incorporating dynamic traffic assignments, more stringent demands are being placed upon the available real time traffic data. In this paper we have proposed mobile agent as a mechanism to handle the traffic problem on road. Mobile software agents can be used to provide the better QoS (Quality of Service) in vehicular ad hoc network to improve the safety application and driver comfort.

cs.NI↗