arXiv subjects

S. K. Sharma

Publications and source records attributed to S. K. Sharma.

12 recordsLinked to original sources

arXiv2021

Spatiotemporal evolution of a self-excited dust density wave in a nanodusty plasma under strong Havnes effect

A broad-spectrum self-excited dust density wave is experimentally studied in a vertically extended nanodusty plasma consisting of in situ grown carbonaceous nanometer sized particles. The nanodusty plasma having high particle density (of the order of 10^12-10^13 m^(-3)) is created with vertical extension up to (40+-0.1) cm and radial extension up to (5+-0.1) cm. The propagation of the self-excited dust density wave under strong Havnes effect is examined over a large axial distance (19+-0.1) cm. Time-resolved Hilbert transformation and Fast Fourier transformation techniques are used to study the spatiotemporal evolution of frequency and wave numbers along three directions from the dust void viz. axial, radial and oblique. The propagation is found to be inhomogeneous throughout the dust cloud. The phase velocity of the wave is estimated to be quite low and decreasing along the direction of propagation. This effect is attributed to the strong reduction of particle charge due to a high Havnes parameter along the propagation direction. By the estimation of average particle charge, ion density and the finite electric field throughout the nanodust cloud, a quantitative analysis of the void formation in nanodusty plasma is presented. New insights are also made regarding wave merging phenomena using time-resolved Hilbert transformation.

physics.plasm-ph↗

arXiv2020

Magnetic correlations in subsystems of the misfit [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$] cobaltate

[Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$], a two dimensional misfit metallic compound, is famous for its rich phases accessed by temperature, $i.e.$ high temperature spin-state transition, metal-insulator transition (MIT) at intermediate temperature ($\sim$ 100 K) and low temperature spin density wave (SDW). It enters into SDW phase below T$_{MIT}$ which becomes long range at 27 K. Information on the independent role of misfit layers (rocksalt/Ca$_2$CoO$_3$ \& triangular/CoO$_2$) in these phases is scarce. By combining a set of complementary macroscopic (DC magnetization and resistivity) and microscopic (neutron diffraction and X-ray absorption fine structure spectroscopy) measurements on pure (CCO) and Tb substituted in the rocksalt layer of CCO (CCO1), magnetic correlations in both subsystems of this misfit compound are unraveled. CCO is found to exhibit glassiness, as well as exchange bias (EB) effects, while CCO1 does not exhibit glassiness, albeit it shows weaker EB effect. By combining local structure investigations from extended X-ray absorption fine structure (EXAFS) spectroscopy and neutron diffraction results on CCO, we confirm that the SDW arises in the CoO$_2$ layer. Our results show that the magnetocrystalline anisotropy associated with the rocksalt layer acts as a source of pinning, which is responsible for EB effect. Ferromagnetic clusters in the Ca$_2$CoO$_3$ affects SDW in CoO$_2$ and ultimately glassiness arises.

cond-mat.mtrl-sci↗

arXiv2020

Satellite Communications in the New Space Era: A Survey and Future Challenges

Satellite communications have recently entered a period of renewed interest motivated by technological advances and nurtured through private investment and ventures. The present survey aims at capturing the state of the art in SatComs, while highlighting the most promising open research topics. Firstly, the main innovation drivers are motivated, such as new constellation types, on-board processing capabilities, nonterrestrial networks and space-based data collection/processing. Secondly, the most promising applications are described i.e. 5G integration, space communications, Earth observation, aeronautical and maritime tracking and communication. Subsequently, an in-depth literature review is provided across five axes: i) system aspects, ii) air interface, iii) medium access, iv) networking, v) testbeds & prototyping. Finally, a number of future challenges and the respective open research topics are described.

eess.SP↗

arXiv2020

Frame of Operators in quaternionic Hilbert spaces

In this paper, we introduce and study frame of operators in quaternionic Hilbert spaces as a generalization of g frames which in turn generalized various notions like Pseduo frames, bounded quasi-projectors and frame of subspaces (fusion frames) in separable quaternionic Hilbert spaces.

math.FA↗

arXiv2019

Riesz bases in quaternionic Hilbert spaces

In this article, we introduce and study Riesz bases in a separable quaternionic Hilbert spaces. Some results on Riesz bases in a separable quaternionic Hilbert spaces are proved. It is also proved that a Riesz basis in a separable quaternionic Hilbert space a frame for the quaternionic Hilbert space. Riesz sequences are defined and equivalence of a Riesz basis and a complete Riesz sequence in a separable quaternionic Hilbert space is proved.

math.FA↗

arXiv2018

Duals of a frame in quaternionic Hilbert spaces

Frames in a separable quaternionic Hilbert space were introduced and studied in [17] to have more applications. In this paper, we extend the study of frames in quaternionic Hilbert spaces and introduce different types of duals of a frame in separable quaternionic Hilbert spaces. As an application, we give the orthogonal projection of $\ell^2(\HH)$ onto the range of analysis operator of the given frame, in terms of elements of canonical dual frame and elements of the frame in quaternionic Hilbert space. Finally, we give an expression for the orthogonal projection in terms of operators related to the frame and its canonical dual frame in quaternionic Hilbert space.

math.FA↗

arXiv2017

Frames in Quaternionic Hilbert Spaces

In this paper, we introduce and study the frames in separable quaternionic Hilbert spaces. Results on the existence of frames in quaternionic Hilbert spaces have been given. Also, a characterization of frame in quaternionic Hilbert spaces in terms of frame operator is given. Finally, a Paley-Wiener type perturbation result for frames in quaternionic Hilbert space has been obtained.

math.FA↗

arXiv2013

Perturbation of FRW Spacetime in NP Formalism

Perturbation of FRW spacetime is carried out in NP formalism. The equation governing the scalar, vector and tensor modes take on a very simple and transparent form. All of them can be combined in one master equation for all helicities. The solutions for the closed, flat and open FRW are analytic continuation of the same functions, so only the solutions in the closed model are described. The scalar equation is the same as that of the conformally coupled massless Klein-Gordon field, the vectorial ones are the same as Maxwell equations, and the tensorial ones are for spin-2 fields. The corresponding eigen-functions are all determined, and in particular, the Green's function and the Lienard-Wiechert type potential also solved. These solutions reduce to the familiar form in flat space.

astro-ph.CO↗

arXiv2013

Electric and Magnetic fields due to Dirac particles in FRW spacetime

Some solutions of the Maxwell equations with Dirac particles for the source in FRW spacetime are discussed. The Green's function of the equation for the radial component of the Maxwell fields, F_{rη} and F_{θϕ} is solved. Green's function is found to reduce to that of Minkowskian spacetime in the appropriate limit. Also, the Lienard-Wiechert type solution is derived. Also, the solutions with the Dirac particle current is also presented. It is found that the F_{rη} is composed of even angular momentum states while the odd states constitue F_{θϕ} .

astro-ph.CO↗

arXiv2011

Effect of Massive Neutrino on Large Scale Structures

Jeans mass calculated with different combination of parameters involved has shown interesting variation with remarkable shifting of position of peak value from x = m/T = 0.5 to 5.5. The standard deviation is 2.217. In particular, using the harmonic mean square velocity, shifts the peak Jeans mass to x = m/T ~ 2, which is remarkably less than previously reported value of 4.2. Different scales of neutrino structures including virialized moments have also been compared.

astro-ph.CO↗

arXiv2009

Structural and magnetic study of a dilute magnetic semiconductor: Fe doped CeO2 nanoparticles

This paper reports the effect of Fe doping on the structure and room temperature ferromagnetism of CeO2 nanoparticles. X-ray diffraction and selective area electron diffraction measurement reflects that Ce1-xFexO2 (x = 0.0 - 0.07) nanoparticles exhibit single phase nature with cubic structure and none of the sample showed the presence of any secondary phase. The mean particle size calculated by using a transmission electron microscopy measurement was found to increase with increase in Fe content. DC magnetization measurements performed at room temperature indicates that all the samples exhibit ferromagnetism. The saturation magnetic moment has been found to increase with an increase in the Fe content.

cond-mat.mtrl-sci↗

arXiv2001

Distributed Data Acquisition System for Pachmarhi Array of Cerenkov Telescopes

Pachmarhi Array of \v Cerenkov Telescopes (PACT) consists of a 5$\times$5 array of \v Cerenkov telescopes deployed over an area of 100 $m$ $\times$ 80 $m$, in the form of a rectangular matrix. The experiment is based on atmospheric \v Cerenkov technique using wavefront sampling technique. Each telescope consists of 7 parabolic mirrors mounted para-axially on an equatorial mount. At the focus of each mirror a fast phototube is mounted. In this experiment a large number of parameters have to be measured and recorded from each of the 175 phototubes in the shortest possible time. Further, the counting rates from each phototube as well as the analog sum of the 7 phototubes from each telescope (royal sum) need to be monitored at regular intervals during the run. In view of the complexity of the system, the entire array is divided into four smaller sectors each of which is handled by an independent field signal processing centre (FSPC) housed in a control room that collects, processes and records information from nearby six telescopes that belong to that sector. The distributed data acquisition system (DDAS) developed for the purpose consists of stand-alone sector data acquisition system (SDAS) in each of the four FSPC's and a master data acquisition system (MDAS). MDAS running in the master signal processing centre (MSPC) records data from each of the 25 telescopes. The data acquisition and monitoring PCs (SDAS and MDAS) are networked through LAN. The entire real time software for DDAS is developed in C under $linux$ environment. Most of the hardware in DDAS are designed and fabricated in-house. The design features and the performance of the entire system along with some other auxiliary systems to facilitate the entire observations will be presented.

astro-ph↗