arXiv subjects

D. Kumar

Publications and source records attributed to D. Kumar.

At least 55 records · Page 3Linked to original sources

arXiv2015

On escaping sets of some families of entire functions and dynamics of composite entire functions

We consider two families of functions $\mathcal{F}=\{f_{{\la},ξ}(z)= e^{-z+\la}+ξ: \la,\,ξ\in\C, \RE{\la}<0, \REξ\geq 1\}$ and $\mathcal{F}'=\{f_{μ,{\ze}}(z)= e^{z+μ}+\ze: μ,\,\ze\in\C, \REμ<0, \RE\ze\leq-1\}$ and investigate the escaping sets of members of the family $\mathcal F$ and $\mathcal F'.$ We also consider the dynamics of composite entire functions and provide conditions for equality of escaping sets of two transcendental entire functions.

math.DS↗

arXiv2015

Fast magnetic field annihilation in the relativistic collisionless regime driven by two ultra-short high-intensity laser pulses

The magnetic quadrupole structure formation during the interaction of two ultra-short high power laser pulses with a collisionless plasma is demonstrated with 2.5-dimensional particle-in-cell simulations. The subsequent expansion of the quadrupole is accompanied by magnetic field annihilation in the ultrarelativistic regime, when the magnetic field can not be sustained by the plasma current. This results in a dominant contribution of the displacement current exciting a strong large scale electric field. This field leads to the conversion of magnetic energy into kinetic energy of accelerated electrons inside the thin current sheet.

physics.plasm-ph↗

arXiv2013

Magnonic Band Structure, Complete Bandgap and Collective Spin Wave Excitation in Nanoscale Two--Dimensional Magnonic Crystals

We present the observation of a complete bandgap and collective spin wave excitation in two-dimensional magnonic crystals comprised of arrays of nanoscale antidots and nanodots, respectively. Considering that the frequencies dealt with here fall in the microwave band, these findings can be used for the development of suitable magnonic metamaterials and spin wave based signal processing. We also present the application of a numerical procedure, to compute the dispersion relations of spin waves for any high symmetry direction in the first Brillouin zone. The results obtained from this procedure has been reproduced and verified by the well established plane wave method for an antidot lattice, when magnetization dynamics at antidot boundaries is pinned. The micromagnetic simulation based method can also be used to obtain iso--frequency countours of spin waves. Iso--frequency contours are analougous of the Fermi surfaces and hence, they have the potential to radicalize our understanding of spin wave dynamics. The physical origin of bands, partial and full magnonic bandgaps has been explained by plotting the spatial distribution of spin wave energy spectral density. Although, unfettered by rigid assumptions and approximations, which afflict most analytical methods used in the study of spin wave dynamics, micromagnetic simulations tend to be computationally demanding. Thus, the observation of collective spin wave excitation in the case of nanodot arrays, which can obviate the need to perform simulations may also prove to be valuable.

cond-mat.mes-hall↗

arXiv2013

δ-doped LaAlO3-SrTiO3 interface: Electrical transport and characterization of the interface potential

Here we investigate LaAlO_3-SrTiO_3 heterostructure withδ-doping of the interface by LaMnO_3 at less than one monolayer. This doping strongly inhibits the formation of mobile electron layer at the interface. This results in giant increase of the resistance and the thermopower of the heterostructure. Several aspects of this phenomena are investigated. A model to calculate the carrier concentration is presented and effect of doping and detailed temperature dependence is analyzed in terms of model parameters and the weak-scattering theory. The large enhancement of thermopower is attributed to the increased spin and orbital entropy originating from the LaMnO_3 mono-layer.

cond-mat.str-el↗

arXiv2013

Resistive Threshold Logic

We report a resistance based threshold logic family useful for mimicking brain like large variable logic functions in VLSI. A universal Boolean logic cell based on an analog resistive divider and threshold logic circuit is presented. The resistive divider is implemented using memristors and provides output voltage as a summation of weighted product of input voltages. The output of resistive divider is converted into a binary value by a threshold operation implemented by CMOS inverter and/or Opamp. An universal cell structure is presented to decrease the overall implementation complexity and number of components. When the number of input variables become very high, the proposed cell offers advantages of smaller area and design simplicity in comparison with CMOS based logic circuits.

cs.ET↗

arXiv2013

Effect of Hole Shape on Spin-Wave Band Structure in One-Dimensional Magnonic Antidot Waveguide

We present the possibility of tuning the spin-wave band structure, particularly the bandgaps in a nanoscale magnonic antidot waveguide by varying the shape of the antidots. The effects of changing the shape of the antidots on the spin-wave dispersion relation in a waveguide have been carefully monitored. We interpret the observed variations by analysing the equilibrium magnetic configuration and the magnonic power and phase distribution profiles during spin-wave dynamics. The inhomogeneity in the exchange fields at the antidot boundaries within the waveguide is found to play a crucial role in controlling the band structure at the discussed length scales. The observations recorded here will be important for future developments of magnetic antidot based magnonic crystals and waveguides.

cond-mat.mes-hall↗

arXiv2013

Intrinsic and extrinsic mirror symmetry breaking in anti-dot spin-wave waveguides

We theoretically study the spin-wave spectra in magnonic waveguides periodically patterned with square anti-dots in nanoscale with pinned magnetization at the edges. We show that the breaking of the mirror symmetry of the waveguide by the structural changes can result in a magnonic band gap closing. These intrinsic symmetry breaking can be compensated by properly chosen asymmetric external bias magnetic field, i.e. in an extrinsic way. As a result the magnonic gaps existing in the ideal symmetric structure can be recovered. The model used for the explanation also suggests that this idea could be generic both for exchange and dipolar interaction regimes of spin-waves and also for other types of waves, e.g., electrons in the graphene ribbons.

cond-mat.mes-hall↗

arXiv2013

Role of rotation and polar cap current on pulsar radio emission and polarization

The perturbations such as rotation and PC--current have been believed to be greatly affecting the pulsar radio emission and polarization. The two effects have not been considered simultaneously in the literature, however, each one of these has been considered separately and deduced the picture by simply superposing them, but such an approach can lead to spurious results. Hence by considering pulsar rotation and PC--current perturbations together instead of one at a time we have developed a single particle curvature radiation model, which is expected to be much more realistic. By simulating a set of typical pulse profiles we have made an attempt to explain most of the observational results on pulsar radio emission and polarization. The model predicts that due to the perturbations leading side component can become either stronger or weaker than the corresponding trailing one in any given cone depending on the passage of sight line and modulation (nonuniform source distribution). Further we find that the phase delay of polarization angle inflection point with respect to the core component greatly depends upon the viewing geometry. The correlation between the sign reversal of circular polarization and the polarization angle swing in the case of core dominated pulsars become obscure once the perturbations and modulation become significant. However the correlation that the negative circular polarization associates with the increasing polarization angle and vice versa shows up very clearly in the case of conal-double pulsars. The `kinky' type distortions in polarization angle swing could be due to the incoherent superposition of modulated emissions in the presence of strong perturbations.

astro-ph.HE↗

arXiv2012

Influence of the polar cap current on pulsar polarization

We have developed a model for the polarization of curvature radiation by taking into account the polar-cap-current induced perturbation on the dipolar magnetic field. We present the effects of the polar cap current on the pulsar radio emission in an artificial case when the rotation effects, such as aberration and retardation, are absent. Our model indicates that the intensity components and the polarization angle inflection point can be shifted to either the leading or the trailing side depending upon the prevailing conditions in the viewing geometry, the non-uniformity in source distribution (modulation), and the polar cap-current-induced perturbation. Also, we find evidence for the origin of symmetric-type circular polarization in addition to the antisymmetric type. Our model predicts a stronger trailing component compared to that on the leading side of a given cone under some specific conditions.

astro-ph.HE↗

arXiv2012

Heliosheath: Diffusion Entropy Analysis and Nonextensivity q-Triplet

In this paper we investigate the scaling behavior, based on Diffusion Entropy Analysis and Standard Deviation Analysis, of the magnetic field strength fluctuations recorded by Voyager-I in the heliosphere. The Voyager-I data set exhibits scaling behavior and may follow Levy-type probability distribution. A general fractional-order spatial and temporal diffusion model could be utilized for the interpretation of this Levy-type behavior in comparison to Gaussian behavior. This result confirms earlier studies of scaling behavior of the heliospheric magnetic field strength fluctuations based on non-extensive statistical mechanics leading to the determination of the nonextensivity q-triplet.

physics.gen-ph↗

arXiv2012

Solar neutrino records: Gauss or non-Gauss is the question

This article discusses the possible variation of the solar neutrino flux over time in the records of Super-Kamiokande-I and the relation to non-equilibrium statistical mechanics and the entropic pathway model. The scaling behavior of the Super-Kamiokande-I time series is investigated utilizing Standard Deviation Analysis and Diffusion Entropy Analysis. The data set exhibit scaling behavior and may follow Levy-type probability distribution function.

physics.gen-ph↗

arXiv2011

Relativistic model on pulsar radio emission and polarization

We have developed a relativistic model for pulsar radio emission and polarization by taking into account of detailed geometry of emission region, rotation and modulation. The sparks activity on the polar cap leads to plasma columns in the emission region and modulated emission. By considering relativistic plasma bunches streaming out along the rotating dipolar field lines as source of curvature radiation, deduced the polarization state of the radiation field in terms of the Stokes parameters. We have simulated a set of typical pulse profiles, and analyzed the role of viewing geometry, rotation and modulation on the pulsar polarization profiles. Our simulations explain most of the diverse behaviors of polarization generally found in pulsar radio profiles. We show that both the `antisymmetric' and `symmetric' types of circular polarization are possible within the frame work of curvature radiation. We also show that the `kinky' nature in the polarization position angle traverses might be due to the rotation and modulation effects. The phase lag of polarization position angle inflection point relative to the phase of core peak also depends up on the rotationally induced asymmetry in the curvature of source trajectory and modulation.

astro-ph.HE↗

arXiv2011

Analytic representations of standard and extended non-resonant thermonuclear functions with depleted tail through the pathway model

The method for the evaluation of the non-resonant thermonuclear function in the Maxwell-Boltzmann case with depleted tail is discussed. Closed forms of the analytical results are obtained in computational format, and written in terms of the H-function in two variables. The standard non-resonant cases are extended to Tsallis reaction rates through the pathway model. Behavior of the depleted non-resonant thermonuclear function is studied. A comparison of the Maxwell-Boltzmann energy distribution with a more general energy distribution called pathway energy distribution is also done.

astro-ph.SR↗

arXiv2011

Analytical results connecting stellar structure parameters and extended reaction rates

Possible modification in the velocity distribution in the non-resonant reaction rates leads to an extended reaction rate probability integral. The closed form representation for these thermonuclear functions are used to obtain the stellar luminosity and neutrino emission rates. The composite parameter {C} that determines the standard nuclear reaction rate through the Maxwell-Boltzmann energy distribution is extended to {C}^* by the extended reaction rates through a more general distribution than the Maxwell-Boltzmann distribution. The new distribution is obtained by the pathway model introduced by Mathai in 2005 [Linear Algebra and Its Applications, 396, 317-328]. Simple analytic models considered by various authors are utilized for evaluating stellar luminosity and neutrino emission rates and are obtained in generalized special functions such as Meijer's G-function and Fox's H-function. The standard and extended non-resonant thermonuclear functions are compared by plotting them. Behavior of the new energy distribution, more general than Maxwell-Boltzmann is also studied.

astro-ph.SR↗

arXiv2011

On extended thermonuclear functions through pathway model

The major problem in the cosmological nucleosynthesis is the evaluation of the reaction rate. The present scenario is that the standard thermonuclear function in the Maxwell-Boltzmann form is evaluated by using various techniques. The Maxwell-Boltzmannian approach to nuclear reaction rate theory is extended to cover Tsallis statistics (Tsallis, 1988) and more general cases of distribution functions. The main purpose of this paper is to investigate in some more detail the extended reaction probability integral in the equilibrium thermodynamic argument and in the cut-off case. The extended reaction probability integrals will be evaluated in closed form for all convenient values of the parameter by means of residue calculus. A comparison of the standard reaction probability integrals with the extended reaction probability integrals is also done.

cond-mat.stat-mech↗

arXiv2010

Fusion yield: Guderley model and Tsallis statistics

The reaction rate probability integral is extended from Maxwell-Boltzmann approach to a more general approach by using the pathway model introduced by Mathai [Mathai A.M.:2005, A pathway to matrix-variate gamma and normal densities, Linear Algebra and Its Applications}, 396, 317-328]. The extended thermonuclear reaction rate is obtained in closed form via a Meijer's G-function and the so obtained G-function is represented as a solution of a homogeneous linear differential equation. A physical model for the hydrodynamical process in a fusion plasma compressed and laser-driven spherical shock wave is used for evaluating the fusion energy integral by integrating the extended thermonuclear reaction rate integral over the temperature. The result obtained is compared with the standard fusion yield obtained by Haubold and John in 1981.[Haubold, H.J. and John, R.W.:1981, Analytical representation of the thermonuclear reaction rate and fusion energy production in a spherical plasma shock wave, Plasma Physics, 23, 399-411]. An interpretation for the pathway parameter is also given.

physics.plasm-ph↗

arXiv2010

Interferometric imaging with the 32 element Murchison Wide-field Array

The Murchison Wide-field Array (MWA) is a low frequency radio telescope, currently under construction, intended to search for the spectral signature of the epoch of re-ionisation (EOR) and to probe the structure of the solar corona. Sited in Western Australia, the full MWA will comprise 8192 dipoles grouped into 512 tiles, and be capable of imaging the sky south of 40 degree declination, from 80 MHz to 300 MHz with an instantaneous field of view that is tens of degrees wide and a resolution of a few arcminutes. A 32-station prototype of the MWA has been recently commissioned and a set of observations taken that exercise the whole acquisition and processing pipeline. We present Stokes I, Q, and U images from two ~4 hour integrations of a field 20 degrees wide centered on Pictoris A. These images demonstrate the capacity and stability of a real-time calibration and imaging technique employing the weighted addition of warped snapshots to counter extreme wide field imaging distortions.

astro-ph.IM↗

arXiv2010

The Murchison Widefield Array

It is shown that the excellent Murchison Radio-astronomy Observatory site allows the Murchison Widefield Array to employ a simple RFI blanking scheme and still calibrate visibilities and form images in the FM radio band. The techniques described are running autonomously in our calibration and imaging software, which is currently being used to process an FM-band survey of the entire southern sky.

astro-ph.IM↗