arXiv subjects

S. K. Pandey

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

At least 19 recordsLinked to original sources

arXiv2023

Electroosmotic flow of a rheological fluid in non-uniform micro-vessels

The paper deals with a theoretical study of electrokinetic flow of a rheological Herschel-Bulkley fluid through a cylindrical tube of variable cross-section. The concern of this study is to analyze combined pressure-driven and electroosmotic flow of Herschel-Bulkley fluid. The wall potential is considered to vary slowly and periodically along the axis of the tube. With reference to flow in the micro-vessels, the problem has been solved using the lubrication theory. The Helmholtz-Smoluchowski (HS) slip boundary condition has been employed in this study. Volumetric flow rate $Q$ is found to be significantly affected by the yield stress parameter $ν$ only if an applied pressure force is active. The linear superposition of flow components separately due to the hydrodynamic and electric force occurs only for a strictly uniform tube. This linear relationship fails if non-uniformity appears in either tube radius or in distribution of the electrokinetic slip boundary condition. Moreover, converging/diverging nature of the mean tube radius plays a crucial role on the fluid transport. For the benefit of readers, along with the original contribution, some applications of external electrical stimulation (ES) in the human body and HS slip velocity, studied in the past by previous researchers have been discussed in the paper.

physics.flu-dyn↗

arXiv2023

A Note on the Koethe Conjecture

In this note we provide some results related to the Koethe conjecture and exhibit that the condition R satisfies the Koethe conjecture given in [2, theorem 2.6 ] is superfluous at least under certain conditions described in this note.

math.RA↗

arXiv2021

Layer dependent electronic structure changes in transition metal dichalcogenides- The microscopic origin

We have examined the electronic structure evolution in transition metal dichalcogenides MX$_2$, where M=Mo,W and X=S,Se and Te. These are generally referred to as van der Waals heterostructures on the one hand, yet one has band gap changes as large as 0.6 eV with thickness in some instances. This does not seem to be consistent with a description where the dominant interactions are van der Waals interactions. Mapping onto a tight binding model allows us to quantify the electronic structure changes which are found to be dictated solely by interlayer hopping interactions. Different environments that an atom encounters could change the Madelung potential and therefore the onsite energies. This could happen while going from monolayer to bilayer as well as in cases where the stackings are different from what is found in 2H structures. These effects are quantitatively found to be negligible, enabling us to quantify the thickness dependent electronic structure changes as arising from interlayer interactions alone.

cond-mat.mes-hall↗

arXiv2019

Evolution of magnetic ordering in FeCr$_2$Se$_{4-x}$Te$_x$; $x$ = 0 -- 4.0

We have systematically studied the magnetic properties of chromium chalcogene compounds FeCr$_2$Se$_{4-x}$Te$_x$. The FeCr2Se4 undergoes antiferromagnetic ordering below 222 K. Substitution of tellurium lowers the antiferromagnetic ordering temperature and leads to short range ferromagnetic cluster behavior towards the tellurium end. Change over from antiferromagnetic to ferrimagnetic like behavior is also reflected in the corresponding transformation from semiconducting to metallic transport behavior. There is a large variation in the Curie-Weiss temperature, effective magnetic moment and ordering temperature (TN / TC) with Te substitution. The electronic band structure calculations suggest antiferromagnetic and ferrimagnetic ground state for the FeCr2Se4 and FeCr2Te4 respectively.

cond-mat.mtrl-sci↗

arXiv2018

Effect of boundary scattering on spin-hall effect

The spin dependent reflection in quasi-two-dimensional electron gas from an impenetrable barrier in presence of Rashba and Dresselhaus spin-orbit coupling is analyzed in detail. It is shown that due to spin-orbit effects the reflected beam split in two beams gives rise to multiple reflection analogous to phenomena birefringence. The interplay between Rashba and Dresselhaus spin-orbit coupling gives rise to anisotropy in Fermi energy surface and a non-zero net spin-polarized current oscillating with two frequencies for all the values of incident angle except at $45^{o}$ when averaged over all components of reflected beam. It is also shown that in over critical region, all the three polarization components as well as net polarization has non-zero values and are exponentially decaying as distance from the barrier increases which in turns spin-accumulation near the barrier is an important consequence of spin-hall effect.

cond-mat.mes-hall↗

arXiv2018

Rheological fluid motion in tube by metachronal wave of cilia

This paper presents a theoretical study of a non-linear rheological fluid transport in an axisymmetric tube by cilia. However, an attempt has been made to explain the role of cilia motion on the transport of fluid through the ductus efferentes of the male reproductive tract. Ostwald-de Waele power-law viscous fluid has been considered to represent the rheological fluid. To analyze pumping by means of a sequence of beat of cilia from row-to-row of cilia in a given row of cells and from one row of cells to the next (metachronal wave movement), we consider the conditions that the corresponding Reynolds number is small enough for inertial effects to be negligible and the wavelength to diameter ratio is large enough for the pressure to be considered uniform over the cross-section. Analyses and computations of the detailed fluid motion reveal that the time-averaged flow rate is dependent on $ε$, a non-dimensional measure involving the mean radius $a$ of the tube and the cilia length. Thus, flow rate significantly varies with the cilia length. Moreover, the flow rate has been reported near to the estimated value $6\times 10^{-3}$$ml/h$ for human efferent ducts if $ε$ is near by 0.4. The estimated value was suggested by Lardner and Shack [1] in human, based on the experimental observations for flow rates in efferent ducts of other animals, e.g., rat, ram and bull. In addition, the nature of the rheological fluid, i.e., the value of the fluid index $n$ strongly influences various flow-governed characteristics. The interesting feature of the paper is that the pumping improves with the thickening behavior for small values of $ε$ or in free pumping ($ΔP=0$) and pumping ($ΔP>0$) regions.

physics.flu-dyn↗

arXiv2018

Systematic Analysis of Low/Hard State RXTE Spectra of GX 339-4 to Constrain the Geometry of the System

One of the popular models for the low/hard state of Black Hole Binaries is that the standard accretion disk is truncated and the hot inner region produces via Comptonization, the hard X-ray flux. This is supported by the value of the high energy photon index, which is often found to be small $\sim$ 1.7 ($<$ 2) implying that the hot medium is seed photons starved. On the other hand, the suggestive presence of a broad relativistic Fe line during the hard state would suggest that the accretion disk is not truncated but extends all the way to the inner most stable circle orbit. In such a case, it is a puzzle why the hot medium would remain photon starved. The broad Fe line should be accompanied by a broad smeared reflection hump at $\sim$ 30 keV and it may be that this additional component makes the spectrum hard and the intrinsic photon index is larger, i.e. $>$ 2. This would mean that the medium is not photon deficient, reconciling the presence of a broad Fe line in the observed hard state. To test this hypothesis, we have analyzed the RXTE observations of GX 339-4 from the four outbursts during 2002-2011 and identify the observations when the system was in the hard state and showed a broad Fe line. We have then attempted to fit these observations with models, which include smeared reflection to understand whether the intrinsic photon index can indeed be large. We find that, while for some observations the inclusion of reflection does increase the photon index, there are hard state observations with broad Fe line that have photon indices less than 2.

astro-ph.HE↗

arXiv2017

Optical Imaging & Spectral Study of FR-I Type Radio Galaxy:CTD 086 (B2 1422+26B)

We present optical imaging and spectroscopic studies of the Fanaroff \& Riley class I (FR I) radio galaxy CTD 086 based on Hubble Space Telescope (HST) and Sloan Digital Sky Survey (SDSS) observations. We use isophote shape analysis to show that there is no stellar disk component within CTD 086 and further that the morphological class of the galaxy is most likely E2. Optical spectroscopy of this galaxy reveals the presence of narrow emission lines only, and thus it qualifies to be termed as a narrow-line radio galaxy (type 2 AGN). We also extract stellar kinematics from the absorption-line spectra of CTD 086 using Penalized Pixel-Fitting method and derive the black hole mass MBH to be equal to (8.8\pm2.4)\times10^{7} Msun.

astro-ph.GA↗

arXiv2016

Centralized group key management scheme for secure multicast communication without re-keying

In the secure group communication, data is transmitted in such a way that only the group members are able to receive the messages. The main problem in the solution using symmetric key is heavy re-keying cost. To reduce re-keying cost tree based architecture is used. But it requires extra overhead to balance the key- tree in order to achieve logarithmic re-keying cost. The main challenging issue in dynamic and secure multimedia multicast communication is to design a centralized group key management scheme with minimal computational, communicational and storages complexities without breaching security issues. Several authors have proposed different centralized group key management schemes, wherein one of them proposes reducing communicational complexity but increases computational and storage costs however another proposes decreasing the computational and storage costs which eventually breaches forward and backward secrecy. In this paper we propose a comparatively more efficient centralized group key management scheme that not only minimize the computational, communicational and storages complexities but also maintaining the security at the optimal level. The message encryptions and decryptions costs are also minimized. Further, we also provide an extended multicast scheme, in which the several requests towards leaving or joining the group can be done by large number of members simultaneously. In order to obtain better performance of multicast encryption, the symmetric-key and asymmetric-key cryptosystems may be combined.

cs.CR↗

arXiv2015

X-Ray Emission from a prominent dust lane lenticular galaxy NGC 5866

We report the multiband imagery with an emphasis on the X-ray emission properties of a prominent dust lane lenticular galaxy NGC 5866. X-ray emission from this galaxy is due to a diffuse component and a substantial contribution from the population of discrete X-ray binary sources. A total of 22 discrete sources have been detected within the optical D25 extent of the galaxy, few of which exhibit spatial association with the globular clusters hosted by this system. Composite spectrum of the diffuse emission from this galaxy was well constrained by a thermal plasma model plus a power law component to represent the emission from unresolved sources, while that of the discrete sources was well fitted by an absorbed power law component of photon index 1.82$\pm$0.14. X-ray color-color plot for the resolved source was used to classify the detected sources. The cumulative X-ray luminosity function of the XRBs is well represented by a power law function of index of Γ ~ 0.82$\pm$0.12. Optical imagery of NGC 5866 revealed a prominent dust lane along the optical major axis of the host with dust extinction properties similar to those of the canonical grains in the Milky Way. The dust grains responsible for the extinction of starlight in NGC 5866 are relatively smaller in size when compared with the canonical grains in the Milky Way and high energetic charged particles seems to be responsible for the modulation of the dust grain size. Spatial correspondence is evident between the dust and other phases of ISM.

astro-ph.GA↗

arXiv2014

Inverse photoemission spectroscopic studies on phase separated La$_{0.2}$Sr$_{0.8}$MnO$_{3}$

We have studied the temperature evolution of the inverse photoemission spectra of phase separated La$_{0.2}$Sr$_{0.8}$MnO$_{3}$. To identify the features in the room temperature experimental spectra, band structure calculations using Korringa-Kohn-Rostoker Green's function method were carried out. We find that the features generated by local moment disorder calculations give a better match with the experimental spectrum. In the insulating phase, we observed unusually an increased intensity at around the Fermi level. This puzzling behaviour is attributed to the shift in the chemical potential towards the conduction band. The present results clearly show the importance of unoccupied electronic states in better understanding of the phase separated systems.

cond-mat.str-el↗

arXiv2014

Evidence of spin lattice coupling in MnTiO$_{3}$: an x-ray diffraction study

Here we investigate the temperature evolution of the structural parameters of a potential magnetoelectric material, MnTiO$_{3}$. The experimental results reveal interesting temperature dependence of the $c/a$ ratio and the Mn-O bonds which can be divided into three regions. In region I (300 K to 200 K), the above parameters are seen to decrease with decrease in temperature due to thermal effect. In the region II (200 K to 95 K), the decrement in the structural parameters are reduced due the competing intra layer antiferromagnetic interaction setting in $\sim$ 200 K. The $c/a$ ratio are seen to display a minima around 140 K. Below 140 K, the short Mn-O bonds increase suggesting the onset of inter layer antiferromagnetic interaction $\sim$ 100 K. In region III (95 K to 23 K), the antiferromagnetic interaction is fully established. The behaviour of the calculated Mn-O bonds based on first principle calculations are in line with the experimental results. This study demonstrates the importance of spin lattice coupling in understanding the magnetic properties of the compound which is expected to be helpful in revealing the origin of magnetically induced ferroelectricity.

cond-mat.str-el↗

arXiv2014

Electronic Spin: Abstract Mathematical or Real Physical Phenomenon

In the description of electron spin obtained through the conventional Copenhagen interpretation of quantum mechanics, the concrete picture of rotation was replaced by an abstract mathematical representation; visualization or visualisability was entirely lost. The work described here takes a step towards restoring this.

quant-ph↗

arXiv2014

Multi-Parameter Decision Support with Data Transmission over GSM/GPRS Network: a Case Study of Landslide Monitoring

The planet Earth has hundreds of impact events, with some occurrences causing both in terms of human casualty as well as economic losses. Such attitudes of earth pushed the frontiers to develop innovative monitoring strategies for the earth system. To make that real, although, will require coherent and real-time data by observing the earth behavior contiguously. Wireless Sensor Network (WSN) appears to be the best suitable infrastructure to sense environmental parameters of our interests. In this event of earth observation, another important issue is the monitoring system with high level of precision. There are different types of sensors to measure the behavioral aspects of earth. The sensors integrated with WSN, provide an accurate and contiguous data for analysis and interpretation. This paper briefly addresses earth observation and areas of critical importance to people and society. A case study has also been carried out for disaster like Landslide in the North Eastern region of India. Application software has been developed for the said study for online data acquisition and analysis with pre-disaster early warning system. The system monitors the changing geotechnical condition of this region using various geo-technical sensors like Rain gauge, In-place Inclinometer, Tilt-meter, Piezo-meter and Crack meter. This paper also touches upon the aspects of data transmission over Global System for Mobile Communication (GSM) / General Packet Radio Service (GPRS) to a remote data center.

cs.NI↗

arXiv2014

Isophotal shapes of early-type galaxies to very faint levels

We report on a study of the isophotal shapes of early-type galaxies, to very faint levels reaching ~ 0.1% of the sky brightness. The galaxies are from the Large Format Camera (LFC) fields obtained using the Palomar 5 m Hale telescope, with integrated exposures ranging from 1 to 4 hours in the SDSS r, i and z bands. The shapes of isophotes of early-type galaxies are important as they are correlated with the physical properties of the galaxies and are influenced by galaxy formation processes. In this paper we report on a sample of 132 E and SO galaxies in one LFC field. We have redshifts for 53 of these, obtained using AAOmega on the Anglo-Australian Telescope. The shapes of early-type galaxies often vary with radius. We derive average values of isophotal shape parameters in four different radial bins along the semi-major axis in each galaxy. We obtain empirical fitting formulae for the probability distribution of the sophotal parameters in each bin and investigate for possible correlations with other global properties of the galaxies. Our main finding is that the isophotal shapes of the inner regions are statistically different from those in the outer regions. This suggests that the outer and inner parts of early-type galaxies have evolved somewhat independently.

astro-ph.GA↗

arXiv2014

Study of Dust and Ionized gas in Early-type Galaxies

We present results of optical broad-band and narrow-band Halpha observations of a sample of forty nearby early-type galaxies. The majority of sample galaxies are known to have dust in various forms viz. dust lanes, nuclear dust and patchy/filamentary dust. A detailed study of dust was performed for 12 galaxies with prominent dust features. The extinction curves for these galaxies run parallel to the Galactic extinction curve, implying that the properties of dust in these galaxies are similar to those of the Milky-Way. The ratio of total to selective extinction (Rv) varies between 2.1 and 3.8, with an average of 2.9 +/- 0.2, fairly close to its canonical value of 3.1 for our Galaxy. The average relative grain size /a_Gal of dust particles in these galaxies turns out to be 1.01 +/- 0.2, while dust mass estimated using optical extinction lies in the range 10^2 to 10^4 M(sun) . The Halpha emission was detected in 23 out of 29 galaxies imaged through narrow- band filters with the Halpha luminosities in the range 10^38 - 10^41 erg s^-1. The mass of the ionized gas is in the range 10^3-10^5 M(sun). The morphology and extent of ionized gas is found similar to those of dust, indicating possible coexistence of dust and ionized gas in these galaxies. The absence of any apparent correlation between blue luminosity and normalized IRAS dust mass is suggestive of merger related origin of dust and gas in these galaxies.

astro-ph.GA↗

arXiv2013

Interpreting the large amplitude X-ray variation of GRS 1915+105 and IGR J17091-3624 as modulations of an accretion disc

Using the flux resolved spectroscopy for the first time, we analyse the RXTE/PCA data of the black hole X-ray binaries GRS 1915+105 and IGR J17091-3624, when both sources show large amplitude, quasi-regular oscillations in 2.0-60.0 keV X-ray light curves. For different observations, we extract spectra during the peak (spectrally soft) and dip (spectrally hard) intervals of the oscillation, and find that their spectra are phenomenologically complex, requiring at least two distinct spectral components. Besides a thermal Comptonization component, we find that the disc emission is better modelled by an index-free multicolour disc blackbody component (p-free disc model) rather than that from a standard accretion disc. While the peak and dip spectra are complex, remarkably, their difference spectra constructed by treating dip spectra as the background spectra of the peak spectra, can be modelled as a single p-free disc component. Moreover, the variability at different time-scales and energy bands of the peak flux level is always greater than or equal to the variability of the dip flux level, which strengthens the possibility that the peak flux level may be due to an independent spectral component added to the dip one. We also verify it using joint spectral analysis of peak and dip spectra with a variable emission component. Our result substantiates the standard hypothesis that the oscillations are due to the limit cycle behaviour of an unstable radiation pressure dominated inner disc. However, in this interpretation, the flux variation of the unstable disc can be several order of magnitudes as expected from some theoretical simulations and need not be fine tuned to match the factor ~10 variation seen between the peak and dip levels.

astro-ph.HE↗

arXiv2013

Properties of unique hard X-ray dips observed from GRS 1915+105 and IGR J17091-3624 and their implications

We report a comprehensive study on spectral and timing properties of hard X-ray dips uniquely observed in some so-called variability classes of the micro-quasars GRS 1915+105 and IGR J17091-3624. These dips are characterized by a sudden decline in the 2.0-60.0 keV X-ray intensity by a factor of 4-12 simultaneous with the increase in hardness ratio by a factor of 2-4. Using 31 observations of GRS 1915+105 with RXTE/PCA, we show that different behavior are observed in different types of variability classes, and we find that a dichotomy is observed between classes with abrupt transitions vs those with smoother evolution. For example, both energy-lag spectra and frequency-lag spectra of hard X-ray dips in classes with abrupt transitions and shorter dip intervals show hard-lag (hard photons lag soft photons), while both lag spectra during hard dips in classes with smoother evolution and longer dip intervals show soft-lag. Both lag time-scales are of the order of 100-600 msec. We also show that timing and spectral properties of hard X-ray dips observed in light curves of IGR J17091-3624 during its 2011 outburst are consistent with the properties of the abrupt transitions in GRS 1915+105 rather than smooth evolution. A global correlation between the X-ray intensity cycle time and hard dip time is observed for both abrupt and smooth transition which may be due to two distinct physical processes whose time-scales are eventually correlated. We discuss implications of our results in the light of some generic models.

astro-ph.HE↗