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A. Sharma

Publications and source records attributed to A. Sharma.

At least 55 records · Page 3Linked to original sources
arXiv2021

Study of Blazar activity in 10 year Fermi-LAT data and implications for TeV neutrino expectations

Blazars are the most active extragalactic gamma-ray sources. They show sporadic bursts of activity, lasting from hours to months. In this work we present a 10-year analysis of a sample of bright sources detected by Fermi-LAT (100 MeV - 300 GeV). Using 2-week binned lightcurves (LC) we estimated the Duty Cycle (DC): fraction of time that the source spends in an active state. The objects present different DC values, with an average of $22.74\%$ and $23.08 \%$ when considering (and not) the Extragalactic Background Light ( EBL). Additionally we study the so called "blazar sequence" trend for the sample of selected blazars in the ten years of data. This analysis constrains a possible counterpart of sub-PeV neutrino emission during the quiescent states, leaving the possibility to explain the observed IceCube signal during the flaring states.

astro-ph.HE↗
arXiv2020

Overdamped Phase Diffusion in hBN Encapsulated Graphene Josephson Junctions

We investigate the zero-bias behavior of Josephson junctions made of encapsulated graphene boron nitride heterostructures in the long ballistic junction regime. For temperatures down to 2.7K, the junctions appear non-hysteretic with respect to the switching and retrapping currents $I_C$ and $I_R$. A small non-zero resistance is observed even around zero bias current, and scales with temperature as dictated by the phase diffusion mechanism. By varying the graphene carrier concentration we are able to confirm that the observed phase diffusion mechanism follows the trend for an overdamped Josephson junction. This is in contrast with the majority of graphene-based junctions which are underdamped and shorted by the environment at high frequencies.

cond-mat.mes-hall↗
arXiv2020

Mean-Field Theory of Inhomogeneous Fluids

The Barker-Henderson perturbation theory is a bedrock of liquid-state physics, providing quantitative predictions for the bulk thermodynamic properties of realistic model systems. However, this successful method has not been exploited for the study of inhomogeneous systems. We develop and implement a first-principles 'Barker-Henderson density functional', thus providing a robust and quantitatively accurate theory for classical fluids in external fields. Numerical results are presented for the hard-core Yukawa model in three dimensions. Our predictions for the density around a fixed test particle and between planar walls are in very good agreement with simulation data. The density profiles for the free liquid vapour interface show the expected oscillatory decay into the bulk liquid as the temperature is reduced towards the triple point, but with an amplitude much smaller than that predicted by the standard mean-field density functional.

cond-mat.soft↗
arXiv2020

Manifestation of the Berry phase in the atomic nucleus $^{213}$Pb

The neutron-rich $^{213}$Pb isotope was produced in the fragmentation of a primary 1 GeV $A$ $^{238}$U beam, separated in FRS in mass and atomic number, and then implanted for isomer decay $γ$-ray spectroscopy with the RISING setup at GSI. A newly observed isomer and its measured decay properties indicate that states in $^{213}$Pb are characterized by the seniority quantum number that counts the nucleons not in pairs coupled to angular momentum $J=0$. The conservation of seniority is a consequence of the Berry phase associated with particle-hole conjugation, which becomes gauge invariant and therefore observable in semi-magic nuclei where nucleons half-fill the valence shell. The $γ$-ray spectroscopic observables in $^{213}$Pb are thus found to be driven by two mechanisms, particle-hole conjugation and seniority conservation, which are intertwined through the Berry phase.

nucl-ex↗
arXiv2020

The HEV Ventilator

HEV is a low-cost, versatile, high-quality ventilator, which has been designed in response to the COVID-19 pandemic. The ventilator is intended to be used both in and out of hospital intensive care units, and for both invasive and non-invasive ventilation. The hardware can be complemented with an external turbine for use in regions where compressed air supplies are not reliably available. The standard modes provided include PC-A/C(Pressure Assist Control),PC-A/C-PRVC(Pressure Regulated Volume Control), PC-PSV (Pressure Support Ventilation) and CPAP (Continuous Positive airway pressure). HEV is designed to support remote training and post market surveillance via a web interface and data logging to complement the standard touch screen operation, making it suitable for a wide range of geographical deployment. The HEV design places emphasis on the quality of the pressure curves and the reactivity of the trigger, delivering a global performance which will be applicable to ventilator needs beyond theCOVID-19 pandemic. This article describes the conceptual design and presents the prototype units together with their performance evaluation.

physics.med-ph↗
arXiv2019

Mini-MALTA: Radiation hard pixel designs for small-electrode monolithic CMOS sensors for the High Luminosity LHC

Depleted Monolithic Active Pixel Sensor (DMAPS) prototypes developed in the TowerJazz 180 nm CMOS imaging process have been designed in the context of the ATLAS upgrade Phase-II at the HL-LHC. The pixel sensors are characterized by a small collection electrode (3 $μ$m) to minimize capacitance, a small pixel size ($36.4\times 36.4$ $μ$m), and are produced on high resistivity epitaxial p-type silicon. The design targets a radiation hardness of $1\times10^{15}$ 1 MeV n$_{eq}$/cm$^{2}$, compatible with the outermost layer of the ATLAS ITK Pixel detector. This paper presents the results from characterization in particle beam tests of the Mini-MALTA prototype that implements a mask change or an additional implant to address the inefficiencies on the pixel edges. Results show full efficiency after a dose of $1\times10^{15}$ 1 MeV n$_{eq}$/cm$^{2}$.

physics.ins-det↗
arXiv2019

Escape rate of transiently active Brownian particle in one dimension

Activity significantly enhances the escape rate of a Brownian particle over a potential barrier. Whereas constant activity has been extensively studied in the past, little is known about the effect of time-dependent activity on the escape rate of the particle. In this paper we study the escape problem for a Brownian particle that is transiently active; the activity decreases rapidly during the escape process. Using the effective equilibrium approach we analytically calculate the escape rate, under the assumption that the particle is either completely passive or fully active when crossing the barrier. We perform numerical simulations of the escape process in one dimension and find good agreement with the theoretical predictions.

cond-mat.soft↗
arXiv2018

Direct measurement of electron numbers created at infrared laser-induced ionization of various gases

In this work, we present temporally resolved measurements of electron numbers created at photoionization of various gases by femtosecond laser pulse at 800 nm wavelength. The experiments were conducted in $O_2$, $Xe$, $Ar$, $N_2$, $Kr$ and $CO$ at room temperature and atmospheric pressure. Elastic microwave scattering was used to directly measure the electron numbers. Numbers of electrons in the range $3*10^8$ to $3*10^{12}$ electrons were produced by the laser pulse energies 100-700 uJ. After the laser pulse, plasma decayed on the time scale varied from 1 to 40 ns depending on the gas type and governed by two competing processes, namely, the creation of new electrons from ionization of the metastable atoms and loss of the electrons due to dissociative recombination and attachment to oxygen.

physics.plasm-ph↗
arXiv2018

Efficient proton acceleration from laser-driven cryogenic hydrogen target of various shapes

We theoretically investigate high energy, collimated proton beam with three dimensional particle in cell simulations of ultrashort petawatt laser interaction with cryogenic hydrogen target of various shapes. Here we show that under appropriate conditions between the laser and target parameters, the protons are accelerated to high energies mainly due to collisionless shock acceleration mechanism combined with TNSA. The dependence of the protonic energy on the laser field, target shape and thickness is reported. It is demonstrated that the irradiation of intense laser (20fs, 2PW) with cryogenic hydrogen target at optimal thickness allows the efficient generation of high energy proton beam (100 MeV) of small divergence. Our results also indicate that diffracted laser field strongly affects the collimation of ions as it passes beside the mass limited target. This approach predicts a possible pathway to control laser driven ion sources.

physics.plasm-ph↗
arXiv2017

Distinct photoluminescence in multilayered van der Waals heterostructures of MoS2/WS2/ReS2 and BN

Van der Waals heterostructures of (TMDL=1/BNL=1-4/TMDL=1/BNL=1-4), [TMD = MoS2, WS2, and ReS2] are grown on c-plane sapphire substrate by pulsed laser deposition under slow kinetic condition. The heterostructure systems show strong emission around 2.3 eV and subsidiary peaks around 2.8, 1.9, 1.7 and 1.5 eV. BN and TMDs forms type-I heterojunction and the emission peaks observed are explained in terms of various band to band recombination processes and considering relative orientation of Brillouin Zones. The emission peak around 2.3eV is promising for solar and photovoltaic application. The observation is almost similar for three different heterostructure systems.

cond-mat.mtrl-sci↗
arXiv2017

Counting the electrons in a multiphoton ionization by elastic scattering of microwaves

Laser induced plasmas have found numerous applications including plasma-assisted combustion, combustion diagnostics, laser induced breakdown spectroscopy, light detection and ranging techniques (LIDAR), microwave guiding, reconfigurable plasma antennae etc. Multiphoton ionization (MPI) is a fundamental first step in high-energy laser-matter interaction and is important for understanding of the mechanism of plasma formation. With the discovery of MPI more than 50 years ago, there were numerous attempts to determine basic physical constants of this process in the direct experiments, namely photoionization rates and cross-sections of the MPI, however, no reliable data is available until today and spread in the literature values often reaches 2-3 orders of magnitude. This is due to inability to conduct absolute measurements of plasma electron numbers generated by MPI which leads to uncertainties and, sometimes, contradictions between the MPI cross-section values utilized by different researchers across the field. Here we report first direct measurement of absolute plasma electron numbers generated at MPI of air and subsequently we precisely determine ionization rate and cross-section of eight-photon ionization of oxygen molecule by 800 nm photons $σ_8=(3.32{\pm}0.3)*10^{-130} W^{-8}m^{16}s^{-1}$. Method is based on the absolute measurement of electron number created by MPI using elastic scattering of microwaves off the plasma volume in Rayleigh regime and establishes a general approach to directly measure and tabulate basic constants of the MPI process for various gases and photon energies.

physics.plasm-ph↗
arXiv2017

Laser-plasma accelerator based single-cycle attosecond undulator source

Laser-plasma accelerators (LPAs), producing high-quality electron beams, provide an opportunity to reduce the size of free-electron lasers (FELs) to only a few meters. A complete system is proposed here, which is based on FEL technology and consists of an LPA, two undulators, and other magnetic devices. The system is capable to generate carrier-envelope phase stable attosecond pulses with engineered waveform. Pulses with up to~60~nJ energy and 90 to~400~attosecond duration in the 30 to 120~nm wavelength range are predicted by numerical simulation. These pulses can be used to investigate ultrafast field-driven electron dynamics in matter.

physics.acc-ph↗
arXiv2017

Pointwise Semi-Slant Warped Product Submanifold in a Lorentzian Paracosymplectic Manifold

Recently Y{ü}ksel et. al. \cite{SY} shows that there doesn't exist any proper semi-slant warped product submanifolds in a Lorentzian paracosymplectic manifold. In the present article, we first define and give preparatory lemmas for a new generalize class of semi-slant submanifolds called pointwise semi-slant submanifolds in a Lorentzian paracosymplectic manifold, and then we ensure by presenting some existence results and characterization theorem that there exist a non-trivial pointwise semi-slant warped product submanifolds in a Lorentzian paracosymplectic manifold counter to warped product semi-slant submanifolds in a Lorentzian paracosymplectic manifold.

math.DG↗
arXiv2017

Study of the plutino object (208996) 2003 AZ84 from stellar occultations: size, shape and topographic features

We present results derived from four stellar occultations by the plutino object (208996) 2003~AZ$_{84}$, detected at January 8, 2011 (single-chord event), February 3, 2012 (multi-chord), December 2, 2013 (single-chord) and November 15, 2014 (multi-chord). Our observations rule out an oblate spheroid solution for 2003~AZ$_{84}$'s shape. Instead, assuming hydrostatic equilibrium, we find that a Jacobi triaxial solution with semi axes $(470 \pm 20) \times (383 \pm 10) \times (245 \pm 8)$~km % axis ratios $b/a= 0.82 \pm 0.05$ and $c/a= 0.52 \pm 0.02$, can better account for all our occultation observations. Combining these dimensions with the rotation period of the body (6.75~h) and the amplitude of its rotation light curve, we derive a density $ρ=0.87 \pm 0.01$~g~cm$^{-3}$ a geometric albedo $p_V= 0.097 \pm 0.009$. A grazing chord observed during the 2014 occultation reveals a topographic feature along 2003~AZ$_{84}$'s limb, that can be interpreted as an abrupt chasm of width $\sim 23$~km and depth $> 8$~km or a smooth depression of width $\sim 80$~km and depth $\sim 13$~km (or an intermediate feature between those two extremes).

astro-ph.EP↗
arXiv2017

Means Moments and Newton's Inequalities

It is shown that Newton's inequalities and the related Maclaurin's inequalities provide several refinements of the fundamental Arithmetic mean - Geometric mean - Harmonic mean inequality in terms of the means and variance of positive real numbers. We also obtain some inequalities involving third and fourth central moments of real numbers.

math.ST↗
arXiv2016

Escape rate of active particles in the effective equilibrium approach

The escape rate of a Brownian particle over a potential barrier is accurately described by the Kramers theory. A quantitative theory explicitly taking the activity of Brownian particles into account has been lacking due to the inherently out-of-equilibrium nature of these particles. Using an effective equilibrium approach [Farage et al., Phys. Rev. E 91, 042310 (2015)] we study the escape rate of active particles over a potential barrier and compare our analytical results with data from direct numerical simulation of the colored noise Langevin equation. The effective equilibrium approach generates an effective potential which, when used as input to Kramers rate theory, provides results in excellent agreement with the simulation data.

cond-mat.soft↗