arXiv subjects

S. Sharma

Publications and source records attributed to S. Sharma.

At least 181 records · Page 10Linked to original sources

arXiv2017

Ultrafast demagnetization in bulk vs thin films: an ab-initio study

We report on {\it ab-initio} simulations of the quantum dynamics of electronic charge and spin when subjected to intense laser pulses. By performing separate calculations for a Ni thin film and bulk Ni, we conclude that surface effects have a dramatic influence on amplifying the laser induced demagnetization. We show that the reason for this amplification is due to increased spin-currents on the surface of the thin film. This enhancement is a direct consequence of the broken symmetry originating from the surface formation. We find that the underlying physics of demagnetization, during the early femtoseconds, for both bulk and thin film is dominated by spin-flips induced by spin-orbit coupling. After the first $\sim 40$ fs this changes in that the dominant cause of demagnetization is the flow of spin-currents, which leads to stronger demagnetization in the film compared to that of the bulk.

cond-mat.mtrl-sci↗

arXiv2017

Bulk physics entwined with a topological surface state

For the IV-VI semiconductor family we derive an exact relation between the microscopic gap edge wave functions of the bulk insulator and the Dirac-Weyl topological surface state wave function, thus obtaining a fully microscopic surface state. We find that the balance of spin-orbit interaction and crystal field in the bulk, and the band bending at the surface, can profoundly influence the surface state spin-momentum locking. As a manifestation of this we predict that the spin texture of the $M$-point Dirac cones of SnTe can be tuned through an unexpectedly rich sequence of spin textures -- warped helical with winding number $\pm 1$, $k_x$ linear, hyperbolic, and $k_y$ linear -- e.g. by tuning the band bending at the surface.

cond-mat.mtrl-sci↗

arXiv2017

Large magnetocrystalline anisotropy in tetragonally distorted Heuslers: a systematic study

With a view to the design of hard magnets without rare earths we explore the possibility of large magnetocrystalline anisotropy energies in Heusler compounds that are unstable with respect to a tetragonal distortion. We consider the Heusler compounds Fe$_2$YZ with Y = (Ni, Co, Pt), and Co$_2$YZ with Y = (Ni, Fe, Pt) where, in both cases, Z = (Al, Ga, Ge, In, Sn). We find that for the Co$_2$NiZ, Co$_2$PtZ, and Fe$_2$PtZ families the cubic phase is always, at $T=0$, unstable with respect to a tetragonal distortion, while, in contrast, for the Fe$_2$NiZ and Fe$_2$CoZ families this is the case for only 2 compounds -- Fe$_2$CoGe and Fe$_2$CoSn. For all compounds in which a tetragonal distortion occurs we calculate the MAE finding remarkably large values for the Pt containing Heuslers, but also large values for a number of the other compounds (e.g. Co$_2$NiGa has an MAE of -2.11~MJ/m$^3$). The tendency to a tetragonal distortion we find to be strongly correlated with a high density of states at the Fermi level in the cubic phase. As a corollary to this fact we observe that upon doping compounds for which the cubic structure is stable such that the Fermi level enters a region of high DOS, a tetragonal distortion is induced and a correspondingly large value of the MAE is then observed.

cond-mat.mtrl-sci↗

arXiv2017

Modified Tsallis and Weibull distributions for multiplicities in e+e- collisions

Multiplicity distributions of charged particles produced in the e^{+}e^{-} collisions at energies ranging from 14 to 91 GeV are studied using Tsallis q-statistics and the recently proposed Weibull distribution functions, in both restricted rapidity windows as well as in full phase space. It is shown that Tsallis $q$-statistics explains the data excellently in all rapidity ranges while the Weibull distribution fails to reproduce the data in full phase space. Modifications to the distributions are proposed to establish manifold improvements in the fitting of the data.

hep-ph↗

arXiv2016

RAVE stars in K2 - I. Improving RAVE red giants spectroscopy using asteroseismology from K2 Campaign 1

We present a set of 87 RAVE stars with detected solar like oscillations, observed during Campaign 1 of the K2 mission (RAVE K2-C1 sample). This dataset provides a useful benchmark for testing the gravities provided in RAVE Data Release 4 (DR4), and is key for the calibration of the RAVE Data Release 5 (DR5). In the present work, we use two different pipelines, GAUFRE (Valentini et al. 2013) and Sp_Ace (Boeche & Grebel 2016), to determine atmospheric parameters and abundances by fixing log(g) to the seismic one. Our strategy ensures highly consistent values among all stellar parameters, leading to more accurate chemical abundances. A comparison of the chemical abundances obtained here with and without the use of seismic log(g) information has shown that an underestimated (overestimated) gravity leads to an underestimated (overestimated) elemental abundance (e.g. [Mg/H] is underestimated by ~0.25 dex when the gravity is underestimated by 0.5 dex). We then perform a comparison between the seismic gravities and the spectroscopic gravities presented in the RAVE DR4 catalogue, extracting a calibration for log(g) of RAVE giants in the colour interval 0.50<(J - Ks)<0.85. Finally, we show a comparison of the distances, temperatures, extinctions (and ages) derived here for our RAVE K2-C1 sample with those derived in RAVE DR4 and DR5.DR5 performs better than DR4 thanks to the seismic calibration, although discrepancies can still be important for objects for which the difference between DR4/DR5 and seismic gravities differ by more than ~0.5 dex. The method illustrated in this work will be used for analysing RAVE targets present in the other K2 campaigns, in the framework of Galactic Archaeology investigations.

astro-ph.GA↗

arXiv2016

The Galah Survey: Classification and diagnostics with t-SNE reduction of spectral information

Galah is an ongoing high-resolution spectroscopic survey with the goal of disentangling the formation history of the Milky Way, using the fossil remnants of disrupted star formation sites which are now dispersed around the Galaxy. It is targeting a randomly selected, magnitude limited ($V \leq 14$) sample of stars, with the goal of observing one million objects. To date, 300,000 spectra have been obtained. Not all of them are correctly processed by parameter estimation pipelines and we need to know about them. We present a semi-automated classification scheme which identifies different types of peculiar spectral morphologies, in an effort to discover and flag potentially problematic spectra and thus help to preserve the integrity of the survey's results. To this end we employ a recently developed dimensionality reduction technique t-SNE (t-distributed Stochastic Neighbour Embedding), which enables us to represent the complex spectral morphology in a two-dimensional projection map while still preserving the properties of the local neighbourhoods of spectra. We find that the majority (178,483) of the 209,533 Galah spectra considered in this study represents normal single stars, whereas 31,050 peculiar and problematic spectra with very diverse spectral features pertaining to 28,579 stars are distributed into 10 classification categories: Hot stars, Cool metal-poor giants, Molecular absorption bands, Binary stars, H$α$/H$β$ emission, H$α$/H$β$ emission superimposed on absorption, H$α$/H$β$ P-Cygni, H$α$/H$β$ inverted P-Cygni, Lithium absorption, and Problematic. Classified spectra with supplementary information are presented in the catalogue, indicating candidates for follow-up observations and population studies of the short-lived phases of stellar evolution.

astro-ph.SR↗

arXiv2016

The K2 Galactic Archaeology Program Data Release 1: Asteroseismic results from Campaign 1

NASA's K2 mission is observing tens of thousands of stars along the ecliptic, providing data suitable for large scale asteroseismic analyses to inform galactic archaeology studies. Its first campaign covered a field near the north galactic cap, a region never covered before by large asteroseismic-ensemble investigations, and was therefore of particular interest for exploring this part of our Galaxy. Here we report the asteroseismic analysis of all stars selected by the K2 Galactic Archaeology Program during the mission's "North Galactic Cap" campaign 1. Our consolidated analysis uses six independent methods to measure the global seismic properties, in particular the large frequency separation, and the frequency of maximum power. From the full target sample of 8630 stars we find about 1200 oscillating red giants, a number comparable with estimates from galactic synthesis modeling. Thus, as a valuable by-product we find roughly 7500 stars to be dwarfs, which provide a sample well suited for galactic exoplanet occurrence studies because they originate from our simple and easily reproducible selection function. In addition, to facilitate the full potential of the data set for galactic archaeology we assess the detection completeness of our sample of oscillating red giants. We find the sample is at least near complete for stars with 40 < numax/microHz < 270, and numax_detec < 2.6*1e6 * 2e-Kp microHz. There is a detection bias against helium core burning stars with numax ~ 30 microHz, affecting the number of measurements of DeltaNu and possibly also numax. Although we can detect oscillations down to Kp = 15, our campaign 1 sample lacks enough faint giants to assess the detection completeness for stars fainter than Kp ~ 14.5.

astro-ph.SR↗

arXiv2016

Thin Film growth of Solid State materials

Magnetron sputtering has also been used to deposit thin films of some materials and it has significant technological importance. A modeling on deposition of epitaxial thin films of Yttrium Stabilized Zirconia (YSZ) was done the diffusion of adatom on the surface were studies. There exists a strong interaction of ions formed in the plasma during the sputtering process. Cu thin films were deposited on Si. Nanocomposite thin film of SiCN showed dendritic growth.

cond-mat.mtrl-sci↗

arXiv2016

Electron-phonon scattering and in-plane electric conductivity in twisted bilayer graphene

We have surveyed the in-plane transport properties of the graphene twist bilayer using (i) a low-energy effective Hamiltonian for the underlying electronic structure, (ii) an isotropic elastic phonon model, and (iii) the linear Boltzmann equation for elastic electron-phonon scattering. We find that transport in the twist bilayer is profoundly sensitive to the rotation angle of the constituent layers. Similar to the electronic structure of the twist bilayer the transport is qualitatively different in three distinct angle regimes. At large angles ($θ> \,\approx\!\!10^\circ$) and at temperatures below an interlayer Bloch-Grüneisen temperature of $\approx 10$~K the conductivity is independent of the twist angle i.e. the layers are fully decoupled. Above this temperature the layers, even though decoupled in the ground state, are re-coupled by electron-phonon scattering and the transport is different both from single layer graphene as well as the Bernal bilayer. In the small angle regime $θ<\,\approx\!\!2^\circ$ the conductivity drops by two orders of magnitude and develops a rich energy dependence, reflecting the complexity of the underlying topological changes (Lifshitz transitions) of the Fermi surface. At intermediate angles the conductivity decreases continuously as the twist angle is reduced, while the energy dependence of the conductivity presents two sharp transitions, that occur at specific angle dependent energies, and that may be related to (i) the well studied van Hove singularity of the twist bilayer and (ii) a Lifshitz transition that occurs when trigonally placed electron pockets decorate the strongly warped Dirac cone. We examine the role of a layer perpendicular electric field finding that it affects the conductivity strongly at low temperatures whereas this effect is washed out by Fermi smearing at room temperatures.

cond-mat.mes-hall↗

arXiv2016

Architecture of the Andromeda: a quantitative analysis of clustering in the inner stellar halo

We present a quantitative measurement of the amount of clustering present in the inner $\sim30$ kpc of the stellar halo of the Andromeda galaxy (M31). For this we analyse the angular positions and radial velocities of the carefully selected Planetary Nebulae (PNe) in the M31 stellar halo. We study the cumulative distribution of pair-wise distances in angular position and line of sight velocity space, and find that the M31 stellar halo contains substantially more stars in the form of close pairs as compared to that of a featureless smooth halo. In comparison to a smoothed/scrambled distribution we estimate that the clustering excess in the M31 inner halo is roughly $40\%$ at maximum and on average $\sim 20\%$. Importantly, comparing against the 11 stellar halo models of \cite{2005ApJ...635..931B}, which were simulated within the context of the $Λ{\rm CDM}$ cosmological paradigm, we find that the amount of substructures in the M31 stellar halo closely resembles that of a typical $Λ{\rm CDM}$ halo.

astro-ph.GA↗

arXiv2016

Stacked Autoencoders for Medical Image Search

Medical images can be a valuable resource for reliable information to support medical diagnosis. However, the large volume of medical images makes it challenging to retrieve relevant information given a particular scenario. To solve this challenge, content-based image retrieval (CBIR) attempts to characterize images (or image regions) with invariant content information in order to facilitate image search. This work presents a feature extraction technique for medical images using stacked autoencoders, which encode images to binary vectors. The technique is applied to the IRMA dataset, a collection of 14,410 x-ray images in order to demonstrate the ability of autoencoders to retrieve similar x-rays given test queries. Using IRMA dataset as a benchmark, it was found that stacked autoencoders gave excellent results with a retrieval error of 376 for 1,733 test images with a compression of 74.61%.

cs.CV↗

arXiv2016

Evidence of asymmetries in the Aldebaran photosphere from multi-wavelength lunar occultations

We have recorded three lunar occultations of Aldebaran (AlfTau) at different telescopes and using various band-passes, from the ultraviolet to the far red. The data have been analyzed using both model-dependent and model-independent methods. The derived uniform-disc angular diameter values have been converted to limb-darkened values using model atmosphere relations, and are found in broad agreement among themselves and with previous literature values. The limb-darkened diameter is about 20.3 milliarcseconds on average . However, we have found indications that the photospheric brightness profile of Aldebaran may have not been symmetric, a finding already reported by other authors for this and for similar late-type stars. At the sampling scale of our brightness profile, between one and two milliarcsecond, the uniform and limb-darkened disc models may not be a good description for Aldebaran. The asymmetries appear to differ with wavelength and over the 137 days time span of our measurements. Surface spots appear as a likely explanation for the differences between observations and the models.

astro-ph.SR↗

arXiv2016

Going beyond k.p theory: a general method for obtaining effective Hamiltonians in both high and low symmetry situations

We provide a method for the generation of effective continuum Hamiltonians that goes beyond the well known k.p method in being equally effective in both high, and low (or no) symmetry situations. Our approach is based on a surprising exact map of the two-centre tight-binding method onto a compact continuum Hamiltonian, with a precise condition given for the hermiticity of the latter object. We apply this method to a broad range of low dimensional systems of both high and low symmetry: graphene, graphdiyne, γ-graphyne, 6,6,12-graphyne, twist bilayer graphene, and partial dislocation networks in Bernal stacked bilayer graphene. For the single layer systems the method yields Hamiltonians for the ideal lattices, as well as a systematic theory for corrections due to deformation. In the case of bilayer graphene we provide a compact expression for an effective field capable of describing any stacking deformation of the bilayer; twist bilayer graphene, as well as the partial dislocation network in AB stacked graphene, emerge as special cases of this field. For the latter system we find (i) charge pooling on the mosaic of AB and AC segments near the Dirac point and (ii) localized current carrying states on the partials with the current density characterized by both intralayer and interlayer components.

cond-mat.mes-hall↗

arXiv2016

Ultrafast Local Magnetization and Demagnetization in Heusler Alloys

With the goal of pushing Spintronic devices towards faster and faster timescales, we demonstrate, using ab-intio time-dependent density functional theory simulations of bulk Heusler compounds subject to ultrashort intense laser pulses, that the local magnetic moment can increase or decrease in a few femtoseconds. This speed is due to the all optical nature of the process, which transfers spin moment from one sublattice to another. This transfer depends on easily tunable laser parameters. By comparing the spin dynamics of a variety of Heusler (or half-Heusler) compounds, we demonstrate that the density of states explains the observed moment transfer; most the physics of inter sublattice moment transfer is due to the flow of spin current which is governed by availability of states above the Fermi level.

cond-mat.mtrl-sci↗

arXiv2016

The curvature of the freeze-out line in heavy ion collisions

We calculate the mean and variance of net-baryon number and net-electric charge distributions from Quantum Chromodynamics (QCD) using a next-to-leading order Taylor expansion in terms of temperature and chemical potentials. We compare these expansions with experimental data from STAR and PHENIX, determine the freeze-out temperature in the limit of vanishing baryon chemical potential, and, for the first time, constrain the curvature of the freeze-out line through a direct comparison between experimental data on net-charge fluctuations and a QCD calculation. We obtain a bound on the curvature coefficient, kappa_2^f < 0.011, that is compatible with lattice QCD results on the curvature of the QCD transition line.

hep-lat↗

arXiv2016

Identification of Globular Cluster Stars in RAVE data II: Extended tidal debris around NGC 3201

We report the identification of extended tidal debris potentially associated with the globular cluster NGC 3201, using the RAVE catalogue. We find the debris stars are located at a distance range of 1-7 kpc based on the forthcoming RAVE distance estimates. The derived space velocities and integrals of motion show interesting connections to NGC 3201, modulo uncertainties in the proper motions. Three stars, which are among the 4 most likely candidates for NGC 3201 tidal debris, are separated by 80 degrees on the sky yet are well matched by the 12 Gyr, [Fe/H] = -1.5 isochrone appropriate for the cluster. This is the first time tidal debris around this cluster has been reported over such a large spatial extent, with implications for the cluster$'$s origin and dynamical evolution.

astro-ph.GA↗

arXiv2015

Charm hadrons above $T_c$

From the analysis of the lattice data on fluctuations and correlations of charm we conclude that open charm meson and baryon-like excitations exist above the QCD crossover temperature, and in fact are the dominant degrees of freedom for thermodynamics in the vicinity of the transition. Charm quarks become the dominant charm degrees of freedom only for temperatures T>200 MeV.

hep-ph↗

arXiv2015

Conserved Charge Fluctuations from Lattice QCD and the Beam Energy Scan

We discuss the next-to-leading order Taylor expansion of ratios of cumulants of net-baryon number fluctuations. We focus on the relation between the skewness ratio, $S_Bσ_B = χ_3^B/χ_1^B$, and the kurtosis ratio, $κ_Bσ_B^2 =χ_4^B/χ_2^B$. We show that differences in these two cumulant ratios are small for small values of the baryon chemical potential. The next-to-leading order correction to $κ_Bσ_B^2$ however is approximately three times larger than that for $S_Bσ_B$. The former thus drops much more rapidly with increasing beam energy, $\sqrt{s_{NN}}$. We argue that these generic patterns are consistent with current data on cumulants of net-proton number fluctuations measured by the STAR Collaboration at $\sqrt{s_{NN}}\ge 19.6$~GeV.

hep-lat↗