arXiv subjects

Y. Wang

Publications and source records attributed to Y. Wang.

At least 415 records · Page 23Linked to original sources

arXiv2015

Dust-acoustic solitary waves and shocks in strongly coupled quantum plasmas

We investigate the propagation characteristics of electrostatic dust-acoustic (DA) solitary waves and shocks in a strongly coupled dusty plasma consisting of intertialess electrons and ions, and strongly coupled inertial charged dust particles. A generalized viscoelastic hydrodynamic model with the effects of electrostatic dust pressure associated with the strong coupling of dust particles, and a quantum hydrodynamic model with the effects of quantum forces associated with the Bohm potential and the exchange-correlation potential for electrons and ions are considered. Both the linear and weakly nonlinear theory of DA waves are studied by the derivation and analysis of dispersion relations as well as Korteweg-de Vries (KdV) and KdV-Burgers (KdVB)-like equations. It is shown that in the kinetic regime ($ωτ_m\gg1$, where $ω$ is the wave frequency and $τ_m$ is the viscoelastic relaxtation time), the amplitude of the DA solitary waves decays slowly with time with the effect of a small amount of dust viscosity. However, the DA shock-like perturbations can be excited in the hydrodynamic regime with $ωτ_m\ll1$. The analytical and numerical solutions of the KdV and KdVB equations are also presented and analyzed with the system parameters.

physics.plasm-ph↗

arXiv2015

Wide-Field InfrarRed Survey Telescope-Astrophysics Focused Telescope Assets WFIRST-AFTA 2015 Report

This report describes the 2014 study by the Science Definition Team (SDT) of the Wide-Field Infrared Survey Telescope (WFIRST) mission. It is a space observatory that will address the most compelling scientific problems in dark energy, exoplanets and general astrophysics using a 2.4-m telescope with a wide-field infrared instrument and an optical coronagraph. The Astro2010 Decadal Survey recommended a Wide Field Infrared Survey Telescope as its top priority for a new large space mission. As conceived by the decadal survey, WFIRST would carry out a dark energy science program, a microlensing program to determine the demographics of exoplanets, and a general observing program utilizing its ultra wide field. In October 2012, NASA chartered a Science Definition Team (SDT) to produce, in collaboration with the WFIRST Study Office at GSFC and the Program Office at JPL, a Design Reference Mission (DRM) for an implementation of WFIRST using one of the 2.4-m, Hubble-quality telescope assemblies recently made available to NASA. This DRM builds on the work of the earlier WFIRST SDT, reported by Green et al. (2012) and the previous WFIRST-2.4 DRM, reported by Spergel et. (2013). The 2.4-m primary mirror enables a mission with greater sensitivity and higher angular resolution than the 1.3-m and 1.1-m designs considered previously, increasing both the science return of the primary surveys and the capabilities of WFIRST as a Guest Observer facility. The addition of an on-axis coronagraphic instrument to the baseline design enables imaging and spectroscopic studies of planets around nearby stars.

astro-ph.IM↗

arXiv2015

Effects of Lifshitz Transition on Charge Transport in Magnetic Phases of Fe-Based Superconductors

The unusual temperature dependence of the resistivity and its in-plane anisotropy observed in the Fe-based superconducting materials, particularly Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, has been a longstanding puzzle. Here we consider the effect of impurity scattering on the temperature dependence of the average resistivity within a simple two-band model of a dirty spin density wave metal. The sharp drop in resistivity below the Néel temperature $T_N$ in the parent compound can only be understood in terms of a Lifshitz transition following Fermi surface reconstruction upon magnetic ordering. We show that the observed resistivity anisotropy in this phase, arising from nematic defect structures, is affected by the Lifshitz transition as well.

cond-mat.supr-con↗

arXiv2015

Glide-Plane Symmetry and Superconducting Gap Structure of Iron-Based Superconductors

We consider the effect of glide-plane symmetry of the Fe-pnictogen/chalcogen layer in Fe-based superconductors on pairing in spin fluctuation models. Recent theories have proposed that so-called $η$-pairing states with nonzero total momentum can be realized and possess exotic properties such as odd parity spin singlet symmetry and time-reversal symmetry breaking. Here we show that $η$ pairing is inevitable when there is orbital weight at the Fermi level from orbitals with even and odd mirror reflection symmetry in $z$; however, by explicit calculation, we conclude that the gap function that appears in observable quantities is identical to that found in earlier, 1 Fe per unit cell pseudocrystal momentum calculations.

cond-mat.supr-con↗

arXiv2015

Carbon p Electron Ferromagnetism in Silicon Carbide

Ferromagnetism can occur in wide-band gap semiconductors as well as in carbon-based materials when specific defects are introduced. It is thus desirable to establish a direct relation between the defects and the resulting ferromagnetism. Here, we contribute to revealing the origin of defect-induced ferromagnetism using SiC as a prototypical example. We show that the long-range ferromagnetic coupling can be attributed to the p electrons of the nearest-neighbor carbon atoms around the VSiVC divacancies. Thus, the ferromagnetism is traced down to its microscopic, electronic origin.

cond-mat.mtrl-sci↗

arXiv2015

Dissipative inertial transport patterns near coherent Lagrangian eddies in the ocean

Recent developments in dynamical systems theory have revealed long-lived and coherent Lagrangian (i.e., material) eddies in incompressible, satellite-derived surface ocean velocity fields. Paradoxically, observed drifting buoys and floating matter tend to create dissipative-looking patterns near oceanic eddies, which appear to be inconsistent with the conservative fluid particle patterns created by coherent Lagrangian eddies. Here we show that inclusion of inertial effects (i.e., those produced by the buoyancy and size finiteness of an object) in a rotating two-dimensional incompressible flow context resolves this paradox. Specifically, we obtain that anticyclonic coherent Lagrangian eddies attract (repel) negatively (positively) buoyant finite-size particles, while cyclonic coherent Lagrangian eddies attract (repel) positively (negatively) buoyant finite-size particles. We show how these results explain dissipative-looking satellite-tracked surface drifter and subsurface float trajectories, as well as satellite-derived \emph{Sargassum} distributions.

physics.ao-ph↗

arXiv2015

Measurement of the Target-Normal Single-Spin Asymmetry in Quasi-Elastic Scattering from the Reaction $^3$He$^\uparrow(e,e^\prime)$

We report the first measurement of the target single-spin asymmetry, $A_y$, in quasi-elastic scattering from the inclusive reaction $^3$He$^{\uparrow}(e,e^\prime)$ on a $^3$He gas target polarized normal to the lepton scattering plane. Assuming time-reversal invariance, this asymmetry is strictly zero for one-photon exchange. A non-zero $A_y$ can arise from the interference between the one- and two-photon exchange processes which is sensitive to the details of the sub-structure of the nucleon. An experiment recently completed at Jefferson Lab yielded asymmetries with high statistical precision at $Q^{2}=$ 0.13, 0.46 and 0.97 GeV$^{2}$. These measurements demonstrate, for the first time, that the $^3$He asymmetry is clearly non-zero and negative with a statistical significance of (8-10)$σ$. Using measured proton-to-$^{3}$He cross-section ratios and the effective polarization approximation, neutron asymmetries of $-$(1-3)% were obtained. The neutron asymmetry at high $Q^2$ is related to moments of the Generalized Parton Distributions (GPDs). Our measured neutron asymmetry at $Q^2=0.97$ GeV$^2$ agrees well with a prediction based on two-photon exchange using a GPD model and thus provides a new, independent constraint on these distributions.

nucl-ex↗

arXiv2015

NANOGrav Constraints on Gravitational Wave Bursts with Memory

Among efforts to detect gravitational radiation, pulsar timing arrays are uniquely poised to detect "memory" signatures, permanent perturbations in spacetime from highly energetic astrophysical events such as mergers of supermassive black hole binaries. The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) observes dozens of the most stable millisecond pulsars using the Arecibo and Green Bank radio telescopes in an effort to study, among other things, gravitational wave memory. We herein present the results of a search for gravitational wave bursts with memory (BWMs) using the first five years of NANOGrav observations. We develop original methods for dramatically speeding up searches for BWM signals. In the directions of the sky where our sensitivity to BWMs is best, we would detect mergers of binaries with reduced masses of $10^9$ $M_\odot$ out to distances of 30 Mpc; such massive mergers in the Virgo cluster would be marginally detectable. We find no evidence for BWMs. However, with our non-detection, we set upper limits on the rate at which BWMs of various amplitudes could have occurred during the time spanned by our data--e.g., BWMs with amplitudes greater than $10^{-13}$ must occur at a rate less than 1.5 yr$^{-1}$.

astro-ph.GA↗

arXiv2015

Galaxy Formation & Dark Matter Modelling in the Era of the Square Kilometre Array

Theoretical galaxy formation models are an established and powerful tool for interpreting the astrophysical significance of observational data, particularly galaxy surveys. Such models have been utilised with great success by optical surveys such as 2dFGRS and SDSS, but their application to radio surveys of cold gas in galaxies has been limited. In this chapter we describe recent developments in the modelling of the cold gas properties in the models, and how these developments are essential if they are to be applied to cold gas surveys of the kind that will be carried out with the SKA. By linking explicitly a galaxy's star formation rate to the abundance of molecular hydrogen in the galaxy rather than cold gas abundance, as was assumed previously, the latest models reproduce naturally many of the global atomic and molecular hydrogen properties of observed galaxies. We review some of the key results of the latest models and highlight areas where further developments are necessary. We discuss also how model predictions can be most accurately compared with observational data, what challenges we expect when creating synthetic galaxy surveys in the SKA era, and how the SKA can be used to test models of dark matter.

astro-ph.GA↗

arXiv2015

High In-content InGaN layers synthesized by plasma-assisted molecular-beam epitaxy: growth conditions, strain relaxation and In incorporation kinetics

We report the interplay between In incorporation and strain relaxation kinetics in high-In-content InxGa1-xN (x = 0.3) layers grown by plasma-assisted molecular-beam epitaxy. For In mole fractions x = 0.13-0.48, best structural and morphological quality is obtained under In excess conditions, at In accumulation limit, and at a growth temperature where InGaN decomposition is active. Under such conditions, in situ and ex situ analysis of the evolution of the crystalline structure with the growth thickness points to an onset of misfit relaxation after the growth of 40 nm, and a gradual relaxation during more than 200 nm which results in an inhomogeneous strain distribution along the growth axis. This process is associated with a compositional pulling effect, i.e. indium incorporation is partially inhibited in presence of compressive strain, resulting in a compositional gradient with increasing In mole fraction towards the surface.

cond-mat.mtrl-sci↗

arXiv2014

Spin injection based on the spin gapless semiconductor(SGS)/semiconductor heterostructures

Spin injection efficiency based on conventional ferromagnet (or half-metallic ferromagnet) /semiconductor is greatly limited by the Schmidt obstacle due to conductivity mismatch, here we proposed that by replacing the metallic injectors with spin gapless semiconductors can significantly reduce the conductive mismatch while conserve high spin polarization. By performing first principles calculations based on superlattice structure, we have studied the representative system of Mn2CoAl/semiconductor spin injector scheme. The results showed that high spin polarization were maintained at the interface in systems of Mn2CoAl/Fe2VAl constructed with (100) interface and Mn2CoAl/GaAs with (110) interface, and the latter is expected to possess long spin diffusion length. Inherited from the spin gapless feature of Mn2CoAl, a pronounced dip was observed around the Fermi level in the majority-spin DOS in both systems, suggesting fast transport of the low-density carriers.

cond-mat.mes-hall↗

arXiv2014

Induced Gravitational Collapse in the BATSE era: the case of GRB 970828

Following the recently established "Binary-driven HyperNova" (BdHN) paradigm, we here interpret GRB 970828 in terms of the four episodes typical of such a model. The "Episode 1", up to 40 s after the trigger time t_0, with a time varying thermal emission and a total energy of E_{iso,1st} = 2.60 x 10^{53} erg, is interpreted as due to the onset of an hyper-critical accretion process onto a companion neutron star, triggered by the companion star, an FeCO core approaching a SN explosion. The "Episode 2", observed up t_0+90 s, is interpreted as a canonical gamma ray burst, with an energy of E^{e^+e^-}_{tot} = 1.60 x 10^{53} erg, a baryon load of B = 7 x 10^{-3} and a bulk Lorentz factor at transparency of Γ= 142.5. From this Episode 2, we infer that the GRB exploded in an environment with a large average particle density \approx 10^3 particles/cm^3 and dense clouds characterized by typical dimensions of (4 ÷8) x 10^{14} cm and δn / n ~ 10. The "Episode 3" is identified from t_0+90 s all the way up to 10^{5-6} s: despite the paucity of the early X-ray data, typical in the BATSE, pre-Swift era, we find extremely significant data points in the late X-ray afterglow emission of GRB 970828, which corresponds to the ones observed in all BdHNe sources. The "Episode 4", related to the Supernova emission, does not appear to be observable in this source, due to the presence of darkening from the large density of the GRB environment, also inferred from the analysis of the Episode 2.

astro-ph.HE↗

arXiv2014

Estimation of the release time of solar energetic particles near the Sun

This paper investigates the onset time of Solar Energetic Particle (SEP) events with numerical simulations, and analyses the accuracy of the Velocity Dispersion Analysis (VDA) method. Using a 3-dimensional focused transport model, we calculate the fluxes of protons observed in the ecliptic at 1 AU in the energy range between 10 MeV and 80 MeV. In particular, three models are used to describe different SEP sources produced by flare or coronal shock, and the effects of particle perpendicular diffusion in the interplanetary space are also studied. We have the following findings: When the observer is disconnected from the source, the effects of perpendicular diffusion in the interplanetary space and particles propagating in the solar atmosphere have significant influence on the VDA results. As a result, although the VDA method is valid with impulsive source duration, low background, and weak scattering in the interplanetary space or fast diffusion in the solar atmosphere, the method is not valid with gradual source duration, high background, or strong scattering.

astro-ph.SR↗

arXiv2014

A detailed X-ray investigation of PSR J2021+4026 and $γ$-Cygni supernova remnant

We have investigated the field around the radio-quiet $γ$-ray pulsar, PSR J2021+4026, with a ~140 ks XMM-Newton observation and a ~56 ks archival Chandra data. Through analyzing the pulsed spectrum, we show that the X-ray pulsation is purely thermal in nature which suggests the pulsation is originated from a hot polar cap with $T\sim3\times10^{6}$ K on the surface of a rotating neutron star. On the other hand, the power-law component that dominates the pulsar emission in the hard band is originated from off-pulse phases, which possibly comes from a pulsar wind nebula. In re-analyzing the Chandra data, we have confirmed the presence of bow-shock nebula which extends from the pulsar to west by ~10 arcsec. The orientation of this nebular feature suggests that the pulsar is probably moving eastward which is consistent with the speculated proper motion by extrapolating from the nominal geometrical center of the supernova remnant (SNR) G78.2+2.1 to the current pulsar position. For G78.2+2.1, our deep XMM-Newton observation also enables a study of the central region and part of the southeastern region with superior photon statistics. The column absorption derived for the SNR is comparable with that for PSR J2021+4026, which supports their association. The remnant emission in both examined regions are in an non-equilibrium ionization state. Also, the elapsed time of both regions after shock-heating is apparently shorter than the Sedov age of G78.2+2.1. This might suggest the reverse shock has reached the center not long ago. Apart from PSR J2021+4026 and G78.2+2.1, we have also serendipitously detected an X-ray flash-like event XMM J202154.7+402855 from this XMM-Newton observation.

astro-ph.HE↗

arXiv2014

Sub-micron period lattice structures of magnetic microtraps for ultracold atoms on an atom chip

We report on the design, fabrication and characterization of magnetic nanostructures to create a lattice of magnetic traps with sub--micron period for trapping ultracold atoms. These magnetic nanostructures were fabricated by patterning a Co/Pd multilayered magnetic film grown on a silicon substrate using high precision e-beam lithography and reactive ion etching. The Co/Pd film was chosen for its small grain size and high remanent magnetization and coercivity. The fabricated structures are designed to magnetically trap $^{87}$Rb atoms above the surface of the magnetic film with 1D and 2D (triangular and square) lattice geometries and sub-micron period. Such magnetic lattices can be used for quantum tunneling and quantum simulation experiments, including using geometries and periods that may be inaccessible with optical lattice.

cond-mat.quant-gas↗

arXiv2014

GRB 130427A and SN 2013cq: A Multi-wavelength Analysis of An Induced Gravitational Collapse Event

We have performed our data analysis of the observations by Swift and Fermi satellites in order to probe the induced gravitational collapse (IGC) paradigm for GRBs associated with supernovae (SNe), in the "terra incognita" of GRB 130427A. We compare and contrast our data analysis with those in the literature. We have verified that the GRB 130427A conforms to the IGC paradigm by examining the power law behavior of the luminosity in the early $10^4$ s of the Swift-XRT observations. This has led to the successful prediction of the occurrence of SN 2013cq and to the identification of the four different episodes of the "binary driven hypernovae" (BdHNe). The exceptional quality of the data has allowed the identification of novel features in Episode 3 including: a) the confirmation and the extension of the existence of the recently discovered "nested structure" in the late X-ray luminosity in GRB 130427A, as well as the identification of a spiky structure at $10^2$ s in the cosmological rest-frame of the source; b) a power law emission of the GeV luminosity light curve and its onset at the end of Episode 2 c) different Lorentz $Γ$ factors for the emitting regions of the X-ray and GeV emissions in this Episode 3. These results make it possible to test the details of the physical and astrophysical regimes at work in the BdHNe: 1) a newly born neutron star and the supernova ejecta, originating in Episode 1, 2) a newly formed black hole originating in Episode 2, and 3) the possible interaction among these components, observable in the standard features of Episode 3.

astro-ph.HE↗

arXiv2014

Measurement of double-polarization asymmetries in the quasi-elastic $^3\vec{\mathrm{He}}(\vec{\mathrm{e}},\mathrm{e}'\mathrm{d})$ process

We present a precise measurement of double-polarization asymmetries in the $^3\vec{\mathrm{He}}(\vec{\mathrm{e}},\mathrm{e}'\mathrm{d})$ reaction. This particular process is a uniquely sensitive probe of hadron dynamics in $^3\mathrm{He}$ and the structure of the underlying electromagnetic currents. The measurements have been performed in and around quasi-elastic kinematics at $Q^2 = 0.25\,(\mathrm{GeV}/c)^2$ for missing momenta up to $270\,\mathrm{MeV}/c$. The asymmetries are in fair agreement with the state-of-the-art calculations in terms of their functional dependencies on $p_\mathrm{m}$ and $ω$, but are systematically offset. Beyond the region of the quasi-elastic peak, the discrepancies become even more pronounced. Thus, our measurements have been able to reveal deficiencies in the most sophisticated calculations of the three-body nuclear system, and indicate that further refinement in the treatment of their two- and/or three-body dynamics is required.

nucl-ex↗

arXiv2014

Antarctic Radio Frequency Albedo and Implications for Cosmic Ray Reconstruction

From an elevation of ~38 km, the balloon-borne ANtarctic Impulsive Transient Antenna (ANITA) is designed to detect the up-coming radio frequency (RF) signal resulting from a sub-surface neutrino-nucleon collision. Although no neutrinos have been discovered thus far, ANITA is nevertheless the only experiment to self-trigger on radio frequency emissions from cosmic-ray induced atmospheric air showers. In the majority of those cases, down-coming RF signals are observed via their reflection from the Antarctic ice sheet and back up to the ANITA interferometer. Estimating the energy scale of the incident cosmic rays therefore requires an estimate of the fractional power reflected at the air-ice interface. Similarly, inferring the energy of neutrinos interacting in-ice from observations of the upwards-directed signal refracting out to ANITA also requires consideration of signal coherence across the interface. By comparing the direct Solar RF signal intensity measured with ANITA to the surface-reflected Solar signal intensity, as a function of incident elevation angle relative to the surface Θ, we estimate the power reflection coefficients R(Θ). We find general consistency between our average measurements and the values of R(Θ) expected from the Fresnel equations, separately for horizontal- vs. vertical-polarizations.

astro-ph.IM↗