arXiv subjects

Hui Zhang

Publications and source records attributed to Hui Zhang.

At least 505 records · Page 28Linked to original sources

arXiv2016

Global Instability of Exo-Moon System Triggered by Photo-Evaporation

Many exoplanets have been found in orbits close to their host stars and thus they are subject to the effects of photo-evaporation. Previous studies have shown that a large portion of exoplanets detected by the Kepler mission have been significantly eroded by photo-evaporation. In this paper, we numerically study the effects of photo-evaporation on the orbital evolution of a hypothesized moon system around a planet. We find that photo-evaporation is crucial to the stability of the moon system. Photo-evaporation can erode the atmosphere of the planet thus leading to significant mass loss. As the planet loses mass, its Hill radius shrinks and its moons increase their orbital semi-major axes and eccentricities. When some moons approach their critical semi-major axes, global instability of the moon system would be triggered, which usually ends up with two, one or even zero surviving moons. Some lost moons could escape from the moon system to become a new planet orbiting the star or run away further to become a free-floating object in the Galaxy. Given the destructive role of photo-evaporation, we speculate that exo-moons are less common for close-in planets ($<0.1$ AU), especially those around M-type stars, because they are more X-ray luminous and thus enhancing photo-evaporation. The lessons we learn in this study may be helpful for the target selection of on-going/future exomoon searching programs.

astro-ph.EP↗

arXiv2016

Stellar Flares in the CSTAR Field: Results from the 2008 Data Set

The Chinese Small Telescope ARray (CSTAR) is the first Chinese astronomical instrument placed in Antarctica. It is a group of four identical, fully automatic $14.5\,\rm{cm}$ telescopes, with an field of view (FOV) of $20\,\rm{deg^2}$ centered on the South Celestial Pole. Placed at Antarctic Dome A, CSTAR is designed to provide high-cadence photometry for site monitoring and variable sources detection. During the 2008 observing season, CSTAR has taken high-precision photometric data for 18,145 stars around the South Celestial Pole. At $i\,=\,7.5$ and $12$, the photometric precision reaches $\sim 8$ mmag and $\sim 30$ mmag with a cadence of 20s or 30s, respectively. Using robust detection method, we have found 15 stellar flares on 13 sources, including two classified variables. We have also found a linear relation between the decay times and the total durations of the stellar flares. The details of all detected flares along with their stellar properties are presented in this work.

astro-ph.SR↗

arXiv2016

Scientific Goals of the Kunlun Infrared Sky Survey (KISS)

The high Antarctic plateau provides exceptional conditions for conducting infrared observations of the cosmos on account of the cold, dry and stable atmosphere above the ice surface. This paper describes the scientific goals behind the first program to examine the time-varying universe in the infrared from Antarctica - the Kunlun Infrared Sky Survey (KISS). This will employ a small (50 cm aperture) telescope to monitor the southern skies in the 2.4um Kdark window from China's Kunlun station at Dome A, on the summit of the Antarctic plateau, through the uninterrupted 4-month period of winter darkness. An earlier paper discussed optimisation of the Kdark filter for the best sensitivity (Li et al 2016). This paper examines the scientific program for KISS. We calculate the sensitivity of the camera for the extrema of observing conditions that will be encountered. We present the parameters for sample surveys that could then be carried out for a range of cadences and sensitivities. We then discuss several science programs that could be conducted with these capabilities, involving star formation, brown dwarfs and hot Jupiters, exoplanets around M dwarfs, the terminal phases of stellar evolution, discovering fast transients as part of multi-wavelength campaigns, embedded supernova searches, reverberation mapping of active galactic nuclei, gamma ray bursts and the detection of the cosmic infrared background. Accepted for publication in PASA, 04/08/16.

astro-ph.IM↗

arXiv2016

Characterization study of a broad-energy germanium detector at CJPL

The ability of background discrimination using pulse shape discrimination (PSD) in broad-energy germanium (BEGe) detectors makes them as competitive candidates for neutrinoless double beta decay (0ν\b{eta}\b{eta}) experiments. The measurements of key parameters for detector modeling in a commercial p-type BEGe detector are presented in this paper. Point-like sources were used to investigate the energy resolution and linearity of the detector. A cylindrical volume source was used for the efficiency calibration. With an assembled device for source positioning, a collimated 133Ba point-like source was used to scan the detector and investigate the active volume. A point-like source of 241Am was used to measure the dead layer thicknesses, which are approximately 0.17 mm on the front and 1.18 mm on the side. The described characterization method will play an important role in the 0ν\b{eta}\b{eta} experiments with BEGe detectors at China JinPing underground Laboratory (CJPL) in the future.

physics.ins-det↗

arXiv2016

The restricted strong convexity revisited: Analysis of equivalence to error bound and quadratic growth

The restricted strong convexity is an effective tool for deriving globally linear convergence rates of descent methods in convex minimization. Recently, the global error bound and quadratic growth properties appeared as new competitors. In this paper, with the help of Ekeland's variational principle, we show the equivalence between these three notions. To deal with convex minimization over a closed convex set and structured convex optimization, we propose a group of modified versions and a group of extended versions of these three notions by using gradient mapping and proximal gradient mapping separately, and prove that the equivalence for the modified and extended versions still holds. Based on these equivalence notions, we establish new asymptotically linear convergence results for the proximal gradient method. Finally, we revisit the problem of minimizing the composition of an affine mapping with a strongly convex differentiable function over a polyhedral set, and obtain a strengthened property of the restricted strong convex type under mild assumptions.

math.OC↗

arXiv2016

The granularity effect in amorphous InGaZnO$_4$ films prepared by rf sputtering method

We systematically investigated the temperature behaviors of the electrical conductivity and Hall coefficient of two series of amorphous indium gallium zinc oxides (a-IGZO) films prepared by rf sputtering method. The two series of films are $\sim$700\,nm and $\sim$25\,nm thick, respectively. For each film, the conductivity increases with decreasing temperature from 300\,K to $T_{\rm max}$, where $T_{\rm max}$ is the temperature at which the conductivity reaches its maximum. Below $T_{\rm max}$, the conductivity decreases with decreasing temperature. Both the conductivity and Hall coefficient vary linearly with $\ln T$ at low temperature regime. The $\ln T$ behaviors of conductivity and Hall coefficient cannot be explained by the traditional electron-electron interaction theory, but can be quantitatively described by the current electron-electron theory due to the presence of granularity. Combining with the scanning electron microscopy images of the films, we propose that the boundaries between the neighboring a-IGZO particles could make the film inhomogeneous and play an important role in the electron transport processes.

cond-mat.mes-hall↗

arXiv2016

Growth and characterization of CaFe$_{1-x}$Co$_x$AsF single crystals by CaAs flux method

Millimeter sized single crystals of CaFe$_{1-x}$Co$_x$AsF were grown using a self-flux method. It is found that high-quality single crystals can be grown from three approaches with different initial raw materials. The chemical compositions and crystal structure were characterized carefully. The c-axis lattice constant is suppressed by the Co substitution. Superconductivity with the critical transition $T_c$ as high as 21 K was confirmed by both the resistivity and magnetic susceptibility measurements in the sample with $x$ = 0.12. Moreover, it is found that $T_c$ can be enhanced for about 1 K under the very small hydrostatic pressure of 0.22 GPa, which is more quickly than that reported in the polycrystalline samples. Our results is a promotion for the physical investigations of 1111 phase iron-pnictide superconductors.

cond-mat.supr-con↗

arXiv2016

On robust width property for Lasso and Dantzig selector

Recently, Cahill and Mixon completely characterized the sensing operators in many compressed sensing instances with a robust width property. The proposed property allows uniformly stable and robust reconstruction of certain solutions from an underdetermined linear system via convex optimization. However, their theory does not cover the Lasso and Dantzig selector models, both of which are popular alternatives in the statistics community. In this letter, we show that the robust width property can be perfectly applied to these two models as well. Our results solve an open problem left by Cahill and Mixon.

cs.IT↗

arXiv2016

Bounds for Batch Codes with Restricted Query Size

We present new upper bounds on the parameters of batch codes with restricted query size. These bounds are an improvement on the Singleton bound. The techniques for derivations of these bounds are based on the ideas in the literature for codes with locality. By employing additional ideas, we obtain further improvements, which are specific for batch codes.

cs.IT↗

arXiv2016

Evaluating consistency of deterministic streamline tractography in non-linearly warped DTI data

Tractography is typically performed for each subject using the diffusion tensor imaging (DTI) data in its native subject space rather than in some space common to the entire study cohort. Despite performing tractography on a population average in a normalized space, the latter is considered less favorably at the \emph{individual} subject level because it requires spatial transformations of DTI data that involve non-linear warping and reorientation of the tensors. Although the commonly used reorientation strategies such as finite strain and preservation of principle direction are expected to result in adequate accuracy for voxel based analyses of DTI measures such as fractional anisotropy (FA), mean diffusivity (MD), the reorientations are not always exact except in the case of rigid transformations. Small imperfections in reorientation at individual voxel level accumulate and could potentially affect the tractography results adversely. This study aims to evaluate and compare deterministic white matter fiber tracking in non-linearly warped DTI against that in native DTI. The data present promising evidence that tractography in non-linear warped DTI data could indeed be a viable and valid option for various statistical analysis of DTI data in a spatially normalized space.

physics.med-ph↗

arXiv2016

MeV Argon ion beam generation with narrow energy spread

Laser driven particle acceleration has shown remarkable progresses in generating multi-GeV electron bunches and 10s of MeV ion beams based on high-power laser facilities. Intense laser pulse offers the acceleration field of 1012 Volt per meter, several orders of magnitude larger than that in conventional accelerators, enabling compact devices. Here we report that a highly-collimated argon ion beam with narrow energy spread is produced by irradiating a 45-fs fully-relativistic laser pulse onto an argon cluster target. The highly-charged (Argon ion with charge state of 16+) heavy ion beam has a minimum absolute energy spread of 0.19 MeV per nucleon at the energy peak of 0.39 MeV per nucleon. we identify a novel scheme from particle-in-cell simulations that greatly reduces the beam energy spread. The laser-driven intense plasma wakefield has a strong modulation on the ion beam in a way that the low energy part is cut off. The pre-accelerated argon ion beam from Coulomb explosion thus becomes more mono-energetic and collimated.

physics.plasm-ph↗

arXiv2016

The effects of electron correlation and spin-orbit coupling in the isovalent Pd-doped superconductor SrPt$_3$P

We present a systematical study on the roles of electron correlation and spin-orbit coupling in the isovalent Pd-doped superconductor SrPt$_3$P. By using solid state reaction method, we fabricated the strong spin-orbit coupling superconductors Sr(Pt$_{1-x}$Pd$_x$)$_3$P with strong electron correlated Pd dopant of the $4d$ orbital. As increasing the isovalent Pd concentrations without introducing any extra electron/hole carriers, the superconducting transition temperature $T_c$ decreases monotonously, which suggests the existence of competition between spin-orbit coupling and electron correlation in the superconducting state. In addition, the electronic band structure calculations demonstrate that the strength of electron susceptibility is suppressed gradually by the Pd dopant suggesting the incompatible relation between spin-orbit coupling and electron correlation, which is also consistent with experimental measurements. Our results provide significant insights in the natures of the interplay between the spin-orbit coupling and the electron correlation in superconductivity, and may pave a way for understanding the mechanism of superconductivity in this 5d-metal-based compound.

cond-mat.supr-con↗

arXiv2015

Convergence of a Fast Explicit Operator Splitting Method for the Molecular Beam Epitaxy Model

A fast explicit operator splitting (FEOS) method for the molecular beam epitaxy model has been presented in [Cheng, et al., Fast and stable explicit operator splitting methods for phase-field models, J. Comput. Phys., submitted]. The original problem is split into linear and nonlinear subproblems. For the linear part, the pseudo-spectral method is adopted; for the nonlinear part, a 33-point difference scheme is constructed. Here, we give a compact center-difference scheme involving fewer points for the nonlinear subproblem. Besides, we analyze the convergence rate of the algorithm. The global error order $\mathcal{O}(τ^2+h^4)$ in discrete $L^2$-norm is proved theoretically and verified numerically. Some numerical experiments show the robustness of the algorithm for small coefficients of the fourth-order term for the one-dimensional case. Besides, coarsening dynamics are simulated in large domains and the $1/3$ power laws are observed for the two-dimensional case.

math.NA↗

arXiv2015

An unconditionally energy stable finite difference scheme for a stochastic Cahn-Hilliard equation

In this work, the MMC-TDGL equation, a stochastic Cahn-Hilliard equation is solved numerically by using the finite difference method in combination with a convex splitting technique of the energy functional. For the non-stochastic case, we develop an unconditionally energy stable difference scheme which is proved to be uniquely solvable. For the stochastic case, by adopting the same splitting of the energy functional, we construct a similar and uniquely solvable difference scheme with the discretized stochastic term. The resulted schemes are nonlinear and solved by Newton iteration. For the long time simulation, an adaptive time stepping strategy is developed based on both first- and second-order derivatives of the energy. Numerical experiments are carried out to verify the energy stability, the efficiency of the adaptive time stepping and the effect of the stochastic term.

math.NA↗

arXiv2015

Mean-field approximation for the chiral soliton in a chiral phase transition

In the mean-field approximation we study the chiral soliton within the linear sigma model in a thermal vacuum. The chiral soliton equations with different boundary conditions are solved at finite temperatures and densities. The solitons are discussed before and after the chiral restoration. We find that the system has soliton solutions even after the chiral restoration and they are very different from those before the chiral restoration, which indicates that the quarks are still bounded after the chiral restoration.

hep-ph↗

arXiv2015

Growth and Characterization of Millimeter-sized Single Crystals of CaFeAsF

High-quality and sizable single crystals are crucial for studying the intrinsic properties of unconventional superconductors, which are lacking in the 1111 phase of the Fe-based superconductors. Here we report the successful growth of CaFeAsF single crystals with the sizes of 1-2 mm using the self-flux method. Owning to the availability of the high-quality single crystals, the structure and transport properties were investigated with a high reliability. The structure was refined by using the single-crystal x-ray diffraction data, which confirms the reports earlier on the basis of powder data. A clear anomaly associated with the structural transition was observed at 121 K from the resistivity, magnetoresistance, and magnetic susceptibility measurements. Another kink-feature at 110 K, most likely an indication of the antiferromagnetic transition, was also detected in the resistivity data. Our results supply a basis to propel the physical investigations on the 1111 phase of the Fe-based superconductors.

cond-mat.supr-con↗

arXiv2015

Discovery of the correlation between peak episodic jet power and X-ray peak luminosity of the soft state in black hole transients

Episodic jets are usually observed in the intermediate state of black hole transients during their X-ray outbursts. Here we report the discovery of a strong positive correlation between the peak radio power of the episodic jet $P_{\rm jet}$ and the corresponding peak X-ray luminosity $L_{\rm x}$ of the soft state (in Eddington units) in a complete sample of the outbursts of black hole transients observed during the RXTE era of which data are available, which follows the relation $\log P_{\rm jet}=(2.2\pm{0.3})+(1.6\pm0.2)\times \log {L_{\rm x}}$. The transient ultra-luminous X-ray source in M31 and HLX-1 in EXO 243-49 fall on the relation if they contain stellar mass black hole and either stellar mass black hole or intermediate mass black hole, respectively. Besides, a significant correlation between the peak power of the episodic jet and the rate-of-increase of the X-ray luminosity $\rm dL_{x}/dt$ during the rising phase of those outbursts is also found, following $\log P_{\rm jet}=(2.0\pm{0.4})+(0.7\pm0.2)\times \log {\rm d}L_{\rm x}/{\rm d}t$. In GX 339$-$4 and H 1743$-$322 in which data for two outbursts are available, measurements of the peak radio power of the episodic jet and the X-ray peak luminosity (and its rate-of-change) shows similar positive correlations between outbursts, which demonstrate the dominant role of accretion over black hole spin in generating episodic jet power. On the other hand, no significant difference is seen among the systems with different measured black hole spin in current sample. This implies that the power of the episodic jet is strongly affected by non-stationary accretion instead of black hole spin characterized primarily by the rate-of-change of the mass accretion rate.

astro-ph.HE↗

arXiv2015

Photometric Variability in the CSTAR Field: Results From the 2008 Data Set

The Chinese Small Telescope ARray (CSTAR) is the first telescope facility built at Dome A, Antarctica. During the 2008 observing season, the installation provided long-baseline and high-cadence photometric observations in the i-band for 18,145 targets within 20 deg2 CSTAR field around the South Celestial Pole for the purpose of monitoring the astronomical observing quality of Dome A and detecting various types of photometric variability. Using sensitive and robust detection methods, we discover 274 potential variables from this data set, 83 of which are new discoveries. We characterize most of them, providing the periods, amplitudes and classes of variability. The catalog of all these variables is presented along with the discussion of their statistical properties.

astro-ph.SR↗