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Xiao Zhang

Publications and source records attributed to Xiao Zhang.

At least 559 records · Page 31Linked to original sources
arXiv2015

Two dimensional photonic quasicrystal edge states protected by second Chern number

Topological physics in photonic systems have attracted great attentions in recent years. In this work, we theoretically study the one and two dimensional photonic quasicrystal resonator lattices characterized by the first and second Chern number, which show exotic boundary states within the photonic energy band gap. In particular, the second Chern number protected edge states has opened up new possibilities for realizing topological physics of dimensions higher than three in photonic systems, which is highly sought for. Such photonic systems can be easily experimentally realized in regular photonic crystal with dielectric rods in air, by varying the radius of the rods, so we propose experiments realizing our predictions.

cond-mat.mes-hall↗
arXiv2015

Supernova Remnant Kesteven 27: Interaction with A Neighbor HI Cloud Viewed by Fermi

We report on the likely detection of gamma-ray emission from the supernova remnant (SNR) Kesteven 27 (Kes 27). We analyze 5.7 yr Fermi Large Area Telescope data of the SNR region and find an unresolved source at a position consistent with the radio brightness peak and the X-ray knot of Kes 27, which is located in the eastern region of the SNR and caused by the interaction with a nearby HI cloud. The source's emission is best fit with a power-law spectrum with a photon index of 2.5 +/- 0.1 and a >0.2 GeV luminosity of 5.8*10^34 erg s^-1 assuming a distance of 4.3 kpc, as derived from radio observations of the nearby HI cloud. Comparing the properties of the source with that of other SNRs that are known to be interacting with nearby high-density clouds, we discuss the origin of the source's emission. The spectral energy distribution of the source can be described by a hadronic model that considers the interaction of energetic protons, escaping from the shock front of Kes 27, with a high-density cloud.

astro-ph.HE↗
arXiv2015

Optimization of Switch Keyboards

Patients with motor control difficulties often "type" on a computer using a switch keyboard to guide a scanning cursor to text elements. We show how to optimize some parts of the design of switch keyboards by casting the design problem as mixed integer programming. A new algorithm to find an optimized design solution is approximately 3600 times faster than a previous algorithm, which was also susceptible to finding a non-optimal solution. The optimization requires a model of the probability of an entry error, and we show how to build such a model from experimental data. Example optimized keyboards are demonstrated.

cs.HC↗
arXiv2015

The positive energy theorem for asymptotically anti-de Sitter spacetimes

We establish the inequality for Henneaux-Teitelboim's total energy-momentum for asymptotically anti-de Sitter initial data sets which are asymptotic to arbitrary $t$-slice in anti-de Sitter spacetime. In particular, when $t=0$, it generalizes Chruściel-Maerten-Tod's inequality in the center of AdS mass coordinates. We also show that the determinant of energy-momentum endomorphism ${\bf Q}$ is the geometric invariant of asymptotically anti-de Sitter spacetimes.

gr-qc↗
arXiv2015

Localization and centrality in networks

Eigenvector centrality is a common measure of the importance of nodes in a network. Here we show that under common conditions the eigenvector centrality displays a localization transition that causes most of the weight of the centrality to concentrate on a small number of nodes in the network. In this regime the measure is no longer useful for distinguishing among the remaining nodes and its efficacy as a network metric is impaired. As a remedy, we propose an alternative centrality measure based on the nonbacktracking matrix, which gives results closely similar to the standard eigenvector centrality in dense networks where the latter is well behaved, but avoids localization and gives useful results in regimes where the standard centrality fails.

cs.SI↗
arXiv2014

Investigations on the charmless decays of $Y(4260)$

Apart from the charmful decay channels of $Y(4260)$, the charmless decay channels of $Y(4260)$ also provide us a good platform to study the nature and the decay mechanism of $Y(4260)$. In this paper, we propose to probe the structure of $Y(4260)$ through the charmless decays $Y(4260)\to VP$ via intermediate $D_1\bar D+c.c.$ meson loops, where $V$ and $P$ stand for light vector and pseudoscalar mesons, respectively. Under the molecule ansatz of $Y(4260)$, the predicted total branching ratio $BR_{VP}$ for all $Y(4260)\to VP$ processes are about $(0.34^{+0.32}_{-0.23})%$ to $(0.75^{+0.72}_{-0.52})%$ with the cutoff parameter $α=2\sim 3$. Numerical results show that the intermediate $D_1 \bar D+c.c.$ meson loops may be a possible transition mechanism in the $Y(4260)\to VP$ decays. These predicted branching ratios are the same order to that of $Y(4260) \to Z_c^+(3900) π^-$, which may be an evidence of $D_1D$ molecule and can be examined by the forthcoming BESIII data in the near future.

hep-ph↗
arXiv2014

Identification of core-periphery structure in networks

Many networks can be usefully decomposed into a dense core plus an outlying, loosely-connected periphery. Here we propose an algorithm for performing such a decomposition on empirical network data using methods of statistical inference. Our method fits a generative model of core-periphery structure to observed data using a combination of an expectation--maximization algorithm for calculating the parameters of the model and a belief propagation algorithm for calculating the decomposition itself. We find the method to be efficient, scaling easily to networks with a million or more nodes and we test it on a range of networks, including real-world examples as well as computer-generated benchmarks, for which it successfully identifies known core-periphery structure with low error rate. We also demonstrate that the method is immune from the detectability transition observed in the related community detection problem, which prevents the detection of community structure when that structure is too weak. There is no such transition for core-periphery structure, which is detectable, albeit with some statistical error, no matter how weak it is.

cs.SI↗
arXiv2014

An XMM-Newton study of the mixed-morphology supernova remnant W28 (G6.4-0.1)

We have performed an XMM-Newton imaging and spectroscopic study of supernova remnant (SNR) W28, a prototype mixed-morphology or thermal composite SNR, believed to be interacting with a molecular cloud. The observed hot X-ray emitting plasma is characterized by low metal abundances, showing no evidence of ejecta. The X-rays arising from the deformed northeast shell consist of a thermal component with a temperature of $\sim0.3$ keV plus a hard component of either thermal (temperature $\sim 0.6$ keV) or non-thermal (photon index $=0.9$-2.4) origin. The X-ray emission in the SNR interior is blobby and the corresponding spectra are best described as the emission from a cold ($kT\sim0.4$ keV) plasma in non-equilibrium ionization with an ionization timescale of $\sim4.3\times 10^{11}$ cm$^{-3}$ s plus a hot ($kT \sim 0.8$ keV) gas in collisional ionization equilibrium. Applying the two-temperature model to the smaller central regions, we find non-uniform interstellar absorption, temperature and density distribution, which indicates that the remnant is evolving in a non-uniform environment with denser material in the east and north. The cloudlet evaporation mechanism can essentially explain the properties of the X-ray emission in the center and thermal conduction may also play a role for length scales comparable to the remnant radius. A recombining plasma model with an electron temperature of $\sim 0.6$ keV is also feasible for describing the hot central gas with the recombination age of the gas estimated at $\sim2.9\times 10^4$ yr.

astro-ph.HE↗
arXiv2014

Fermi Large Area Telescope observations of the supernova remnant HESS J1731-347

Context: HESS J1731-347 has been identified as one of the few TeV-bright shell-type supernova remnants (SNRs). These remnants are dominated by nonthermal emission, and the nature of TeV emission has been continuously debated for nearly a decade. Aims: We carry out the detailed modeling of the radio to gamma-ray spectrum of HESS J1731-347 to constrain the magnetic field and energetic particles sources, which we compare with those of the other TeV-bright shell-type SNRs explored before. Methods: Four years of data from Fermi Large Area Telescope (LAT) observations for regions around this remnant are analyzed, leading to no detection correlated with the source discovered in the TeV band. The Markov Chain Monte Carlo method is used to constrain parameters of one-zone models for the overall emission spectrum. Results: Based on the 99.9% upper limits of fluxes in the GeV range, one-zone hadronic models with an energetic proton spectral slope greater than 1.8 can be ruled out, which favors a leptonic origin for the gamma-ray emission, making this remnant a sibling of the brightest TeV SNR RX J1713.7-3946, the Vela Junior SNR RX J0852.0-4622, and RCW 86. The best-fit leptonic model has an electron spectral slope of 1.8 and a magnetic field of about 30 muG, which is at least a factor of 2 higher than those of RX J1713.7-3946 and RX J0852.0-4622, posing a challenge to the distance estimate and/or the energy equipartition between energetic electrons and the magnetic field of this source. A measurement of the shock speed will address this challenge and has implications on the magnetic field evolution and electron acceleration driven by shocks of SNRs.

astro-ph.HE↗
arXiv2014

Mode-Locked Chip Laser using Waveguide Arrays

We demonstrate theoretically that robust mode-locking can be achieved on a semiconductor chip with a waveguide array architecture. The waveguide arrays are used as an ideal saturable absorption mechanism for initial noise start-up as well as pulse shaping and stabilization. The cavity gain is provided by an injection current and forward biasing of the semiconductor material. The technology can be integrated directly with semiconductor architectures and technologies, thus allowing for the potential of an on-chip, broadband device.

physics.optics↗
arXiv2013

How to enhance the dynamic range of excitatory-inhibitory excitable networks

We investigate the collective dynamics of excitatory-inhibitory excitable networks in response to external stimuli. How to enhance dynamic range, which represents the ability of networks to encode external stimuli, is crucial to many applications. We regard the system as a two-layer network (E-Layer and I-Layer) and explore the criticality and dynamic range on diverse networks. Interestingly, we find that phase transition occurs when the dominant eigenvalue of E-layer's weighted adjacency matrix is exactly one, which is only determined by the topology of E-Layer. Meanwhile, it is shown that dynamic range is maximized at critical state. Based on theoretical analysis, we propose an inhibitory factor for each excitatory node. We suggest that if nodes with high inhibitory factors are cut out from I-Layer, dynamic range could be further enhanced. However, because of the sparseness of networks and passive function of inhibitory nodes, the improvement is relatively small compared tooriginal dynamic range. Even so, this provides a strategy to enhance dynamic range.

q-bio.NC↗
arXiv2013

The likely Fermi Detection of the Supernova Remnant RCW 103

We report on the results from our $γ$-ray analysis of the supernova remnant (SNR) RCW 103 region. The data were taken with the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. An extended source is found at a position consistent with that of RCW 103, and its emission was only detected above 1 GeV (10$σ$ significance), having a power-law spectrum with a photon index of 2.0$\pm$0.1. We obtain its 1--300 GeV spectrum, and the total flux gives a luminosity of 8.3$\times 10^{33}$ erg s$^{-1}$ at a source distance of 3.3 kpc. Given the positional coincidence and property similarities of this source with other SNRs, we identify it as the likely Fermi $γ$-ray counterpart to RCW 103. Including radio measurements of RCW 103, the spectral energy distribution (SED) is modeled by considering emission mechanisms based on both hadronic and leptonic scenarios. We find that models in the two scenarios can reproduce the observed SED, while in the hadronic scenario the existence of SNR--molecular-cloud interaction is suggested as a high density of the target protons is required.

astro-ph.HE↗
arXiv2013

Spectra of random graphs with community structure and arbitrary degrees

Using methods from random matrix theory researchers have recently calculated the full spectra of random networks with arbitrary degrees and with community structure. Both reveal interesting spectral features, including deviations from the Wigner semicircle distribution and phase transitions in the spectra of community structured networks. In this paper we generalize both calculations, giving a prescription for calculating the spectrum of a network with both community structure and an arbitrary degree distribution. In general the spectrum has two parts, a continuous spectral band, which can depart strongly from the classic semicircle form, and a set of outlying eigenvalues that indicate the presence of communities.

cs.SI↗
arXiv2013

Molecular Environments of SNRs

There are about 70 Galactic supernova remnants (SNRs) that are now confirmed or suggested to be in physical contact with molecular clouds (MCs) with six kinds of evidence of multiwavelength observations. Recent detailed CO-line spectroscopic mappings of a series of SNRs reveal them to be in cavities of molecular gas, implying the roles the progenitors may have played. We predict a linear correlation between the wind bubble sizes of main-sequence OB stars in a molecular environment and the stellar masses and discuss its implication for supernova progenitors. The molecular environments of SNRs can serve as a good probe for the gamma-rays arising from the hadronic interaction of the accelerated protons, and this paper also discusses the gamma-ray emission from MCs illuminated by diffusive protons that escape from SNR shocks.

astro-ph.GA↗
arXiv2012

On the Hadronic Gamma-ray Emission from Tycho's Supernova Remnant

Hadronic gamma-ray emission from supernova remnants (SNRs) is an important tool to test shock acceleration of cosmic ray protons. Tycho is one of nearly a dozen Galactic SNRs which are suggested to emit hadronic gamma-ray emission. Among them, however, it is the only one in which the hadronic emission is proposed to arise from the interaction with low-density (~0.3 cm^{-3}) ambient medium. Here we present an alternative hadronic explanation with a modest conversion efficiency (of order 1%) for this young remnant. With such an efficiency, a normal electron-proton ratio (of order 10^{-2}) is derived from the radio and X-ray synchrotron spectra and an average ambient density that is at least one-order-of-magnitude higher is derived from the hadronic gamma-ray flux. This result is consistent with the multi-band evidence of the presence of dense medium from the north to the east of the Tycho SNR. The SNR-cloud association, in combination with the HI absorption data, helps to constrain the so-far controversial distance to Tycho and leads to an estimate of 2.5 kpc.

astro-ph.HE↗
arXiv2012

Superconducting state in the metastable binary bismuthide Rh3Bi14 single crystals

We report detailed magnetic, transport and thermodynamic properties of metastable Rh3Bi14 single crystals in superconducting and normal state. We show that Rh3Bi14 is nearly isotropic, weak to intermediately coupled BCS superconductor, whereas the electronic resistivity above superconducting Tc = 2.94 K is dominated by the phonon scattering in the large unit cell with pores filled by Bi atoms. Superconductivity is strongly influenced by the nature of atoms that fill the voids in the crystal structure.

cond-mat.supr-con↗
arXiv2012

Complex Orthogonal Matching Pursuit and Its Exact Recovery Conditions

In this paper, we present new results on using orthogonal matching pursuit (OMP), to solve the sparse approximation problem over redundant dictionaries for complex cases (i.e., complex measurement vector, complex dictionary and complex additive white Gaussian noise (CAWGN)). A sufficient condition that OMP can recover the optimal representation of an exactly sparse signal in the complex cases is proposed both in noiseless and bound Gaussian noise settings. Similar to exact recovery condition (ERC) results in real cases, we extend them to complex case and derivate the corresponding ERC in the paper. It leverages this theory to show that OMP succeed for k-sparse signal from a class of complex dictionary. Besides, an application with geometrical theory of diffraction (GTD) model is presented for complex cases. Finally, simulation experiments illustrate the validity of the theoretical analysis.

cs.IT↗