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Cheng Li

Publications and source records attributed to Cheng Li.

At least 487 records · Page 27Linked to original sources

Residual Attention Network for Image Classification

In this work, we propose "Residual Attention Network", a convolutional neural network using attention mechanism which can incorporate with state-of-art feed forward network architecture in an end-to-end training fashion. Our Residual Attention Network is built by stacking Attention Modules which generate attention-aware features. The attention-aware features from different modules change adaptively as layers going deeper. Inside each Attention Module, bottom-up top-down feedforward structure is used to unfold the feedforward and feedback attention process into a single feedforward process. Importantly, we propose attention residual learning to train very deep Residual Attention Networks which can be easily scaled up to hundreds of layers. Extensive analyses are conducted on CIFAR-10 and CIFAR-100 datasets to verify the effectiveness of every module mentioned above. Our Residual Attention Network achieves state-of-the-art object recognition performance on three benchmark datasets including CIFAR-10 (3.90% error), CIFAR-100 (20.45% error) and ImageNet (4.8% single model and single crop, top-5 error). Note that, our method achieves 0.6% top-1 accuracy improvement with 46% trunk depth and 69% forward FLOPs comparing to ResNet-200. The experiment also demonstrates that our network is robust against noisy labels.

cs.CV↗

Budgeted Batch Bayesian Optimization With Unknown Batch Sizes

Parameter settings profoundly impact the performance of machine learning algorithms and laboratory experiments. The classical grid search or trial-error methods are exponentially expensive in large parameter spaces, and Bayesian optimization (BO) offers an elegant alternative for global optimization of black box functions. In situations where the black box function can be evaluated at multiple points simultaneously, batch Bayesian optimization is used. Current batch BO approaches are restrictive in that they fix the number of evaluations per batch, and this can be wasteful when the number of specified evaluations is larger than the number of real maxima in the underlying acquisition function. We present the Budgeted Batch Bayesian Optimization (B3O) for hyper-parameter tuning and experimental design - we identify the appropriate batch size for each iteration in an elegant way. To set the batch size flexible, we use the infinite Gaussian mixture model (IGMM) for automatically identifying the number of peaks in the underlying acquisition functions. We solve the intractability of estimating the IGMM directly from the acquisition function by formulating the batch generalized slice sampling to efficiently draw samples from the acquisition function. We perform extensive experiments for both synthetic functions and two real world applications - machine learning hyper-parameter tuning and experimental design for alloy hardening. We show empirically that the proposed B3O outperforms the existing fixed batch BO approaches in finding the optimum whilst requiring a fewer number of evaluations, thus saving cost and time.

cs.LG↗

The Chern Numbers of Interaction-stretched Monopoles in Spinor Bose Condensates

Using the Dirac and the Yang monopole in spinor condensates as examples, we show that interactions can stretch the point singularity of a monopole into an extended manifold, whose shape is strongly influenced by the sign of interaction. The singular manifold will cause the first and second Chern number to assume non-integer values when it intersects the surface on which the Chern numbers are calculated. This leads to a gradual decrease of the Chern numbers as the monopole moves away from the surface of integration, instead of the sudden jump characteristic of a point monopole. A gradual change in $C_2$ has in fact been observed in the recent experiment by Spielman's group at NIST. By measuring the range of non-integer values of the Chern numbers as the monopole moves away from the surface of integration along different directions, one can map out the shape of the singular manifold in the parameter space.

cond-mat.quant-gas↗

Enhanced circular photogalvanic effect in HgTe quantum wells in the heavily inverted regime

Thanks to the strong spin-orbit interaction (SOI), HgTe-based quantum wells (QWs) exhibit very rich spin-related properties. But the full descriptions of them are beyond the simple parabolic band models and conventional Rashba and Dresselhaus SOI terms, as a result of the strong interband coupling of the narrow gap band structures. Here, we develop a theoretical method to calculate the circular photogalvanic effect (CPGE) in Hg$_{0.3}$Cd$_{0.7}$Te/HgTe/Hg$_{0.3}$Cd$_{0.7}$Te quantum wells (HgTe QWs) based on the realistic eight-band $\mathbf{k}\cdot\mathbf{p}$ model with density matrix formalism. Our method could take account of the unusual band structures and SOIs of HgTe QWs, therefore can be used to calculate the CPGE currents in HgTe QWs with non-parabolic, Dirac-like and inverted energy dispersions. The microscopic origin of CPGE and the interplay effect of structure inversion asymmetry (SIA) and bulk inversion asymmetry (BIA) is also investigated. In addition, this method is extended to study the pure spin currents (PSCs) in HgTe QWs injected by linearly polarized light at normal incidence. Our calculation results support the following findings: (i) In the inverted phase regime, the energy dispersion of heavily inverted HgTe QWs could be strongly distorted, lead to a significant enhancement of CPGE at a certain range of energy spectrum. (ii) The interplay of SIA and BIA could lead to the CPGE currents anisotropically dependent on the azimuth angle of oblique incident light. (iii) The PSC $j_{y'}^{x'}$ ($x'\parallel[110]$ and $y'\parallel[\bar{1}10]$) produced by [110]-linearly-polarized light could change sign with HgTe QW transformed from normal phase to inverted phase. These findings might be utilized in developing the HgTe-based infrared/terahertz optoelectronic and spintronic devices.

cond-mat.mes-hall↗

SDSS-IV MaNGA: variation of the stellar initial mass function in spiral and early-type galaxies

We perform Jeans anisotropic modeling (JAM) on elliptical and spiral galaxies from the MaNGA DR13 sample. By comparing the stellar mass-to-light ratios estimated from stellar population synthesis (SPS) and from JAM, we find a similar systematic variation of the initial mass function (IMF) as in the earlier $\rm ATLAS^{3D}$ results. Early type galaxies (elliptical and lenticular) with lower velocity dispersions within one effective radius are consistent with a Chabrier-like IMF while galaxies with higher velocity dispersions are consistent with a more bottom heavy IMF such as the Salpeter IMF. Spiral galaxies have similar systematic IMF variations, but with slightly different slopes and larger scatters, due to the uncertainties caused by higher gas fractions and extinctions for these galaxies. Furthermore, we examine the effects of stellar mass-to-light ratio gradients on our JAM modeling, and find that the trends from our results becomes stronger after considering the gradients.

astro-ph.GA↗

Bar-induced central star formation as revealed by integral field spectroscopy from CALIFA

We investigate the recent star formation history (SFH) in the inner region of 57 nearly face-on spiral galaxies selected from the Calar Alto Legacy Integral Field Area (CALIFA) survey. For each galaxy we use the integral field spectroscopy from CALIFA to obtain two-dimensional maps and radial profiles of three parameters that are sensitive indicators of the recent SFH: the 4000Å break (D$_n$(4000)), and the equivalent width of H$δ$ absorption (EW(H$δ_A$)) and H$α$ emission (EW(H$α$)). We have also performed photometric decomposition of bulge/bar/disk components based on SDSS optical image. We identify a class of 17 "turnover" galaxies whose central region present significant drop in D$_n$(4000), and most of them correspondingly show a central upturn in EW(H$δ_A$) and EW(H$α$). This indicates that the central region of the turnover galaxies has experienced star formation in the past 1-2 Gyr, which makes the bulge younger and more star-forming than surrounding regions. We find almost all (15/17) the turnover galaxies are barred, while only half of the barred galaxies in our sample (15/32) are classified as a turnover galaxy. This finding provides strong evidence in support of the theoretical expectation that the bar may drive gas from the disc inward to trigger star formation in galaxy center, an important channel for the growth/rejuvenation of pseudobulges in disc galaxies.

astro-ph.GA↗

Behavior of Oil under Breaking Waves by a Two-phase SPH Model

A two-phase Smoothed Particle Hydrodynamics (SPH) model has been developed on the basis of GPUSPH, which is an open-source implementation of the weakly compressible SPH method on graphics processing units, to investigate oil dispersion under breaking waves. By assuming that the multiple phases are immiscible, the two-phase model solves the same set of governing equations for both phases. Density in each phase is preserved by renormalization, and the harmonic mean of viscosities is used in the transition zone. Interfacial surface tension effect between the oil and the water is considered by a numerical surface tension model. The model is first used to simulate a single oil drop rising through still water. The numerical model predicts realistic shape change of the oil drop during the rising process caused by the buoyancy force. Next it is applied to reproduce a laboratory experiment on oil dispersion under breaking waves conducted at Johns Hopkins University. Several high-speed cameras were used to record the interaction between breaking waves and the oil. Comparison with the laboratory measurements shows that GPUSPH is able to reproduce well the pre- & post-breaking wave in the laboratory, however, oil dispersion predicted by GPUSPH only match part of the laboratory observation. Several factors (e.g., 3D & chaotic nature of breaking waves, numerical setup) cause the discrepancy.

physics.ao-ph↗

Deriving a multivariate CO-to-H$_2$ conversion function using the [CII]/CO(1-0) ratio and its application to molecular gas scaling relations

We present Herschel PACS observations of the [CII] 158 micron emission line in a sample of 24 intermediate mass (9<logM$_\ast$/M$_\odot$<10) and low metallicity (0.4< Z/Z$_\odot$<1.0) galaxies from the xCOLD GASS survey. Combining them with IRAM CO(1-0) measurements, we establish scaling relations between integrated and molecular region [CII]/CO(1-0) luminosity ratios as a function of integrated galaxy properties. A Bayesian analysis reveals that only two parameters, metallicity and offset from the star formation main sequence, $Δ$MS, are needed to quantify variations in the luminosity ratio; metallicity describes the total dust content available to shield CO from UV radiation, while $Δ$MS describes the strength of this radiation field. We connect the [CII]/CO luminosity ratio to the CO-to-H$_2$ conversion factor and find a multivariate conversion function $α_{CO}$, which can be used up to z~2.5. This function depends primarily on metallicity, with a second order dependence on $Δ$MS. We apply this to the full xCOLD GASS and PHIBSS1 surveys and investigate molecular gas scaling relations. We find a flattening of the relation between gas mass fraction and stellar mass at logM$_\ast$/M$_\odot$<10. While the molecular gas depletion time varies with sSFR, it is mostly independent of mass, indicating that the low L$_{CO}$/SFR ratios long observed in low mass galaxies are entirely due to photodissociation of CO, and not to an enhanced star formation efficiency.

astro-ph.GA↗

SDSS IV MaNGA: Discovery of an $H_α$ blob associated with a dry galaxy pair -- ejected gas or a `dark' galaxy candidate?

We report the discovery of a mysterious giant $H_α$ blob that is $\sim 8$ kpc away from the main MaNGA target 1-24145, one component of a dry galaxy merger, identified in the first-year SDSS-IV MaNGA data. The size of the $H_α$ blob is $\sim$ 3-4 kpc in radius, and the $H_α$ distribution is centrally concentrated. However, there is no optical continuum counterpart in deep broadband images reaching $\sim$26.9 mag arcsec$^{-2}$ in surface brightness. We estimate that the masses of ionized and cold gases are $3.3 \times 10^{5}$ $\rm M_{\odot}$ and $< 1.3 \times 10^{9}$ $\rm M_{\odot}$, respectively. The emission-line ratios indicate that the $H_α$ blob is photoionized by a combination of massive young stars and AGN. Furthermore, the ionization line ratio decreases from MaNGA 1-24145 to the $H_α$ blob, suggesting that the primary ionizing source may come from MaNGA 1-24145, likely a low-activity AGN. Possible explanations of this $H_α$ blob include AGN outflow, the gas remnant being tidally or ram-pressure stripped from MaNGA 1-24145, or an extremely low surface brightness (LSB) galaxy. However, the stripping scenario is less favoured according to galaxy merger simulations and the morphology of the $H_α$ blob. With the current data, we can not distinguish whether this $H_α$ blob is ejected gas due to a past AGN outburst, or a special category of `ultra-diffuse galaxy' (UDG) interacting with MaNGA 1-24145 that further induces the gas inflow to fuel the AGN in MaNGA 1-24145.

astro-ph.GA↗

SDSS-IV MaNGA: The Impact of Diffuse Ionized Gas on Emission-line Ratios, Interpretation of Diagnostic Diagrams, and Gas Metallicity Measurements

Diffuse Ionized Gas (DIG) is prevalent in star-forming galaxies. Using a sample of 365 nearly face-on star-forming galaxies observed by MaNGA, we demonstrate how DIG in star-forming galaxies impacts the measurements of emission line ratios, hence the interpretation of diagnostic diagrams and gas-phase metallicity measurements. At fixed metallicity, DIG-dominated low Hα surface brightness regions display enhanced [SII]/Hα, [NII]/Hα, [OII]/Hβ, and [OI]/Hα. The gradients in these line ratios are determined by metallicity gradients and Hα surface brightness. In line ratio diagnostic diagrams, contamination by DIG moves HII regions towards composite or LI(N)ER-like regions. A harder ionizing spectrum is needed to explain DIG line ratios. Leaky HII region models can only shift line ratios slightly relative to HII region models, and thus fail to explain the composite/LI(N)ER line ratios displayed by DIG. Our result favors ionization by evolved stars as a major ionization source for DIG with LI(N)ER-like emission. DIG can significantly bias the measurement of gas metallicity and metallicity gradients derived using strong-line methods. Metallicities derived using N2O2 are optimal because they exhibit the smallest bias and error. Using O3N2, R23, N2=[NII]/Hα, and N2S2Hα (Dopita et al. 2016) to derive metallicities introduces bias in the derived metallicity gradients as large as the gradient itself. The strong-line method of Blanc et al. (2015; IZI hereafter) cannot be applied to DIG to get an accurate metallicity because it currently contains only HII region models which fail to describe the DIG.

astro-ph.GA↗

Deep Memory Networks for Attitude Identification

We consider the task of identifying attitudes towards a given set of entities from text. Conventionally, this task is decomposed into two separate subtasks: target detection that identifies whether each entity is mentioned in the text, either explicitly or implicitly, and polarity classification that classifies the exact sentiment towards an identified entity (the target) into positive, negative, or neutral. Instead, we show that attitude identification can be solved with an end-to-end machine learning architecture, in which the two subtasks are interleaved by a deep memory network. In this way, signals produced in target detection provide clues for polarity classification, and reversely, the predicted polarity provides feedback to the identification of targets. Moreover, the treatments for the set of targets also influence each other -- the learned representations may share the same semantics for some targets but vary for others. The proposed deep memory network, the AttNet, outperforms methods that do not consider the interactions between the subtasks or those among the targets, including conventional machine learning methods and the state-of-the-art deep learning models.

cs.CL↗

SOI RF Switch for Wireless Sensor Network

The objective of this research was to design a 0-5 GHz RF SOI switch, with 0.18um power Jazz SOI technology by using Cadence software, for health care applications. This paper introduces the design of a RF switch implemented in shunt-series topology. An insertion loss of 0.906 dB and an isolation of 30.95 dB were obtained at 5 GHz. The switch also achieved a third order distortion of 53.05 dBm and 1 dB compression point reached 50.06dBm. The RF switch performance meets the desired specification requirements.

cs.OH↗

The Galactic extinction and reddening from the South Galactic Cap U-band Sky Survey: u band galaxy number counts and $u-r$ color distribution

We study the integral Galactic extinction and reddening based on the galaxy catalog of the South Galactic Cap U-band Sky Survey (SCUSS), where $u$ band galaxy number counts and $u-r$ color distribution are used to derive the Galactic extinction and reddening respectively. We compare these independent statistical measurements with the reddening map of \citet{Schlegel1998}(SFD) and find that both the extinction and reddening from the number counts and color distribution are in good agreement with the SFD results at low extinction regions ($E(B-V)^{SFD}<0.12$ mag). However, for high extinction regions ($E(B-V)^{SFD}>0.12$ mag), the SFD map overestimates the Galactic reddening systematically, which can be approximated by a linear relation $ΔE(B-V)= 0.43[E(B-V)^{SFD}-0.12$]. By combing the results of galaxy number counts and color distribution together, we find that the shape of the Galactic extinction curve is in good agreement with the standard $R_V=3.1$ extinction law of \cite{ODonnell1994}.

astro-ph.GA↗

SCUSS u-Band Emission as a Star-Formation-Rate Indicator

We present and analyze the possibility of using optical ${\it u}$-band luminosities to estimate star-formation rates (SFRs) of galaxies based on the data from the South Galactic Cap ${\it u }$ band Sky Survey (SCUSS), which provides a deep ${\it u}$-band photometric survey covering about 5000 $deg^2$ of the South Galactic Cap. Based on two samples of normal star-forming galaxies selected by the BPT diagram, we explore the correlations between ${\it u}$-band, H$α$, and IR luminosities by combing SCUSS data with the Sloan Digital Sky Survey (SDSS) and ${\it Wide}$-${\it field\ Infrared\ Survey\ Explorer}$ (${\it WISE}$). The attenuation-corrected ${\it u}$-band luminosities are tightly correlated with the Balmer decrement-corrected H$α$ luminosities with an rms scatter of $\sim$ 0.17 dex. The IR-corrected ${\it u }$ luminosities are derived based on the correlations between the attenuation of ${\it u}$-band luminosities and ${\it WISE}$ 12 (or 22) $μ$m luminosities, and then calibrated with the Balmer-corrected H$α$ luminosities. The systematic residuals of these calibrations are tested against the physical properties over the ranges covered by our sample objects. We find that the best-fitting nonlinear relations are better than the linear ones and recommended to be applied in the measurement of SFRs. The systematic deviations mainly come from the pollution of old stellar population and the effect of dust extinction; therefore, a more detailed analysis is needed in the future work.

astro-ph.GA↗

Simple, Scalable and Accurate Posterior Interval Estimation

There is a lack of simple and scalable algorithms for uncertainty quantification. Bayesian methods quantify uncertainty through posterior and predictive distributions, but it is difficult to rapidly estimate summaries of these distributions, such as quantiles and intervals. Variational Bayes approximations are widely used, but may badly underestimate posterior covariance. Typically, the focus of Bayesian inference is on point and interval estimates for one-dimensional functionals of interest. In small scale problems, Markov chain Monte Carlo algorithms remain the gold standard, but such algorithms face major problems in scaling up to big data. Various modifications have been proposed based on parallelization and approximations based on subsamples, but such approaches are either highly complex or lack theoretical support and/or good performance outside of narrow settings. We propose a very simple and general posterior interval estimation algorithm, which is based on running Markov chain Monte Carlo in parallel for subsets of the data and averaging quantiles estimated from each subset. We provide strong theoretical guarantees and illustrate performance in several applications.

stat.CO↗

Coupling-Enhanced Broadband Mid-Infrared Light Absorption in Graphene Plasmonic Nanostructures

Plasmons in graphene nanostructures show great promise for mid-infrared applications ranging from a few to tens of microns. However, mid-infrared plasmonic resonances in graphene nanostructures are usually weak and narrow-banded, limiting their potential in light manipulation and detection. Here we investigate the coupling among graphene plasmonic nanostructures and further show that by engineering the coupling, enhancement of light-graphene interaction strength and broadening of spectral width can be achieved simultaneously. Leveraging the concept of coupling, we demonstrate a hybrid 2-layer graphene nanoribbon array which shows 5 to 7% extinction within the entire 8 to 14 μm (~700 to 1250 cm-1) wavelength range, covering one of the important atmosphere "infrared transmission windows". Such coupled hybrid graphene plasmonic nanostructures may find applications in infrared sensing and free-space communications.

cond-mat.mes-hall↗

Optimal Kernel Estimation of Spot Volatility of Stochastic Differential Equations

Kernel Estimation is one of the most widely used estimation methods in non-parametric Statistics, having a wide-range of applications, including spot volatility estimation of stochastic processes. The selection of bandwidth and kernel function is of great importance, especially for the finite sample settings commonly encountered in econometric applications. In the context of spot volatility estimation, most of the proposed selection methods are either largely heuristic or just formally stated without any feasible implementation. In this work, an objective method of bandwidth and kernel selection is proposed, under some mild conditions on the volatility, which not only cover classical Brownian motion driven dynamics but also some processes driven by long-memory fractional Brownian motions or other Gaussian processes. We characterize the leading order terms of the Mean Squared Error, which are also ratified by central limit theorems for the estimation error. As a byproduct, an approximated optimal bandwidth is then obtained in closed form. This result allows us to develop a feasible plug-in type bandwidth selection procedure, for which, as a sub-problem, we propose a new estimator of the volatility of volatility. The optimal selection of kernel function is also discussed. For Brownian Motion type volatilities, the optimal kernel function is proved to be the exponential kernel. For fractional Brownian motion type volatilities, numerical results to compute the optimal kernel are devised and, for the deterministic volatility case, explicit optimal kernel functions of different orders are derived. Simulation studies further confirm the good performance of the proposed methods.

math.ST↗

Efficient Electrical Control of Thin-Film Black Phosphorus Bandgap

Recently rediscovered black phosphorus is a layered semiconductor with promising electronic and photonic properties. Dynamic control of its bandgap can enable novel device applications and allow for the exploration of new physical phenomena. However, theoretical investigations and photoemission spectroscopy experiments performed on doped black phosphorus through potassium adsorption indicate that in its few-layer form, an exceedingly large electric field in the order of several volts per nanometer is required to effectively tune its bandgap, making the direct electrical control unfeasible. Here we demonstrate the tuning of bandgap in intrinsic black phosphorus using an electric field directly and reveal the unique thickness-dependent bandgap tuning properties, arising from the strong interlayer electronic-state coupling. Furthermore, leveraging a 10-nm-thick black phosphorus in which the field-induced potential difference across the film dominates over the interlayer coupling, we continuously tune its bandgap from ~300 to below 50 milli-electron volts, using a moderate displacement field up to 1.1 volts per nanometer. Such dynamic tuning of bandgap may not only extend the operational wavelength range of tunable black phosphorus photonic devices, but also pave the way for the investigation of electrically tunable topological insulators and topological nodal semimetals.

cond-mat.mes-hall↗