arXiv subjects

Liang Chen

Publications and source records attributed to Liang Chen.

At least 487 records · Page 27Linked to original sources

arXiv2016

High Performance WSe2 Field-Effect Transistors via Controlled Formation of In-Plane Heterojunctions

Monolayer WSe2 is a two dimensional (2D) semiconductor with a direct bandgap, and it has been recently explored as a promising material for electronics and optoelectronics. Low field effect mobility is the main constraint preventing WSe2 from becoming one of the competing channel materials for field-effect transistors (FETs). Recent results have demonstrated that chemical treatments can modify the electrical properties of transition metal dichalcogenides (TMDCs) including MoS2 and WSe2. Here, we report that controlled heating in air significantly improves device performance of WSe2 FETs in terms of on-state currents and field-effect mobilities. Specifically, after heating at optimized conditions, chemical vapor deposition grown monolayer WSe2 FETs showed an average FET mobility of 31 cm2/Vs and on/off current ratios up to 5*108. For few-layer WSe2 FETs, after the same treatment applied, we achieved a high mobility up to 92 cm2/Vs. These values are significantly higher than FETs fabricated using as-grown WSe2 flakes without heating treatment, demonstrating the effectiveness of air heating on the performance improvements of WSe2 FETs. The underlying chemical processes involved during air heating and the formation of in-plane heterojunctions of WSe2 and WO3-x, which is believed to be the reason for the improved FET performance, were studied by spectroscopy and transmission electron microscopy. We further demonstrated that by combining air heating method developed in this work with supporting 2D materials on BN substrate, we achieved a noteworthy field effect mobility of 83 cm2/Vs for monolayer WSe2 FETs. This work is a step towards controlled modification of the properties of WSe2 and potentially other TMDCs, and may greatly improve device performance for future applications of 2D materials in electronics and optoelectronics.

cond-mat.mtrl-sci↗

arXiv2016

Object Recognition Based on Amounts of Unlabeled Data

This paper proposes a novel semi-supervised method on object recognition. First, based on Boost Picking, a universal algorithm, Boost Picking Teaching (BPT), is proposed to train an effective binary-classifier just using a few labeled data and amounts of unlabeled data. Then, an ensemble strategy is detailed to synthesize multiple BPT-trained binary-classifiers to be a high-performance multi-classifier. The rationality of the strategy is also analyzed in theory. Finally, the proposed method is tested on two databases, CIFAR-10 and CIFAR-100. Using 2% labeled data and 98% unlabeled data, the accuracies of the proposed method on the two data sets are 78.39% and 50.77% respectively.

cs.CV↗

arXiv2016

An Efficient Inexact Symmetric Gauss-Seidel Based Majorized ADMM for High-Dimensional Convex Composite Conic Programming

In this paper, we propose an inexact multi-block ADMM-type first-order method for solving a class of high-dimensional convex composite conic optimization problems to moderate accuracy. The design of this method combines an inexact 2-block majorized semi-proximal ADMM and the recent advances in the inexact symmetric Gauss-Seidel (sGS) technique for solving a multi-block convex composite quadratic programming whose objective contains a nonsmooth term involving only the first block-variable. One distinctive feature of our proposed method (the sGS-imsPADMM) is that it only needs one cycle of an inexact sGS method, instead of an unknown number of cycles, to solve each of the subproblems involved.With some simple and implementable error tolerance criteria, the cost for solving the subproblems can be greatly reduced, and many steps in the forward sweep of each sGS cycle can often be skipped, which further contributes to the efficiency of the proposed method. Global convergence as well as the iteration complexity in the non-ergodic sense is established.Preliminary numerical experiments on some high-dimensional linear and convex quadratic SDP problems with a large number of linear equality and inequality constraints are also provided. The results show that for the vast majority of the tested problems, the sGS-imsPADMM is 2 to 3 times faster than the directly extended multi-block ADMM with the aggressive step-length of 1.618, which is currently the benchmark among first-order methods for solving multi-block linear and quadratic SDP problems though its convergence is not guaranteed.

math.OC↗

arXiv2016

A Note on the Convergence of ADMM for Linearly Constrained Convex Optimization Problems

This note serves two purposes. Firstly, we construct a counterexample to show that the statement on the convergence of the alternating direction method of multipliers (ADMM) for solving linearly constrained convex optimization problems in a highly influential paper by Boyd et al. [Found. Trends Mach. Learn. 3(1) 1-122 (2011)] can be false if no prior condition on the existence of solutions to all the subproblems involved is assumed to hold. Secondly, we present fairly mild conditions to guarantee the existence of solutions to all the subproblems and provide a rigorous convergence analysis on the ADMM, under a more general and useful semi-proximal ADMM (sPADMM) setting considered by Fazel et al. [SIAM J. Matrix Anal. Appl. 34(3) 946-977 (2013)], with a computationally more attractive large step-length that can even exceed the practically much preferred golden ratio of $(1+\sqrt{5})/2$.

math.OC↗

arXiv2016

Feature-Area Optimization: A Novel SAR Image Registration Method

This letter proposes a synthetic aperture radar (SAR) image registration method named Feature-Area Optimization (FAO). First, the traditional area-based optimization model is reconstructed and decomposed into three key but uncertain factors: initialization, slice set and regularization. Next, structural features are extracted by scale invariant feature transform (SIFT) in dual-resolution space (SIFT-DRS), a novel SIFT-Like method dedicated to FAO. Then, the three key factors are determined based on these features. Finally, solving the factor-determined optimization model can get the registration result. A series of experiments demonstrate that the proposed method can register multi-temporal SAR images accurately and efficiently.

cs.CV↗

arXiv2015

Testing General Relativity with Present and Future Astrophysical Observations

One century after its formulation, Einstein's general relativity has made remarkable predictions and turned out to be compatible with all experimental tests. Most of these tests probe the theory in the weak-field regime, and there are theoretical and experimental reasons to believe that general relativity should be modified when gravitational fields are strong and spacetime curvature is large. The best astrophysical laboratories to probe strong-field gravity are black holes and neutron stars, whether isolated or in binary systems. We review the motivations to consider extensions of general relativity. We present a (necessarily incomplete) catalog of modified theories of gravity for which strong-field predictions have been computed and contrasted to Einstein's theory, and we summarize our current understanding of the structure and dynamics of compact objects in these theories. We discuss current bounds on modified gravity from binary pulsar and cosmological observations, and we highlight the potential of future gravitational wave measurements to inform us on the behavior of gravity in the strong-field regime.

gr-qc↗

arXiv2015

Neutrino oscillation from the beam with Gaussian-like energy distribution

A recent neutrino experiment at Daya Bay gives superior data of the distribution of the prompt energy. In this paper, the energy distribution presented in the experiment is simulated by applying a Gaussian-like packet to the neutrino wave function received by the detector. We find that the wave packet of neutrinos is expanded during the propagation. As a result, the mixing angle $θ_{13}$ is more difficult to be measured than $θ_{12}$ and $θ_{23}$ in long baseline experiments. Some other propagation properties, such as the time evaluation of the survival probability, the neutrino oscillation and the $CP$ violation, are also studied with the employment of the coherent state method. When the Gaussian packet width increases, the amplitude of the neutrino oscillation decreases, whereas the oscillation period increases gradually.

hep-ph↗

arXiv2015

Discovery of gamma-ray emission from a strongly lobe-dominated quasar 3C 275.1

We systematically analyze the 6-year {\it Fermi}/LAT data of the lobe-dominated quasars (LDQs) in the complete LDQ sample from 3CRR survey and report the discovery of high-energy $γ$-ray emission from 3C 275.1. The $γ$-ray emission of 3C 207 is confirmed and significant variability of the lightcurve is identified. We do not find statistically significant $γ$-ray emission from other LDQs. 3C 275.1 is the known $γ$-ray quasar with the lowest core dominance parameter (i.e., $R=0.11$). We also show that both the northern radio hotspot and parsec jet models can reasonably reproduce the $γ$-ray data. The parsec jet model, however, is favored by the potential $γ$-ray variability at the timescale of months. We suggest that some dimmer $γ$-ray LDQs will be detected in the future and LDQs could contribute non-negligibly to the extragalactic $γ$-ray background.

astro-ph.HE↗

arXiv2015

Chemical Vapor Deposition Growth of Monolayer WSe2 with Tunable Device Characteristics and Growth Mechanism Study

Semiconducting transition metal dichalcogenides (TMDCs) have attracted a lot of attention recently, because of their interesting electronic, optical, and mechanical properties. Among large numbers of TMDCs, monolayer of tungsten diselenides (WSe2) is of particular interest since it possesses a direct band gap and tunable charge transport behaviors, which make it suitable for a variety of electronic and optoelectronic applications. Direct synthesis of large domains of monolayer WSe2 and their growth mechanism studies are important steps toward applications of WSe2. Here, we report systematical studies on ambient pressure chemical vapor deposition (CVD) growth of monolayer and few layer WSe2 flakes directly on silica substrates. The WSe2 flakes were characterized using optical microscopy, atomic force microscopy, Raman spectroscopy, and photoluminescence spectroscopy. We investigated how growth parameters, with emphases on growth temperatures and durations, affect the sizes, layer numbers, and shapes of as-grown WSe2 flakes. We also demonstrated that transport properties of CVD-grown monolayer WSe2, similar to mechanically-exfoliated samples, can be tuned into either p-type or ambipolar electrical behavior, depending on the types of metal contacts. These results deepen our understandings on the vapor phase growth mechanism of WSe2, and may benefit the uses of these CVD-grown monolayer materials in electronic and optoelectronics.

cond-mat.mtrl-sci↗

arXiv2015

The Kondo Temperature of a Two-dimensional Electron Gas with Rashba Spin-orbit Coupling

We use the Hirsch-Fye quantum Monte Carlo method to study the single magnetic impurity problem in a two-dimensional electron gas with Rashba spin-orbit coupling. We calculate the spin susceptibility for various values of spin-orbit coupling, Hubbard interaction, and chemical potential. The Kondo temperatures for different parameters are estimated by fitting the universal curves of spin susceptibility. We find that the Kondo temperature is almost a linear function of Rashba spin-orbit energy when the chemical potential is close to the edge of the conduction band. When the chemical potential is far away from the band edge, the Kondo temperature is independent of the spin-orbit coupling. These results demonstrate that, for single impurity problem in this system, the most important reason to change the Kondo temperature is the divergence of density of states near the band edge, and the divergence is induced by the Rashba spin-orbit coupling.

cond-mat.str-el↗

arXiv2015

Study of digital pulse shape discrimination method for n-γ separation of EJ-301 liquid scintillation detector

A digital pulse shape discrimination system based on a programmable module NI-5772 has been established and tested with EJ-301 liquid scintillation detector. The module was operated by means of running programs developed in LabVIEW with the sampling frequency up to 1.6GS/s. Standard gamma sources 22Na, 137Cs and 60Co were used to calibrate the EJ-301 liquid scintillation detector, and the gamma response function has been obtained. Digital algorithms for charge comparison method and zero-crossing method have been developed. The experimental results showed that both digital signal processing (DSP) algorithms could discriminate neutrons from gamma-rays. Moreover, the zero-crossing method shows better n-γ discrimination at 80 keVee and lower, whereas the charge comparison method gives better results at higher thresholds. In addition, the figure-of-merit (FOM) of two different dimension detectors were extracted at 9 energy thresholds, and it was found that the smaller one presented a better n-γ separation property for fission neutrons.

physics.ins-det↗

arXiv2014

Screw-Dislocation-Driven Growth of Two-Dimensional Few-Layer and Pyramid-Like WSe2 by Sulfur-Assisted Chemical Vapor Deposition

Two-dimensional (2D) layered tungsten diselenides (WSe2) material has recently drawn a lot of attention due to its unique optoelectronic properties and ambipolar transport behavior. However, direct chemical vapor deposition (CVD) synthesis of 2D WSe2 is not as straightforward as other 2D materials due to the low reactivity between reactants in WSe2 synthesis. In addition, the growth mechanism of WSe2 in such CVD process remains unclear. Here we report the observation of a screw-dislocation-driven (SDD) spiral growth of 2D WSe2 flakes and pyramid-like structures using a sulfur-assisted CVD method. Few-layer and pyramid-like WSe2 flakes instead of monolayer were synthesized by introducing a small amount of sulfur as a reducer to help the selenization of WO3, which is the precursor of tungsten. Clear observations of steps, helical fringes, and herring-bone contours under atomic force microscope characterization reveal the existence of screw dislocations in the as-grown WSe2. The generation and propagation mechanisms of screw dislocations during the growth of WSe2 were discussed. Back-gated field-effect transistors were made on these 2D WSe2 materials, which show on/off current ratios of 106 and mobility up to 44 cm2/Vs.

cond-mat.mtrl-sci↗

arXiv2014

Constraints on minimum electron Lorentz factor and matter content of jets for a sample of bright Fermi blazars

We fit the (quasi-)simultaneous multi-waveband spectral energy distributions (SEDs) for a sample of low-synchrotron-peaked (LSP) blazars with a one-zone leptonic model. The seed photons that predominantly come from broad line region (BLR) and infrared (IR) molecular torus are considered respectively in external Compton process. We find that the modeling with IR seed photons is systematically better than that with BLR photons based on a $χ^2$ test, which suggest that $γ$-ray emitting region most possibly stay outside the BLR. The minimum electron Lorentz factor, $γ_{\rm min}$, is constrained from the modeling for these LSP blazars with good soft X-ray data (ranges from 5 to 160 with a median value of 55), which plays a key role in jet power estimation. Assuming one-to-one ratio of proton and electron, we find that the jet power for LSP blazars is systematically higher than that of FR II radio galaxies at given 151 MHz radio luminosity, $L_{\rm 151MHz}$, even though FR IIs are regarded as same as LSP blazars in unification scheme except the jet viewing angle. The possible reason is that there are some $e^{\pm}$ pairs in the jet of these blazars. If this is the case, we find the number density of $e^{\pm}$ pairs should be several times higher than that of $e^{-}-p$ pairs by assuming the jet power is the same for LSP blazars and FR IIs at given $L_{\rm 151MHz}$.

astro-ph.HE↗

arXiv2014

The study of neutron spectra in water bath from Pb target irradiated by 250MeV/u protons

The spallation neutrons were produced by the irradiation of Pb with 250 MeV protons. The Pb target was surrounded by water which was used to slow down the emitted neutrons. The moderated neutrons in the water bath were measured by using the resonance detectors of Au, Mn and In with Cd cover. According to the measured activities of the foils, the neutron flux at different resonance energy were deduced and the epithermal neutron spectra were proposed. Corresponding results calculated with the Monte Carlo code MCNPX were compared with the experimental data to check the validity of the code.

physics.ins-det↗

arXiv2014

Is BZB J1450+5201 the most distant $γ$-ray BL Lacertae object?

BL Lacertae (BL Lac) objects at high redshifts ($z\geq 2$) are rarely detected. Through careful analysis of the SDSS spectrum, BZB J1450+5201 is confirmed to be a high-$z$ BL Lac object with $z\geq$ 2.471 by identifying the Ly$α$ 1216 and CIV 1548/1550 absorption lines. This indicates that BZB J1450+5201 is the most distant BL Lac object discovered to date. Careful analysis of the five-year \fermi data of 2FGL J1451.0+5159 shows that its $γ$-ray emission is robust with confidence level of 6.2$σ$ at 1-3 GeV and 6.7$σ$ at 3-10 GeV, and that the confusion of bright neighbor is negligible, which can not be fixed in the analysis of the two-year data. Meanwhile, 2FGL J1451.0+5159 is confirmed to be associated with BZB J1450+5201 using the five-year data. The analysis of multiwavelength data, from radio to $γ$-ray energies, indicates BZB J1450+5201 is an intermediate synchrotron peaked (ISP) source and consistent with distributions of other ISP sources at lower redshifts in the second LAT AGN catalog. The pure SSC model seems to be disfavoured, while scattering of weak external emission plus SSC process can provide a satisfactory description of the broadband emission.

astro-ph.HE↗

arXiv2014

Towards Cross-layer Reliability Analysis of Transient and Permanent Faults

Due to the increasing complexity of Multi-Processor Systems on Chip (MPSoCs), system-level design methodologies have got a lot of attention in recent years. However, the significant gap between the system-level reliability analysis and the level where the actual faults occur necessitates a cross-layer approach in which the sufficient data about the effects of faults at low levels are passed to the system level. So far, the cross-layer reliability analysis techniques focus on a specific type of faults, e.g., either permanent or transient faults. In this work, we aim at proposing a cross-layer reliability analysis which considers different fault types concurrently and connects reliability analysis techniques at different levels of abstraction using adapters.

cs.DC↗

arXiv2014

The curvature of spectral energy distribution of blazars

The SED of blazars show significant curvature. In this paper, we study the curvature properties for a large sample of Fermi/LAT bright blazars based on quasi-simultaneous SED. Both SEDs of synchrotron and inverse Compton (IC) components are fitted by a log-parabolic law in log_v-log_vfv diagram. The second-degree term of log-parabola measures the curvature of SED. We find a statistically significant correlation between synchrotron peak frequency and its curvature. This result is in agreement with the theoretical prediction, and confirms previous studies, which dealt with single source with various epoch observations or a small sample. If a broken power-law is employed to fit the SED, the difference between the two spectral indexes (alpha2-alpha1) can be considered as a "surrogate" of the SED curvature. We collect spectral parameters of a sample blazars from literature, and find a correlation between the synchrotron peak frequency and the spectral difference. We do not find a significant correlation between the IC peak frequency and its curvature, which may be caused by complicated seed photon field. It is also found that the synchrotron curvatures are on average larger than that of IC curvatures, and there is no correlation between these two parameters. As suggested by previous works in literature, both the log-parabolic law of SED and above correlation can be explained by statistical and/or stochastic particle accelerations. Stochastic particle acceleration predicts a different slope of the correlation from that of statistical one, and our result seems favor stochastic acceleration mechanisms and emission processes. Some of other evidences also seem to support that the electron energy distribution (and/or synchrotron SED) may be log-parabolic, which include SED modeling, particle acceleration simulation, and comparisons between some predictions and empirical relations/correlations.

astro-ph.HE↗

arXiv2014

Smart Streaming for Online Video Services

Bandwidth consumption is a significant concern for online video service providers. Practical video streaming systems usually use some form of HTTP streaming (progressive download) to let users download the video at a faster rate than the video bitrate. Since users may quit before viewing the complete video, however, much of the downloaded video will be "wasted". To the extent that users' departure behavior can be predicted, we develop smart streaming that can be used to improve user QoE with limited server bandwidth or save bandwidth cost with unlimited server bandwidth. Through measurement, we extract certain user behavior properties for implementing such smart streaming, and demonstrate its advantage using prototype implementation as well as simulations.

cs.MM↗