arXiv subjects

Lei Liu

Publications and source records attributed to Lei Liu.

At least 343 records · Page 19Linked to original sources

arXiv2020

Identification of splicing edges in tampered image based on Dichromatic Reflection Model

Imaging is a sophisticated process combining a plenty of photovoltaic conversions, which lead to some spectral signatures beyond visual perception in the final images. Any manipulation against an original image will destroy these signatures and inevitably leave some traces in the final forgery. Therefore we present a novel optic-physical method to discriminate splicing edges from natural edges in a tampered image. First, we transform the forensic image from RGB into color space of S and o1o2. Then on the assumption of Dichromatic Reflection Model, edges in the image are discovered by composite gradient and classified into different types based on their different photometric properties. Finally, splicing edges are reserved against natural ones by a simple logical algorithm. Experiment results show the efficacy of the proposed method.

cs.CV↗

arXiv2020

Testing the Agreement of Trees with Internal Labels

The input to the agreement problem is a collection $P = \{T_1, T_2, \dots , T_k\}$ of phylogenetic trees, called input trees, over partially overlapping sets of taxa. The question is whether there exists a tree $T$, called an agreement tree, whose taxon set is the union of the taxon sets of the input trees, such that for each $i \in \{1, 2, \dots , k\}$, the restriction of $T$ to the taxon set of $T_i$ is isomorphic to $T_i$. We give a $O(n k (\sum_{i \in [k]} d_i + \log^2(nk)))$ algorithm for a generalization of the agreement problem in which the input trees may have internal labels, where $n$ is the total number of distinct taxa in $P$, $k$ is the number of trees in $P$, and $d_i$ is the maximum number of children of a node in $T_i$.

cs.DS↗

arXiv2020

Universal mechanical exfoliation of large-area 2D crystals

Two-dimensional (2D) materials provide extraordinary opportunities for exploring phenomena arising in atomically thin crystals. Beginning with the first isolation of graphene, mechanical exfoliation has been a key to provide high-quality 2D materials but despite improvements it is still limited in yield, lateral size and contamination. Here we introduce a contamination-free, one-step and universal Au-assisted mechanical exfoliation method and demonstrate its effectiveness by isolating 40 types of single-crystalline monolayers, including elemental 2D crystals, metal-dichalcogenides, magnets and superconductors. Most of them are of millimeter-size and high-quality, as shown by transfer-free measurements of electron microscopy, photo spectroscopies and electrical transport. Large suspended 2D crystals and heterojunctions were also prepared with high-yield. Enhanced adhesion between the crystals and the substrates enables such efficient exfoliation, for which we identify a common rule that underpins a universal route for producing large-area monolayers and thus supports studies of fundamental properties and potential application of 2D materials.

cond-mat.mes-hall↗

arXiv2019

Enhanced Electronic Transport in Disordered Hyperuniform Two-Dimensional Amorphous Silica

Disordered hyperuniformity (DHU) is a recently proposed new state of matter, which has been observed in a variety of classical and quantum many-body systems. DHU systems are characterized by vanishing infinite-wavelength density fluctuations and are endowed with unique novel physical properties. Here we report the first discovery of disordered hyperuniformity in atomic-scale 2D materials, i.e., amorphous silica composed of a single layer of atoms, based on spectral-density analysis of high-resolution transmission electron microscope images. Subsequent simulations suggest that the observed DHU is closely related to the strong topological and geometrical constraints induced by the local chemical order in the system. Moreover, we show via large-scale density functional theory calculations that DHU leads to almost complete closure of the electronic band gap compared to the crystalline counterpart, making the material effectively a metal. This is in contrast to the conventional wisdom that disorder generally diminishes electronic transport and is due to the unique electron wave localization induced by the topological defects in the DHU state.

cond-mat.dis-nn↗

arXiv2019

A catalog of newly identified star clusters in GAIA DR2

We present the Star cluster Hunting Pipeline (SHiP) which can identify star clusters in GAIA DR2 data, and establish a star cluster catalog for the Galactic disk. A Friend of Friend based cluster finder method is used to identify star clusters using 5-dimensional stellar parameters, $l, b, \varpi, μ_α\cosδ$, and $μ_δ$. Our new catalog contains 2443 star cluster candidates identified from disk stars located within $|b|=25^\circ$ and with $G<18$ mag. An automatic isochrone fitting scheme is applied to all cluster candidates. With a combination of parameters obtained from isochrone fitting, we classify cluster candidates into three classes (Class 1, 2 and 3). Class 1 clusters are the most probable star cluster candidates with the most stringent criteria. Most of these clusters are nearby (within 4 kpc). Our catalog is cross-matched with three Galactic star cluster catalogs, Kharchenko et al. (2013), Cantat-Gaudin et al. (2018, 2019), and Bica et al. (2019). The proper motion and parallax of matched star clusters are in good agreement with these earlier catalogs. We discover 76 new star cluster candidates that are not listed in these 3 catalogs. The majority of these are clusters older than log(age/yr)=8.0, and are located in the inner disk with $|b|<5^\circ$. The recent discovery of new star clusters suggests that current Galactic star cluster catalogs are still incomplete. Among the Class 1 cluster candidates, we find 56 candidates for star cluster groups. The pipeline, the catalog and the member list containing all candidates star clusters and star cluster groups have been made publicly available.

astro-ph.GA↗

arXiv2019

Hyperspectral Image Classification Based on Adaptive Sparse Deep Network

Sparse model is widely used in hyperspectral image classification.However, different of sparsity and regularization parameters has great influence on the classification results.In this paper, a novel adaptive sparse deep network based on deep architecture is proposed, which can construct the optimal sparse representation and regularization parameters by deep network.Firstly, a data flow graph is designed to represent each update iteration based on Alternating Direction Method of Multipliers (ADMM) algorithm.Forward network and Back-Propagation network are deduced.All parameters are updated by gradient descent in Back-Propagation.Then we proposed an Adaptive Sparse Deep Network.Comparing with several traditional classifiers or other algorithm for sparse model, experiment results indicate that our method achieves great improvement in HSI classification.

eess.IV↗

arXiv2019

Bandgap closing at the screw dislocations of WS2 spirals

Van der Waals (vdW) layered transition metal dichalcogenides (TMDCs) materials are emerging as one class of quantum materials holding novel optical and electronic properties. In particular, the bandgap tunability attractive for nanoelectronics technology have been observed up to 1.1 eV when applying dielectric screening or grain boundary engineering. Here we present the experimental observation of bandgap closing at the center of the screw dislocation-driven WS2 spiral pyramid by means of scanning tunneling spectroscopy, which is validated by first-principle calculations. The observed giant bandgap modulation is attributed to the presence of dangling bonds induced by the W-S broken and the enhanced localized stress in the core of the dislocation. Achieving this metallic state and the consequent vertical conducting channel presents a pathway to 3D-interconnected vdW heterostructure devices based on emergent semiconducting TMDCs.

cond-mat.mes-hall↗

arXiv2019

Vehicular Edge Computing and Networking: A Survey

As one key enabler of Intelligent Transportation System (ITS), Vehicular Ad Hoc Network (VANET) has received remarkable interest from academia and industry. The emerging vehicular applications and the exponential growing data have naturally led to the increased needs of communication, computation and storage resources, and also to strict performance requirements on response time and network bandwidth. In order to deal with these challenges, Mobile Edge Computing (MEC) is regarded as a promising solution. MEC pushes powerful computational and storage capacities from the remote cloud to the edge of networks in close proximity of vehicular users, which enables low latency and reduced bandwidth consumption. Driven by the benefits of MEC, many efforts have been devoted to integrating vehicular networks into MEC, thereby forming a novel paradigm named as Vehicular Edge Computing (VEC). In this paper, we provide a comprehensive survey of state-of-art research on VEC. First of all, we provide an overview of VEC, including the introduction, architecture, key enablers, advantages, challenges as well as several attractive application scenarios. Then, we describe several typical research topics where VEC is applied. After that, we present a careful literature review on existing research work in VEC by classification. Finally, we identify open research issues and discuss future research directions.

eess.SP↗

arXiv2019

A New Insight into GAMP and AMP

A concise expectation propagation (EP) based message passing algorithm (MPA) is derived for the general measurement channel. By neglecting some high-order infinitesimal terms, the EP-MPA is proven to be equivalent to the Generalized Approximate Message Passing (GAMP), which exploits central limit theorem and Taylor expansion to simplify the belief propagation process. Furthermore, for additive white gaussian noise measurement channels, EP-MPA is proven to be equivalent to the AMP. Such intrinsic equivalence between EP and GAMP/AMP offers a new insight into GAMP and AMP via a unified message passing rule for non-linear processing, and may provide clues towards building new MPAs in solving more general non-linear problems.

cs.IT↗

arXiv2019

Space very long baseline interferometry in China

Space very long baseline interferometry (VLBI) has unique applications in high-resolution imaging of fine structure of astronomical objects and high-precision astrometry due to the key long space-Earth or space-space baselines beyond the Earth's diameter. China has been actively involved in the development of space VLBI in recent years. This review briefly summarizes China's research progress in space VLBI and the future development plan.

astro-ph.IM↗

arXiv2019

Compressing $Θ$-chain in slit geometry

When compressed in a slit of width $D$, a $Θ$-chain that displays the scaling of size $R_0$ (diameter) with respect to the number of monomers $N$, $R_0\sim aN^{1/2}$, expands in the lateral direction as $R_{\parallel}\sim a N^ν(a/D)^{2ν-1}$. Provided that the $Θ$ condition is strictly maintained throughout the compression, the well-known scaling exponent of $Θ$-chain in 2 dimensions, $ν=4/7$, is anticipated in a perfect confinement. However, numerics shows that upon increasing compression from $R_0/D<1$ to $R_0/D\gg 1$, $ν$ gradually deviates from $ν=1/2$ and plateaus at $ν=3/4$, the exponent associated with the self-avoiding walk in two dimensions. Using both theoretical considerations and numerics, we argue that it is highly nontrivial to maintain the $Θ$ condition under confinement because of two major effects. First, as the dimension is reduced from 3 to 2 dimensions, the contributions of higher order virial terms, which can be ignored in 3 dimensions at large $N$, become significant. Second and more importantly, the geometrical confinement, which is regarded as an applied external field, alters the second virial coefficient ($B_2$) changes from $B_2=0$ ($Θ$ condition) in free space to $B_2>0$ (good-solvent condition) in confinement. Our study provides practical insight into how confinement affects the conformation of a single polymer chain.

cond-mat.soft↗

arXiv2019

Stacking-Dependent Spatial Charge Separation in Graphitic Carbonic Nitride layers

We reveal the existence of stacking-dependent spatial(SDS) charge separation in graphitic carbonic nitride (g-C3N4) layers, with the density functional theory (DFT) calculations. In g-C3N4 bilayers, such SDS charge separation is found in particular effective for top valence bands that can drive electrons 100% away from one layer to the other. However, for bottom conduction ones, it results in little charge redistribution between layers. As spatial charge separation naturally suppresses the electron-hole recombination, that makes g-C3N4 layers with proper stacking much more efficient for harvesting solar energy in photovoltaic or photocatalytic applications. The SDS charge separation has been understood as a result of the inter-layer quantum entanglement from those g-C3N4 band electrons, whose unique chirality and phases in corner-atom-shared C6N10 units are relatively isolated and in tune only through the corner N atoms. The SDS charge separation in g-C3N4 may lead to an intrinsic way, i.e. without alien dopings, interfaces or electrical fields, to manipulate charge carriers in semiconducting materials. That may lead to new physics in the future optoelectronics or electronics of two-dimensional (2D) materials, such as realizing the layer-selected charge transport through the bilayer or multi-layer 2D materials.

cond-mat.mtrl-sci↗

arXiv2019

A simple rule for finding Dirac Cones in Bilayered Perovskites

We propose a simple rule for finding Dirac cone electronic states in solids, that is neglecting those lattice atoms inert to the particular electronic bands, and pursuing the two dimensional (2D) graphene-like quasi-atom lattices with s- and p-bindings by considering the equivalent atom groups in the unit cell as quasi-atoms. With CsPbBr$_3$ and Cs$_3$Bi$_2$Br$_9$ bilayers as examples, we demonstrate the effectiveness and generality of this rule with the density functional theory (DFT) calculations. We demonstrate that both bilayers have Dirac cones around the Fermi level and reveal that their corresponding Fermi velocities can reach as high as $\sim$ 0.2$\times$10$^6$m/s. That makes these new 2D layered materials very promising in making new ultra-fast ionic electronic devices.

cond-mat.mtrl-sci↗

arXiv2019

Towards Obtaining 2D and 3D and 1D PtPN with Pentagonal Pattern

We apply an alloying strategy to single-layer PtN$_2$ and PtP$_2$, aiming to obtain a single-layer Pt-P-N alloy with a relatively low formation energy with reference to its bulk structure. We perform structure search based on a cluster-expansion method and predict single-layer and bulk PtPN consisting of pentagonal networks. The formation energy of single-layer PtPN is significantly lower in comparison with that of single-layer PtP$_2$. The predicted bulk structure of PtPN adopts a structure that is similar to the pyrite structure. We also find that single-layer pentagonal PtPN, unlike PtN$_2$ and PtP$_2$, exhibits a sizable, direct PBE band gap of 0.84 eV. Furthermore, the band gap of single-layer pentagonal PtPN calculated with the hybrid density functional theory is 1.60 eV, which is within visible light spectrum and promising for optoelectronics applications. In addition to predicting PtPN in the 2D and 3D forms, we study the flexural rigidity and electronic structure of PtPN in the nanotube form. We find that single-layer PtPN has similar flexural rigidity to that of single-layer carbon and boron nitride nanosheets and that the band gaps of PtPN nanotubes depend on their radii. Our work shed light on obtaining an isolated 2D planar, pentagonal PtPN nanosheet from its 3D counterpart and on obtaining 1D nanotubes with tunable bandgaps.

cond-mat.mtrl-sci↗

arXiv2019

Multiplicity of closed characteristics on $P$-symmetric compact convex hypersurfaces in $\mathbb{R}^{2n}$

There is a long standing conjecture that there are at least $n$ closed characteristics for any compact convex hypersurface $Σ$ in $\mathbb{R}^{2n}$, and the symmetric case, i.e. $Σ=-Σ$, has already been proved by C. Liu, Y. Long and C. Zhu in [Math. Ann., 323(2002), pp. 201-215]. In this paper, we extend the result in that paper to the $P$-symmetric case $Σ=PΣ$ for a certain class of symplectic matrix $P$, and prove that there are at least $[\frac{3n}{4}]$ closed characteristics on $Σ$ for any positive integer $n$, where $[a]:=\sup\{l\in\mathbb{Z},l\leq a\}$. To obtain our result, the key problem is to estimate (3.13) in which the method is based on the theorem called Common Index Jump Theorem. By using the Bott-type iteration formulas of Maslov index and Maslov-type index for a certain kind of iteration symplectic path, we provide the some new estimations (4.9-4.11), which are not considered in other papers.

math.DS↗

arXiv2019

Fixed-Symbol Aided Random Access Scheme for Machine-to-Machine Communications

The massiveness of devices in crowded Machine-to-Machine (M2M) communications brings new challenges to existing random-access (RA) schemes, such as heavy signaling overhead and severe access collisions. In order to reduce the signaling overhead, we propose a fixed-symbol aided RA scheme where active devices access the network in a grant-free method, i.e., data packets are directly transmitted in randomly chosen slots. To further address the access collision which impedes the activity detection, one fixed symbol is inserted into each transmitted data packet in the proposed scheme. An iterative message passing based activity detection (MP-AD) algorithm is performed upon the received signal of this fixed symbol to detect the device activity in each slot. In addition, the deep neural network-aided MP-AD (DNN-MP-AD) algorithm is further designed to alleviate the correlation problem of the iterative message passing process. In the DNN-MP-AD algorithm, the iterative message passing process is transferred from a factor graph to a DNN. Weights are imposed on the messages in the DNN and further trained to improve the accuracy of the device activity detection. Finally, numerical simulations are provided for the throughput of the proposed RA scheme, the accuracy of the proposed MP-AD algorithm, as well as the improvement brought by the DNN-MP-AD algorithm.

cs.IT↗

arXiv2019

DENet: A Universal Network for Counting Crowd with Varying Densities and Scales

Counting people or objects with significantly varying scales and densities has attracted much interest from the research community and yet it remains an open problem. In this paper, we propose a simple but an efficient and effective network, named DENet, which is composed of two components, i.e., a detection network (DNet) and an encoder-decoder estimation network (ENet). We first run DNet on an input image to detect and count individuals who can be segmented clearly. Then, ENet is utilized to estimate the density maps of the remaining areas, where the numbers of individuals cannot be detected. We propose a modified Xception as an encoder for feature extraction and a combination of dilated convolution and transposed convolution as a decoder. In the ShanghaiTech Part A, UCF and WorldExpo'10 datasets, our DENet achieves lower Mean Absolute Error (MAE) than those of the state-of-the-art methods.

cs.CV↗

arXiv2019

Harmonic maps with free boundary from degenerating bordered Riemann surfaces

We study the blow-up analysis and qualitative behavior for a sequence of harmonic maps with free boundary from degenerating bordered Riemann surfaces with uniformly bounded energy. With the help of Pohozaev type constants associated to harmonic maps defined on degenerating collars, including vertical boundary collars and horizontal boundary collars, we establish a generalized energy identity.

math.DG↗