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Yuan Wang

Publications and source records attributed to Yuan Wang.

At least 271 records · Page 15Linked to original sources
arXiv2018

Confirmation Of Two Galactic Supernova Remnant Candidates Discovered By THOR

Anderson et al. (2017) identified seventy six candidate supernova remnants (SNRs) using data from The HI, OH, Recombination line survey of the Milky Way (THOR). The spectral index and polarization properties can help distinguish between SNRs and H II regions, which are often confused. We confirm two SNR candidates using spectral index data and morphology. However, we observe that the fractional linear polarization cannot distinguish between SNRs and H II regions, likely due to contamination by diffuse Galactic synchrotron emission. We also comment on the association of SNR candidates with pulsars through geometric and age considerations.

astro-ph.GA↗
arXiv2018

Observation of acoustic spin

Unlike optical waves, acoustic waves in fluids are described by scalar pressure fields, and therefore are considered spinless. Here, we demonstrate experimentally the existence of spin in acoustics. In the interference of two acoustic waves propagating perpendicularly to each other, we observed the spin angular momentum in free space as a result of the rotation of local particle velocity. We successfully measured the acoustic spin, and spin induced torque acting on a lossy acoustic meta-atom that results from absorption of the spin angular momentum. The acoustic spin is also observed in the evanescent field of a guided mode traveling along a metamaterial waveguide. We found spin-momentum locking in acoustic waves whose propagation direction is determined by the sign of spin. The observed acoustic spin could open a new door in acoustics and their applications for the control of wave propagation and particle rotation.

physics.class-ph↗
arXiv2018

A Submillimeter Burst of S255IR~SMA1 - The Rise And Fall Of Its Luminosity

Temporal photometric variations at near infrared to submillimeter wavelengths have been found in low-mass young stellar objects. These phenomena are generally interpreted as accretion events of star-disk systems with varying accretion rates. There is growing evidence suggesting that similar luminosity flaring also occurs in high-mass star/cluster-forming regions. We report in this Letter the rise and fall of the 900 $μ$m continuum emission and the newly found 349.1 GHz methanol maser emission in the massive star forming region S255IR~SMA1 observed with the Submillimeter Array and the Atacama Large Millimeter/submillimeter Array. The level of flux variation at a factor of $\sim$ 2 at the submillimeter band and the relatively short 2-year duration of this burst suggest that the event is probably similar to those milder and more frequent minor bursts seen in 3D numerical simulations.

astro-ph.GA↗
arXiv2018

Big enterprise registration data imputation: Supporting spatiotemporal analysis of industries in China

Big, fine-grained enterprise registration data that includes time and location information enables us to quantitatively analyze, visualize, and understand the patterns of industries at multiple scales across time and space. However, data quality issues like incompleteness and ambiguity, hinder such analysis and application. These issues become more challenging when the volume of data is immense and constantly growing. High Performance Computing (HPC) frameworks can tackle big data computational issues, but few studies have systematically investigated imputation methods for enterprise registration data in this type of computing environment. In this paper, we propose a big data imputation workflow based on Apache Spark as well as a bare-metal computing cluster, to impute enterprise registration data. We integrated external data sources, employed Natural Language Processing (NLP), and compared several machine-learning methods to address incompleteness and ambiguity problems found in enterprise registration data. Experimental results illustrate the feasibility, efficiency, and scalability of the proposed HPC-based imputation framework, which also provides a reference for other big georeferenced text data processing. Using these imputation results, we visualize and briefly discuss the spatiotemporal distribution of industries in China, demonstrating the potential applications of such data when quality issues are resolved.

cs.CY↗
arXiv2018

An Event-based Diffusion LMS Strategy

We consider a wireless sensor network consists of cooperative nodes, each of them keep adapting to streaming data to perform a least-mean-squares estimation, and also maintain information exchange among neighboring nodes in order to improve performance. For the sake of reducing communication overhead, prolonging batter life while preserving the benefits of diffusion cooperation, we propose an energy-efficient diffusion strategy that adopts an event-based communication mechanism, which allow nodes to cooperate with neighbors only when necessary. We also study the performance of the proposed algorithm, and show that its network mean error and MSD are bounded in steady state. Numerical results demonstrate that the proposed method can effectively reduce the network energy consumption without sacrificing steady-state network MSD performance significantly.

eess.SP↗
arXiv2018

Magnetically-defined topological edge plasmons in edgeless electron gas

Topological materials bear gapped excitations in bulk yet protected gapless excitations at boundaries. Magnetoplasmons (MPs), as high-frequency density excitations of two-dimensional electron gas (2DEG) in a perpendicular magnetic field, embody a prototype of band topology for bosons. The time-reversal-breaking magnetic field opens a topological gap for bulk MPs up to the cyclotron frequency; topologically-protected edge magnetoplasmons (EMPs) bridge the bulk gap and propagate unidirectionally along system's boundaries. However, all the EMPs known to date adhere to physical edges where the electron density terminates abruptly. This restriction has made device application extremely difficult. Here we demonstrate a new class of topological edge plasmons -- domain-boundary magnetoplasmons (DBMPs), within a uniform edgeless 2DEG. Such DBMPs arise at the domain boundaries of an engineered sign-changing magnetic field and are protected by the difference of gap Chern numbers (+/-1) across the magnetic domains. They propagate unidirectionally along the domain boundaries and are immune to domain defects. Moreover, they exhibit wide tunability in the microwave frequency range under an applied magnetic field or gate voltage. Our study opens a new direction to realize high-speed reconfigurable topological devices.

cond-mat.mtrl-sci↗
arXiv2018

Rational curves on hypersurfaces

Let $(X,D)$ be a pair where $X$ is a projective variety. We study in detail how the behavior of rational curves on $X$ as well as the positivity of $-(K_X+D)$ and $D$ influence the behavior of rational curves on $D$. In particular we give criteria for uniruledness and rational connectedness of components of $D$.

math.AG↗
arXiv2018

On the characterization of abelian varieties for log pairs in zero and positive characteristic

Let $(X,Δ)$ be a pair. We study how the condition $κ(K_X + Δ)=0$ causes surjectivity or birationality of the Albanese map and the Albanese morphism of $X$ in both characteristic $0$ and characteristic $p > 0$. In particular in characteristic $0$ we generalize Kawamata's result to the cases of log canonial pairs, and in characteristic $p>0$ we generalize a result of Hacon-Patakfalvi to the cases of log pairs. Moreover we show that if $X$ is a normal projective threefold in characteristic $p>0$, the coefficients of the components of $Δ$ are $\le 1$ and $-(K_X+Δ)$ is semiample, then the Albanese morphism of $X$ is surjective under reasonable assumptions on $p$ and the singularities of the general fibers of the Albanese morphism. This is a positive characteristic analog in dimension 3 of a result of Zhang on a conjecture of Demailly-Peternell-Schneider.

math.AG↗
arXiv2018

First Data Release of the ESO-ARO Public Survey SAMPLING --- SMT "All-sky" Mapping of PLanck Interstellar Nebulae in the Galaxy

We make the first data release (DR1) of the ongoing ESO Public Survey SAMPLING (http://dx.doi.org/10.7910/DVN/0L8NHX). DR1 comprises of 124 fields distributed in $70^\circ < l < 216^\circ$, $-35^\circ < b < 25^\circ$. The 12CO and 13CO (2-1) cubes are gridded in $8"$ pixels, with an effective resolution of $36"$. The channel width is 0.33 km/s and the RMS noise is $T_{\rm mb}<0.2$ K. Once completed, SAMPLING and complementary surveys will initiate the first major step forward to characterize molecular clouds and star formation on truly Galactic scales.

astro-ph.GA↗
arXiv2017

State Space LSTM Models with Particle MCMC Inference

Long Short-Term Memory (LSTM) is one of the most powerful sequence models. Despite the strong performance, however, it lacks the nice interpretability as in state space models. In this paper, we present a way to combine the best of both worlds by introducing State Space LSTM (SSL) models that generalizes the earlier work \cite{zaheer2017latent} of combining topic models with LSTM. However, unlike \cite{zaheer2017latent}, we do not make any factorization assumptions in our inference algorithm. We present an efficient sampler based on sequential Monte Carlo (SMC) method that draws from the joint posterior directly. Experimental results confirms the superiority and stability of this SMC inference algorithm on a variety of domains.

cs.LG↗
arXiv2017

Topological Distances between Networks and Its Application to Brain Imaging

This paper surveys various distance measures for networks and graphs that were introduced in persistent homology. The scope of the paper is limited to network distances that were actually used in brain networks but the methods can be easily adapted to any weighted graph in other fields. The network version of Gromov-Hausdorff, bottleneck, kernel distances are introduced. We also introduce a recently developed KS-test like distance based on monotonic topology features such as the zeroth Betti number. Numerous toy examples and the result of applying many different distances to the brain networks of different clinical status and populations are given.

q-bio.QM↗
arXiv2017

Vanishing on toric surfaces

We prove the Kawamata-Viehweg vanishing and another Kodaira-type vanishing for projective toric surfaces over arbitrary fields.

math.AG↗
arXiv2017

Unidirectional Localization of Photons

Artificial defects embedded in periodic structures are important foundation for creating localized states with vast range of applications in condensed matter physics, photonics and acoustics. In photonics, localized states are extensively used to confine and manipulate photons. Up to now, all the proposed localized states are reciprocal and restricted by time reversal symmetry. Consequently, localization is bidirectional and photons at the allowed passband in the otherwise forbidden stop band are confined irrespective of the direction of incident beam. In this report, by embedding a single defect in a one-dimensional spatiotemporally modulated photonic lattice, we demonstrate that it is possible to have localization of photon only in one direction. In a spatiotemporally modulated photonics lattice, a time dependent potential generates an effective magnetic biasing, which breaks the reciprocity. Moreover, in such moving lattices the dispersion relation obtains a shift depending on the direction of effective magnetic biasing. A static defect synthesized in a temporally modulated lattice will generate a spatial localization of light in the bandgap. However, due to the shift of the bandgap the localization occurs in different frequencies depending on the direction of incident field. We envisage that this phenomenon might has impact not only in photonics but also other areas of physics and engineering such as condensed matter and acoustics, opens the doors for designing new types of devices such as non-reciprocal traps, sensors, unidirectional tunable filters, and might result in unconventional transports such as unidirectional lasing. Despite its applications, our proposal, namely a defect sate in a driven system, can be considered as a pedagogical example of Floquet problem with analytical solution.

physics.optics↗
arXiv2017

Discovery of intrinsic ferromagnetism in 2D van der Waals crystals

It has been long hoped that the realization of long-range ferromagnetic order in two-dimensional (2D) van der Waals (vdW) crystals, combined with their rich electronic and optical properties, would open up new possibilities for magnetic, magnetoelectric and magneto-optic applications. However, in 2D systems, the long-range magnetic order is strongly hampered by thermal fluctuations which may be counteracted by magnetic anisotropy, according to the Mermin-Wagner theorem. Prior efforts via defect and composition engineering, and proximity effect only locally or extrinsically introduce magnetic responses. Here we report the first experimental discovery of intrinsic long-range ferromagnetic order in pristine Cr2Ge2Te6 atomic layers by scanning magneto-optic Kerr microscopy. In such a 2D vdW soft ferromagnet, for the first time, an unprecedented control of transition temperature of ~ 35% - 57% enhancement is realized via surprisingly small fields (<= 0.3 Tesla in this work), in stark contrast to the stiffness of the transition temperature to magnetic fields in the three-dimensional regime. We found that the small applied field enables an effective anisotropy far surpassing the tiny magnetocrystalline anisotropy, opening up a sizable spin wave excitation gap. Confirmed by renormalized spin wave theory, we explain the phenomenon and conclude that the unusual field dependence of transition temperature constitutes a hallmark of 2D soft ferromagnetic vdW crystals. Our discovery of 2D soft ferromagnetic Cr2Ge2Te6 presents a close-to-ideal 2D Heisenberg ferromagnet for studying fundamental spin behaviors, and opens the door for exploring new applications such as ultra-compact spintronics.

cond-mat.mes-hall↗
arXiv2017

A Multitask Diffusion Strategy with Optimized Inter-Cluster Cooperation

We consider a multitask estimation problem where nodes in a network are divided into several connected clusters, with each cluster performing a least-mean-squares estimation of a different random parameter vector. Inspired by the adapt-then-combine diffusion strategy, we propose a multitask diffusion strategy whose mean stability can be ensured whenever individual nodes are stable in the mean, regardless of the inter-cluster cooperation weights. In addition, the proposed strategy is able to achieve an asymptotically unbiased estimation, when the parameters have same mean. We also develop an inter-cluster cooperation weights selection scheme that allows each node in the network to locally optimize its inter-cluster cooperation weights. Numerical results demonstrate that our approach leads to a lower average steady-state network mean-square deviation, compared with using weights selected by various other commonly adopted methods in the literature.

eess.SY↗
arXiv2017

L1188: a promising candidate of cloud-cloud collision triggering the formation of the low- and intermediate-mass stars

We present a new large-scale (4 square degrees) simultaneous $^{12}$CO, $^{13}$CO, and C$^{18}$O ($J$=1$-$0) mapping of L1188 with the PMO 13.7-m telescope. Our observations have revealed that L1188 consists of two nearly orthogonal filamentary molecular clouds at two clearly separated velocities. Toward the intersection showing large velocity spreads, we find several bridging features connecting the two clouds in velocity, and an open arc structure which exhibits high excitation temperatures, enhanced $^{12}$CO and $^{13}$CO emission, and broad $^{12}$CO line wings. This agrees with the scenario that the two clouds are colliding with each other. The distribution of young stellar object (YSO) candidates implies an enhancement of star formation in the intersection of the two clouds. We suggest that a cloud-cloud collision happened in L1188 about 1~Myr ago, possibly triggering the formation of low- and intermediate-mass YSOs in the intersection.

astro-ph.SR↗
arXiv2016

High Thermoelectric Powerfactor in Single and Few-Layer MoS$_2$

The quest for high-efficiency heat-to-electricity conversion has been one of the major driving forces towards renewable energy production for the future. Efficient thermoelectric devices require high voltage generation from a temperature gradient and a large electrical conductivity, while maintaining a low thermal conductivity. For a given thermal conductivity and temperature, the thermoelectric powerfactor is determined by the electronic structure of the material. Low dimensionality (1D and 2D) opens new routes to high powerfactor due to the unique density of states (DOS) of confined electrons and holes. 2D transition metal dichalcogenide (TMDC) semiconductors represent a new class of thermoelectric materials not only due to such confinement effects, but especially due to their large effective masses and valley degeneracies. Here we report a powerfactor of $MoS_2$ as large as $8.5 mWm^{-1}K^{-2}$ at room temperature, which is amongst the highest measured in traditional, gapped thermoelectric materials. To obtain these high powerfactors, we perform thermoelectric measurements on few-layer $MoS_2$ in the metallic regime, which allows us to access the 2D DOS near the conduction band edge and exploit the effect of 2D confinement on electron scattering rates, which result in a large Seebeck coefficient. The demonstrated high, electronically modulated powerfactor in 2D TMDCs holds promise for efficient thermoelectric energy conversion.

cond-mat.mtrl-sci↗