A Gaussian Bayesian model to identify spatio-temporal causalities for air pollution based on urban big data

Identifying the causalities for air pollutants and answering questions, such as, where do Beijing's air pollutants come from, are crucial to inform government decision-making. In this paper, we identify the spatio-temporal (ST) causalities among air pollutants at different locations by mining the urban big data. This is challenging for two reasons: 1) since air pollutants can be generated locally or dispersed from the neighborhood, we need to discover the causes in the ST space from many candidate locations with time efficiency; 2) the cause-and-effect relations between air pollutants are further affected by confounding variables like meteorology. To tackle these problems, we propose a coupled Gaussian Bayesian model with two components: 1) a Gaussian Bayesian Network (GBN) to represent the cause-and-effect relations among air pollutants, with an entropy-based algorithm to efficiently locate the causes in the ST space; 2) a coupled model that combines cause-and-effect relations with meteorology to better learn the parameters while eliminating the impact of confounding. The proposed model is verified using air quality and meteorological data from 52 cities over the period Jun 1st 2013 to May 1st 2015. Results show superiority of our model beyond baseline causality learning methods, in both time efficiency and prediction accuracy. © 2016 IEEE.postprintLink_to_subscribed_fulltex

Universal fractal structures in the weak interaction of solitary waves in generalized nonlinear Schr\"{o}dinger equations

Weak interactions of solitary waves in the generalized nonlinear Schr\"{o}dinger equations are studied. It is first shown that these interactions exhibit similar fractal dependence on initial conditions for different nonlinearities. Then by using the Karpman-Solov'ev method, a universal system of dynamical equations is derived for the velocities, amplitudes, positions and phases of interacting solitary waves. These dynamical equations contain a single parameter, which accounts for the different forms of nonlinearity. When this parameter is zero, these dynamical equations are integrable, and the exact analytical solutions are derived. When this parameter is non-zero, the dynamical equations exhibit fractal structures which match those in the original wave equations both qualitatively and quantitatively. Thus the universal nature of fractal structures in the weak interaction of solitary waves is analytically established. The origin of these fractal structures is also explored. It is shown that these structures bifurcate from the initial conditions where the solutions of the integrable dynamical equations develop finite-time singularities. Based on this observation, an analytical criterion for the existence and locations of fractal structures is obtained. Lastly, these analytical results are applied to the generalized nonlinear Schr\"{o}dinger equations with various nonlinearities such as the saturable nonlinearity, and predictions on their weak interactions of solitary waves are made.Comment: 22pages, 15 figure

Topological Properties of Spatial Coherence Function

Topology of the spatial coherence function is considered in details. The phase singularity (coherence vortices) structures of coherence function are classified by Hopf index and Brouwer degree in topology. The coherence flux quantization and the linking of the closed coherence vortices are also studied from the topological properties of the spatial coherence function.Comment: 9 page

Experimental observation of chiral phonons in monolayer WSe2

Chirality characterizes an object that is not identical to its mirror image. In condensed matter physics, Fermions have been demonstrated to obtain chirality through structural and time-reversal symmetry breaking. These systems display unconventional electronic transport phenomena such as the quantum Hall effect and Weyl semimetals. However, for bosonic collective excitations in atomic lattices, chirality was only theoretically predicted and has never been observed. We experimentally show that phonons can exhibit intrinsic chirality in monolayer tungsten diselenide, whose lattice breaks the inversion symmetry and enables inequivalent electronic K and -K valley states. The time-reversal symmetry is also broken when we selectively excite the valley polarized holes by circularly polarized light. Brillouin-zone-boundary phonons are then optically created by the indirect infrared absorption through the hole-phonon interactions. The unidirectional intervalley transfer of holes ensures that only the phonon modes in one valley are excited. We found that such photons are chiral through the transient infrared circular dichroism, which proves the valley phonons responsible to the indirect absorption has non-zero pseudo-angular momentum. From the spectrum we further deduce the energy transferred to the phonons that agrees with both the first principle calculation and the double-resonance Raman spectroscopy. The chiral phonons have significant implications for electron-phonon coupling in solids, lattice-driven topological states, and energy efficient information processing

Single Cell Gap Transflective Liquid Crystal Display with Slanted Reflector Above Transmissive Pixels

Single cell gap transflective liquid crystal display which provides that the backlight traverses the reflective pixel portion twice and thereby follows a path similar to that of the ambient light. A slant reflector is built on the path of the back light to reflect the transmitted light to the reflective portion so that the back light and ambient light follow similar paths

Laser sintering of graphene nanoplatelets encapsulated polyamide powders

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this recordThis paper presents a comprehensive study in the fabrication and safety of new nanocomposite powders for laser sintering. The nanocomposite powder is based on a core-shell structure where nanoparticles (graphene nanoplatelets, GNP) are encapsulated on the surface of the polymeric particles (polyamides PA12) in a thin layer of poly(vinyl alcohol) (PVA). Powder rheology data as well as SEM and TEM showed that GNP was dispersed well in the PVA coating and improved flow. Half time crystallisation kinetics was used to determine differences induced in the polymer and the laser sintering process by the presence of GNP. Nanosafety aspects, critical in a manufacturing environment, are also considered here and exposure monitoring tests were carried out. Results confirmed a low nanoparticle air exposure and therefore confirmed the successful surface encapsulation of the GNP in nanocomposite powders. The laser sintered 0.1GNP/PVA-PA12 parts showed enhanced mechanical properties in tensile, compression and 3-point bending test.Engineering and Physical Sciences Research Council (EPSRC

Topology of Knotted Optical Vortices

Optical vortices as topological objects exist ubiquitously in nature. In this paper, by making use of the $\phi$-mapping topological current theory, we investigate the topology in the closed and knotted optical vortices. The topological inner structure of the optical vortices are obtained, and the linking of the knotted optical vortices is also given.Comment: 11 pages, no figures, accepted by Commun. Theor. Phys. (Beijing, P. R. China

Structural Diversity of Class 1 integrons and their associated gene cassettes in Klebsiella pneumoniae isolates from a hospital in China

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Static Synchronous Generator Model: A New Perspective to Investigate Dynamic Characteristics and Stability Issues of Grid-Tied PWM Inverter

With increasing penetration of the renewable energy, the grid-tied PWM inverters need to take corresponding responsibilities for the security and stability of future grid, behaving like conventional rotational synchronous generator (RSG). Therefore, recognizing the inherent relationship and intrinsic differences between inverters and RSGs is essential for such target. By modeling the typical electromechanical transient of grid-tied PWM inverters, this paper first proves that PWM inverters and RSGs are similar in physical mechanism and equivalent in mathematical model, and the concept of static synchronous generator (SSG) is thereby developed. Furthermore, the comprehensive comparison between RSG and SSG is carried out in detail, and their inherent relation is built. Based on these findings, the rationality and feasibility of migrating the concepts, tools, and methods of RSG stability analysis to investigate the dynamic behaviors and stability issues of SSG is therefore confirmed. Taking stability issues as an example, the criteria of small signal and transient stability of a typical grid-tied PWM inverter is put forward to demonstrate the significance of the developed SSG model (including synchronizing coefficient, damping coefficient, inertia constant, and power-angle curve), providing clear physical interpretation on the dynamic characteristics and stability issues. The developed SSG model promotes grid-friendly integration of renewable energy to future grid and stimulates interdisciplinary research between power electronics and power system