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Search Results: 1 - 10 of 431962 matches for " Jian J Wang "
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Quantum thermal transport in nanostructures
Jian-Sheng Wang,Jian Wang,J. T. Lu
Physics , 2008, DOI: 10.1140/epjb/e2008-00195-8
Abstract: In this colloquia review we discuss methods for thermal transport calculations for nanojunctions connected to two semi-infinite leads served as heat-baths. Our emphases are on fundamental quantum theory and atomistic models. We begin with an introduction of the Landauer formula for ballistic thermal transport and give its derivation from scattering wave point of view. Several methods (scattering boundary condition, mode-matching, Piccard and Caroli formulas) of calculating the phonon transmission coefficients are given. The nonequilibrium Green's function (NEGF) method is reviewed and the Caroli formula is derived. We also give iterative methods and an algorithm based on a generalized eigenvalue problem for the calculation of surface Green's functions, which are starting point for an NEGF calculation. A systematic exposition for the NEGF method is presented, starting with the fundamental definitions of the Green's functions, and ending with equations of motion for the contour ordered Green's functions and Feynman diagrammatic expansion. In the later part, we discuss the treatments of nonlinear effects in heat conduction, including a phenomenological expression for the transmission, NEGF for phonon-phonon interactions, molecular dynamics (generalized Langevin) with quantum heat-baths, and electron-phonon interactions. Some new results are also shown. We also briefly review the experimental status of the thermal transport measurements in nanostructures.
Quantum phonon transport of molecular junctions amide-linked with carbon nanotubes: a first-principle study
J. T. Lü,Jian-Sheng Wang
Physics , 2008, DOI: 10.1103/PhysRevB.78.235436
Abstract: Quantum phonon transport through benzene and alkane chains amide-linked with single wall carbon nanotubes (SWCNTs) is studied within the level of density functional theory. The force constant matrices are obtained from standard quantum chemistry software. The phonon transmission and thermal conductance are from the nonequilibrium Green's function and the mode-matching method. We find that the ballistic thermal conductance is not sensitive to the compression or stretching of the molecular junction. The terminating groups of the SWCNTs at the cutting edges only influence the thermal conductance quantitatively. The conductance of the benzene and alkane chains shows large difference. Analysis of the transmission spectrum shows that (i) the low temperature thermal conductance is mainly contributed by the SWCNT transverse acoustic modes, (ii) the degenerate phonon modes show different transmission probability due to the presence of molecular junction, (iii) the SWCNT twisting mode can hardly be transmitted by the alkane chain. As a result, the ballistic thermal conductance of alkane chains is larger than that of benzene chains below 38 K, while it is smaller at higher temperature.
Correlated electron-phonon transport from molecular dynamics with quantum baths
J. T. Lu,Jian-Sheng Wang
Physics , 2008, DOI: 10.1088/0953-8984/21/2/025503
Abstract: Based on generalized quantum Langevin equations for the tight-binding wave function amplitudes and lattice displacements, electron and phonon quantum transport are obtained exactly using molecular dynamics (MD) in the ballistic regime. The electron-phonon interactions can be handled with a quasi-classical approximation. Both charge and energy transport and their interplay can be studied. We compare the MD results with that of a fully quantum mechanical nonequilibrium Green's function (NEGF) approach for the electron currents. We find a ballistic to diffusive transition of the electron conduction in one dimensional chains as the chain length increases.
Coupled electron and phonon transport in one-dimensional atomic junctions
J. T. Lü,Jian-Sheng Wang
Physics , 2007, DOI: 10.1103/PhysRevB.76.165418
Abstract: Employing the nonequilibrium Green's function method, we develop a fully quantum mechanical model to study the coupled electron-phonon transport in one-dimensional atomic junctions in the presence of a weak electron-phonon interaction. This model enables us to study the electronic and phononic transport on an equal footing. We derive the electrical and energy currents of the coupled electron-phonon system and the energy exchange between them. As an application, we study the heat dissipation in current carrying atomic junctions within the self-consistent Born approximation, which guarantees energy current conservation. We find that the inclusion of phonon transport is important in determining the heat dissipation and temperature change of the atomic junctions.
Electromagnetic Modelling of Raman Enhancement from Nanoscale Structures as a Means to Predict the Efficacy of SERS Substrates
Richard J. C. Brown,Jian Wang,Martin J. T. Milton
Journal of Nanomaterials , 2007, DOI: 10.1155/2007/12086
Abstract: The requirement to optimise the balance between signal enhancement and reproducibility in surface enhanced Raman spectroscopy (SERS) is stimulating the development of novel substrates for enhancing Raman signals. This paper describes the application of finite element electromagnetic modelling to predict the Raman enhancement produced from a variety of SERS substrates with differently sized, spaced and shaped morphologies with nanometre dimensions. For the first time, a theoretical comparison between four major generic types of SERS substrate (including metal nanoparticles, structured surfaces, and sharp tips) has been performed and the results are presented and discussed. The results of the modelling are consistent with published experimental data from similar substrates.
Electronic States in Two-Dimensional Triangular Cobalt Oxides: Role of Electronic Correlation
Liang-Jian Zou,J. -L. Wang,Z. Zeng
Physics , 2003, DOI: 10.1103/PhysRevB.69.132505
Abstract: We obtain the electronic states and structures of two-dimensional cobalt oxides, Na$_{x}$CoO$_{2}$ (x=0, 0.35, 0.5 and 0.75) by utilizing the full-potential linear muffin-tin orbitals (FP-LMTO) methods, from which some essential electronic interaction parameters are estimated: the bare on-site Coulomb interaction of cobalt U$_{dd}$=7.5 eV renormalizes to 5 eV for x=0.35, the $pd$ hybridizations t$_{pd\sigma}$ and t$_{pd\pi}$ are -1.40 and 0.70 eV, respectively. The density of states at E$_{F}$ decreases from 6-7 states/eV in the local density approximation (LDA) to about 1.0 states/eV in the LDA+U scheme. The role of the intercalation of water molecules and the microscopic mechanism of the superconductivity in Na$_{0.35}$CoO$_{2}$$\cdot$mH$_{2}$O is discussed.
A Multi-Model Approach for Uncertainty Propagation and Model Calibration in CFD Applications
Jian-xun Wang,Christopher J. Roy,Heng Xiao
Physics , 2015,
Abstract: Proper quantification and propagation of uncertainties in computational simulations are of critical importance. This issue is especially challenging for CFD applications. A particular obstacle for uncertainty quantifications in CFD problems is the large model discrepancies associated with the CFD models used for uncertainty propagation. Neglecting or improperly representing the model discrepancies leads to inaccurate and distorted uncertainty distribution for the Quantities of Interest. High-fidelity models, being accurate yet expensive, can accommodate only a small ensemble of simulations and thus lead to large interpolation errors and/or sampling errors; low-fidelity models can propagate a large ensemble, but can introduce large modeling errors. In this work, we propose a multi-model strategy to account for the influences of model discrepancies in uncertainty propagation and to reduce their impact on the predictions. Specifically, we take advantage of CFD models of multiple fidelities to estimate the model discrepancies associated with the lower-fidelity model in the parameter space. A Gaussian process is adopted to construct the model discrepancy function, and a Bayesian approach is used to infer the discrepancies and corresponding uncertainties in the regions of the parameter space where the high-fidelity simulations are not performed. The proposed multi-model strategy combines information from models with different fidelities and computational costs, and is of particular relevance for CFD applications, where a hierarchy of models with a wide range of complexities exists. Several examples of relevance to CFD applications are performed to demonstrate the merits of the proposed strategy. Simulation results suggest that, by combining low- and high-fidelity models, the proposed approach produces better results than what either model can achieve individually.
Study on the Production Technology, Elasticities and Market Structure Impact on Profit Formation  [PDF]
Jian Wang, Xuejun Zheng
Modern Economy (ME) , 2012, DOI: 10.4236/me.2012.36094
Abstract: The size of the profit in a firm or a production system not only depends on the quantity of inputs and outputs, but also depends on the market structure that means the market is perfect competition or imperfect competition. In general, the relationship between output and inputs can be defined as the production structure, which is usually decided by production technology. Therefore, under the market economy system, the production structure regulation has to follow the market structure variation. Here we assume that production technology is a C-D function, and then to determine the effects of different market structures, which we find, they are in close contacted with both production and market structures, especially some variations of elasticities. Through out a series of deduction and equilibrium analysis, the restricted conditions of the maximum profit have been found. Therefore, the consequences show that the values of elasticities have taken an important role in profit obtained for producer, the profits in a perfect competition market hardly depends on market demand elasticity, in which production elasticity requires rather small. However, in the imperfect competition market, monopoly make both price of demand and production elasticities impact on the profit. Those also prove that market monopoly factors make production lose efficiency, or lead to the market failure. In the actual process of production and management, production elasticities and related market information should be strengthened for measurement, which will be useful for analysis price fluctuation risk and management decision.
The Numerical Simulation of Aerodynamic Noise Generated by CRH3 Train’s Head Surface  [PDF]
Fanguo Kong, Jian Wang
Open Journal of Applied Sciences (OJAppS) , 2015, DOI: 10.4236/ojapps.2015.58049
Abstract: In order to solve the increasingly serious problem of railway noise which caused by the train’s speed-up, especially the problem of the dominant aerodynamic noise of the high-speed train, it’s necessary to have a numerical simulation analysis for the CRH3 train’s three dimensional flow model. Setting monitoring points in the positions that the surface curvature changes significantly, using the Large Eddy Simulation Method (LES) to have a transient simulation for the CRH3 train which is in the speed of 300 km/h and 350 km/h, applying the acoustics theory of Lighthill-Curle to predict the aerodynamic noise caused by the head of the CRH3 train. The generation and distribution of the train’s aerodynamic noise are analyzed, so as to provide some reasonable suggestions for the design of the train body.
Application of Improved Deep Auto-Encoder Network in Rolling Bearing Fault Diagnosis  [PDF]
Jian Di, Leilei Wang
Journal of Computer and Communications (JCC) , 2018, DOI: 10.4236/jcc.2018.67005
Abstract: Since the effectiveness of extracting fault features is not high under traditional bearing fault diagnosis method, a bearing fault diagnosis method based on Deep Auto-encoder Network (DAEN) optimized by Cloud Adaptive Particle Swarm Optimization (CAPSO) was proposed. On the basis of analyzing CAPSO and DAEN, the CAPSO-DAEN fault diagnosis model is built. The model uses the randomness and stability of CAPSO algorithm to optimize the connection weight of DAEN, to reduce the constraints on the weights and extract fault features adaptively. Finally, efficient and accurate fault diagnosis can be implemented with the Softmax classifier. The results of test show that the proposed method has higher diagnostic accuracy and more stable diagnosis results than those based on the DAEN, Support Vector Machine (SVM) and the Back Propagation algorithm (BP) under appropriate parameters.
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