arXiv subjects

Qing Li

Publications and source records attributed to Qing Li.

590 records · Page 33Linked to original sources

arXiv2016

Non-orthogonal multiple-relaxation-time lattice Boltzmann method for incompressible thermal flows

In this paper, a non-orthogonal multiple-relaxation-time (MRT) lattice Boltzmann (LB) method for simulating incompressible thermal flows is presented. In the method, the incompressible Navier-Stokes equations and temperature equation are solved separately by two different MRT-LB equations, which are developed based on non-orthogonal basis vectors obtained from the combinations of the lattice velocity components. The macroscopic governing equations of incompressible thermal flows can be recovered from the method through the Chapman-Enskog analysis in the incompressible limit. Numerical simulations of several typical two-dimensional problems are carried out to validate the proposed method. It is found that the present results are in good agreement with the analytical solutions and/or other numerical results reported in the literature. Furthermore, the non-orthogonal MRT-LB model shows better numerical stability in comparison with the BGK-LB model, and the grid convergence tests indicate that the present MRT-LB method has a second-order convergence rate in space.

physics.comp-ph↗

arXiv2016

A multiple-relaxation-time lattice Boltzmann model for simulating incompressible axisymmetric thermal flows in porous media

In this paper, a multiple-relaxation-time (MRT) lattice Boltzmann (LB) model is developed for simulating incompressible axisymmetric thermal flows in porous media at the representative elementary volume (REV) scale. In the model, a D2Q9 MRT-LB equation is proposed to solve the flow field in addition to the D2Q5 LB equation for the temperature field. The source terms of the model are simple and contain no velocity and temperature gradient terms. The generalized axisymmetric Navier-Stokes equations for axisymmetric flows in porous media are correctly recovered from the MRT-LB model through the Chapman-Enskog analysis in the moment space. The present model is validated by numerical simulations of several typical axisymmetric thermal problems in porous media. The numerical results agree well with the data reported in the literature, demonstrating the effectiveness and accuracy of the present MRT-LB model for simulating axisymmetric thermal flows in porous media.

physics.comp-ph↗

arXiv2016

Interference Evidence for Rashba-Type Spin-Split on Semimetallic WTe2 Surface

Semimetallic tungsten ditelluride (WTe2) displays an extremely large non-saturating magnetoresistance (XMR), which is the subject of intense interest. This phenomenon is thought to arise from the combination of perfect n-p charge compensation with low carrier densities in WTe2 and presumably details of its band structure. Recently, "spin texture" induced by strong spin-orbital coupling (SOC) has been observed in WTe2 by angle-resolved photoemission spectroscopy (ARPES). This provides a mechanism for protecting backscattering for the states involved and thus was proposed to play an important role in the XMR of WTe2. Here, based on our density functional calculations for bulk WTe2, we found a strong Rashba spin-orbit effect in the calculated band structure due to its non-centrosymmetric structure. This splits bands and two-fold spin degeneracy of bands is lifted. A prominent Umklapp interference pattern (a spectroscopic feature with involving reciprocal lattice vectors) can be observed by scanning tunneling microscopic (STM) measurements on WTe2 surface at 4.2 K. This differs distinctly from the surface atomic structure demonstrated at 77 K. The energy dependence of Umklapp interference shows a strong correspondence with densities of states integrated from ARPES measurement, manifesting a fact that the bands are spin-split on the opposites side of Gamma point. Spectroscopic survey reveals the ratio of electron/hole asymmetry changes alternately with lateral locations along b axis, providing a microscopic picture for double-carrier transport of semimetallic WTe2. The calculated band structure and Fermi surface is further supported by our ARPES results and Shubnikov-de Haas (SdH) oscillations measurements.

cond-mat.mtrl-sci↗

arXiv2015

Lattice Boltzmann methods for multiphase flow and phase-change heat transfer

Over the past few decades, tremendous progress has been made in the development of particle-based discrete simulation methods versus the conventional continuum-based methods. In particular, the lattice Boltzmann (LB) method has evolved from a theoretical novelty to a ubiquitous, versatile and powerful computational methodology for both fundamental research and engineering applications. It is a kinetic-based mesoscopic approach that bridges the microscales and macroscales, which offers distinctive advantages in simulation fidelity and computational efficiency. Applications of the LB method have been found in a wide range of disciplines including physics, chemistry, materials, biomedicine and various branches of engineering. The present work provides a comprehensive review of the LB method for thermofluids and energy applications, focusing on multiphase flows, thermal flows and thermal multiphase flows with phase change. The review first covers the theoretical framework of the LB method, revealing the existing inconsistencies and defects as well as common features of multiphase and thermal LB models. Recent developments in improving the thermodynamic and hydrodynamic consistency, reducing the spurious currents, enhancing the numerical stability, etc., are highlighted. These efforts have put the LB method on a firmer theoretical foundation with enhanced LB models that can achieve larger liquid-gas density ratio, higher Reynolds number and flexible surface tension. Examples of applications are provided in fuel cells and batteries, droplet collision, boiling heat transfer and evaporation, and energy storage. Finally, further developments and future prospect of the LB method are outlined for thermofluids and energy applications.

physics.comp-ph↗

arXiv2015

Efficient and low-noise single-photon-level frequency conversion interfaces using silicon nanophotonics

Optical frequency conversion has applications ranging from tunable light sources to telecommunications-band interfaces for quantum information science. Here, we demonstrate efficient, low-noise frequency conversion on a nanophotonic chip through four-wave-mixing Bragg scattering in compact (footprint < 0.5 x 10^-4 cm^2) Si3N4 microring resonators. We investigate three frequency conversion configurations: (1) spectral translation over a few nanometers within the 980 nm band, (2) upconversion from 1550 nm to 980 nm, and (3) downconversion from 980 nm to 1550 nm. With conversion efficiencies ranging from 25 % for the first process to > 60 % for the last two processes, a signal conversion bandwidth > 1 GHz, < 60 mW of continuous-wave pump power needed, and background noise levels between a few fW and a few pW, these devices are suitable for quantum frequency conversion of single photon states from InAs quantum dots. Simulations based on coupled mode equations and the Lugiato-Lefever equation are used to model device performance, and show quantitative agreement with measurements.

physics.optics↗

arXiv2015

On $L_{n}$-Injective Modules and $L_{n}$-Injective Dimensions

Let $R$ be a ring, and $n$ a fixed nonnegative integer. An $R$-module $W$ is called $L_{n}$-injective if ${\rm Ext}_{R}^{1}(M,W)=0$ for any $R$-module $M$ with flat dimension at most $n$. In this paper, we prove first that ($\mathcal{F}_{n},\mathcal{L}_{n}$) is a complete hereditary cotorsion theory, where $\mathcal{F}_n$ (resp. $\mathcal{L}_n$) denotes the class of all $R$-modules with flat dimension at most $n$ (resp. $L_{n}$-injective $R$-modules). Then we introduce the $L_{n}$-injective dimension of a module and $L_n$-global dimension of a ring. Finally, over rings with weak global dimension $\leq n$, perfect rings, and $L_n$-hereditary rings, more properties and applications of $L_{n}$-injective modules, $L_{n}$-injective dimensions of modules and $\mathcal{L}_{n}$-global dimensions of rings are given.

math.RA↗

arXiv2015

Anisotropic Magnetotransport and Exotic Longitudinal Linear Magnetoresistance in WTe2 Crystals

WTe2 semimetal, as a typical layered transition-metal dichalcogenide, has recently attracted much attention due to the extremely large, non-saturating parabolic magnetoresistance in perpendicular field. Here, we report a systematic study of the angular dependence of the magnetoresistance in WTe2 single crystal. The violation of the Kohler rule and a significant anisotropic magnetotransport behavior in different magnetic field directions are observed. Surprisingly, when the applied field is parallel to the tungsten chains of WTe2, an exotic large longitudinal linear magnetoresistance as high as 1200% at 15 T and 2 K is identified. Violation of the Kohler rule in transverse magnetoresistance can be understood based on a dual effect of the excitons formation and thermal activation, while large longitudinal linear magnetoresistance reflects perfectly the scattering and nesting of quasi-1D nature of this balanced hole-electron system. Our work will stimulate studies of such double-carrier correlated material and corresponding quantum physics.

cond-mat.mtrl-sci↗

arXiv2014

HCRS: A hybrid clothes recommender system based on user ratings and product features

Nowadays, online clothes-selling business has become popular and extremely attractive because of its convenience and cheap-and-fine price. Good examples of these successful Web sites include Yintai.com, Vancl.com and Shop.vipshop.com which provide thousands of clothes for online shoppers. The challenge for online shoppers lies on how to find a good product from lots of options. In this article, we propose a collaborative clothes recommender for easy shopping. One of the unique features of this system is the ability to recommend clothes in terms of both user ratings and clothing attributes. Experiments in our simulation environment show that the proposed recommender can better satisfy the needs of users.

cs.AI↗

arXiv2014

Mobility Improvement and Temperature Dependence in MoSe2 Field-Effect Transistors on Parylene-C Substrate

We report low temperature scanning tunneling microscopy characterization of MoSe2 crystals, and the fabrication and electrical characterization of MoSe2 field-effect transistors on both SiO2 and parylene-C substrates. We find that the multilayer MoSe2 devices on parylene-C show a room temperature mobility close to the mobility of bulk MoSe2 (100 cm2V-1s-1 - 160 cm2V-1s-1), which is significantly higher than that on SiO2 substrate (~50 cm2V-1s-1). The room temperature mobility on both types of substrates are nearly thickness independent. Our variable temperature transport measurements reveal a metal-insulator transition at a characteristic conductivity of e2/h. The mobility of MoSe2 devices extracted from the metallic region on both SiO2 and parylene-C increases up to ~ 500 cm2V-1s-1 as the temperature decreases to ~ 100 K, with the mobility of MoSe2 on SiO2 increasing more rapidly. In spite of the notable variation of charged impurities as indicated by the strongly sample dependent low temperature mobility, the mobility of all MoSe2 devices on SiO2 converges above 200 K, indicating that the high temperature (> 200 K) mobility in these devices is nearly independent of the charged impurities. Our atomic force microscopy study of SiO2 and parylene-C substrates further rule out the surface roughness scattering as a major cause of the substrate dependent mobility. We attribute the observed substrate dependence of MoSe2 mobility primarily to the surface polar optical phonon scattering originating from the SiO2 substrate, which is nearly absent in MoSe2 devices on parylene-C substrate.

cond-mat.mtrl-sci↗

arXiv2013

A multiple-relaxation-time lattice Boltzmann model for convection heat transfer in porous media

In this paper, a two-dimensional (2D) multiple-relaxation-time (MRT) lattice Boltzmann (LB) model is developed for simulating convection heat transfer in porous media at the representative elementary volume scale. In the model, a MRT-LB equation is used to simulate the flow field, while another MRT-LB equation is employed to simulate the temperature field. The effect of the porous media is considered by introducing the porosity into the equilibrium moments, and adding a forcing term to the MRT-LB equation of the flow field in the moment space. The present MRT-LB model is validated by numerical simulations of several 2D convection problems in porous media. The numerical results are in good agreement with the well-documented data reported in the literature.

physics.comp-ph↗

arXiv2013

Microscopic and Spectroscopic Evidence for a Slater Metal-Insulator Transition in Sr2IrO4

Layered 5d transition metal oxides (TMOs) have attracted significant interest in recent years because of the rich physical properties induced by the interplay between spin-orbit coupling, bandwidth and on-site Coulomb repulsion. In Sr2IrO4, this interplay opens a gap near the Fermi energy and stabilizes a Jeff=1/2 spin-orbital entangled insulating state at low temperatures. Whether this metal-insulating transition (MIT) is Mott-type (electronic-correlation driven) or Slater-type (magnetic-order driven) has been under intense debate. We address this issue via spatially resolved imaging and spectroscopic studies of the Sr2IrO4 surface using scanning tunneling microscopy/spectroscopy (STM/S). The STS results clearly illustrate the opening of the (~150-250 meV) insulating gap at low temperatures, in qualitative agreement with our density-functional theory (DFT) calculations. More importantly, the measured temperature dependence of the gap width coupled with our DFT+dynamical mean field theory (DMFT) results strongly support the Slater-type MIT scenario in Sr2IrO4. The STS data further reveal a pseudogap structure above the Neel temperature, presumably related to the presence of antiferromagnetic fluctuations.

cond-mat.str-el↗

arXiv2012

Temperature-composition Phase Diagrams for Ba1-xSrxFe2As2 and Ba0.5Sr0.5(Fe1-yCoy)2As2

Single crystals of mixed alkaline earth metal iron arsenide materials of Ba1-xSrxFe2As2 and Ba0.5Sr0.5(Fe1-yCoy)2As2 are synthesized via the self-flux method. Ba1-xSrxFe2As2 display spin-density wave features (TN) at temperatures intermediate to the parent materials, x = 0 and 1, with TN(x) following an approximately linear trend. Cobalt doping of the 1 to 1 Ba:Sr mixture, Ba0.5Sr0.5(Fe1-yCoy)2As2, results in a superconducting dome with maximum transition temperature of TC = 19 K at y = 0.092, close to the maximum transition temperatures observed in unmixed A(Fe1-yCoy)2As2; however, an annealed crystal with y = 0.141 showed a TC increase from 11 to 16 K with a decrease in Sommerfeld coefficient from 2.58(2) to 0.63(2) mJ/(K2 mol atom). For the underdoped y = 0.053, neutron diffraction results give evidence that TN and structural transition (To) are linked at 78 K, with anomalies observed in magnetization, resistivity and heat capacity data, while a superconducting transition at TC ~ 6 K is seen in resistivity and heat capacity data. Scanning tunneling microscopy measurements for y = 0.073 give Dynes broadening factor of 1.15 and a superconducting gap of 2.37 meV with evidence of surface inhomogeneity.

cond-mat.supr-con↗

arXiv2012

A unified approach to mode splitting and scattering loss in high-Q whispering-gallery-mode microresonators

Current theoretical treatment of mode splitting and scattering loss resulting from sub-wavelength scatterers attached to the surface of high-quality-factor whispering-gallery-mode microresonators is not satisfactory. Different models have been proposed for two distinct scatterer regimes, i.e., a-few- and many-scatterers. In addition, many experimental results seem difficult to understand within the existing theoretical framework. Here we develop a unified approach that applies to an arbitrary number of scatterers, which reveals the applicable conditions and the limits of the existing theoretical models. Moreover, many new understandings on mode splitting and scattering loss have been achieved, which are supported by numerical and experimental evidences. Such a unified approach is essential for the fundamental studies as well as the practical applications of mode splitting and scattering loss in high-quality-factor whispering-gallery-mode microresonators.

physics.optics↗

arXiv2010

A Dynamic Model of Streamer Coupling for High Pressure Discharges

A streamer coupling theory is developed to describe the formation of homogenous emission, and the high moving speed of emission patterns in high pressure discharges. By considering the effects of both electron diffusion and electronic drift in the streamer head, the minimum required preionization level $n_{\rm min}$ for the formation of streamer coupling is found to depend on electric field strength, gas pressure and electron temperature. The homogeneity and moving speed of the emission pattern in streamer coupling head increase with preionization level $n_0$, when $n_0 > n_{\rm min}$. The predicted results for atmospheric helium plasma indicate $n_{\rm min} \sim 10^5~{\rm cm^{-3}}$ and moving speed of $10^4 - 10^6$ m/s, in agreement with experiments.

physics.plasm-ph↗