arXiv subjects

Xiaoming Wang

Publications and source records attributed to Xiaoming Wang.

At least 73 records · Page 4Linked to original sources

arXiv2019

A stabilized second order exponential time differencing multistep method for thin film growth model without slope selection

In this paper, a stabilized second order in time accurate linear exponential time differencing (ETD) scheme for the no-slope-selection thin film growth model is presented. An artificial stabilizing term $Aτ^2\frac{\partialΔ^2 u}{\partial t}$ is added to the physical model to achieve energy stability, with ETD-based multi-step approximations and Fourier collocation spectral method applied in the time integral and spatial discretization of the evolution equation, respectively. Long time energy stability and detailed $\ell^{\infty}(0,T; \ell^2)$ error analysis are provided based on the energy method, with a careful estimate of the aliasing error. In addition, numerical experiments are presented to demonstrate the energy decay and convergence rate.

math.NA↗

arXiv2019

An ensemble of random graphs with identical degree distribution

Degree distribution, or equivalently called degree sequence, has been commonly used to be one of most significant measures for studying a large number of complex networks with which some well-known results have been obtained. By contrast, in this paper, we report a fact that two arbitrarily chosen networks with identical degree distribution can have completely different other topological structure, such as diameter, spanning trees number, pearson correlation coefficient, and so forth. Besides that, for a given degree distribution (as power-law distribution with exponent $γ=3$ discussed here), it is reasonable to ask how many network models with such a constraint we can have. To this end, we generate an ensemble of this kind of random graphs with $P(k)\sim k^{-γ}$ ($γ=3$), denoted as graph space $\mathcal{N}(p,q,t)$ where probability parameters $p$ and $q$ hold on $p+q=1$, and indirectly show the cardinality of $\mathcal{N}(p,q,t)$ seems to be large enough in the thermodynamics limit, i.e., $N\rightarrow\infty$, by varying values of $p$ and $q$. From the theoretical point of view, given an ultrasmall constant $p_{c}$, perhaps only graph model $N(1,0,t)$ is small-world and other are not in terms of diameter. And then, we study spanning trees number on two deterministic graph models and obtain both upper bound and lower bound for other members. Meanwhile, for arbitrary $p(\neq1)$, we prove that graph model $N(p,q,t)$ does go through two phase transitions over time, i.e., starting by non-assortative pattern and then suddenly going into disassortative region, and gradually converging to initial place (non-assortative point). Among of them, one "null" graph model is built.

physics.soc-ph↗

arXiv2019

Positivity-preserving, energy stable numerical schemes for the Cahn-Hilliard equation with logarithmic potential

We present and analyze finite difference numerical schemes for the Allen Cahn/Cahn-Hilliard equation with a logarithmic Flory Huggins energy potential. Both the first order and second order accurate temporal algorithms are considered. In the first order scheme, we treat the nonlinear logarithmic terms and the surface diffusion term implicitly, and update the linear expansive term and the mobility explicitly. We provide a theoretical justification that, this numerical algorithm has a unique solution such that the positivity is always preserved for the logarithmic arguments. In particular, our analysis reveals a subtle fact: the singular nature of the logarithmic term around the values of $-1$ and 1 prevents the numerical solution reaching these singular values, so that the numerical scheme is always well-defined as long as the numerical solution stays similarly bounded at the previous time step. Furthermore, an unconditional energy stability of the numerical scheme is derived, without any restriction for the time step size. The unique solvability and the positivity-preserving property for the second order scheme are proved using similar ideas, in which the singular nature of the logarithmic term plays an essential role. For both the first and second order accurate schemes, we are able to derive an optimal rate convergence analysis, which gives the full order error estimate. The case with a non-constant mobility is analyzed as well. We also describe a practical and efficient multigrid solver for the proposed numerical schemes, and present some numerical results, which demonstrate the robustness of the numerical schemes.

math.NA↗

arXiv2017

First-Principles Understanding of the Electronic Band Structure of Copper-Antimony Halide Perovskite: The Effect of Magnetic Ordering

We report the understanding of the electronic band structure of $Cs_4CuSb_2Cl_{12}$ perovskite through first-principles density-functional theory calculations. We find that the most stable state has the antiferromagnetic configuration where each $[CuCl_6]$ octahedral chain along the [010] direction is antiferromagnetic. The reasonable band structure of the compound can be obtained only if both the correct magnetic order and the improved exchange interaction of the Cu $\it{d}$ electrons are taken into account.

cond-mat.mtrl-sci↗

arXiv2017

Parity-Forbidden Transitions and Their Impacts on the Optical Absorption Properties of Lead-Free Metal Halide Perovskites and Double Perovskites

Using density-functional theory calculations, we analyze the optical absorption properties of lead (Pb)-free metal halide perovskites (AB$^{2+}$X$_3$) and double perovskites (AB$^+$B$^{3+}$X$_6$) (A = Cs or monovalent organic ion, B$^{2+}$ = non-Pb divalent metal, B$^+$ = monovalent metal, B$^{3+}$ = trivalent metal, X = halogen). We show that, if B$^{2+}$ is not Sn or Ge, Pb-free metal halide perovskites exhibit poor optical absorptions because of their indirect bandgap nature. Among the nine possible types of Pb-free metal halide double perovskites, six have direct bandgaps. Of these six types, four show inversion symmetry-induced parity-forbidden or weak transitions between band edges, making them not ideal for thin-film solar cell application. Only one type of Pb-free double perovskite shows optical absorption and electronic properties suitable for solar cell applications, namely those with B$^+$ = In, Tl and B$^{3+}$ = Sb, Bi. Our results provide important insights for designing new metal halide perovskites and double perovskites for optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2017

Electronic Band Structures and Excitonic Properties of Delafossites: A $\textit{GW}$-BSE study

We report the band structures and excitonic properties of delafossites $CuMO_2$ (M = Al, Ga, In, Sc, Y, Cr) calculated using the state-of-the-art $\textit{GW}$-BSE approach. We find that all the delafossites are indirect band gap semiconductors with large exciton binding energies, varying from 0.24 to 0.44 eV. The lowest and strongest exciton, mainly contributed from either Cu 3$\textit{d}$ $\rightarrow$ Cu 3$\textit{p}$ (Al, Ga, In) or Cu 3$\textit{d}$ $\rightarrow$ M 3$\textit{d}$ (M = Sc, Y, Cr) transitions, is always located at $L$ point of the rhombohedral Brillouin zone. Taking the electron-hole effect into account, our theoretical band gaps exhibit nice agreement with experiments.

cond-mat.mtrl-sci↗

arXiv2016

Charge transfer at hybrid inorganic-organic interfaces

Organic dopants are frequently used to surface-dope inorganic semiconductors. The resulted hybrid inorganic-organic materials have a crucial role in advanced functional materials and semiconductor devices. In this article, we study charge transfer at hybrid silicon-molecule interfaces theoretically. The idea is to filter out the best molecular acceptors to dope silicon with hole densities as high as 1013 cm-2. Here, we use a combinatorial algorithm merging chemical hardness method and first-principles DFT and GW calculations. We start by using the chemical hardness method which is simple and fast to narrow down our search for molecular dopants. Then, for the most optimistic candidates, we perform first-principles DFT calculations and discuss the necessity of GW corrections. This screening approach is quite general and applicable to other hybrid interfaces.

cond-mat.mtrl-sci↗

arXiv2016

Uniquely solvable and energy stable decoupled schemes for Cahn-Hilliard-Stokes-Darcy system for two-phase flows in karstic geometry

We propose and analyze two novel decoupled numerical schemes for solving the Cahn-Hilliard-Stokes-Darcy (CHSD) model for two-phase flows in karstic geometry. In the first numerical scheme, we explore a fractional step method (operator splitting) to decouple the phase-field (Cahn-Hilliard equation) from the velocity field (Stokes-Darcy fluid equations). To further decouple the Stokes-Darcy system, we introduce a first order pressure stabilization term in the Darcy solver in the second numerical scheme so that the Stokes system is decoupled from the Darcy system and hence the CHSD system can be solved in a fully decoupled manner. We show that both decoupled numerical schemes are uniquely solvable, energy stable, and mass conservative. Ample numerical results are presented to demonstrate the accuracy and efficiency of our schemes.

math.NA↗

arXiv2016

Self-aligning concave relativistic plasma mirror with adjustable focus

We report an experimental-computational study of the optical properties of plasma mirrors (PMs) at the incident laser frequency when irradiated directly at relativistic intensity (1e18 < I_0 < 1e19 W/cm^2) by near-normally incident (4 degree), high-contrast, 30 fs, 800 nm laser pulses. We find that such relativistic PMs are highly reflective (0.6 to 0.8), and focus a significant fraction of reflected light to intensity as large as 10I_0 at distance f as small 25 microns from the PM, provided that pre-pulses do not exceed 1e14 W/cm^2 prior to 20 ps before arrival of the main pulse peak. Particle-in-cell simulations show that focusing results from denting of the reflecting surface by light pressure combined with relativistic transparency, and that reflectivity and f can be adjusted by controlling pre-plasma length L over the range 0.5 < L < 3 microns. Pump-probe reflectivity measurements show the PM's focusing properties evolve on a ps time scale.

physics.plasm-ph↗

arXiv2016

High Thermoelectric Power Factor in Graphene/hBN Devices

Fast and controllable cooling at nanoscales requires a combination of highly efficient passive cooling and active cooling. While passive cooling in graphene-based devices is quite effective due to graphene's extraordinary heat-conduction, active cooling has not been considered feasible due to graphene's low thermoelectric power factor. Here we show that the thermoelectric performance of graphene can be significantly improved by using hBN substrates instead of SiO2. We find the room temperature efficiency of active cooling, as gauged by the power factor times temperature, reaches values as high as 10.35 W/mK, corresponding to more than doubling the highest reported room temperature bulk power factors, 5 W/mK in YbAl3, and quadrupling the best 2D power factor, 2.5 W/mK, in MoS2. We further show that in these devices the electron-hole puddles region is significantly reduced. This enables fast gate-controlled switching of the Seebeck coefficient polarity for application in n- and p-type integrated active cooling devices.

cond-mat.mes-hall↗

arXiv2016

On Douglas general $(α,β)$-metrics

Douglas metrics are metrics with vanishing Douglas curvature which is an important projective invariant in Finsler geometry. To find more Douglas metrics, in this paper we consider a class of Finsler metrics called general $(α,β)$-metrics, which are defined by a Riemannian metric $α=\sqrt{a_{ij}(x)y^iy^j}$ and a $1$-form $β=b_i(x)y^i$. We obtain the differential equations that characterizes these metrics with vanishing Douglas curvature. By solving the equivalent PDEs, the metrics in this class are totally determined. Then many new Douglas metrics are constructed.

math.DG↗

arXiv2016

Convergence Analysis and Error Estimates for a Second Order Accurate Finite Element Method for the Cahn-Hilliard-Navier-Stokes System

In this paper, we present a novel second order in time mixed finite element scheme for the Cahn-Hilliard-Navier-Stokes equations with matched densities. The scheme combines a standard second order Crank-Nicholson method for the Navier-Stokes equations and a modification to the Crank-Nicholson method for the Cahn-Hilliard equation. In particular, a second order Adams-Bashforth extrapolation and a trapezoidal rule are included to help preserve the energy stability natural to the Cahn-Hilliard equation. We show that our scheme is unconditionally energy stable with respect to a modification of the continuous free energy of the PDE system. Specifically, the discrete phase variable is shown to be bounded in $\ell^\infty \left(0,T;L^\infty\right)$ and the discrete chemical potential bounded in $\ell^\infty \left(0,T;L^2\right)$, for any time and space step sizes, in two and three dimensions, and for any finite final time $T$. We subsequently prove that these variables along with the fluid velocity converge with optimal rates in the appropriate energy norms in both two and three dimensions.

math.NA↗

arXiv2016

Design of efficient vdW thermionic heterostructures from first principles

This work is the first step towards understanding thermionic transport properties of graphene/phosphorene/graphene van der Waals heterostructures in contact with gold electrodes by using density functional theory based first principles calculations combined with real space Green's function formalism. We show that for monolayer phosphorene in the heterostructure, quantum tunneling dominates the transport. By adding more phosphorene layers, one can switch from tunneling dominated transport to thermionic dominated transport, resulting in transporting more heat per charge carrier, thus, enhancing the cooling coefficient of performance. The thermionic coefficient of performance for the proposed device is 18.5 at 600 K corresponding to an equivalent ZT of 0.13, which is significant for nanoscale devices.

cond-mat.mtrl-sci↗

arXiv2015

Study on Sparse Representation based Classification for Biometric Verification

In this paper, we propose a multimodal verification system integrating face and ear based on sparse representation based classification (SRC). The face and ear query samples are first encoded separately to derive sparsity-based match scores, and which are then combined with sum-rule fusion for verification. Apart from validating the encouraging performance of SRC-based multimodal verification, this paper also dedicates to provide a clear understanding about the characteristics of SRC-based biometric verification. To this end, two sparsity-based metrics, i.e. spare coding error (SCE) and sparse contribution rate (SCR), are involved, together with face and ear unimodal SRC-based verification. As for the issue that SRC-based biometric verification may suffer from heavy computational burden and verification accuracy degradation with increase of enrolled subjects, we argue that it could be properly resolved by exploiting small random dictionary for sparsity-based score computation, which consists of training samples from a limited number of randomly selected subjects. Experimental results demonstrate the superiority of SRC-based multimodal verification compared to the state-of-the-art multimodal methods like likelihood ratio (LLR), support vector machine (SVM), and the sum-rule fusion methods using cosine similarity, meanwhile the idea of using small random dictionary is feasible in both effectiveness and efficiency.

cs.CV↗

arXiv2015

Compact tunable Compton x-ray source from laser-plasma accelerator and plasma mirror

We present an in-depth experimental-computational study of the parameters necessary to optimize a tunable, quasi-monoenergetic, efficient, low-background Compton backscattering (CBS) x-ray source that is based on the self-aligned combination of a laser-plasma accelerator (LPA) and a plasma mirror (PM). The main findings are: (1) an LPA driven in the blowout regime by 30 TW, 30 fs laser pulses producesnot only a high-quality, tunable, quasi-monoenergetic electron beam, but also a high-quality, relativistically intense (a0~1) spent drive pulse that remains stable in profile and intensity over the LPA tuning range. (2) A thin plastic film near the gas jet exit retro-reflects the spent drive pulse efficiently into oncoming electrons to produce CBS x-rays without detectable bremsstrahlung background. Meanwhile anomalous far-field divergence of the retro-reflected light demonstrates relativistic "denting" of the PM. Exploiting these optimized LPA and PM conditions, we demonstrate quasi-monoenergetic (50% FWHM energy spread), tunable (75 to 200 KeV) CBS x-rays, characteristics previously achieved only on more powerful laser systems by CBS of a split-off, counter-propagating pulse. Moreover, laser-to-x-ray photon conversion efficiency ~6e12 exceeds that of any previous LPA-based quasi-monoenergetic Compton source. Particle-in-cell simulations agree well with the measurements.

physics.plasm-ph↗

arXiv2014

A second order in time, uniquely solvable, unconditionally stable numerical scheme for Cahn-Hilliard-Navier-Stokes equation

We propose a novel second order in time numerical scheme for Cahn-Hilliard-Navier- Stokes phase field model with matched density. The scheme is based on second order convex-splitting for the Cahn-Hilliard equation and pressure-projection for the Navier-Stokes equation. We show that the scheme is mass-conservative, satisfies a modified energy law and is therefore unconditionally stable. Moreover, we prove that the scheme is uncondition- ally uniquely solvable at each time step by exploring the monotonicity associated with the scheme. Thanks to the weak coupling of the scheme, we design an efficient Picard iteration procedure to further decouple the computation of Cahn-Hilliard equation and Navier-Stokes equation. We implement the scheme by the mixed finite element method. Ample numerical experiments are performed to validate the accuracy and efficiency of the numerical scheme.

math.NA↗

arXiv2014

Energy Efficiency of Cross-Tier Base Station Cooperation in Heterogeneous Cellular Networks

Heterogeneous cellular networks (HetNets) are to be deployed for future wireless communication to meet the ever-increasing mobile traffic demand. However, the dense and random deployment of small cells and their uncoordinated operation raise important concerns about energy efficiency. Base station (BS) cooperation is set to play a key role in managing interference in the HetNets. In this paper, we consider BS cooperation in the downlink HetNets where BSs from different tiers within the respective cooperative clusters jointly transmit the same data to a typical user, and further optimize the energy efficiency performance. First, based on the proposed clustering model, we derive the spatial average rate using tools from stochastic geometry. Furthermore, we formulate a power minimization problem with a minimum spatial average rate constraint and derive an approximate result of the optimal received signal strength (RSS) thresholds. Building upon these results, we effectively address the problem of how to design appropriate RSS thresholds, taking into account the trade-off between spatial average rate and energy efficiency. Simulations show that our proposed clustering model is more energy-saving than the geometric clustering model, and under our proposed clustering model, deploying a two-tier HetNet is significantly more energy-saving compared to a macro-only network.

cs.IT↗

arXiv2014

Existence and uniqueness of global weak solutions to a Cahn-Hilliard-Stokes-Darcy system for two phase incompressible flows in karstic geometry

We study the well-posedness of a coupled Cahn-Hilliard-Stokes-Darcy system which is a diffuse-interface model for essentially immiscible two phase incompressible flows with matched density in a karstic geometry. Existence of finite energy weak solution that is global in time is established in both 2D and 3D. Weak-strong uniqueness property of the weak solutions is provided as well.

math.AP↗