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High thermal conductivity of suspended few-layer hexagonal boron nitride sheets

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Nano Research 2014年第8期7卷 1232-1240页

作者： Haiqing Zhou Jixin Zhu Zheng Liu Zheng Yan Xiujun Fan Jian Lin Gunuk Wang Qingyu Yan Ting Yu Pulickel M. Ajayan James M. Tour Department of Chemistry Rice University 6100 Main Street Houston Texas 77005 USA Division of Physics and Applied Physics School of Physical and Mathematical Sciences Nanyang Technological University 537371 Singapore Department of Materials Science and NanoEngineering Rice University 6100 Main Street Houston Texas 77005 USA School of Materials Science and Engineering Nanyang Technological University Nanyang Avenue 639798 Singapore College of Electronic Information and Control Engineering Beijing University of Technology Beijing I00124 China The Smalley Institute for Nanoscale Science and Technology Rice University 6100 Main Street Houston Texas 77005 USA

The thermal conduction of suspended few-layer hexagonal boron nitride （h-BN） sheets was experimentally investigated using a noncontact micro-Raman spectroscopy method. The first-order temperature coefficients for monolayer （1L）, bilayer （2L） and nine-layer （9L） h-BN sheets were measured to be -（3.41 ± 0.12）× 10-2, -（3.15 ± 0.14） × 10-2 and -（3.78 ±0.16）× 10-2 cm-1.K-1, respectively. The room-temperature thermal conductivity of few-layer h-BN sheets was found to be in the range from 227 to 280 W.m-1-K-1, which is comparable to that of bulk h-BN, indicating their potential use as important components to solve heat dissipation problems in thermal management configurations.

关键词： two-dimensional hexagonal boron nitride(h-BN) thermal conductivity Raman spectroscopy

Probe Knots and Hopf Insulators with Ultracold Atoms

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Chinese Physics Letters 2018年第1期35卷 36-40页

作者： Dong-Ling Deng Sheng-Tao Wang Kai Sun L.-M.Duan Department of Physics University of MichiganAnn ArborMichigan 48109USA Condensed Matter Theory Center and Joint Quantum Institute Department of PhysicsUniversity of MarylandCollege ParkMD 20742-4111USA Center for Quantum Information IIISTsinghua UniversityBeijing 100084 Department of Physics Harvard UniversityCambridgeMassachusetts 02138USA

Knots and links are fascinating and intricate topological *** influence spans from DNA and molecular chemistry to vortices in superfluid helium,defects in liquid crystals and cosmic strings in the early *** we find that knotted structures also exist in a peculiar class of three-dimensional topological insulators—the Hopf *** particular,we demonstrate that the momentum-space spin textures of Hopf insulators are twisted in a nontrivial way,which implies the presence of various knot and link *** further illustrate that the knots and nontrivial spin textures can be probed via standard time-of-flight images in cold atoms as preimage contours of spin orientations in stereographic *** extracted Hopf invariants,knots,and links are validated to be robust to typical experimental *** work establishes the existence of knotted structures in Hopf insulators,which may have potential applications in spintronics and quantum information processing.

关键词： Superfluid helium

First demonstration of an all-solid-state optical cryocooler

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Light(Science & Applications) 2018年第1期7卷 922-931页

作者： Markus P.Hehlen Junwei Meng Alexander R.Albrecht Eric R.Lee Aram Gragossian Steven P.Love Christopher E.Hamilton Richard I.Epstein Mansoor Sheik-Bahae Los Alamos National Laboratory P.O.Box 1663Los AlamosNM 87545USA Department of Physics&Astronomy University of New MexicoAlbuquerqueNM 87131USA

Solid-state optical refrigeration uses anti-Stokes fluorescence to cool macroscopic objects to cryogenic temperatures without *** such as Yb3+-doped YLiF4(YLF:Yb)have previously been laser-cooled to 91 *** this study,we show for the first time laser cooling of a payload connected to a cooling crystal.A YLF:Yb crystal was placed inside a Herriott cell and pumped with a 1020-nm laser(47 W)to cool a HgCdTe sensor that is part of a working Fourier Transform Infrared(FTIR)spectrometer to 135 *** first demonstration of an all-solid-state optical cryocooler was enabled by careful control of the various desired and undesired heat *** heating of the payload was minimized by using a single-kink YLF thermal link between the YLF:Yb cooling crystal and the copper coldfinger that held the HgCdTe *** adhesive-free bond between YLF and YLF:Yb showed excellent thermal *** laser-cooled assembly was then supported by silica aerogel cylinders inside a vacuum clamshell to minimize undesired conductive and radiative heat loads from the warm *** structure can serve as a baseline for future optical cryocooler devices.

关键词： optical pumped desired

Thickness-dependent patterning of MoS2 sheets with well-oriented triangular pits by heating in air

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Nano Research 2013年第10期6卷 703-711页

作者： Haiqing Zhou Fang Yu Yuanyue Liu Xiaolong Zou Chunxiao Cong Caiyu Qiu Ting Yu Zheng Yan Xiaonan Shen Lianfeng Sun Boris I. Yakobson James M. Tour Division of Physics and Applied Physics School of Physical and Mathematical Sciences Nanyang Technological University 637371Singapore Department of Chemistry Rice University 6100 Main Street Houston Texas 77005 USA The Smalley Institute for Nanoscale Science and Technology Rice University 6100 Main Street Houston Texas 77005 USA Department of Mechanical Engineering and Materials Science Rice University 6100 Main Street Houston Texas 77005 USA National Center for Nanoscience and Technology Beijing 100190 China

Patterning ultrathin MoS2 layers with regular edges or controllable shapes is appealing since the properties of MoS2 sheets are sensitive to the edge structures. In this work, we have introduced a simple, effective and well-controlled technique to etch layered MoS2 sheets with well-oriented equilateral triangular pits by simply heating the samples in air. The anisotropic oxidative etching is greatly affected by the surrounding temperature and the number of MoS2 layers, whereby the pit sizes increase with the increase of surrounding temperature and the number of MoS2 layers. First-principles computations have been performed to explain the formation mechanism of the triangular pits. This technique offers an alternative avenue to engineering the structure of MoS2 sheets.

关键词： layered MoS2 oxidative etching thickness-dependent triangular pits

Correlation between types of defects/vacancies of Bi2S3 nanostructures and their transient photocurrent

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Nano Research 2017年第7期10卷 2405-2414页

作者： Mingyang Liu Luqing Wang Pei Dong Liangliang Dong Xifan Wang Jarin Joyner Xiangjian Wan Boris I. Yakobson Robert Vajtai Pulickel Ajayan PolSpanos Department of Materials Science and Nanoengineeriing Rice University Houston TX 77005 USA Department of Chemistry Rice University Houston TX 77005 USA College of Chemistry Nankai University Tianjin 300071 China

Crystalline nanostructures possess defects/vacancies that affect their physical and chemical properties. In this regard, the electronic structure of materials can be effectively regulated through defect engineering; therefore, the correlation between defects/vacancies and the properties of a material has attracted extensive attention. Here, we report the synthesis of Bi2S3 microspheres by nanorod assemblies with exposed {211} facets, and the investigation of the types and concentrations of defects/vacancies by means of positron annihilation spectrometry. Our studies revealed that an increase in the calcined temperature, from 350 to 400 ℃, led the predominant defect/vacancy densities to change from isolated bismuth vacancies （VBi） to septuple Bi3＋-sulfur vacancy associates （VBiBiBiSSSS）. Furthermore, the concentration of septuple BiB＋-sulfur vacancy associates increased as the calcined temperature was increased from 400 to 450 ℃. The characterized transient photocurrent spectrum demonstrates that the photocurrent values closely correlate with the types and concentrations of the predominant defects/vacancies. Our theoretical computation, through first principles, showed that VBiBiBiSSSS strongly absorbs I2（sol）, easily desorbs I-（sol）, and enhances the electrocatalytic activity of the nanostructures.

关键词： Bi2S3 microspheres nanorod assembly defects/vacancies positron annihilationspectrometry transient photocurrent

Artificial Boundary Conditions for Nonlocal Heat Equations on Unbounded Domain

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Communications in Computational Physics 2017年第1期21卷 16-39页

作者： Wei Zhang Jiang Yang Jiwei Zhang Qiang Du Beijing Computational Science Research Centre BeijingP.R.China Department of Mathematics Columbia UniversityNew YorkNY 10027USA

This paper is concerned with numerical approximations of a nonlocal heat equation define on an infinite *** classes of artificial boundary conditions(ABCs)are designed,namely,nonlocal analog Dirichlet-to-Neumann-type ABCs(global in time)and high-order Pad´e approximate ABCs(local in time).These ABCs reformulate the original problem into an initial-boundary-value(IBV)problem on a bounded *** the global ABCs,we adopt a fast evolution to enhance computational efficiency and reduce memory *** order fully discrete schemes,both second-order in time and space,are given to discretize two reduced *** numerical experiments are carried out to show the accuracy and efficiency of the proposed methods.

关键词： Artificial boundary conditions nonlocal models Pad´e approximation nonlocal heat equations artificial boundary method

Lax-Oleinik-Type Formulas and Efficient Algorithms for Certain High-Dimensional Optimal Control Problems

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Communications on Applied Mathematics and Computation 2024年第2期6卷 1428-1471页

作者： Paula Chen Jerome Darbon Tingwei Meng Division of Applied Mathematics Brown UniversityProvidenceRIUSA Department of Mathematics UCLALos AngelesCAUSA

Two of the main challenges in optimal control are solving problems with state-dependent running costs and developing efficient numerical solvers that are computationally tractable in high *** this paper,we provide analytical solutions to certain optimal control problems whose running cost depends on the state variable and with constraints on the *** also provide Lax-Oleinik-type representation formulas for the corresponding Hamilton-Jacobi partial differential equations with state-dependent ***,we present an efficient,grid-free numerical solver based on our representation formulas,which is shown to scale linearly with the state dimension,and thus,to overcome the curse of *** existing optimization methods and the min-plus technique,we extend our numerical solvers to address more general classes of convex and nonconvex initial *** demonstrate the capabilities of our numerical solvers using implementations on a central processing unit(CPU)and a field-programmable gate array(FPGA).In several cases,our FPGA implementation obtains over a 10 times speedup compared to the CPU,which demonstrates the promising performance boosts FPGAs can *** numerical results show that our solvers have the potential to serve as a building block for solving broader classes of high-dimensional optimal control problems in real-time.

关键词： Optimal control Hamilton-Jacobi partial differential equations Grid-free numerical methods High dimensions Field-programmable gate arrays(FPGAs)

Nonlinear terahertz devices utilizing semiconducting plasmonic metamaterials

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Light(Science & Applications) 2016年第1期5卷 760-766页

作者： Huseyin R Seren Jingdi Zhang George R Keiser Scott J Maddox Xiaoguang Zhao Kebin Fan Seth R Bank Xin Zhang Richard D Averitt Laboratory for Microsystems Technology Department of Mechanical EngineeringBoston UniversityBostonMA 02215USA Department of Physics Boston UniversityBostonMA 02215USA Department of Physics UC San DiegoLa JollaCA 92093USA School of Engineering Brown UniversityProvidenceRI 02912USA Microelectronics Research Center The University of Texas at AustinAustinTX 78758USA

The development of responsive metamaterials has enabled the realization of compact tunable photonic devices capable of manipulating the amplitude,polarization,wave vector and frequency of *** of semiconductors into the active regions of metallic resonators is a proven approach for creating nonlinear metamaterials through optoelectronic control of the semiconductor carrier ***-free subwavelength resonant semiconductor structures offer an alternative approach to create dynamic *** present InAs plasmonic disk arrays as a viable resonant metamaterial at terahertz ***,InAs plasmonic disks exhibit a strong nonlinear response arising from electric field-induced intervalley scattering,resulting in a reduced carrier mobility thereby damping the plasmonic *** demonstrate nonlinear perfect absorbers configured as either optical limiters or saturable absorbers,including flexible nonlinear absorbers achieved by transferring the disks to polyimide *** plasmonic metamaterials show potential for use in ultrafast terahertz(THz)optics and for passive protection of sensitive electromagnetic devices.

关键词： nonlinear absorbers nonlinear metamaterials plasmonic semiconductor metamaterials terahertz metamaterials transfer printing

Global Solutions to Nonconvex Problems by Evolution of Hamilton-Jacobi PDEs

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Communications on Applied Mathematics and Computation 2024年第2期6卷 790-810页

作者： Howard Heaton Samy Wu Fung Stanley Osher Typal Research Typal LLCLos AngelesUSA Department of Applied Mathematics and Statistics Colorado School of MinesGoldenUSA Department Mathematics UCLALos AngelesUSA

Computing tasks may often be posed as optimization *** objective functions for real-world scenarios are often nonconvex and/or ***-of-the-art methods for solving these problems typically only guarantee convergence to local *** work presents Hamilton-Jacobi-based Moreau adaptive descent(HJ-MAD),a zero-order algorithm with guaranteed convergence to global minima,assuming continuity of the objective *** core idea is to compute gradients of the Moreau envelope of the objective(which is"piece-wise convex")with adaptive smoothing *** of the Moreau envelope(i.e.,proximal operators)are approximated via the Hopf-Lax formula for the viscous Hamilton-Jacobi *** numerical examples illustrate global convergence.

关键词： Global optimization Moreau envelope Hamilton-Jacobi Hopf-Lax-Cole-Hopf Proximals Zero-order optimization