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Observation of Topological Links Associated with Hopf Insulators in a solid-state Quantum Simulator

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Chinese Physics Letters 2017年第6期34卷 14-18页

作者：袁新星何丽王胜涛邓东灵王飞连文倩王歆张楚珩张慧丽常秀英段路明 Center for Quantum Information Institute for Interdisciplinary Information SciencesTsinghua University Department of Physics University of Michigan condensed matter theory Center and Joint Quantum institute Department of PhysicsUniversity of Maryland

Hopf insulators are intriguing three-dimensional topological insulators characterized by an integer topological invariant. They originate from the mathematical theory of Hopf fibration and epitomize the deep connection between knot theory and topological phases of matter, which distinguishes them from other classes of topological insulators. Here, we implement a model Hamiltonian for Hopf insulators in a solid-state quantum simulator and report the first experimental observation of their topological properties, including nontrivial topological links associated with the Hopf fibration and the integer-valued topological invariant obtained from a direct tomographic measurement. Our observation of topological links and Hopf fibration in a quantum simulator opens the door to probe rich topological properties of Hopf insulators in experiments. The quantum simulation and probing methods are also applicable to the study of other intricate three-dimensional topological model Hamiltonians.

关键词： Observation of Topological Links Associated with Hopf Insulators in a solid-state Quantum Simulator

Coexistence of plastic and partially diffusive phases in a helium-methane compound

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National Science Review 2020年第10期7卷 1540-1547页

作者： Hao Gao Cong Liu Andreas Hermann Richard J.Needs Chris J.Pickard Hui-Tian Wang Dingyu Xing Jian Sun National Laboratory of solid state Microstructures School of Physics and Collaborative Innovation Center of Advanced MicrostructuresNanjing University Centre for Science at Extreme Conditions and The School of Physics and Astronomy The University of Edinburgh theory of condensed matter Group Cavendish Laboratory Department of Materials Science & Metallurgy University of Cambridge Advanced institute for Materials Research Tohoku University

Helium and methane are major components of giant icy planets and are abundant in the universe. However,helium is the most inert element in the periodic table and methane is one of the most hydrophobic molecules, thus whether they can react with each other is of fundamental importance. Here, our crystal structure searches and first-principles calculations predict that a He;CH;compound is stable over a wide range of pressures from 55 to 155 GPa and a HeCH;compound becomes stable around 105 GPa. As nice examples of pure van der Waals crystals, the insertion of helium atoms changes the original packing of pure methane molecules and also largely hinders the polymerization of methane at higher pressures. After analyzing the diffusive properties during the melting of He;CH;at high pressure and high temperature, in addition to a plastic methane phase, we have discovered an unusual phase which exhibits coexistence of diffusive helium and plastic methane. In addition, the range of the diffusive behavior within the helium-methane phase diagram is found to be much narrower compared to that of previously predicted helium-water compounds. This may be due to the weaker van der Waals interactions between methane molecules compared to those in helium-water compounds, and that the helium-methane compound melts more easily.

关键词： crystal structure prediction ab initio molecular dynamics ab initio calculations high pressure and high temperature melting and phase transition collective motion

Composition dependent elastic modulus and phase stability of Ni_2MnGa based ferromagnetic shape memory alloys

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Science China(Technological Sciences) 2012年第2期55卷 295-305页

作者： HU QingMiao LUO HuBin LI ChunMei VITOS Levente YANG Rui Shenyang National Laboratory for Materials Science Institute of Metal ResearchChinese Academy of SciencesShenyang 110016China Applied Materials Physics Department of Materials Science and EngineeringRoyal Institute of TechnologyStockholm SE-10044Sweden condensed matter theory Group Physics DepartmentUppsala UniversityUppsala SE-75120Sweden Research institute for solid state Physics and optics P.O.Box 49Budapest H-1525Hungary

Ni2MnGa based ferromagnetic alloys are ideal candidates for applications such as actuators, magnetic refrigerators or magne-tostrictive transducers due to their attractive properties such as magnetic field induced shape memory effect and large magnetocaloric effect. The properties of these alloys （e.g., the martensitic transformation temperature TM） sensitively depend on the composition. Understanding the composition dependence of these properties so as to design the alloy as desired is one of the main research topics in this area. In recent years, we have investigated the composition dependent elastic modulus and phase stability of Ni2MnGa-based alloys by using a first-principles method, in hope of clarifying their connection to the properties of these alloys. In this article, we review the main results of our investigations. We show that the tetragonal shear modulus C＇ is a better predictor of the composition dependent TM than the number of valence electrons per atom （e/a） since the general TM-C＇ correlation works for some of the alloys for which the TM-ela correlation fails, although there exist several cases for which both the general TM-C＇ and TM-ela correlations break down. Employing the experimentally determined modulation function, the complex 5-layer modulated （5M） structure of the martensite of Ni2MnGa and the Al-doping effect on it are studied. We find that the shuffle and shear of the 5M structure are linearly coupled. The relative stability of the austenite and the marten- sites is examined by comparing their total energies. The non-modulated martensite β＇″ with the tetragonality of the unit cell c/a〉1 is shown to be globally stable whereas the 5M martensite with c/a〈1 is metastable. The critical Al atomic fraction over which the martensitic transformation between the 5M martensite and austenite cannot occur is predicted to be 0.26, in reason- able agreement with experimental findings.

关键词： NieMnGa martensitic transformation elastic modulus phase stability density functional theory

Strong coupling superconductivity and prominent superconducting fluctuations in the new superconductor Bi_4O_4S_3

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Science China(Physics,Mechanics & Astronomy) 2013年第11期56卷 2019-2025页

作者： LI Sheng YANG Huan FANG DeLong WANG ZhenYu TAO Jian DING XiaXin WEN HaiHu Center for Superconducting Physics and Materials National Laboratory of Solid State Microstructures and Department of PhysicsNanjing University National Laboratory for Superconductivity Institute of Physics and National Laboratory for Condensed Matter PhysicsChinese Academy of Sciences

Electric transport and scanning tunneling spectrum(STS)have been investigated on polycrystalline samples of the new superconductor Bi4O4S3.A weak insulating behavior in the resistive curve has been induced in the normal state when the superconductivity is suppressed by applying a magnetic ***,a kink appears on the temperature dependence of resistivity near 4 K at all high magnetic fields above 1 T when the bulk superconductivity is completely *** kink associated with the upper critical field as well as the wide range of excess conductance at low fields and high temperatures is explained as the possible evidence of strong superconducting *** the tunneling spectra,a superconducting gap of about 3 meV is frequently observed yielding a ratio of 2Δ/kB TC^*** value is much larger than the one predicted by the BCS theory in the weak coupling regime(2Δ/kB TC^3.53),which suggests the strong coupling superconductivity in the present ***,the gapped feature persists on the spectra until 14 K in the STS measurement,which suggests a prominent fluctuation region of *** a superconducting fluctuation can survive at very high magnetic fields,which are far beyond the critical fields for bulk superconductivity as inferred both from electric transport and tunneling measurements.

关键词： BiS2-based superconductor strong coupling superconductivity superconducting fluctuation

Aperture-averaged scintillation index and fade statistics in weak oceanic turbulence

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Chinese Physics B 2021年第6期30卷 320-328页

作者： Hao Wang Fu-Zeng Kang Xuan Wang Wei Zhao Shu-Wei Sun state Key Laboratory of Transient optics and Photonics Xi'an Institute of Optics and Precision MechanicsChinese Academy of SciencesXi'an 710119China University of Chinese Academy of Sciences Beijing 100049China Beijing National Laboratory for condensed matter Physics Institute of PhysicsChinese Academy of SciencesBeijing 100190China Songshan Lake Materials Laboratory Dongguan 523808China

With the rapid demand for underwater optical communication(UOC), studies of UOC degradation by oceanic turbulence have attached increasing attention worldwide and become a research hot-spot in recent years. Previous studies used a simplified and inaccurate oceanic turbulence spectrum, in which the eddy diffusivity ratio between temperature and salinity is assumed to be unity and the outer scale of turbulence is assumed to be infinite. However, both assumptions are not true in most of the actual marine environments. In this paper, based on the Rytov theory in weak turbulence, we derive analytical expressions of "the aperture-averaged scintillation index"(SI) for both plane and spherical waves, which can clearly demonstrate how SI is influenced by several key factors in UOC. Then, typical fade statistics of the UOC system in weak turbulence is discussed including the probability of fade, the expected number of fades per time, the mean fade time,signal-to-noise ratio and bit error rate. Our results show that spherical wave is preferable in the UOC system in weak turbulence compared to plane wave, and the aperture-averaged effect has a significant impact on UOC system's *** results can be used to determine those key parameters for designing the UOC system over reasonable ranges.

关键词： underwater optical communication(UOC) aperture-averaged scintillation index fade statistics

Enhancing precision in fs-laser material processing by simultaneous spatial and temporal focusing

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Light(Science & Applications) 2014年第1期3卷 223-230页

作者： Robert Kammel Roland Ackermann Jens Thomas Jorg Gotte Stefan Skupin Andreas Tunnermann Stefan Nolte institute of Applied Physics Abbe Center of PhotonicsFriedrich-Schiller-Universitat Jena07743 JenaGermany Department of Physics Colorado School of MinesGoldenCO 80401USA Max Planck institute for the Physics of Complex Systems 01187 DresdenGermany institute of condensed matter theory and solid state optics Abbe Center of PhotonicsFriedrich-Schiller-Universitat Jena07743 JenaGermany Universitede Bordeaux–CNRS–CEA Centre Lasers Intenses et ApplicationsUMR 510733405 TalenceFrance Fraunhofer institute for Applied optics and Precision Engineering 07745 JenaGermany

In recent years,femtosecond(fs)-lasers have evolved into a versatile tool for high precision micromachining of transparent materials because nonlinear absorption in the focus can result in refractive index modifications or material ***,when high pulse energies or low numerical apertures are required,nonlinear side effects such as self-focusing,filamentation or white light generation can decrease the modification *** this paper,we apply simultaneous spatial and temporal focusing(SSTF)to overcome these *** main advantage of SSTF is that the ultrashort pulse is only formed at the focal plane,thereby confining the intensity distribution strongly to the focal volume and suppressing detrimental nonlinear side ***,we investigate the optical breakdown within a water cell by pump-probe shadowgraphy,comparing conventional focusing and SSTF under equivalent focusing *** plasma formation is well confined for low pulse energies,2 mJ,but higher pulse energies lead to the filamentation and break-up of the disruptions for conventional focusing,thereby decreasing the modification *** contrast,plasma induced by SSTF stays well confined to the focal plane,even for high pulse energies up to 8 mJ,preventing extended filaments,side branches or break-up of the ***,while conventional focusing leads to broadband supercontinuum generation,only marginal spectral broadening is observed using *** experimental findings are in excellent agreement with numerical simulations of the nonlinear pulse propagation and interaction ***,SSTF appears to be a powerful tool to control the processing of transparent materials,e.g.,for precise ophthalmic fs-surgery.

关键词： filamentation fs-laser surgery laser-induced optical breakdown materials processing plasma shadowgraphy

High-throughput theoretical design of lithium battery materials

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Chinese Physics B 2016年第1期25卷 97-105页

作者：凌仕刚高健肖睿娟陈立泉 Beijing National Laboratory for condensed matter Physics.institute of Physics.Chinese Academy of Sciences Beijing 100190China

The rapid evolution of high-throughput theoretical design schemes to discover new lithium battery materials is re- viewed, including fiigh-capacity cathodes, low-strain cathodes, anodes, solid state eleclrolytes, and electrolyte additives. With tfie development of efficient theoretical methods and inexpensive computers, high-throughput theoretical calculations have played an increasingly important role in the discovery of new malerials. With the help of automatic simnlation flow, many types of materials can be screened, optimized and designed from a structural database according to specific search criteria. In advanced cell technology, new materials for next generation lithium batteries are of great significance to achieve perlbmmnce, and some representative criteria are： higher energy density, better safety, and faster charge/discharge speed.

关键词： lithium battery materials high-throughput calculations density functional theory virtual screen- ing

Design of a solid-Core Large-Mode-Area Bragg Fiber

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Chinese Physics Letters 2015年第5期32卷 51-54页

作者：李璐庞利辉周志广张爱东何建丽司金海林傲祥 state Key Laboratory of Transient optics and Photonics Xi'an Institute of Optics and Precision Mechanics Chinese Academy of Sciences Xi'an 710119 Beijing National Laboratory for condensed matter Physics Institute of Physics Chinese Academy of Sciences Beijing 100190 Research Center of Laser Fusion China Academy of Engineering Physics Mianyang 621900 School of Electronic and Information Engineering Xi'an Jiaotong University Xi 'an 710049

We report the design and simulation results of a solid-core Bragg fiber with 3-bilayer periodic cladding. The simulation results present single mode bandgap guidance, a large effective area of -400 μm2 around 1.08 μm, a very low bend loss of 0.038dB/m at 1.08μm even under tight bend radius （R = 4cm）, and excellent beam quality. The results indicate that the proposed fiber could be a competitive solution for high-power fiber laser applications.

关键词： BF Design of a solid-Core Large-Mode-Area Bragg Fiber LMA

Modeling electronic and optical properties of Ⅲ–Ⅴ quantum dots-selected recent developments

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Light(Science & Applications) 2022年第1期11卷 13-26页

作者： Alexander Mittelstädt Andrei Schliwa Petr Klenovský institute for solid state Physics Technical University of BerlinHardenbergstrasse 36D-10623 BerlinGermany Department of condensed matter Physics Faculty of ScienceMasaryk UniversityKotlářská267/261137 BrnoCzech Republic Czech Metrology institute Okružní3163800 BrnoCzech Republic

Electronic properties of selected quantum dot(QD)systems are surveyed based on the multi-band k·p method,which we benchmark by direct comparison to the empirical tight-binding algorithm,and we also discuss the newly developed"linear combination of quantum dot orbitals"***,we focus on two major complexes:First,the role of antimony incorporation in InGaAs/GaAs submonolayer QDs and In1−xGax AsySb1−y/GaP QDs,and second,the theory of QD-based quantum cascade lasers and the related prospect of room temperature lasing.

关键词： quantum optical lasing