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Twistronics in graphene-based van der Waals structures

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Chinese physics B 2020年第11期29卷 1-16页

作者： Ya-Ning Ren Yu Zhang Yi-Wen Liu Lin He center for Advanced Quantum Studies Department of PhysicsBeijing Normal UniversityBeijing 100875China

The electronic properties of van der Waals(vd W) structures can be substantially modified by the moiré superlattice potential, which strongly depends on the twist angle among the compounds. In twisted bilayer graphene(TBG), two lowenergy Van Hove singularities(VHSs) move closer with decreasing twist angles and finally become highly non-dispersive flat bands at the magic angle(~1.1°). When the Fermi level lies within the flat bands of the TBG near the magic angle,Coulomb interaction is supposed to exceed the kinetic energy of the electrons, which can drive the system into various strongly correlated phases. Moreover, the strongly correlated states of flat bands are also realized in other graphene-based vd W structures with an interlayer twist. In this article, we mainly review the recent scanning tunneling microscopy(STM)advances on the strongly correlated physics of the magic-angle TBG(MATBG) and the small-angle twisted multilayer graphene. Lastly we will give out a perspective of this field.

关键词： twisted bilayer graphene correlated states superconductivity

Attractive electron-electron interaction induced by geometric phase in a Bloch band

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Science China(physics,Mechanics & Astronomy) 2020年第2期63卷 146-149页

作者： JunRen Shi Qian Niu International center for Quantum Materials Peking UniversityBeijing 100871China Collaborative Innovation center of Quantum Matter Beijing 100871China department of physics University of Texas at AustinAustin TX 78712USA

Attractive interaction between electrons(or neutral fermions)is responsible for superconductivity(or superfluidity).In condensed matter systems,attractive interaction is usually induced by the boson-exchange mechanism[1].Indeed,in the celebrated Bardeen-Cooper-Schreiffer(BCS)theory,electrons develop attractive interaction by exchanging phonons[2].Subsequent studies show that other collective excitations such as charge density waves[3]and spin fluctuations[4]can also induce attractive *** bosonexchange mechanism,together with the concept of Cooperpair,is considered to be the cornerstone of the modern theory of superconductivity.

关键词： interaction. interaction attractive

First-Principles Study on the Elastic Properties of Platinum Nitride

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Chinese physics Letters 2005年第10期22卷 2637-2638页

作者：范长增孙力玲王渊旭魏尊杰刘日平曾松岩王文魁 National Laboratory for superconductivity Institute of Physics Chinese Academy of Sciences Beijing 100080 department of Material Science and Engineering Harbin Institute of Technology Harbin 150001 tlaterial Science and Engineering Yanshan University Qinhuangdao 066004 Computational Materials Science center National Institute for Materials Science Tsukuba 305-0044 Japan

Elastic properties of platinum nitride （PtN） are studied by first-principles calculations with the fully relativistic full potential linearized augmented plane-wave （LAPW） method, the plane-wave ultrasoft pseudopotential （PWPP） and the projector-augmented wave （PAW） methods. The results reveal that：（1） the scalar relativistic scheme is sufficient to treat the valence electronic structure, i.e. the spin-orbit effect has little effect on the bulk modulus value of platinum nitride; （2） the all-electron full potential method is no more accurate than the pseudopotential and PAW-based methods when calculating the lattice constant and bulk modulus properties of the platinum nitride; （3） platinum nitride in zinc-blende structure is unstable and its crystal structure is still an open problem.

关键词： TOTAL-ENERGY CALCULATIONS WAVE BASIS-SET

Using Helmholtz resonator arrays to improve dipole transmission efficiency in waveguide

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Chinese physics B 2019年第9期28卷 265-269页

作者： Liwei Wang Li Quan Feng Qian Xiaozhou Liu Key Laboratory of Modern Acoustics(Ministry of Education) Institute of Acoustics and School of PhysicsCollaborative Innovation Center of Advanced MicrostructuresNanjing UniversityNanjing 210093China department of Electrical and Computer Engineering The University of Texas at AustinAustinTexas 78713USA College of physics & Electronic Engineering Changshu Institute of TechnologyChangshu 215500China

It is well known that the radiation efficiency of an acoustic dipole is very low, increasing the radiation efficiency of an acoustic dipole is a difficult task, especially in an ordinary *** addition, current acoustic superlenses all utilize in-phase sources to do the super-resolution imaging, it is almost impossible to realize super-resolution imaging of an acoustic *** this paper, after using the Helmholtz resonator arrays(HRAs) which are placed at the upper and lower surfaces of the waveguide, we observe a large dipole radiation efficiency at the certain frequency, which gives a method to observe an acoustic dipole in the far field and offers a novel model which is promising to realize the superlens with a source of an acoustic *** discuss how the arrangement of HRAs affects the transmission of the acoustic dipole.

关键词： dipole source waveguide Helmholtz resonator arrays double-negative area

Protonation induced high-T_c phases in iron-based superconductors evidenced by NMR and magnetization measurements

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Science Bulletin 2018年第1期63卷 11-16页

作者： Yi Cui Gehui Zhang Haobo Li Hai Lin Xiyu Zhu Hai-Hu Wen Guoqing Wang Jinzhao Sun Mingwei Ma Yuan Li Dongliang Gong Tao Xie Yanhong Gu Shiliang Lie Huiqian Luo Pu Yu Weiqiang Yu department of physics Renmin University of China Beijing 100872 China State Key Laboratory of Low Dimensional Quantum physics and department of physics Tsinghua University Beijing 100084 China center for Superconducting physics and Materials National Laboratory of Solid State Microstructures and Department of Physics Collaborative Innovation Center for Advanced Microstructures Nanjing University Nanjing 210093 China International center for Quantum Materials School of Physics Peking University Beijing 100871 China Collaborative Innovation center of Quantum Matter Beijing 100871 China Beijing National Laboratory for Condensed Matter physics Institute of Physics Chinese Academy of Sciences Beijing 100190 China University of Chinese Academy of Sdences Beijing 100049 China

Chemical substitution during growth is a well-established method to manipulate electronic states of quantum materials, and leads to rich spectra of phase diagrams in cuprate and iron-based superconductors. Here we report a novel and generic strategy to achieve nonvolatile electron doping in series of(i.e.11 and 122 structures) Fe-based superconductors by ionic liquid gating induced protonation at room temperature. Accumulation of protons in bulk compounds induces superconductivity in the parent compounds, and enhances the Tclargely in some superconducting ones. Furthermore, the existence of proton in the lattice enables the first proton nuclear magnetic resonance(NMR) study to probe directly superconductivity. Using Fe S as a model system, our NMR study reveals an emergent high-Tcphase with no coherence peak which is hard to measure by NMR with other isotopes. This novel electric-fieldinduced proton evolution opens up an avenue for manipulation of competing electronic states(*** insulators), and may provide an innovative way for a broad perspective of NMR measurements with greatly enhanced detecting resolution.

关键词： Ion liquid gating Protonation NMR Iron-based superconductors

Late-Stage Vortical Structures and Eddy Motions in a Transitional Boundary Layer

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Chinese physics Letters 2010年第2期27卷 161-164页

作者：刘小兵陈政清刘超群 Wind Engineering Research center Hunan University Changsha 410082 department of Mathematics University of Texas at Arlington Arlington Texas 76019 USA

A high-order direct numerical simulation of flow transition over a flat-plate at a free stream Mach number 0.5 is carried out. Formation and development of three-dimensional vortical structures, typically shown as λ-vortices, hairpin vortices and ring-like vortices, are observed. Numerical results show that there is a strong downdraft motion of fluid excited by every ring-like vortex in the late-stage of the transition process. At two sides of the vortical structure centerline, the downdraft motions induced by the ring-like vortex and the rotating legs superimpose. This is responsible for the appearance of a high-speed streak associated with the positive spike observed in a previous investigation and the appearance of a high-shear layer in the near wall region.

关键词： Fluid dynamics

Nanoscale superconductivity of ?-Ga islands grown by molecular beam epitaxy

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Science China(physics,Mechanics & Astronomy) 2015年第10期58卷 89-93页

作者： ZHANG HuiMin PENG JunPing GUAN JiaQi LI Zhi SONG CanLi WANG LiLi HE Ke MA XuCun XUE QiKun Institute of physics Chinese Academy of Sciences State Key Laboratory of Low-Dimensional Quantum physics Department of Physics Tsinghua University Collaborative Innovation center for Quantum Matter

We report on the preparation and superconductivity of metastable γ-Ga islands on Si(111) substrate. The Ga grows in a typical Volmer-Weber mode at a low temperature of 110 K, resulting in formation of Ga nanoislands at various sizes with the identical γ-phase. In-situ low temperature scanning tunneling spectra reveal quantized electronic states in ultrathin Ga islands. It is found that both the lateral size and thickness of the Ga islands strongly suppress the superconductivity. Due to substantial surface energy contribution, the transition temperature Tc scales inversely with the island thickness and the minimum thickness for the occurrence of superconductivity is calculated to be two monolayers.

关键词： γ-Ga superconductivity size effects molecular beam epitaxy

superconductivity in Pd-Intercalated Ternary Rare-Earth Polychalcogenide NdSeTe2

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Chinese physics Letters 2015年第11期32卷 140-143页

作者：王培培薛面起龙雨佳赵凌霄蔡瑶杨槐馨李建奇任治安陈根富 Institute of physics and Beijing National Laboratory for Condensed Matter physics Chinese Academy of Sciences department of physics Renmin University of China Collaborative Innovation center of Quantum Matter

We synthesize a set of Pd-doped polycrystalline samples PdxNdSeTe2 and measure their physical properties. Compared with pure NdSeTe2, the charge density wave （CDW） order is continuously suppressed with the Pdintercalation. Bulk superconductivity first appears at x = 0.06 with Tc nearly 2.5I〈, coexisting with a CDW transition at 176K. Further Pd-doping enhances Tc, until it reaches the maximum value 2.84K at x=0.1, meanwhile the CDW transition vanishes. The upper critical field for the optimal doping sample ***2 is determined from the R-H measurement, which is estimated to be 0.6 T. These results provide another kind of ideal compound for studying the interplay between CDW and superconductivity systematically.

关键词： Pd Te Nd superconductivity in Pd-Intercalated Ternary Rare-Earth Polychalcogenide NdSeTe2

Radiopaque nano and polymeric materials for atherosclerosis imaging, embolization and other catheterization procedures

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Acta Pharmaceutica Sinica B 2018年第3期8卷 360-370页

作者： Li Tian Linfeng Lu James Feng Marites P.Melancon department of Interventional Radiology The University of Texas MD Anderson Cancer Center Duke University Graduate School of Biomedical Sciences University of Texas Health Science Center at Houston

A review of radiopaque nano and polymeric materials for atherosclerosis imaging and catheterization procedures is presented in this paper. Cardiovascular diseases(CVDs) are the leading cause of death in the US with atherosclerosis as a significant contributor for mortality and morbidity. In this review paper, we discussed the physics of radiopacity and X-ray/CT, clinically used contrast agents,and the recent progress in the development of radiopaque imaging agents and devices for the diagnosis and treatment of CVDs. We focused on radiopaque imaging agents for atherosclerosis, radiopaque embolic agents and drug eluting beads, and other radiopaque medical devices related to catheterization procedures to treat CVDs. Common strategies of introducing radiopacity in the polymers, together with examples of their applications in imaging and medical devices, are also presented.

关键词： Radiopaque contrast nanoparticles and materials X-ray CT Vascular diseases Atherosclerosis Plaque Stenosis Embolization Inferior vena cava filters