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中国科学:物理学、力学、天文学英文版 2023年第9期66卷 79-85页

作者： Dinghui Wang Huaiqiang Wang dingyu xing Haijun Zhang National Laboratory of Solid State Microstructures School of PhysicsNanjing UniversityNanjing 210093China Collaborative Innovation Center of Advanced Microstructures Nanjing UniversityNanjing 210093China

Layered van der Waals(vdW) topological materials, especially the recently discovered MnBi_(2)Te_(4)-family magnetic topological insulators(TIs), have aroused great attention. However, there has been a serious debate about whether the surface states are gapped or gapless for antiferromagnetic(AFM) TI MnBi_(2)Te_(4), which is crucial to the prospect of various magnetic topological phenomena. Here, a minimal three-Dirac-fermion approach is developed to generally describe topological surface states of nonmagnetic/magnetic vdW TIs under the modulation of the interlayer vdW gap. In particular, this approach is applied to address the controversial issues concerning the surface states of vdW AFM TIs. Remarkably, topologically protected gapless Dirac-cone surface states are found to arise due to a small expansion of the interlayer vdW gap on the surface, when the Chern number equals zero for the surface ferromagnetic layer;while the surface states remain gapped in all other cases. These results are further confirmed by our first-principles calculations on AFM TI MnBi_(2)Te_(4). The theorectically discovered gapless Dirac-cone states provide a unique mechanism for understanding the puzzle of the experimentally observed gapless surface states in MnBi_(2)Te_(4).This work also provides a promising way for experiments to realize the intrinsic magnetic quantum anomalous Hall efect in MnBi_(2)Te_(4) films with a large energy gap.

关键词： magnetic topological insulator van der Waals gap Dirac-fermion model gapless surface state

MAGUS: machine learning and graph theory assisted universal structure searcher

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National Science Review 2023年第7期10卷 213-233页

作者： Junjie Wang Hao Gao Yu Han Chi Ding Shuning Pan Yong Wang Qiuhan Jia Hui-Tian Wang dingyu xing Jian Sun National Laboratory of Solid State Microstructures School of Physics and Collaborative Innovation Center of Advanced Microstructures Nanjing University

Crystal structure predictions based on first-principles calculations have gained great success in materials science and solid state physics.However,the remaining challenges still limit their applications in systems with a large number of atoms,especially the complexity of conformational space and the cost of local optimizations for big systems.Here,we introduce a crystal structure prediction method,MAGUS,based on the evolutionary algorithm,which addresses the above challenges with machine learning and graph theory.Techniques used in the program are summarized in detail and benchmark tests are provided.With intensive tests,we demonstrate that on-the-fly machine-learning potentials can be used to significantly reduce the number of expensive first-principles calculations,and the crystal decomposition based on graph theory can efficiently decrease the required configurations in order to find the target structures.We also summarized the representative applications of this method on several research topics,including unexpected compounds in the interior of planets and their exotic states at high pressure and high temperature(superionic,plastic,partially diffusive state,etc.);new functional materials(superhard,high-energy-density,superconducting,photoelectric materials),etc.These successful applications demonstrated that MAGUS code can help to accelerate the discovery of interesting materials and phenomena,as well as the significant value of crystal structure predictions in general.

关键词： crystal structure searching materials design ab-initio calculations density functional theory high-pressure phase transition

Anomalous Andreev reflection on a torus-shaped Fermi surface

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Science China(Physics,Mechanics & Astronomy) 2021年第6期64卷 2-7页

作者： Wei Luo Wei Chen dingyu xing National Laboratory of Solid State Microstructures School of PhysicsNanjing UniversityNanjing 210093China School of Science Jiangxi University o f Science and TechnologyGanzhou 341000China Collaborative Innovation Center of Advanced Microstructures Nanjing UniversityNanjing 210093China

Andreev reflection(AR)refers to the electron-hole conversion at the normal metal-superconductor interface.In a threedimensional metal with a spherical Fermi surface,retro(specular)AR can occur with the sign reversal of all three(a single)components of particle velocity.Here,we predict a novel type of AR with the inversion of two velocity components,dubbed"anomalous Andreev reflection"(AAR),which can be realized in a class of materials with a torus-shaped Fermi surface,such as doped nodal line semimetals.For its toroidal circle perpendicular to the interface,the Fermi torus doubles the AR channels and generates multiple AR processes.In particular,the AAR and retro AR are found to dominate electron transport in the light and heavy doping regimes,respectively.We show that the AAR visibly manifests itself as a ridge structure in the spatially resolved nonlocal conductance,in contrast to the peak structure for the retro AR.Our work opens a new avenue for the AR spectroscopy and offers a clear transport signature of the torus-shaped Fermi surface.

关键词： nodal line semimetal Andreev reflection Fermi surface

Tunable dynamical magnetoelectric effect in antiferromagnetic topological insulator MnBi_(2)Te_(4) films

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npj Computational Materials 2021年第1期7卷 1101-1106页

作者： Tongshuai Zhu Huaiqiang Wang Haijun Zhang dingyu xing National Laboratory of Solid State Microstructures School of PhysicsNanjing UniversityNanjingChina Department of Physics University of ZurichZurichSwitzerland Collaborative Innovation Center of Advanced Microstructures Nanjing UniversityNanjingChina

Axion was postulated as an elementary particle to solve the strong charge conjugation and parity puzzle,and later axion was also considered to be a possible component of dark matter in the universe.However,the existence of axions in nature has not been confirmed.Interestingly,axions arise out of pseudoscalar fields derived from the Chern–Simons theory in condensed matter physics.In antiferromagnetic insulators,the axion field can become dynamical due to spin-wave excitations and exhibits rich exotic phenomena,such as axion polariton.However,antiferromagnetic dynamical axion insulator has yet been experimentally identified in realistic materials.Very recently,MnBi_(2)Te_(4)was discovered to be an antiferromagnetic topological insulator with a quantized static axion field protected by inversion symmetry P and magnetic-crystalline symmetry S.Here,we studied MnBi_(2)Te_(4)films in which both the P and S symmetries are spontaneously broken and found that substantially enhanced dynamical magnetoelectric effects could be realized through tuning the thickness of MnBi_(2)Te_(4)films,temperature,or element substitutions.Our results show that thin films of MnBi_(2)Te_(4)and related compounds could provide a promising material platform to experimentally study axion electrodynamics.

关键词： materials dynamical insulator

A novel superhard tungsten nitride predicted by machine-learning accelerated crystal structure search

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Science Bulletin 2018年第13期63卷 817-824页

作者： Kang Xia Hao Gao Cong Liu Jianan Yuan Jian Sun Hui-Tian Wang dingyu xing National Laboratory of Solid State Microstructures School of Physics and Collaborative Innovation Center of Advanced MicrostructuresNanjing University

Transition metal nitrides have been suggested to have both high hardness and good thermal stability with large potential application value,but so far stable superhard transition metal nitrides have not been synthesized.Here,with our newly developed machine-learning accelerated crystal structure searching method,we designed a superhard tungsten nitride,h-WN_6,which can be synthesized at pressure around65 GPa and quenchable to ambient pressure.This h-WN_6is constructed with single-bonded armchair-like N_6rings and presents ionic-like features,which can be formulated as W^(2.4+)N_6^(2.4à).It has a band gap of1.6 e V at 0 GPa and exhibits an abnormal gap broadening behavior under pressure.Excitingly,this h-WN_6is found to be the hardest among transition metal nitrides known so far(Vickers hardness around57 GPa)and also has a very high melting temperature(around 1,900 K).Additionally,the good gravimetric(3.1 k J/g)and volumetric(28.0 k J/cm^3)energy densities make this nitrogen-rich compound a potential high-energy-density material.These predictions support the designing rules and may stimulate future experiments to synthesize superhard and high-energy-density material.

关键词：金属氮化物水晶结构预言机器学习搜索钨热稳定性

High-pressure phases of Weyl semimetals NbP, NbAs, TaP,and TaAs

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Science China(Physics,Mechanics & Astronomy) 2018年第3期61卷 47-55页

作者： ZhaoPeng Guo PengChao Lu Tong Chen JueFei Wu Jian Sun dingyu xing National Laboratory of Solid State Microstructures School of Physics and Collaborative Innovation Center of Advanced MicrostructuresNanjing University

In this study, we used the crystal structure search method and first-principles calculations to systematically explore the highpressure phase diagrams of the TaAs family(NbP, NbAs, TaP, and TaAs). Our calculation results show that NbAs and TaAs have similar phase diagrams, the same structural phase transition sequence I4_1 md→P6 m2→P2_1/c→Pm3 m, and slightly different transition pressures. The phase transition sequence of NbP and TaP differs somewhat from that of NbAs and TaAs, in which new structures emerge, such as the Cmcm structure in NbP and the Pmmn structure in TaP. Interestingly, we found that in the electronic structure of the high-pressure phase P6 m2-NbAs, there are coexistingWeyl points and triple degenerate points, similar to those found in high-pressure P6 m2-TaAs.

关键词： TaAs NbP 高压半金属水晶结构阶段转变计算系统搜索方法

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

Metallic Aluminum Suboxides with Ultrahigh Electrical Conductivity at High Pressure

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Research 2023年第2期2022卷 25-32页

作者： Tianheng Huang Cong Liu Junjie Wang Shuning Pan Yu Han Chris J.Pickard Ravit Helled Hui-Tian Wang dingyu xing Jian Sun National Laboratory of Solid State Microstructures School of PhysicsAnd Collaborative Innovation Center of Advanced MicrostructuresNanjing UniversityNanjing 210093China Department of Materials Science Metallurgy University of Cambridge27 Charles Babbage RoadCambridge CB3 OFSUK Advanced Institute for Materials Research Tohoku University2-1-1 KatahiraAobaSendai 980-8577Japan Institute for Computational Science Center for Theoretical Astrophysics CosmologyUniversity of ZurichSwitzerland

Aluminum,as the most abundant metallic elemental content in the Earth's crust,usually exists in the form of alumina(Al_(2)O_(3)).However,the oxidation state of aluminum and the crystal structures of aluminum oxides in... 详细信息

关键词： oxides Conductivity interior

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钾氨电子化合物中的准二维自旋-派尔斯相变

Science Bulletin 2024年第8期69卷 1027-1036页

作者：丁驰鲁清郭照芃黄天衡王俊杰韩瑜邢定钰孙建 National Laboratory of Solid State Microstructures School of Physics and Collaborative Innovation Center of Advanced MicrostructuresNanjing UniversityNanjing 210093China

Electron-phonon interactions and electron-electron correlations represent two crucial facets of condensed matter physics.For instance,in a half-filled spin-1/2 anti-ferromagnetic chain,the lattice dimerization induced by electron-nucleus interaction can be intensified by onsite Coulomb repulsion,resulting in a spin-Peierls state.Through first-principles calculations and crystal structure prediction methods,we have identified that under mild pressures,potassium and ammonia can form stable compounds:R3m K(NH_(3))_(2),Pm3 m K(NH_(3))_(2),and Cm K_(2)(NH_(3))_(3).Our predictions suggest that the R3 m K(NH_(3))_(2)exhibits electride characteristics,marked by the formation of interstitial anionic electrons(IAEs)in the interlayer space.These IAEs are arranged in quasi-two-dimensional triangular arrays.With increasing pressure,the electronic van-Hove singularity shifts toward the Fermi level,resulting in an augmented density of states and the onset of both Peierls and magnetic instabilities.Analyzing these instabilities,we determine that the ground state of the R3 m K(NH_(3))_(2)is the dimerized P2_(1)/m phase with zigzag-type anti-ferromagnetic IAEs.This state can be described by the triangular-lattice antiferromagnetic Heisenberg model with modulated magnetic interactions.Furthermore,we unveil the coexistence and positive interplay between magnetic and Peierls instability,constituting a scenario of spin-Peierls instability unprecedented in realistic 2D materials,particularly involving IAEs.This work provides valuable insights into the coupling of IAEs with the adjacent lattice and their spin correlations in quantum materials.

关键词： Quasi-two-dimensional Interstitial anionic electrons Potassium ammonia compounds Spin-Peierls transition Antiferromagnetism

Composite topological nodal lines penetrating the Brillouin zone in orthorhombic AgF_(2)

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npj Computational Materials 2019年第1期5卷 694-701页

作者： Dexi Shao Huaiqiang Wang Tong Chen Pengchao Lu Qinyan Gu Li Sheng dingyu xing Jian Sun National Laboratory of Solid State Microstructures and Department of Physics Nanjing University210093 NanjingChina Collaborative Innovation Center of Advanced Microstructures 210093 NanjingChina

It has recently been found that nonsymmorphic symmetries can bring many exotic band crossings.Here,based on symmetry analysis,we predict that materials with time-reversal symmetry in the space group of Pbca(No.61)possess rich symmetry-enforced band crossings,including nodal surfaces,fourfold degenerate nodal lines and hourglass Dirac loops,which appear in triplets as ensured by the cyclic permutation symmetry.We take Pbca AgF_(2) as an example in real systems and studied its band structures with ab initio calculations.Specifically,in the absence of spin-orbit coupling(SOC),besides the above-mentioned band degeneracies,this system features a nodal chain and a nodal armillary sphere penetrating the Brillouin zone(BZ).While with SOC,we find a new configuration of the hourglass Dirac loop/chain with the feature traversing the BZ,which originates from the splitting of a Dirac loop confined in the BZ.Furthermore,guided by the bulk-surface correspondence,we calculated the surface states to explore these bulk nodal phenomena.The evolution of these interesting nodal phenomena traversing the BZ under two specific uniaxial strains is also discussed.

关键词： nodal symmetry Brillouin