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Negative magnetoresistance in Weyl semimetals NbAs and NbP： Intrinsic chiral anomaly and extrinsic effects

Frontiers of physics 2017年第3期12卷 19-28页

作者： Yupeng Li Zhen Wang Pengshan Li Xiaojun Yang Zhixuan Shen Feng Sheng Xiaodong Li Yunhao Lu Yi Zheng Zhu-An Xu Department of Physics Zhejiang University Hangzhou 310027 China State Key Lab of Silicon Materials Zhejiang University Hangzhou 310027 China Institute of High Energy Physics Chinese Academy of Sciences Beijing 100049 China Zhejiang California International NanoSystems Institute Zhejiang University Hangzhou 310058 China Collaborative Innovation Centre of Advanced Microstructures Nanjing 210093 China

chiral anomaly-induced negative magnetoresistance （NMR） has been widely used as critical transport evidence for the existence of Weyl fermions in topological semimetals. In this mini-review, we discuss the general observation of NMR phenomena in non-centrosymmetric NbP and NbAs. We show that NMR can arise from the intrinsic chiral anomaly of Weyl fermions and/or extrinsic effects, such as the superimposition of Hall signals; field-dependent inhomogeneous current flow in the bulk, i.e., current jetting; and weak localization （WL） of coexistent trivial carriers. The WLmcontrolled NMR is heavily dependent on sample quality and is characterized by a pronounced crossover from positive to negative MR growth at elevated temperatures, resulting from the competition between the phase coherence time and the spin-orbital scattering constant of the bulk trivial pockets. Thus, the correlation between the NMR and the chiral anomaly need to be scrutinized without the support of complimentary techniques. Because of the lifting of spin degeneracy, the spin orientations of Weyl fermions are either parallel or antiparallel to the momentum, which is a unique physical property known as helicity. The conservation of helicity provides strong protection for the transport of Weyl fermions, which can only be effectively scattered by magnetic impurities. Chemical doping with magnetic and non-magnetic impurities is thus more convincing than the NMR method for detecting the existence of Weyl fermions.

关键词： Weyl semimetals chiral anomaly negative magnetoresistance extrinsic effects

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Symmetry transformation of nonlinear optical current of tilted Weyl nodes and application to ferromagnetic MnBi_(2)Te_(4)

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Chinese Physics B 2024年第4期33卷 160-171页

作者：卢倬成冯济 International Center for Quantum Materials School of PhysicsPeking UniversityBeijing 100871China

A Weyl node is characterized by its chirality and *** develop a theory of how nth-order nonlinear optical conductivity behaves under transformations of anisotropic tensor and tilt, which clarifies how chirality-dependent and-independent parts of optical conductivity transform under the reversal of tilt and *** on this theory, we propose ferromagnetic Mn Bi2Te4as a magnetoelectrically regulated, terahertz optical device, by magnetoelectrically switching the chiralitydependent and-independent DC *** results are useful for creating nonlinear optical devices based on the topological Weyl semimetals.

关键词： nonlinear optical response topological Weyl semimetals chiral anomaly

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Quantum transport in topological semimetals under magnetic fields

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Frontiers of physics 2017年第3期12卷 43-60页

作者： Hai-Zhou Lu Shun-Qing Shen Department of Physics The University of Hong Kong Pokfulam Road Hong Kong China

Topological semimetals are three-dimensional topological states of matter, in which the conduction and valence bands touch at a finite number of points, i.e., the Weyl nodes. Topological semimetals host paired monopoles and antimonopoles of Berry curvature at the Weyl nodes and topologically protected Fermi arcs at certain surfaces. We review our recent works on quantum transport in topo- logical semimetals, according to the strength of the magnetic field. At weak magnetic fields, there are competitions between the positive magnetoresistivity induced by the weak anti-localization effect and negative magnetoresistivity related to the nontrivial Berry curvature. We propose a fitting formula for the magnetoconductivity of the weak anti-localization. We expect that the weak localization may be induced by inter-valley effects and interaction effect, and occur in double-Weyl semimetals. For the negative magnetoresistance induced by the nontrivial Berry curvature in topological semimetals, we show the dependence of the negative magnetoresistance on the carrier density. At strong magnetic fields, specifically, in the quantum limit, the magnetoconductivity depends on the type and range of the scattering potential of disorder. The high-field positive magnetoconductivity nmy not be a com- pelling signature of the chiral anomaly. For long-range Gaussian scattering potential and half filling, the magnetoconductivity can be linear in the quantum limit. A minimal conductivity is found at the Weyl nodes although the density of states vanishes there.

关键词： topological semimetal magnetoconductivity magnetoresistance localization,anti-localization chiral anomaly

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Doping-enhanced robustness of anomaly-related magnetoresistance in WTe_(2±α)flakes

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Chinese Physics B 2023年第4期32卷 634-638页

作者：孟建超陈鑫祥邵婷娜刘明睿姜伟民张子涛熊昌民窦瑞芬聂家财 Department of Physics Beijing Normal UniversityBeijing 100875China College of Science Inner Mongolia University of TechnologyHohhot 010051China State Key Laboratory of Luminescence and Applications Changchun Institute of OpticsFine Mechanics and PhysicsChinese Academy of SciencesChangchun 130033China

We study systematically the negative magnetoresistance(MR)effect in WTe_(2±α)flakes with different thicknesses and doping *** negative MR is sensitive to the relative orientation between electrical-/magnetic-field and crystallographic orientation of WTe_(2±α).The analysis proves that the negative MR originates from chiral anomaly and is *** entropy mobility spectrum is used to analyze the electron and hole concentrations in the flake *** is found that the negative MR observed in WTe_(2±α)flakes with low doping concentration is small,and the high doping concentration is *** doping-induced disorder obviously inhibits the positive MR,so the negative MR can be more easily *** a word,we introduce disorder to suppress positive MR by doping,and successfully obtain the negative MR in WTe_(2±α)flakes with different thicknesses and doping concentrations,which indicates that the chiral anomaly effect in WTe_(2)is robust.

关键词： Weyl semimetal WTe_(2±α)flakes doping chiral anomaly robustness

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Nonlinear current response and electric quantum oscillations in the Dirac semimetal Cd_(3)As_(2)

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Chinese Physics B 2023年第8期32卷 104-108页

作者：崔浩楠吴泽南王建坤祝光宇杨佳洁郑文壮廖志敏王硕林本川俞大鹏 State Key Laboratory for Mesoscopic Physics and Frontiers Science Center for Nano-optoelectronics School of PhysicsPeking UniversityBeijing 100871China Shenzhen Institute for Quantum Science and Engineering Southern University of Science and TechnologyShenzhen 518055China International Quantum Academy Shenzhen 518048China Guangdong Provincial Key Laboratory of Quantum Science and Engineering Southern University of Science and TechnologyShenzhen 518055China

chiral anomaly is a distinct quantum anomaly associated with chiral fermions in Dirac or Weyl *** use of negative magnetoresistance(negative MR)as a signature for this anomaly remains contentious,as trivial mechanisms such as current jetting and weak localization can also induce negative *** this study,we report a novel nonlinear behavior of the chiral anomaly in the longitudinal direction,which we observed by applying parallel current and magnetic field to the Dirac semimetal Cd_(3)A_(s_(2)).This nonlinear characteristic peaks at an intermediate magnetic field of approximately5 T,displaying a resistance-increasing property concomitant with strengthening of the current *** angledependence experiments,we were able to rule out trivial factors,such as thermal effects,geometric artifacts,and ***,additional electric quantum oscillations were observed when the direct current(DC)was applied as high as300μ*** an unusual phenomenon is ascribed to the formation of quantized levels due to Bloch oscillation in the high DC regime,suggesting that an oscillatory density distribution may arise as the electric field *** non-Ohmic electric quantum oscillations open a new avenue for exploring chiral anomaly and other nontrivial topological properties,which is also one of the salient features of nonequilibrium steady states in condensed matter physics.

关键词： nonlinear response electric quantum oscillations chiral anomaly topological semimetals

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Negative magnetoresistance in Dirac semimetal Cd_(3)As_(2)with in-plane magnetic field perpendicular to current

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Chinese Physics B 2023年第7期32卷 181-185页

作者：崔浩楠祝光宇王建坤杨佳洁郑文壮林本川廖志敏王硕俞大鹏 State Key Laboratory for Mesoscopic Physics and Frontiers Science Center for Nano-optoelectronics School of PhysicsPeking UniversityBeijing 100871China Shenzhen Institute for Quantum Science and Engineering Southern University of Science and TechnologyShenzhen 518055China International Quantum Academy Shenzhen 518048China Guangdong Provincial Key Laboratory of Quantum Science and Engineering Southern University of Science and TechnologyShenzhen 518055China

Topological insulators and semimetals have exotic surface and bulk states with massless Dirac or Weyl fermions,demonstrating microscopic transport phenomenon based on relativistic *** anomaly induced negative magnetoresistance(negative MR)under parallel magnetic field and current has been used as a probable evidence ofWeyl fermions in recent *** we report a novel negative MR result with mutually perpendicular in-plane magnetic field and current in Cd_(3)As_(2)*** negative MR has a considerable value of-16%around 1.5 K and could persist to room temperature of 300 K with value of-1%.The gate tuning and angle dependence of the negative MR demonstrate the mechanism of the observed negative MR is different from the chiral *** current paths induced by charge puddles and disorder might be involved to produce such considerable negative *** results indicate the negative MR effect in topological semimetals involves synergistic effects of many mechanisms besides chiral anomaly.

关键词： negative magnetoresistance chiral anomaly topological semimetals quantum transport

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