The field of two-dimensional topological semimetals,which emerged at the intersection of two-dimensional materials and topological materials,has been rapidly developing in recent years.In this article,we briefly revie...The field of two-dimensional topological semimetals,which emerged at the intersection of two-dimensional materials and topological materials,has been rapidly developing in recent years.In this article,we briefly review the progress in this field.Our focus is on the basic concepts and notions,in order to convey a coherent overview of the field.Some material examples are discussed to illustrate the concepts.We discuss the outstanding problems in the field that need to be addressed in future research.展开更多
Topological materials have aroused great interest in recent years,especially when magnetism is involved.Pressure can effectively tune the topological states and possibly induce superconductivity.Here we report the hig...Topological materials have aroused great interest in recent years,especially when magnetism is involved.Pressure can effectively tune the topological states and possibly induce superconductivity.Here we report the high-pressure study of topological semimetals XCd_(2)Sb_(2)(X=Eu and Yb),which have the same crystal structure.In antiferromagnetic(AFM)Weyl semimetal EuCd_(2)Sb_(2),the Néel temperature(TN)increases from 7.4 K at ambient pressure to 50.9 K at 14.9 GPa.When pressure is above 14.9 GPa,the AFM peak of resistance disappears,indicating a non-magnetic state.In paramagnetic Dirac semimetal candidate YbCd_(2)Sb_(2),pressure-induced superconductivity appears at 1.94 GPa,then Tc reaches to a maximum of 1.67 K at 5.22 GPa and drops to zero at about 30 GPa,displaying a dome-shaped temperature–pressure phase diagram.High-pressure x-ray diffraction measurement demonstrates that a crystalline-to-amorphous phase transition occurs at about 16 GPa in YbCd_(2)Sb_(2),revealing the robustness of pressure-induced superconductivity against structural instability.Similar structural phase transition may also occur in EuCd_(2)Sb_(2),causing the disappearance of magnetism.Our results show that XCd_(2)Sb_(2)(X=Eu and Yb)is a novel platform for exploring the interplay among magnetism,topology,and superconductivity.展开更多
We propose a square optical lattice in which some of neighbor hoppings have a Peierls phase.The Peierls phase makes the lattice have a special band structure and induces the existence of Dirac points in the Brillouin ...We propose a square optical lattice in which some of neighbor hoppings have a Peierls phase.The Peierls phase makes the lattice have a special band structure and induces the existence of Dirac points in the Brillouin zone,which means that topological semimetals exist in the system.The Dirac points move with the change of the Peierls phase and the Dirac cones are anisotropic for some vales of the Peierls phase.The lattice has a novel hidden symmetry,which is a composite antiunitary symmetry composed of a translation operation,a sublattice exchange,a complex conjugation,and a local U(1)gauge transformation.We prove that the Dirac points are protected by the hidden symmetry and perfectly explain the moving of Dirac points with the change of the Peierls phase based on the hidden symmetry protection.展开更多
Materials featuring topological energy bands and nontrivial surface states hold significant promise in unlocking unprecedented opportunities for innovating electrocatalytic mechanism.However,it remains a challenge to ...Materials featuring topological energy bands and nontrivial surface states hold significant promise in unlocking unprecedented opportunities for innovating electrocatalytic mechanism.However,it remains a challenge to realize superior topological catalysts which can carry both high catalytic activity and excellent catalytic stability.Here,we propose that a family of Ni-based binary materials hosting fantasying topological conjunct-nodalpoint state and a large nontrivial energy window(NEWD)represents an ideal choice for such superior topological catalysts in hydrogen evolution reaction.The presence of conjunct-nodal-points ensures long Fermi arcs on the surface,thereby enabling an extremely high catalytic activity.The NEWD plays a crucial role in stabilizing the high catalytic activity against external perturbations,such as strain and electron/hole injection.The roles for conjunctnodal-points and NEWD are substantiated by the observable weakening of catalytic performance during topological phase transitions,which result in the removal of the conjunct-nodal-points,NEWD and their corresponding long Fermi arcs.Our work unveils a hidden mechanism and opens a feasible route for developing superior quantum catalysts from novel topology point of view.展开更多
We review our most recent research on quantum transport,organizing the review according to the intensity of the magnetic field and focus mostly on topological semimetals and topological insulators.We first describe th...We review our most recent research on quantum transport,organizing the review according to the intensity of the magnetic field and focus mostly on topological semimetals and topological insulators.We first describe the phenomenon of quantum transport when a magnetic field is not present.We introduce the nonlinear Hall effect and its theoretical descriptions.Then,we discuss Coulomb instabilities in 3D higher-order topological insulators.Next,we pay close attention to the surface states and find a function to identify the axion insulator in the antiferromagnetic topological insulator MnBi2Te4.Under weak magnetic fields,we focus on the decaying Majorana oscillations which has the correlation with spin−orbit coupling.In the section on strong magnetic fields,we study the helical edge states and the one-sided hinge states of the Fermi-arc mechanism,which are relevant to the quantum Hall effect.Under extremely large magnetic fields,we derive a theoretical explanation of the negative magnetoresistance without a chiral anomaly.Then,we show how magnetic responses can be used to detect relativistic quasiparticles.Additionally,we introduce the 3D quantum Hall effect’s charge-density wave mechanism and compare it with the theory of 3D transitions between metal and insulator driven by magnetic fields.展开更多
Spin–momentum locking is a key feature of the topological surface state, which plays an important role in spintronics.The electrical detection of current-induced spin polarization protected by the spin–momentum lock...Spin–momentum locking is a key feature of the topological surface state, which plays an important role in spintronics.The electrical detection of current-induced spin polarization protected by the spin–momentum locking in nonmagnetic systems provides a new platform for developing spintronics, while previous studies were mostly based on magnetic materials.In this study, the spin transport measurement of Dirac semimetal Cd_(3)As_(2) was studied by three-terminal geometry, and a hysteresis loop signal with high resistance and low resistance state was observed. The hysteresis was reversed by reversing the current direction, which illustrates the spin–momentum locking feature of Cd_(3)As_(2). Furthermore, we realized the on–off states of the spin signals through electric modulation of the Fermi arc via the three-terminal configuration, which enables the great potential of Cd_(3)As_(2) in spin field-effect transistors.展开更多
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 monopol...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.展开更多
Prototypical three-dimensional(3D)topological Dirac semimetals(DSMs),such as Cd3As2 and Na3Bi,contain electrons that obey a linear momentum-energy dispersion with different Fermi velocities along the three orthogonal...Prototypical three-dimensional(3D)topological Dirac semimetals(DSMs),such as Cd3As2 and Na3Bi,contain electrons that obey a linear momentum-energy dispersion with different Fermi velocities along the three orthogonal momentum dimensions.Despite being extensively studied in recent years,the inherent Fermi velocity anisotropy has often been neglected in the theoretical and numerical studies of 3D DSMs.Although this omission does not qualitatively alter the physics of light-driven massless quasiparticles in 3D DSMs,it does quantitatively change the optical coefficients which can lead to nontrivial implications in terms of nanophotonics and plasmonics applications.Here we study the linear optical response of 3D DSMs for general Fermi velocity values along each direction.Although the signature conductivity-frequency scaling,σ(ω)∝ω,of 3D Dirac fermion is well-protected from the Fermi velocity anisotropy,the linear optical response exhibits strong linear dichroism as captured by the universal extinction ratio scaling law,Λi j=(vi/v j)^2(where i=j denotes the three spatial coordinates x,y,z,and vi is the i-direction Fermi velocity),which is independent of frequency,temperature,doping,and carrier scattering lifetime.For Cd3As2 and Na3Bi3,an exceptionally strong extinction ratio larger than 15 and covering a broad terahertz window is revealed.Our findings shed new light on the role of Fermi velocity anisotropy in the optical response of Dirac semimetals and open up novel polarization-sensitive functionalities,such as photodetection and light modulation.展开更多
A Weyl node is characterized by its chirality and tilt.We develop a theory of how nth-order nonlinear optical conductivity behaves under transformations of anisotropic tensor and tilt, which clarifies how chirality-de...A Weyl node is characterized by its chirality and tilt.We 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 chirality.Built on this theory, we propose ferromagnetic Mn Bi2Te4as a magnetoelectrically regulated, terahertz optical device, by magnetoelectrically switching the chiralitydependent and-independent DC photocurrents.These results are useful for creating nonlinear optical devices based on the topological Weyl semimetals.展开更多
Recently, the Dirac and Weyl semimetals have attracted extensive attention in condensed matter physics due to both the fundamental interest and the potential application of a new generation of electronic devices. Here...Recently, the Dirac and Weyl semimetals have attracted extensive attention in condensed matter physics due to both the fundamental interest and the potential application of a new generation of electronic devices. Here we review the exotic electrical transport phenomena in Dirac andWeyl semimetals. Section 1 is a brief introduction to the topological semimetals(TSMs). In Section 2 and Section 3, the intriguing transport phenomena in Dirac semimetals(DSMs) andWeyl semimetals(WSMs) are reviewed, respectively. The most widely studied Cd_3A_(s2) and the TaAs family are selected as representatives to show the typical properties of DSMs and WSMs, respectively. Beyond these systems, the advances in other TSM materials,such as ZrTe_5 and the MoTe_2 family, are also introduced. In Section 4, we provide perspectives on the study of TSMs especially on the magnetotransport investigations.展开更多
Topological semimetals are a new type of matter with one-dimensional Fermi lines or zero-dimensional Weyl or Dirac points in momentum space. Here using first-principles calculations, we find that the non-centrosymmetr...Topological semimetals are a new type of matter with one-dimensional Fermi lines or zero-dimensional Weyl or Dirac points in momentum space. Here using first-principles calculations, we find that the non-centrosymmetric PbTaS2 is a topological nodal line semimetal. In the absence of spin-orbit coupling (SOC), one band inversion happens around a high symmetrical H point, which leads to forming a nodal line. The nodal line is robust and protected against gap opening by mirror reflection symmetry even with the inclusion of strong SOC. In addition, it also hosts exotic drumhead surface states either inside or outside the projected nodal ring depending on surface termination. The robust bulk nodal lines and drumhead-like surface states with SOC in PbTaS2 make it a potential candidate material for exploring the freakish properties of the topological nodal line fermions in condensed matter systems.展开更多
Various novel physical properties have emerged in Dirac electronic systems, especially the topological characters pro- tected by symmetry. Current studies on these systems have been greatly promoted by the intuitive c...Various novel physical properties have emerged in Dirac electronic systems, especially the topological characters pro- tected by symmetry. Current studies on these systems have been greatly promoted by the intuitive concepts of Berry phase and Berry curvature, which provide precise definitions of the topological phases. In this topical review, transport properties of topological insulator (Bi2Se3), topological Dirac semimetal (Cd3As2), and topological insulator-graphene heterojunc- tion are presented and discussed. Perspectives about transport properties of two-dimensional topological nontrivial systems, including topological edge transport, topological valley transport, and topological Weyl semimetals, are provided.展开更多
Extremely large magnetoresistance(XMR)has been explored in many nonmagnetic topologically nontrivial/trivial semimetals,while it is experimentally ambiguous which mechanism should be responsible in a specific material...Extremely large magnetoresistance(XMR)has been explored in many nonmagnetic topologically nontrivial/trivial semimetals,while it is experimentally ambiguous which mechanism should be responsible in a specific material due to the complex electronic structures.In this paper,the magnetoresistance origin of single crystal CaAl4 with C2/m structure at low temperature is investigated,exhibiting unsaturated magnetoresistance of~3000%at 2.5 K and 14 T as the fingerprints of XMR materials.By the combination of ARPES and the first-principles calculations,we elaborate multiband features and anisotropic Fermi surfaces,which can explain the mismatch of isotropic two-band model.Although the structural phase transition from I4/mmm to C2/m has been recognized,the subtle impact on electronic structure is revealed by our ARPES measurements.Considering that both charge compensation and potential topologically nontrivial band structure exist in CaAl4,our findings report CaAl4 as a new reference material for exploring the XMR phenomena.展开更多
Topological insulators and semimetals have exotic surface and bulk states with massless Dirac or Weyl fermions,demonstrating microscopic transport phenomenon based on relativistic theory.Chiral anomaly induced negativ...Topological insulators and semimetals have exotic surface and bulk states with massless Dirac or Weyl fermions,demonstrating microscopic transport phenomenon based on relativistic theory.Chiral anomaly induced negative magnetoresistance(negative MR)under parallel magnetic field and current has been used as a probable evidence ofWeyl fermions in recent years.Here we report a novel negative MR result with mutually perpendicular in-plane magnetic field and current in Cd_(3)As_(2)nanowires.The 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 anomaly.Percolating current paths induced by charge puddles and disorder might be involved to produce such considerable negative MR.Our results indicate the negative MR effect in topological semimetals involves synergistic effects of many mechanisms besides chiral anomaly.展开更多
Chiral anomaly is a distinct quantum anomaly associated with chiral fermions in Dirac or Weyl semimetals.The use of negative magnetoresistance(negative MR)as a signature for this anomaly remains contentious,as trivial...Chiral anomaly is a distinct quantum anomaly associated with chiral fermions in Dirac or Weyl semimetals.The 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 MR.In 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 source.Through angledependence experiments,we were able to rule out trivial factors,such as thermal effects,geometric artifacts,and anisotropy.Furthermore,additional electric quantum oscillations were observed when the direct current(DC)was applied as high as300μA.Such 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 increases.The 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.展开更多
Topological nodal-line semimetals attract growing research attention in the photonic and optoelectronic fields due to their unique topological energy-level bands and fascinating nonlinear optical responses.Here,to the...Topological nodal-line semimetals attract growing research attention in the photonic and optoelectronic fields due to their unique topological energy-level bands and fascinating nonlinear optical responses.Here,to the best of our knowledge,we first report the saturable absorption property of topological nodal-line semimetal HfGeTe and the related pulse modulation in passively Q-switched visible lasers.Few-layer HfGeTe demonstrates outstanding saturable absorption properties in the visible-light band,yielding the saturation intensities of 7.88,12.66,and 6.64μJ/cm^(2)at 515,640,and 720 nm,respectively.Based on an as-prepared few-layer HfGeTe optical switch and a Pr:LiYF_(4)gain medium,Q-switched visible lasers are also successfully achieved at 522,640,and 720 nm.The minimum pulse widths of the green,red,and deep-red pulsed lasers are150,125.5,and 420 ns,respectively.Especially for the green and red pulsed laser,the obtained pulse width is smaller than those of the low-dimensional layered materials.Our work sheds light on the application potential of topological nodal-line semimetals in the generation of visible pulsed lasers.展开更多
Topological semimetals are newly discovered states of quantum matter, which have extended the con- cept of topological states from insulators to metals and attracted great research interest in recent years. In general...Topological semimetals are newly discovered states of quantum matter, which have extended the con- cept of topological states from insulators to metals and attracted great research interest in recent years. In general, there are three kinds of topological semimetals, namely Dirac semimetals, Weyl semimet- als, and nodal line semimetals. Nodal line semimetals can be considered as precursor states for other topological states. For example, starting from such nodal line states, the nodal line structure might evolve into Weyl points, convert into Dirac points, or become a topological insulator by introducing the spin-orbit coupling (SOC) or mass term. In this review paper, we introduce theoretical materials that show the nodal line semimetal state, including the all-carbon Mackay-Terrones crystal (MTC), anti-perovskite Cu3PdN, pressed black phosphorus, and the CaP3 family of materials, and we present the design principles for obtaining such novel states of matter.展开更多
Three-dimensional(3D)topological insulators(TIs)have been studied for approximately fifteen years,but those made from group-IV elements,especially Ge and Sn,seem particularly attractive owing to their nontoxicity,siza...Three-dimensional(3D)topological insulators(TIs)have been studied for approximately fifteen years,but those made from group-IV elements,especially Ge and Sn,seem particularly attractive owing to their nontoxicity,sizable intrinsic spin–orbit coupling(SOC)strength and natural compatibility with the current semiconductor industry.However,group-IV elemental TIs have rarely been reported,except for the low temperature phase ofα-Sn under strain.Here,based on first-principles calculations,we propose new allotropes of Ge and Sn,named T5-Ge/Sn,as desirable TIs.These new allotropes are also highly anisotropic Dirac semimetals if the SOC is turned off.To the best of our knowledge,T5-Ge/Sn are the first 3D allotropes of Ge/Sn that possess topological states in their equilibrium states at room temperature.Additionally,their isostructures of C and Si are metastable indirect and direct semiconductors.Our work not only reveals two promising TIs,but more profoundly,we justify the advantages of group-IV elements as topological quantum materials(TQMs)for fundamental research and potential practical applications,and thus reveal a new direction in the search for desirable TQMs.展开更多
Nontrivial topological behaviors in superconducting materials provide resourceful ground for the emergence and study of unconventional quantum states.Charge doping by the controlled intercalation of donor atoms is an ...Nontrivial topological behaviors in superconducting materials provide resourceful ground for the emergence and study of unconventional quantum states.Charge doping by the controlled intercalation of donor atoms is an efficient route for enhancing/inducement of superconducting and topological behaviors in layered topological insulators and semimetals.Herein,we enhanced the superconducting temperature of TaSe_(2) by 20-folds(~3 K)through Sn atoms intercalation.Using first-principles calculations,we demonstrated the existence of nontrivial topological features.Sn_(0.5)TaSe_(2) displays topological nodal lines around the K high symmetry point in the Brillouin zone,with drumhead-like shaped surface states protected by inversion symmetry.Altogether,the coexistence of these properties makes Sn_(0.5)TaSe_(2) a potential candidate for topological superconductivity.展开更多
Subject Code:A04With the support by the National Natural Science Foundation of China,a collaborative research team led by Prof.Ding Hong(丁洪),Prof.Qian Tian(钱天),and Prof.Shi Youguo(石友国)from the Institute of Phys...Subject Code:A04With the support by the National Natural Science Foundation of China,a collaborative research team led by Prof.Ding Hong(丁洪),Prof.Qian Tian(钱天),and Prof.Shi Youguo(石友国)from the Institute of Physics,Chinese Academy of Sciences(IOP,CAS)has made a major breakthrough in the study展开更多
基金supported by the Singapore Ministry of Education AcRF Tier 2(Grant No.MOE2019-T2-1-101).
文摘The field of two-dimensional topological semimetals,which emerged at the intersection of two-dimensional materials and topological materials,has been rapidly developing in recent years.In this article,we briefly review the progress in this field.Our focus is on the basic concepts and notions,in order to convey a coherent overview of the field.Some material examples are discussed to illustrate the concepts.We discuss the outstanding problems in the field that need to be addressed in future research.
基金the National Natural Science Foundation of China(Grant No.12174064)the Shanghai Municipal Science and Technology Major Project(Grant No.2019SHZDZX01)the research fund from the State Key Laboratory of Surface Physics and Department of Physics,Fudan University(Grant No.KF202009)。
文摘Topological materials have aroused great interest in recent years,especially when magnetism is involved.Pressure can effectively tune the topological states and possibly induce superconductivity.Here we report the high-pressure study of topological semimetals XCd_(2)Sb_(2)(X=Eu and Yb),which have the same crystal structure.In antiferromagnetic(AFM)Weyl semimetal EuCd_(2)Sb_(2),the Néel temperature(TN)increases from 7.4 K at ambient pressure to 50.9 K at 14.9 GPa.When pressure is above 14.9 GPa,the AFM peak of resistance disappears,indicating a non-magnetic state.In paramagnetic Dirac semimetal candidate YbCd_(2)Sb_(2),pressure-induced superconductivity appears at 1.94 GPa,then Tc reaches to a maximum of 1.67 K at 5.22 GPa and drops to zero at about 30 GPa,displaying a dome-shaped temperature–pressure phase diagram.High-pressure x-ray diffraction measurement demonstrates that a crystalline-to-amorphous phase transition occurs at about 16 GPa in YbCd_(2)Sb_(2),revealing the robustness of pressure-induced superconductivity against structural instability.Similar structural phase transition may also occur in EuCd_(2)Sb_(2),causing the disappearance of magnetism.Our results show that XCd_(2)Sb_(2)(X=Eu and Yb)is a novel platform for exploring the interplay among magnetism,topology,and superconductivity.
文摘We propose a square optical lattice in which some of neighbor hoppings have a Peierls phase.The Peierls phase makes the lattice have a special band structure and induces the existence of Dirac points in the Brillouin zone,which means that topological semimetals exist in the system.The Dirac points move with the change of the Peierls phase and the Dirac cones are anisotropic for some vales of the Peierls phase.The lattice has a novel hidden symmetry,which is a composite antiunitary symmetry composed of a translation operation,a sublattice exchange,a complex conjugation,and a local U(1)gauge transformation.We prove that the Dirac points are protected by the hidden symmetry and perfectly explain the moving of Dirac points with the change of the Peierls phase based on the hidden symmetry protection.
基金financially supported by the National Natural Science Foundation of China(No.12274112)funded by the Overseas Scientists Sponsorship Program of Hebei Province(No.C20210330)+1 种基金the State Key Laboratory of Reliability and Intelligence of Electrical Equipment of Hebei University of Technology(No.EERI_PI2020009)S&T Program of Hebei(No.225676163GH)。
文摘Materials featuring topological energy bands and nontrivial surface states hold significant promise in unlocking unprecedented opportunities for innovating electrocatalytic mechanism.However,it remains a challenge to realize superior topological catalysts which can carry both high catalytic activity and excellent catalytic stability.Here,we propose that a family of Ni-based binary materials hosting fantasying topological conjunct-nodalpoint state and a large nontrivial energy window(NEWD)represents an ideal choice for such superior topological catalysts in hydrogen evolution reaction.The presence of conjunct-nodal-points ensures long Fermi arcs on the surface,thereby enabling an extremely high catalytic activity.The NEWD plays a crucial role in stabilizing the high catalytic activity against external perturbations,such as strain and electron/hole injection.The roles for conjunctnodal-points and NEWD are substantiated by the observable weakening of catalytic performance during topological phase transitions,which result in the removal of the conjunct-nodal-points,NEWD and their corresponding long Fermi arcs.Our work unveils a hidden mechanism and opens a feasible route for developing superior quantum catalysts from novel topology point of view.
基金This work was supported by the National Key R&D Program of China(No.2022YFA1403700)the National Natural Science Foundation of China(No.11925402)+1 种基金Guangdong province(Nos.2020KCXTD001 and 2016ZT06D348)the Science,Technology and Innovation Commission of Shenzhen Municipality(Nos.ZDSYS20170303165926217,JAY20170412152620376,and KYTDPT20181011104202253).
文摘We review our most recent research on quantum transport,organizing the review according to the intensity of the magnetic field and focus mostly on topological semimetals and topological insulators.We first describe the phenomenon of quantum transport when a magnetic field is not present.We introduce the nonlinear Hall effect and its theoretical descriptions.Then,we discuss Coulomb instabilities in 3D higher-order topological insulators.Next,we pay close attention to the surface states and find a function to identify the axion insulator in the antiferromagnetic topological insulator MnBi2Te4.Under weak magnetic fields,we focus on the decaying Majorana oscillations which has the correlation with spin−orbit coupling.In the section on strong magnetic fields,we study the helical edge states and the one-sided hinge states of the Fermi-arc mechanism,which are relevant to the quantum Hall effect.Under extremely large magnetic fields,we derive a theoretical explanation of the negative magnetoresistance without a chiral anomaly.Then,we show how magnetic responses can be used to detect relativistic quasiparticles.Additionally,we introduce the 3D quantum Hall effect’s charge-density wave mechanism and compare it with the theory of 3D transitions between metal and insulator driven by magnetic fields.
基金Project supported by the National Key Research and Development Program of China (Grant Nos.2020YFA0309300 and 2022YFA1403700)the National Natural Science Foundation of China (Grant Nos.12004158,12074162,and 91964201)+2 种基金the Key-Area Research and Development Program of Guangdong Province (Grant No.2018B030327001)Guangdong Provincial Key Laboratory (Grant No.2019B121203002)Guangdong Basic and Applied Basic Research Foundation (Grant No.2022B1515130005)。
文摘Spin–momentum locking is a key feature of the topological surface state, which plays an important role in spintronics.The electrical detection of current-induced spin polarization protected by the spin–momentum locking in nonmagnetic systems provides a new platform for developing spintronics, while previous studies were mostly based on magnetic materials.In this study, the spin transport measurement of Dirac semimetal Cd_(3)As_(2) was studied by three-terminal geometry, and a hysteresis loop signal with high resistance and low resistance state was observed. The hysteresis was reversed by reversing the current direction, which illustrates the spin–momentum locking feature of Cd_(3)As_(2). Furthermore, we realized the on–off states of the spin signals through electric modulation of the Fermi arc via the three-terminal configuration, which enables the great potential of Cd_(3)As_(2) in spin field-effect transistors.
文摘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.
基金Project supported by Singapore Ministry of Education(MOE)Tier 2 Grant No.(2018-T2-1-007)USA ONRG Grant No.(N62909-19-1-2047)+2 种基金JL is supported by MOE PhD RSS.KJAO acknowledges the funding support of Xiamen University Malaysia Research Fund,Grant Nos.XMUMRF/2019-C3/IECE/0003 and XMUMRF/2020-C5/IENG/0025the Ministry of Higher Education Malaysia under the Fundamental Research Grant No.Scheme,Grant No.FRGS/1/2019/TK08/XMU/02CZ acknowledges the funding support by the Australian Research Council(Grant No.DP160101474).
文摘Prototypical three-dimensional(3D)topological Dirac semimetals(DSMs),such as Cd3As2 and Na3Bi,contain electrons that obey a linear momentum-energy dispersion with different Fermi velocities along the three orthogonal momentum dimensions.Despite being extensively studied in recent years,the inherent Fermi velocity anisotropy has often been neglected in the theoretical and numerical studies of 3D DSMs.Although this omission does not qualitatively alter the physics of light-driven massless quasiparticles in 3D DSMs,it does quantitatively change the optical coefficients which can lead to nontrivial implications in terms of nanophotonics and plasmonics applications.Here we study the linear optical response of 3D DSMs for general Fermi velocity values along each direction.Although the signature conductivity-frequency scaling,σ(ω)∝ω,of 3D Dirac fermion is well-protected from the Fermi velocity anisotropy,the linear optical response exhibits strong linear dichroism as captured by the universal extinction ratio scaling law,Λi j=(vi/v j)^2(where i=j denotes the three spatial coordinates x,y,z,and vi is the i-direction Fermi velocity),which is independent of frequency,temperature,doping,and carrier scattering lifetime.For Cd3As2 and Na3Bi3,an exceptionally strong extinction ratio larger than 15 and covering a broad terahertz window is revealed.Our findings shed new light on the role of Fermi velocity anisotropy in the optical response of Dirac semimetals and open up novel polarization-sensitive functionalities,such as photodetection and light modulation.
基金Project supported by the National Key R&D Program of China (Grant Nos.2018YFA, 0305601, and 2021YFA1400100)the National Natural Science Foundation of China (Grant Nos.12274003, 11725415, and 11934001)the Innovation Program for Quantum Science and Technology (Grant No.2021ZD0302600)。
文摘A Weyl node is characterized by its chirality and tilt.We 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 chirality.Built on this theory, we propose ferromagnetic Mn Bi2Te4as a magnetoelectrically regulated, terahertz optical device, by magnetoelectrically switching the chiralitydependent and-independent DC photocurrents.These results are useful for creating nonlinear optical devices based on the topological Weyl semimetals.
基金Project supported by the National Basic Research Program of China(Grant Nos.2018YFA0305604,2017YFA0303300,and 2013CB934600)the Research Fund for the Doctoral Program of Higher Education(RFDP)of China(Grant No.20130001110003)+2 种基金the Open Project Program of the Pulsed High Magnetic Field Facility(Grant No.PHMFF2015002) at the Huazhong University of Science and Technologythe Open Research Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics,Tsinghua University(Grant No.KF201703)the Key Research Program of the Chinese Academy of Sciences(Grant No.XDPB08-2)
文摘Recently, the Dirac and Weyl semimetals have attracted extensive attention in condensed matter physics due to both the fundamental interest and the potential application of a new generation of electronic devices. Here we review the exotic electrical transport phenomena in Dirac andWeyl semimetals. Section 1 is a brief introduction to the topological semimetals(TSMs). In Section 2 and Section 3, the intriguing transport phenomena in Dirac semimetals(DSMs) andWeyl semimetals(WSMs) are reviewed, respectively. The most widely studied Cd_3A_(s2) and the TaAs family are selected as representatives to show the typical properties of DSMs and WSMs, respectively. Beyond these systems, the advances in other TSM materials,such as ZrTe_5 and the MoTe_2 family, are also introduced. In Section 4, we provide perspectives on the study of TSMs especially on the magnetotransport investigations.
基金Supported by the National Natural Science Foundation of China under Grant No 11504366the National Basic Research Program of China under Grant Nos 2015CB921503 and 2016YFE0110000
文摘Topological semimetals are a new type of matter with one-dimensional Fermi lines or zero-dimensional Weyl or Dirac points in momentum space. Here using first-principles calculations, we find that the non-centrosymmetric PbTaS2 is a topological nodal line semimetal. In the absence of spin-orbit coupling (SOC), one band inversion happens around a high symmetrical H point, which leads to forming a nodal line. The nodal line is robust and protected against gap opening by mirror reflection symmetry even with the inclusion of strong SOC. In addition, it also hosts exotic drumhead surface states either inside or outside the projected nodal ring depending on surface termination. The robust bulk nodal lines and drumhead-like surface states with SOC in PbTaS2 make it a potential candidate material for exploring the freakish properties of the topological nodal line fermions in condensed matter systems.
基金Project supported by the Ministry of Science and Technology of China(Grant Nos.2016YFA0300802 and 2013CB934600)the National Natural Science Foundation of China(Grant Nos.11274014 and 11234001)
文摘Various novel physical properties have emerged in Dirac electronic systems, especially the topological characters pro- tected by symmetry. Current studies on these systems have been greatly promoted by the intuitive concepts of Berry phase and Berry curvature, which provide precise definitions of the topological phases. In this topical review, transport properties of topological insulator (Bi2Se3), topological Dirac semimetal (Cd3As2), and topological insulator-graphene heterojunc- tion are presented and discussed. Perspectives about transport properties of two-dimensional topological nontrivial systems, including topological edge transport, topological valley transport, and topological Weyl semimetals, are provided.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 11774421, 12074425, 11874422, 11574391, and 11774424)the National Key Research and Development Program of China (Grant No. 2019YFA0308602)+1 种基金the Fundamental Research Funds for the Central Universities,Chinathe Research Funds of Renmin University of China (Grant Nos. 19XNLG13, 18XNLG14, and 19XNLG18)
文摘Extremely large magnetoresistance(XMR)has been explored in many nonmagnetic topologically nontrivial/trivial semimetals,while it is experimentally ambiguous which mechanism should be responsible in a specific material due to the complex electronic structures.In this paper,the magnetoresistance origin of single crystal CaAl4 with C2/m structure at low temperature is investigated,exhibiting unsaturated magnetoresistance of~3000%at 2.5 K and 14 T as the fingerprints of XMR materials.By the combination of ARPES and the first-principles calculations,we elaborate multiband features and anisotropic Fermi surfaces,which can explain the mismatch of isotropic two-band model.Although the structural phase transition from I4/mmm to C2/m has been recognized,the subtle impact on electronic structure is revealed by our ARPES measurements.Considering that both charge compensation and potential topologically nontrivial band structure exist in CaAl4,our findings report CaAl4 as a new reference material for exploring the XMR phenomena.
基金This work was supported by the National Natural Science Foundation of China(Grant Nos.12004158,12074162,and 91964201)the National Key Research and Development Program of China(Grant Nos.2022YFA1403700 and 2020YFA0309300)+2 种基金the Key-Area Research and Development Program of Guangdong Province,China(Grant No.2018B030327001)Guangdong Provincial Key Laboratory(Grant No.2019B121203002)Guangdong Basic and Applied Basic Research Foundation(Grant No.2022B1515130005).
文摘Topological insulators and semimetals have exotic surface and bulk states with massless Dirac or Weyl fermions,demonstrating microscopic transport phenomenon based on relativistic theory.Chiral anomaly induced negative magnetoresistance(negative MR)under parallel magnetic field and current has been used as a probable evidence ofWeyl fermions in recent years.Here we report a novel negative MR result with mutually perpendicular in-plane magnetic field and current in Cd_(3)As_(2)nanowires.The 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 anomaly.Percolating current paths induced by charge puddles and disorder might be involved to produce such considerable negative MR.Our results indicate the negative MR effect in topological semimetals involves synergistic effects of many mechanisms besides chiral anomaly.
基金supported by the National Natural Science Foundation of China(Grant Nos.12074162,12004158,and 91964201)the National Key Research and Development Program of China(Grant Nos.2022YFA1403700 and 2020YFA0309300)+2 种基金the Key-Area Research and Development Program of Guangdong Province(Grant No.2018B030327001)Guangdong Provincial Key Laboratory(Grant No.2019B121203002)Guangdong Basic and Applied Basic Research Foundation(Grant No.2022B1515130005)。
文摘Chiral anomaly is a distinct quantum anomaly associated with chiral fermions in Dirac or Weyl semimetals.The 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 MR.In 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 source.Through angledependence experiments,we were able to rule out trivial factors,such as thermal effects,geometric artifacts,and anisotropy.Furthermore,additional electric quantum oscillations were observed when the direct current(DC)was applied as high as300μA.Such 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 increases.The 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.
基金supported by the National Key Research and Development Program of China(No.2021YFB3601504)the National Natural Science Foundation of China(Nos.52025021and 92163207)+1 种基金the Natural Science Foundation of Shandong Province(No.ZR2022LLZ005)the Future Plans of Young Scholars at Shandong University。
文摘Topological nodal-line semimetals attract growing research attention in the photonic and optoelectronic fields due to their unique topological energy-level bands and fascinating nonlinear optical responses.Here,to the best of our knowledge,we first report the saturable absorption property of topological nodal-line semimetal HfGeTe and the related pulse modulation in passively Q-switched visible lasers.Few-layer HfGeTe demonstrates outstanding saturable absorption properties in the visible-light band,yielding the saturation intensities of 7.88,12.66,and 6.64μJ/cm^(2)at 515,640,and 720 nm,respectively.Based on an as-prepared few-layer HfGeTe optical switch and a Pr:LiYF_(4)gain medium,Q-switched visible lasers are also successfully achieved at 522,640,and 720 nm.The minimum pulse widths of the green,red,and deep-red pulsed lasers are150,125.5,and 420 ns,respectively.Especially for the green and red pulsed laser,the obtained pulse width is smaller than those of the low-dimensional layered materials.Our work sheds light on the application potential of topological nodal-line semimetals in the generation of visible pulsed lasers.
文摘Topological semimetals are newly discovered states of quantum matter, which have extended the con- cept of topological states from insulators to metals and attracted great research interest in recent years. In general, there are three kinds of topological semimetals, namely Dirac semimetals, Weyl semimet- als, and nodal line semimetals. Nodal line semimetals can be considered as precursor states for other topological states. For example, starting from such nodal line states, the nodal line structure might evolve into Weyl points, convert into Dirac points, or become a topological insulator by introducing the spin-orbit coupling (SOC) or mass term. In this review paper, we introduce theoretical materials that show the nodal line semimetal state, including the all-carbon Mackay-Terrones crystal (MTC), anti-perovskite Cu3PdN, pressed black phosphorus, and the CaP3 family of materials, and we present the design principles for obtaining such novel states of matter.
基金This project was funded by China Postdoctoral Science Foundation(Grant No.2019M660107)the National Natural Science Foundation of China(Grant No.11804039).
文摘Three-dimensional(3D)topological insulators(TIs)have been studied for approximately fifteen years,but those made from group-IV elements,especially Ge and Sn,seem particularly attractive owing to their nontoxicity,sizable intrinsic spin–orbit coupling(SOC)strength and natural compatibility with the current semiconductor industry.However,group-IV elemental TIs have rarely been reported,except for the low temperature phase ofα-Sn under strain.Here,based on first-principles calculations,we propose new allotropes of Ge and Sn,named T5-Ge/Sn,as desirable TIs.These new allotropes are also highly anisotropic Dirac semimetals if the SOC is turned off.To the best of our knowledge,T5-Ge/Sn are the first 3D allotropes of Ge/Sn that possess topological states in their equilibrium states at room temperature.Additionally,their isostructures of C and Si are metastable indirect and direct semiconductors.Our work not only reveals two promising TIs,but more profoundly,we justify the advantages of group-IV elements as topological quantum materials(TQMs)for fundamental research and potential practical applications,and thus reveal a new direction in the search for desirable TQMs.
基金the financial support in part by the National Key R&D Program of China(No.2017YFA0303500)the National Natural Science Foundation of China(NSFC)(Nos.U1932201,11574280,and 21727801)+2 种基金NSFC-MAECI(No.51861135202)International Partnership Program of CAS(No.211134KYSB20190063)CAS Collaborative Innovation Program of Hefei Science Center(No.2019HSC-CIP002).
文摘Nontrivial topological behaviors in superconducting materials provide resourceful ground for the emergence and study of unconventional quantum states.Charge doping by the controlled intercalation of donor atoms is an efficient route for enhancing/inducement of superconducting and topological behaviors in layered topological insulators and semimetals.Herein,we enhanced the superconducting temperature of TaSe_(2) by 20-folds(~3 K)through Sn atoms intercalation.Using first-principles calculations,we demonstrated the existence of nontrivial topological features.Sn_(0.5)TaSe_(2) displays topological nodal lines around the K high symmetry point in the Brillouin zone,with drumhead-like shaped surface states protected by inversion symmetry.Altogether,the coexistence of these properties makes Sn_(0.5)TaSe_(2) a potential candidate for topological superconductivity.
文摘Subject Code:A04With the support by the National Natural Science Foundation of China,a collaborative research team led by Prof.Ding Hong(丁洪),Prof.Qian Tian(钱天),and Prof.Shi Youguo(石友国)from the Institute of Physics,Chinese Academy of Sciences(IOP,CAS)has made a major breakthrough in the study