Metal–insulator transition(MIT)is one of the most conspicuous phenomena in correlated electron systems.However such a transition has rarely been induced by an external magnetic field as the field scale is normally to...Metal–insulator transition(MIT)is one of the most conspicuous phenomena in correlated electron systems.However such a transition has rarely been induced by an external magnetic field as the field scale is normally too small compared with the charge gap.We present the observation of a magnetic-field-driven MIT in a magnetic semiconductorβ-EuP 3.Concomitantly,we find a colossal magnetoresistance in an extreme way:the resistance drops billionfold at 2 K in a magnetic field less than 3 T.We ascribe this striking MIT as a field-driven transition from an antiferromagnetic and paramagnetic insulator to a spin-polarized topological semimetal,in which the spin configuration of Eu 2+cations and spin-orbital coupling play a crucial role.As a phosphorene-bearing compound whose electrical properties can be controlled by the application of field,β-EuP 3 may serve as a tantalizing material in the basic research and even future electronics.展开更多
Most topological insulators(TIs)discovered today in spinful systems can be transformed from topological semimetals(TSMs)with vanishing bulk gap via introducing the spin-orbit coupling(SOC),which manifests the intrinsi...Most topological insulators(TIs)discovered today in spinful systems can be transformed from topological semimetals(TSMs)with vanishing bulk gap via introducing the spin-orbit coupling(SOC),which manifests the intrinsic links between the gapped topological insulator phases and the gapless TSMs.Recently,we have discovered a family of TSMs in time-reversal invariant spinless systems,which host butterfly-like nodal-lines(NLs)consisting of a pair of identical concentric intersecting coplanar ellipses(CICE).In this Communication,we unveil the intrinsic link between this exotic class of nodal-line semimetals(NLSMs)and a Z_(4)=2 topological crystalline insulator(TCI),by including substantial SOC.We demonstrate that in three space groups(i.e.,Pbam(No.55),P4/mbm(No.127),and P4_(2)/mbc(No.135)),the TCI supports a fourfold Dirac fermion on the(001)surface protected by two glide symmetries,which originates from the intertwined drumhead surface states of the CICE NLs.The higher order topology is further demonstrated by the emergence of one-dimensional helical hinge states,indicating the discovery of a higher order topological insulator protected by a glide symmetry.展开更多
The nonlinear Hall effect(NLHE)is a phenomenon which could produce a transverse Hall voltage in a time-reversal-invariant material.Here,we report the real space characterizations of NLHE evaluated through quantum tran...The nonlinear Hall effect(NLHE)is a phenomenon which could produce a transverse Hall voltage in a time-reversal-invariant material.Here,we report the real space characterizations of NLHE evaluated through quantum transport in TaIrTe4 nanoribbon without the explicit Berry curvature dipole(BCD)information.We first characterize the NLHE in both transverse confined directions in global-level measurement.The impact of quantum confinement in NLHE is evaluated by adjusting the width of nanoribbons.Then,the probing area is trimmed to the atomic scale to evaluate the local texture,where we discover its patterns differ among the probed neighboring atomic groups.The analysis of charge distribution reveals the connections between NLHE’s local patterns and its non-centrosymmetric nature,rendering nearly an order of Hall voltage enhancement through probe positioning.Our work paves the way to expand the range of NLHE study and unveil its physics in more versatile material systems.展开更多
Protected surface states arising fromnon-trivial bandstructure topology in semimetals can potentially enable advanced device functionalities in compute,memory,interconnect,sensing,and communication.This necessitates a...Protected surface states arising fromnon-trivial bandstructure topology in semimetals can potentially enable advanced device functionalities in compute,memory,interconnect,sensing,and communication.This necessitates a fundamental understanding of surface-state transport in nanoscale topological semimetals.Here,we investigate quantum transport in a prototypical topological semimetal NbAs to evaluate the potential of this class of materials for beyond-Cu interconnects in highly-scaled integrated circuits.Using density functional theory(DFT)coupled with non-equilibrium Green’s function(NEGF)calculations,we show that the resistance-areaRAproduct in NbAs films decreases with decreasing thickness at the nanometer scale,in contrast to a nearly constant RA product in ideal Cu films.This anomalous scaling originates from the disproportionately large number of surface conduction states which dominate the ballistic conductance by up to 70%in NbAs thin films.We also show that this favorable RA scaling persists even in the presence of surface defects,in contrast to RA sharply increasing with reducing thickness for films of conventional metals,such as Cu,in the presence of surface defects.These results underscore the potential of topological semimetals as future back-end-of-line(BEOL)interconnect metals.展开更多
We propose a novel class of two-dimensional(2D)Dirac materials in the MX family(M=Be,Mg,Zn and Cd,X=Cl,Br and I),which exhibit graphene-like band structures with linearly-dispersing Dirac-cone states over large energy...We propose a novel class of two-dimensional(2D)Dirac materials in the MX family(M=Be,Mg,Zn and Cd,X=Cl,Br and I),which exhibit graphene-like band structures with linearly-dispersing Dirac-cone states over large energy scales(0.8–1.8 eV)and ultra-high Fermi velocities comparable to graphene.Spin-orbit coupling opens sizable topological band gaps so that these compounds can be effectively classified as quantum spin Hall insulators.The electronic and topological properties are found to be highly tunable and amenable to modulation via anion-layer substitution and vertical electric field.Electronic structures of several members of the family are shown to host a Van-Hove singularity(VHS)close to the energy of the Dirac node.The enhanced density-of-states associated with these VHSs could provide a mechanism for inducing topological superconductivity.The presence of sizable band gaps,ultra-high carrier mobilities,and small effective masses makes the MX family promising for electronics and spintronics applications.展开更多
The search for novel materials with new functionalities and applications potential is continuing to intensify.Quantum anomalous Hall(QAH)effect was recently realized in magnetic topological insulators(TIs)but only at ...The search for novel materials with new functionalities and applications potential is continuing to intensify.Quantum anomalous Hall(QAH)effect was recently realized in magnetic topological insulators(TIs)but only at extremely low temperatures.Here,based on our first-principles electronic structure calculations,we predict that chemically functionalized Ⅲ-Bi honeycombs can support large-gap QAH insulating phases.Specifically,we show that functionalized AlBi and TlBi films harbor QAH insulator phases.GaBi and InBi are identified as semimetals with non-zero Chern number.Remarkably,TlBi exhibits a robust QAH phase with a band gap as large as 466 meV in a buckled honeycomb structure functionalized on one side.Furthermore,the electronic spectrum of a functionalized TlBi nanoribbon with zigzag edge is shown to possess only one chiral edge band crossing the Fermi level within the band gap.Our results suggest that Ⅲ-Bi honeycombs would provide a new platform for developing potential spintronics applications based on the QAH effect.展开更多
Emergent Dirac fermion states underlie many intriguing properties of graphene,and the search for them constitutes one strong motivation to explore two-dimensional(2D)allotropes of other elements.Phosphorene,the ultrat...Emergent Dirac fermion states underlie many intriguing properties of graphene,and the search for them constitutes one strong motivation to explore two-dimensional(2D)allotropes of other elements.Phosphorene,the ultrathin layers of black phosphorous,has been a subject of intense investigations recently,and it was found that other group-Va elements could also form 2D layers with similar puckered lattice structure.Here,by a close examination of their electronic band structure evolution,we discover two types of Dirac fermion states emerging in the low-energy spectrum.One pair of(type-I)Dirac points is sitting on high-symmetry lines,while two pairs of(type-II)Dirac points are located at generic k-points,with different anisotropic dispersions determined by the reduced symmetries at their locations.Such fully-unpinned(type-II)2D Dirac points are discovered for the first time.In the absence of spin-orbit coupling(SOC),we find that each Dirac node is protected by the sublattice symmetry from gap opening,which is in turn ensured by any one of three point group symmetries.The SOC generally gaps the Dirac nodes,and for the type-I case,this drives the system into a quantum spin Hall insulator phase.We suggest possible ways to realise the unpinned Dirac points in strained phosphorene.展开更多
基金Supported by the National Natural Science Foundation of China(Grant Nos.U1832214 and 11774007)the National Key R&D Program of China(Grant No.2018YFA0305601)+1 种基金the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDB28000000)The experimental and theoretical work at Princeton University was supported by the Gordon and Betty Moore Foundation(Grant Nos.GBMF4547 and GBMF9461/Hasan)。
文摘Metal–insulator transition(MIT)is one of the most conspicuous phenomena in correlated electron systems.However such a transition has rarely been induced by an external magnetic field as the field scale is normally too small compared with the charge gap.We present the observation of a magnetic-field-driven MIT in a magnetic semiconductorβ-EuP 3.Concomitantly,we find a colossal magnetoresistance in an extreme way:the resistance drops billionfold at 2 K in a magnetic field less than 3 T.We ascribe this striking MIT as a field-driven transition from an antiferromagnetic and paramagnetic insulator to a spin-polarized topological semimetal,in which the spin configuration of Eu 2+cations and spin-orbital coupling play a crucial role.As a phosphorene-bearing compound whose electrical properties can be controlled by the application of field,β-EuP 3 may serve as a tantalizing material in the basic research and even future electronics.
基金The work at CSUN was supported by NSF-Partnership in Research and Education in Materials(PREM)Grant No.DMR-1828019H.L.acknowledges the support by the Ministry of Science and Technology(MOST)in Taiwan under grant number MOST 109-2112-M-001-014-MY3+2 种基金The work of J.L.M.has been supported by Spanish Science Ministry grant PGC2018-094626-B-C21(MCIU/AEI/FEDER,EU)and Basque Government grant IT979-16M.G.V.thanks support from DFG INCIEN2019-000356 from Gipuzkoako Foru AldundiaM.G.V.and M.I.acknowledges the Spanish Ministerio de Ciencia e Innovacion(grant number PID2019-109905GB-C21).
文摘Most topological insulators(TIs)discovered today in spinful systems can be transformed from topological semimetals(TSMs)with vanishing bulk gap via introducing the spin-orbit coupling(SOC),which manifests the intrinsic links between the gapped topological insulator phases and the gapless TSMs.Recently,we have discovered a family of TSMs in time-reversal invariant spinless systems,which host butterfly-like nodal-lines(NLs)consisting of a pair of identical concentric intersecting coplanar ellipses(CICE).In this Communication,we unveil the intrinsic link between this exotic class of nodal-line semimetals(NLSMs)and a Z_(4)=2 topological crystalline insulator(TCI),by including substantial SOC.We demonstrate that in three space groups(i.e.,Pbam(No.55),P4/mbm(No.127),and P4_(2)/mbc(No.135)),the TCI supports a fourfold Dirac fermion on the(001)surface protected by two glide symmetries,which originates from the intertwined drumhead surface states of the CICE NLs.The higher order topology is further demonstrated by the emergence of one-dimensional helical hinge states,indicating the discovery of a higher order topological insulator protected by a glide symmetry.
基金the National University of Singapore is supported by MOE-2017-T2-2-114,MOE-2019-T2-2-215,and FRC-A-8000194-01-00Northeastern University was supported by the Air Force Office of Scientific Research under award number FA9550-20-1-0322,and it benefited from the computational resources of Northeastern University’s Advanced Scientific Computation Center(ASCC)and the Discovery Cluster+4 种基金The work at Nanyang Technological University was supported by the National Research Foundation,Singapore under its Fellowship Award(NRF-NRFF13-2021-0010)the Nanyang Assistant Professorship grant(NTUSUG)G.C.Liang would also like to thank the financial support from the National Science and Technology Council(NSTC)under grant number NSTC 112-2112-M-A49-047-MY3the Co-creation Platform of the Industry-Academia Innovation School,NYCU,under the framework of the National Key Fields Industry-University Cooperation and Skilled Personnel Training Act,from the Ministry of Education(MOE)and industry partners in ROCH.Lin acknowledges the support by the National Science and Technology Council(NSTC)under grant number MOST 111-2112-M-001-057-MY3.
文摘The nonlinear Hall effect(NLHE)is a phenomenon which could produce a transverse Hall voltage in a time-reversal-invariant material.Here,we report the real space characterizations of NLHE evaluated through quantum transport in TaIrTe4 nanoribbon without the explicit Berry curvature dipole(BCD)information.We first characterize the NLHE in both transverse confined directions in global-level measurement.The impact of quantum confinement in NLHE is evaluated by adjusting the width of nanoribbons.Then,the probing area is trimmed to the atomic scale to evaluate the local texture,where we discover its patterns differ among the probed neighboring atomic groups.The analysis of charge distribution reveals the connections between NLHE’s local patterns and its non-centrosymmetric nature,rendering nearly an order of Hall voltage enhancement through probe positioning.Our work paves the way to expand the range of NLHE study and unveil its physics in more versatile material systems.
基金the National University of Singapore was supported by MOE-2017-T2-2-114,MOE-2019-T2-2-215,and FRC-A-8000194-01-00supported by 2030 Cross-Generation Young Scholars Program from the Science and Technology Council(MOST111-2628-M-006-003-MY3)Cheng Kung University,and the Center for Theoretical Sciences.
文摘Protected surface states arising fromnon-trivial bandstructure topology in semimetals can potentially enable advanced device functionalities in compute,memory,interconnect,sensing,and communication.This necessitates a fundamental understanding of surface-state transport in nanoscale topological semimetals.Here,we investigate quantum transport in a prototypical topological semimetal NbAs to evaluate the potential of this class of materials for beyond-Cu interconnects in highly-scaled integrated circuits.Using density functional theory(DFT)coupled with non-equilibrium Green’s function(NEGF)calculations,we show that the resistance-areaRAproduct in NbAs films decreases with decreasing thickness at the nanometer scale,in contrast to a nearly constant RA product in ideal Cu films.This anomalous scaling originates from the disproportionately large number of surface conduction states which dominate the ballistic conductance by up to 70%in NbAs thin films.We also show that this favorable RA scaling persists even in the presence of surface defects,in contrast to RA sharply increasing with reducing thickness for films of conventional metals,such as Cu,in the presence of surface defects.These results underscore the potential of topological semimetals as future back-end-of-line(BEOL)interconnect metals.
基金This work was supported by the U.S.Department of Energy,Office of Science,Basic Energy Sciences,under Award#DE-SC0019275.It benefitted from the supercomputing resources of the National Energy Research Scientific Computing Center(NERSC),a U.S.Department of Energy Office of Science User Facility operated under Contract No.DE-AC02-05CH11231,and Temple University’s HPC resources supported in part by the National Science Foundation through major research instrumentation grant number 1625061 and by the US Army Research Laboratory under contract number W911NF-16-2-0189.S.X.D.and Y.-F.Z.acknowledge support from the National Key Research and Development Program of China(No.2016YFA0202300)Strategic Priority Research Program(No.XDB30000000)+1 种基金the National Natural Science Foundation of China(No.61888102)the International Partnership Program of the Chinese Academy of Sciences(No.112111KYSB20160061).
文摘We propose a novel class of two-dimensional(2D)Dirac materials in the MX family(M=Be,Mg,Zn and Cd,X=Cl,Br and I),which exhibit graphene-like band structures with linearly-dispersing Dirac-cone states over large energy scales(0.8–1.8 eV)and ultra-high Fermi velocities comparable to graphene.Spin-orbit coupling opens sizable topological band gaps so that these compounds can be effectively classified as quantum spin Hall insulators.The electronic and topological properties are found to be highly tunable and amenable to modulation via anion-layer substitution and vertical electric field.Electronic structures of several members of the family are shown to host a Van-Hove singularity(VHS)close to the energy of the Dirac node.The enhanced density-of-states associated with these VHSs could provide a mechanism for inducing topological superconductivity.The presence of sizable band gaps,ultra-high carrier mobilities,and small effective masses makes the MX family promising for electronics and spintronics applications.
基金support from the National Center for Theoretical Sciences and the Ministry of Science and Technology of Taiwan under Grants Nos.MOST-104-2112-M-110-002-MY3 and MOST-103-2112-M-110-008-MY3the support under NSYSU-NKMU JOINT RESEARCH PROJECT#105-P005 and#106-P005+3 种基金supported by the US Department of Energy(DOE),Office of Science,Basic Energy Sciences grant number DE-FG02-07ER46352(core research)benefited from Northeastern University’s Advanced Scientific Computation Center(ASCC),the NERSC supercomputing center through DOE grant number DE-AC02-05CH11231support(applications to layered materials)from the DOE EFRC:Center for the Computational Design of Functional Layered Materials(CCDM)under DE-SC0012575the Singapore National Research Foundation for support under NRF Award No.NRFNRFF2013-03.
文摘The search for novel materials with new functionalities and applications potential is continuing to intensify.Quantum anomalous Hall(QAH)effect was recently realized in magnetic topological insulators(TIs)but only at extremely low temperatures.Here,based on our first-principles electronic structure calculations,we predict that chemically functionalized Ⅲ-Bi honeycombs can support large-gap QAH insulating phases.Specifically,we show that functionalized AlBi and TlBi films harbor QAH insulator phases.GaBi and InBi are identified as semimetals with non-zero Chern number.Remarkably,TlBi exhibits a robust QAH phase with a band gap as large as 466 meV in a buckled honeycomb structure functionalized on one side.Furthermore,the electronic spectrum of a functionalized TlBi nanoribbon with zigzag edge is shown to possess only one chiral edge band crossing the Fermi level within the band gap.Our results suggest that Ⅲ-Bi honeycombs would provide a new platform for developing potential spintronics applications based on the QAH effect.
基金supported by NSFC(Grant No.11374009,61574123 and 21373184)the National Key Basic Research Program of China(2012CB825700)+5 种基金SUTD-SRG-EPD2013062Singapore MOE Academic Research Fund Tier 1(SUTD-T1-2015004)A*STAR SERC 122-PSF-0017 and AcRF R-144-000-310-112support by Singapore National Research Foundation under NRF Award No.NRF-NRFF2013-03Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund(the second phase)support from SR16000 supercomputing resources of the Center for Computational Materials Science,Tohoku University.
文摘Emergent Dirac fermion states underlie many intriguing properties of graphene,and the search for them constitutes one strong motivation to explore two-dimensional(2D)allotropes of other elements.Phosphorene,the ultrathin layers of black phosphorous,has been a subject of intense investigations recently,and it was found that other group-Va elements could also form 2D layers with similar puckered lattice structure.Here,by a close examination of their electronic band structure evolution,we discover two types of Dirac fermion states emerging in the low-energy spectrum.One pair of(type-I)Dirac points is sitting on high-symmetry lines,while two pairs of(type-II)Dirac points are located at generic k-points,with different anisotropic dispersions determined by the reduced symmetries at their locations.Such fully-unpinned(type-II)2D Dirac points are discovered for the first time.In the absence of spin-orbit coupling(SOC),we find that each Dirac node is protected by the sublattice symmetry from gap opening,which is in turn ensured by any one of three point group symmetries.The SOC generally gaps the Dirac nodes,and for the type-I case,this drives the system into a quantum spin Hall insulator phase.We suggest possible ways to realise the unpinned Dirac points in strained phosphorene.
