The hybridization gap in strained-layer InAs/In_(x)Ga_(1−x) Sb quantum spin Hall insulators(QSHIs)is significantly enhanced compared to binary InAs/GaSb QSHI structures,where the typical indium composition,x,ranges be...The hybridization gap in strained-layer InAs/In_(x)Ga_(1−x) Sb quantum spin Hall insulators(QSHIs)is significantly enhanced compared to binary InAs/GaSb QSHI structures,where the typical indium composition,x,ranges between 0.2 and 0.4.This enhancement prompts a critical question:to what extent can quantum wells(QWs)be strained while still preserving the fundamental QSHI phase?In this study,we demonstrate the controlled molecular beam epitaxial growth of highly strained-layer QWs with an indium composition of x=0.5.These structures possess a substantial compressive strain within the In_(0.5)Ga_(0.5)Sb QW.Detailed crystal structure analyses confirm the exceptional quality of the resulting epitaxial films,indicating coherent lattice structures and the absence of visible dislocations.Transport measurements further reveal that the QSHI phase in InAs/In_(0.5)Ga_(0.5)Sb QWs is robust and protected by time-reversal symmetry.Notably,the edge states in these systems exhibit giant magnetoresistance when subjected to a modest perpendicular magnetic field.This behavior is in agreement with the𝑍2 topological property predicted by the Bernevig–Hughes–Zhang model,confirming the preservation of topologically protected edge transport in the presence of enhanced bulk strain.展开更多
In moiré-patterned van der Waals structures of transition metal dichalcogenides,correlated insulators can form under integer and fractional fillings,whose transport properties are governed by various quasiparticl...In moiré-patterned van der Waals structures of transition metal dichalcogenides,correlated insulators can form under integer and fractional fillings,whose transport properties are governed by various quasiparticle excitations including holons,doublons and interlayer exciton insulators.Here we theoretically investigate the nearest-neighbor inter-site hoppings of holons and interlayer exciton insulators.Our analysis indicates that these hopping strengths are significantly enhanced compared to that of a single carrier.The underlying mechanism can be attributed to the strong Coulomb interaction between carriers at different sites.For the interlayer exciton insulator consisting of a holon and a carrier in different layers,we have also obtained its effective Bohr radius and energy splitting between the ground and the first-excited states.展开更多
We investigate the interplay between the pseudogap state and d-wave superconductivity in the two-dimensional doped Hubbard model by employing an eight-site cluster dynamical mean-field theory method.By tuning electron...We investigate the interplay between the pseudogap state and d-wave superconductivity in the two-dimensional doped Hubbard model by employing an eight-site cluster dynamical mean-field theory method.By tuning electron hopping parameters,the strong-coupling pseudogap in the two-dimensional Hubbard model can be either enhanced or suppressed in the doped Mott insulator regime.We find that in underdoped cases,the closing of pseudogap leads to a significant enhancement of superconductivity,indicating competition between the two in the underdoped regime.In contrast,at large dopings,suppressing the pseudogap is accompanied by a concurrent decrease in the superconducting transition temperature Tc,which can be attributed to a reduction in antiferromagnetic correlations behind both the pseudogap and superconductivity.We elucidate this evolving relationship between pseudogap and superconductivity across different doping regimes.展开更多
Ferroelectric topological insulators realized in heterostructures of two topologically trivial two-dimensional materials have recently attracted significant interest. Using first-principles calculations combined with ...Ferroelectric topological insulators realized in heterostructures of two topologically trivial two-dimensional materials have recently attracted significant interest. Using first-principles calculations combined with topological quantum chemistry, we investigate bilayer α-In_(2) Se_(3)(2 L-In_(2) Se_(3)) in van der Waals heterostructures with XSe(X = Ga, In, Tl) substrates within space group P 3m1(No. 156). We show that the emergence of ferroelectricity-driven topological phase transitions in these systems is dictated by fundamental symmetry principles rather than material-specific effects. The band bending at the XSe/2 L-In_(2) Se_(3) interface enables topological band inversions, with higher-electron-affinity substrates such as GaSe and TlSe favoring the transition. Remarkably, GaSe/2 L-In_(2) Se_(3) exhibits a reversible transition between topological and trivial insulating phases upon polarization switching, while TlSe/2 L-In_(2) Se_(3) undergoes sequential transitions from a topological insulator to a trivial insulator and eventually to a metallic state. This multistate manipulation highlights a viable route for designing tunable, low-power, multi-functional electronic devices.展开更多
The topological phases and edge states of a topological Euler insulator on a triangular lattice is studied.Differently from two-band Chern insulators,a topological Euler insulator is a kind of three-band model,describ...The topological phases and edge states of a topological Euler insulator on a triangular lattice is studied.Differently from two-band Chern insulators,a topological Euler insulator is a kind of three-band model,described by the Euler number not the Chern number.The spin textures of a topological Euler insulator in the momentum space is like a Néel-type skyrmion.It is found that the topological edge states exist in the band gap of the topological Euler insulator,and the topological Euler insulator can be transformed into a topological metal without the topological phase transition.展开更多
The exploration of topological phases remains a cutting-edge research frontier,driven by their promising potential for next-generation electronic and quantum technologies.In this work,we employ first-principles calcul...The exploration of topological phases remains a cutting-edge research frontier,driven by their promising potential for next-generation electronic and quantum technologies.In this work,we employ first-principles calculations and tightbinding modeling to systematically investigate the topological properties of freestanding two-dimensional(2D)honeycomb Bi,HgTe,and Al_(2)O_(3)(0001)-supported HgTe.Remarkably,all three systems exhibit coexistence of intrinsic first-and higher-order topological insulator states,induced by spin-orbit coupling(SOC).These states manifest as topologically protected gapless edge states in one-dimensional(1D)nanoribbons and symmetry-related corner states in zero-dimensional(0D)nanoflakes.Furthermore,fractional electron charges may accumulate at the corners of armchair-edged nanoflakes.Among these materials,HgTe/Al_(2)O_(3)(0001)is particularly promising due to its experimentally feasible atomic configuration and low-energy corner states.Our findings highlight the importance of exploring higher-order topological phases in quantum spin Hall insulators and pave the way for new possibilities in device applications.展开更多
Higher-order band topology not only enriches our understanding of topological phases but also unveils pioneering lower-dimensional boundary states,which harbors substantial potential for next-generation device applica...Higher-order band topology not only enriches our understanding of topological phases but also unveils pioneering lower-dimensional boundary states,which harbors substantial potential for next-generation device applications.The distinct electronic configurations and tunable attributes of two-dimensional materials position them as a quintessential platform for the realization of second-order topological insulators(SOTIs).This article provides an overview of the research progress in SOTIs within the field of two-dimensional electronic materials,focusing on the characterization of higher-order topological properties and the numerous candidate materials proposed in theoretical studies.These endeavors not only enhance our understanding of higher-order topological states but also highlight potential material systems that could be experimentally realized.展开更多
Atmospheric particle adsorption on insulator surfaces,coupled with humid environments,significantly affects contamination flashover,necessitating a clear understanding of the electric field distribution on insulator s...Atmospheric particle adsorption on insulator surfaces,coupled with humid environments,significantly affects contamination flashover,necessitating a clear understanding of the electric field distribution on insulator surfaces with adsorbed particles.This is crucial for accurately assessing insulator safety and informing critical decision-making.Although previous research has demonstrated that particle arrangement significantly influences the electric field distribution around transmission lines,an in-depth analysis of its effects on insulator surfaces remains lacking.To address this gap,this study establishes a composite insulator model to examine how three types of spherical contamination layers affect the electric field distribution on insulator surfaces under varying environmental conditions.The results reveal that in dry environments,the electric field strength at the apex of single-particle contamination layers increases with the particle size and relative permittivity.For the double-particle contamination layers,the electric field intensity on the insulator surface decreases as the particle spacing increases,and larger particles are more likely to attract smaller charged particles.For triple-particle contamination layers arranged in a triangular pattern,the maximum surface field strength is nearly double that of the chain-arranged particles.Furthermore,within the chain-arranged triple-particle contamination layers,a large-small-large size arrangement has a more pronounced impact on the surface electric field than a small-large-small size arrangement.In humid environments,the surface electric field strength of insulators decreases with increasing contamination levels.These findings are of significant theoretical and practical importance for ensuring the safe operation of power systems.展开更多
Recent studies have successfully demonstrated high-Tc superconductivity in bilayer nickelate La3Ni2O7.However,research on modulating the structural and transport characteristics of La3Ni2O7 films by applying“chemical...Recent studies have successfully demonstrated high-Tc superconductivity in bilayer nickelate La3Ni2O7.However,research on modulating the structural and transport characteristics of La3Ni2O7 films by applying“chemical”compressive pressure through cation substitution is still limited.Here,we address this issue in the La_(3−x)Nd_(x)Ni_(2)O_(7)(x=0,1.0,1.5,2.0,and 2.5)thin film samples.It was found that using Nd3+with a smaller radius instead of La3+can reduce the c-axis lattice constant and shift the metal-insulator transition(MIT)temperature TMIT.To probe the origin of the MIT at cryogenic temperatures,experimental measurements of magnetoresistance were conducted,and theoretical analysis was carried out using the Kondo model,Hikami-Larkin-Nagaoka equation,and other methods.The results indicate that as Nd doping rises,the contributions of the Kondo effect and two-dimensional weak localization(WL)first decrease and then increase.The total contribution of WL and the Kondo effect in the mid-doped La_(1.5)Nd_(1.5)Ni_(2)O_(7)sample was the smallest,which to some extent explains the changes in TMIT.The Kondo effect dominates in other La_(3−x)Nd_(x)Ni_(2)O_(7)(x=0,1.0,2.0,and 2.5)samples.This work demonstrates that cation doping has a significant impact on bilayer nickelates,providing experimental evidence for understanding the physical mechanism of the MIT in bilayer nickelates.展开更多
The cubic pyrochlore Tl_(2)Ru_(2)O_(7) undergoes concurrently a metal–insulator transition (MIT) and a first-order structural transition at T_(MIT)≈120 K,below which the system was found to form one-dimensional spin...The cubic pyrochlore Tl_(2)Ru_(2)O_(7) undergoes concurrently a metal–insulator transition (MIT) and a first-order structural transition at T_(MIT)≈120 K,below which the system was found to form one-dimensional spin-one Haldane chains associated with an orbital ordering of Ru-4d electrons.With an aim to tune and access distinct ground states with strong entanglements of multiple degrees of freedom,i.e.,spin,orbital,charge,and lattice,we utilize a high-pressure approach to regulate the MIT of this system.Our detailed resistivityρ(T) measurements on the polycrystalline Tl_(2)Ru_(2)O_(7) samples under various hydrostatic pressures indeed reveal an unusual evolution of the electronic ground states.At first,the MIT is suppressed monotonically from 120 K at ambient to about 70 K at 1.5 GPa and then vanishes suddenly at about 1.8 GPa without achieving a metallic ground state.Meanwhile,the system evolves into a semiconducting ground state with magnitude ofρ(T) in the entire temperature range enhanced gradually by further increasing pressure.Prior to the abrupt disappearance of MIT,a new electronic order manifested as a kink-like anomaly inρ(T) emerges at T_(0)>T_(MIT) at 1.2 GPa and it continues to increase with pressure,producing a tricritical-point-like behavior in the T–P phase diagram of Tl_(2)Ru_(2)O_(7).The presence of two successive transitions at T_(0 )and T_(MIT )in the pressure range 1.2–1.5 GPa indicates an inhomogeneous electronic state nearby the tricritical point.At P≥3 GPa,another broad anomaly emerges inρ(T) at T_(1)>T_(0),and T_(1)continuously increases with pressure,dividing the semiconductingρ(T) into distinct thermally activated regions.These rich phenomena in the pressurized Tl_(2)Ru_(2)O_(7) should originate from the complex interplay of strongly entangled multiple quantum degrees of freedom in the system near the localized-to-itinerant crossover regime.展开更多
The excitonic insulator(EI)is a more than 60-year-old theoretical proposal that is still elusive.It is a purely quantum phenomenon involving the spontaneous generation of excitons in quantum mechanics and the spontane...The excitonic insulator(EI)is a more than 60-year-old theoretical proposal that is still elusive.It is a purely quantum phenomenon involving the spontaneous generation of excitons in quantum mechanics and the spontaneous condensation of excitons in quantum statistics.At this point,the excitons represent the ground state rather than the conventional excited state.Thus,the scarcity of candidate materials is a key factor contributing to the lack of recognized EI to date.In this review,we begin with the birth of EI,presenting the current state of the field and the main challenges it faces.We then focus on recent advances in the discovery and design of EIs based on the first-principles Bethe-Salpeter scheme,in particular the dark-exciton rule guided screening of materials.It not only opens up new avenues for realizing excitonic instability in direct-gap and wide-gap semiconductors,but also leads to the discovery of novel quantum states of matter such as half-EIs and spin-triplet EIs.Finally,we will look ahead to possible research pathways leading to the first recognized EI,both theoretically and computationally.展开更多
Topological insulators represent a new phase of matter,characterized by conductive surfaces,while their bulk remains insulating.When the dimension of the system exceeds that of the topological state by at least two,th...Topological insulators represent a new phase of matter,characterized by conductive surfaces,while their bulk remains insulating.When the dimension of the system exceeds that of the topological state by at least two,the insulators are classified as higher-order topological insulators(HOTI).The appearance of higher-order topological states,such as corner states,can be explained by the filling anomaly,which leads to the fractional spectral charges in the unit cell.Previously reported fractional charges have been quite limited in number and size.In this work,based on the two-dimensional(2D)Su-Schrieffer-Heeger model lattice,we demonstrated a new class of HOTIs with adjustable fractional charges that can take any value ranging from 0 to 1,achieved by utilizing the Lorentz transformation.Furthermore,this transformation generates novel bound-state-in-continuum-like corner states,even when the lattice is in a topological trivial phase,offering a new approach to light beam localization.This work paves the way for fabricating HOTIs with diverse corner states that offer promising applicative potential.展开更多
In this investigation,we delve into the interplay between strong interactions and intricate topological configurations,leading to emergent quantum states such as magnetic topological insulators.The crux of our researc...In this investigation,we delve into the interplay between strong interactions and intricate topological configurations,leading to emergent quantum states such as magnetic topological insulators.The crux of our research centers on elucidating how lattice symmetry modulates antiferromagnetic quantum Hall phenomena.Utilizing the spinful Harper-Hofstadter model enriched with a next-nearest-neighbor(NNN)hopping term,we discern a half-filling bandgap,paving the way for the manifestation of a quantum Hall insulator characterized by a Chern number,C=2.Upon integrating a checkerboardpatterned staggered potential(△)and the Hubbard interaction(U),the system exhibits complex dynamical behaviors.Marginal NNN hopping culminates in a Ne′el antiferromagnetic Mott insulator.In contrast,intensified hopping results in stripe antiferromagnetic configurations.Moreover,in the regime of limited NNN hopping,a C=1 Ne′el antiferromagnetic quantum Hall insulator emerges.A salient observation pertains to the manifestation of a C=1 antiferromagnetic quantum Hall insulator when spin-flip mechanisms are not offset by space group symmetries.These findings chart a pathway for further explorations into antiferromagnetic Quantum Hall States.展开更多
The thickness dependence of linearly polarized light-induced momentum anisotropy and the inverse spin Hall effect(PISHE)in topological insulator(TI)Bi_(2)Te_(3)films has been investigated.A significant enhancement of ...The thickness dependence of linearly polarized light-induced momentum anisotropy and the inverse spin Hall effect(PISHE)in topological insulator(TI)Bi_(2)Te_(3)films has been investigated.A significant enhancement of the PISHE signal is observed in the 12-quintuple-layer(QL)Bi_(2)Te_(3)film compared with that of the 3-and 5-QL samples,whereas a minimal value of photoinduced momentum anisotropy is found in the 12-QL sample.The photoinduced momentum anisotropy and the PISHE in Bi_(2)Te_(3)films are more than three and two orders of magnitude larger than those in Bi2Se3 films grown on SrTiO_(3)substrates,respectively.The 3-QL sample exhibits a sinusoidal dependence of the PISHE current on the light spot position,while the 5-QL and 12-QL samples show aW-shaped dependence,which arises from the different angles between the coordinate axis x and the in-plane crystallographic axis of the Bi_(2)Te_(3)films.Our findings demonstrate the critical role of film thickness in modulating both the photoinduced momentum anisotropy and the PISHE current,thereby suggesting a thickness-engineering strategy for designing novel optoelectronic devices based on TIs.展开更多
We investigate the localization and topological properties of the Haldane model under the influence of random flux and Anderson disorder. Our localization analysis reveals that random flux induces a transition from in...We investigate the localization and topological properties of the Haldane model under the influence of random flux and Anderson disorder. Our localization analysis reveals that random flux induces a transition from insulating to metallic states, while Anderson localization only arises under the modulation of Anderson disorder. By employing real-space topological invariant methods, we demonstrates that the system undergoes topological phase transitions under different disorder manipulations, whereas random flux modulation uniquely induces topological Anderson insulator phases, with the potential to generate states with opposite Chern numbers. These findings highlight the distinct roles of disorder in shaping the interplay between topology and localization, providing insights into stabilizing topological states and designing robust topological quantum materials.展开更多
We theoretically study the effect of a uniform orbital magnetic field on spin waves in a triangular lattice tetrahedral antiferromagnetic insulator without spin–orbit coupling. Through symmetry analysis and microscop...We theoretically study the effect of a uniform orbital magnetic field on spin waves in a triangular lattice tetrahedral antiferromagnetic insulator without spin–orbit coupling. Through symmetry analysis and microscopic calculation, we show that the optical spin wave mode at the Brillouin zone center can acquire a small orbital magnetic moment, although it exhibits no magnetic moment from the Zeeman coupling. Our results are potentially applicable to intercalated van der Waals materials and twisted double-bilayer graphene.展开更多
Electron-hole interactions play a crucial role in determining the optoelectronic properties of materials,and in lowdimensional systems this is especially true due to the decrease of screening.In this review,we focus o...Electron-hole interactions play a crucial role in determining the optoelectronic properties of materials,and in lowdimensional systems this is especially true due to the decrease of screening.In this review,we focus on one unique quantum phase induced by the electron-hole interaction in two-dimensional systems,known as“exciton insulators”(EIs).Although this phase of matter has been studied for more than half a century,suitable platforms for its stable realization remain scarce.We provide an overview of the strategies to realize EIs in accessible materials and structures,along with a discussion on some unique properties of EIs stemming from the band structures of these materials.Additionally,signatures in experiments to distinguish EIs are discussed.展开更多
We report a systematic study on the transport properties of(Bi_(0.2)Sb_(0.8))_(2)Te_(3)and(Bi_(0.4)Sb_(0.6))_(2)Te_(3)nanoplates with a thickness of about 6 nm grown by chemical vapor deposition(CVD)on Si/SiO_(2)subst...We report a systematic study on the transport properties of(Bi_(0.2)Sb_(0.8))_(2)Te_(3)and(Bi_(0.4)Sb_(0.6))_(2)Te_(3)nanoplates with a thickness of about 6 nm grown by chemical vapor deposition(CVD)on Si/SiO_(2)substrate.We achieve a significant ambipolar field effect in the two samples with different compositions by applying back-gate voltage,successfully tuning the Fermi level across the Dirac point of surface states.It is found that the Hall resistance exhibits strong non-linear behavior and magnetic field induced sign change of the slope when the Fermi level is near the Dirac point,indicating the coexistence of n-type and p-type carriers.Moreover,this coincides with the striking crossover from weak antilocalization(WAL)to linear magnetoresistance(LMR).These gate and temperature dependent magneto-transport studies provide a deeper insight into the nature of LMR and WAL in topological materials.展开更多
In recent years,the study of higher-order topological states and their material realizations has become a research frontier in topological condensed matter physics.We demonstrate that twisted bilayer graphene with sma...In recent years,the study of higher-order topological states and their material realizations has become a research frontier in topological condensed matter physics.We demonstrate that twisted bilayer graphene with small twist angles behaves as a second-order topological insulator possessing topological corner charges.Using a tight-binding model,we compute the topological band indices and corner states of finite-sized twisted bilayer graphene flakes.It is found that for any small twist angle,whether commensurate or incommensurate,the gaps both below and above the flat bands are associated with nontrivial topological indices.Our results not only extend the concept of second-order band topology to arbitrary small twist angles but also confirm the existence of corner states at acute-angle corners.展开更多
The flashover of insulator strings occurring at normal working voltages undercontaminated/polluted conditions, obviously deserves serious consideration. Though much researchhas been gone into pollution-induced flashov...The flashover of insulator strings occurring at normal working voltages undercontaminated/polluted conditions, obviously deserves serious consideration. Though much researchhas been gone into pollution-induced flashover phenomena but grey areas still exist in ourknowledge. In the present experimental study the breakdown (flashover) voltages across gaps oninsulator top surfaces and gaps between sheds (on the underside of an insulator), also the flashoverstudies on a single unit and a 3-unit insulator strings were carried out. An attempt has been madeto correlate the values obtained for all the cases. From the present investigation it was found thatresistance measurement of individual units of a polluted 3-unit string before and after flashoverindicates that strongly differing resistances could be the cause of flashover of ceramic discinsulator strings.展开更多
基金supported by the Strategic Priority Research Program of Chinese Academy of Sciences (Grant Nos.XDB28000000 and XDB0460000)the Quantum Science and Technology-National Science and Technology Major Project (Grant No.2021ZD0302600)the National Key Research and Development Program of China(Grant No.2024YFA1409002)。
文摘The hybridization gap in strained-layer InAs/In_(x)Ga_(1−x) Sb quantum spin Hall insulators(QSHIs)is significantly enhanced compared to binary InAs/GaSb QSHI structures,where the typical indium composition,x,ranges between 0.2 and 0.4.This enhancement prompts a critical question:to what extent can quantum wells(QWs)be strained while still preserving the fundamental QSHI phase?In this study,we demonstrate the controlled molecular beam epitaxial growth of highly strained-layer QWs with an indium composition of x=0.5.These structures possess a substantial compressive strain within the In_(0.5)Ga_(0.5)Sb QW.Detailed crystal structure analyses confirm the exceptional quality of the resulting epitaxial films,indicating coherent lattice structures and the absence of visible dislocations.Transport measurements further reveal that the QSHI phase in InAs/In_(0.5)Ga_(0.5)Sb QWs is robust and protected by time-reversal symmetry.Notably,the edge states in these systems exhibit giant magnetoresistance when subjected to a modest perpendicular magnetic field.This behavior is in agreement with the𝑍2 topological property predicted by the Bernevig–Hughes–Zhang model,confirming the preservation of topologically protected edge transport in the presence of enhanced bulk strain.
基金support by the National Natural Sci-ence Foundation of China(Grant No.12274477)the De-partment of Science and Technology of Guangdong Provincein China(Grant No.2019QN01X061)。
文摘In moiré-patterned van der Waals structures of transition metal dichalcogenides,correlated insulators can form under integer and fractional fillings,whose transport properties are governed by various quasiparticle excitations including holons,doublons and interlayer exciton insulators.Here we theoretically investigate the nearest-neighbor inter-site hoppings of holons and interlayer exciton insulators.Our analysis indicates that these hopping strengths are significantly enhanced compared to that of a single carrier.The underlying mechanism can be attributed to the strong Coulomb interaction between carriers at different sites.For the interlayer exciton insulator consisting of a holon and a carrier in different layers,we have also obtained its effective Bohr radius and energy splitting between the ground and the first-excited states.
基金supported by the National Natural Science Foundation of China(Grant Nos.12274472,12494594,12494591,and 92165204)National Key Research and Development Program of China(Grant No.2022YFA1402802)+2 种基金Guangdong Provincial Key Laboratory of Magnetoelectric Physics and Devices(Grant No.2022B1212010008)Guangdong Fundamental Research Center for Magnetoelectric Physics(Grant No.2024B0303390001)Guangdong Provincial Quantum Science Strategic Initiative(Grant No.GDZX2401010)。
文摘We investigate the interplay between the pseudogap state and d-wave superconductivity in the two-dimensional doped Hubbard model by employing an eight-site cluster dynamical mean-field theory method.By tuning electron hopping parameters,the strong-coupling pseudogap in the two-dimensional Hubbard model can be either enhanced or suppressed in the doped Mott insulator regime.We find that in underdoped cases,the closing of pseudogap leads to a significant enhancement of superconductivity,indicating competition between the two in the underdoped regime.In contrast,at large dopings,suppressing the pseudogap is accompanied by a concurrent decrease in the superconducting transition temperature Tc,which can be attributed to a reduction in antiferromagnetic correlations behind both the pseudogap and superconductivity.We elucidate this evolving relationship between pseudogap and superconductivity across different doping regimes.
基金supported by the National Natural Science Foundation of China (Grant Nos.11874141,12174059,and 11604134)。
文摘Ferroelectric topological insulators realized in heterostructures of two topologically trivial two-dimensional materials have recently attracted significant interest. Using first-principles calculations combined with topological quantum chemistry, we investigate bilayer α-In_(2) Se_(3)(2 L-In_(2) Se_(3)) in van der Waals heterostructures with XSe(X = Ga, In, Tl) substrates within space group P 3m1(No. 156). We show that the emergence of ferroelectricity-driven topological phase transitions in these systems is dictated by fundamental symmetry principles rather than material-specific effects. The band bending at the XSe/2 L-In_(2) Se_(3) interface enables topological band inversions, with higher-electron-affinity substrates such as GaSe and TlSe favoring the transition. Remarkably, GaSe/2 L-In_(2) Se_(3) exhibits a reversible transition between topological and trivial insulating phases upon polarization switching, while TlSe/2 L-In_(2) Se_(3) undergoes sequential transitions from a topological insulator to a trivial insulator and eventually to a metallic state. This multistate manipulation highlights a viable route for designing tunable, low-power, multi-functional electronic devices.
基金supported by the National Natural Science Foundation of China(Grants Nos.12174288 and 12274326)the National Key R&D Program of China(Grant No.2021YFA1400602)。
文摘The topological phases and edge states of a topological Euler insulator on a triangular lattice is studied.Differently from two-band Chern insulators,a topological Euler insulator is a kind of three-band model,described by the Euler number not the Chern number.The spin textures of a topological Euler insulator in the momentum space is like a Néel-type skyrmion.It is found that the topological edge states exist in the band gap of the topological Euler insulator,and the topological Euler insulator can be transformed into a topological metal without the topological phase transition.
基金supported by the Program for Science and Technology Innovation Team in Zhejiang Province,China(Grant No.2021R01004)the Six Talent Peaks Project of Jiangsu Province,China(Grant No.2019-XCL-081)the Startup Funding of Ningbo University and Yongjiang Recruitment Project(Grant No.432200942).
文摘The exploration of topological phases remains a cutting-edge research frontier,driven by their promising potential for next-generation electronic and quantum technologies.In this work,we employ first-principles calculations and tightbinding modeling to systematically investigate the topological properties of freestanding two-dimensional(2D)honeycomb Bi,HgTe,and Al_(2)O_(3)(0001)-supported HgTe.Remarkably,all three systems exhibit coexistence of intrinsic first-and higher-order topological insulator states,induced by spin-orbit coupling(SOC).These states manifest as topologically protected gapless edge states in one-dimensional(1D)nanoribbons and symmetry-related corner states in zero-dimensional(0D)nanoflakes.Furthermore,fractional electron charges may accumulate at the corners of armchair-edged nanoflakes.Among these materials,HgTe/Al_(2)O_(3)(0001)is particularly promising due to its experimentally feasible atomic configuration and low-energy corner states.Our findings highlight the importance of exploring higher-order topological phases in quantum spin Hall insulators and pave the way for new possibilities in device applications.
基金supported by the National Natu-ral Science Foundation of China(Grants No.12174220 and No.12074217)the Shandong Provincial Science Foundation for Excellent Young Scholars(Grant No.ZR2023YQ001)+1 种基金the Taishan Young Scholar Program of Shandong Provincethe Qilu Young Scholar Pro-gram of Shandong University.
文摘Higher-order band topology not only enriches our understanding of topological phases but also unveils pioneering lower-dimensional boundary states,which harbors substantial potential for next-generation device applications.The distinct electronic configurations and tunable attributes of two-dimensional materials position them as a quintessential platform for the realization of second-order topological insulators(SOTIs).This article provides an overview of the research progress in SOTIs within the field of two-dimensional electronic materials,focusing on the characterization of higher-order topological properties and the numerous candidate materials proposed in theoretical studies.These endeavors not only enhance our understanding of higher-order topological states but also highlight potential material systems that could be experimentally realized.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.12064034 and 11562017)the Leading Talents Program of Science and Technology Innovation in Ningxia Hui Autonomous Region,China(Grant No.2020GKLRLX08)the Natural Science Foundation of Ningxia Hui Autonomous Region,China(Grant No.2024AAC05040)。
文摘Atmospheric particle adsorption on insulator surfaces,coupled with humid environments,significantly affects contamination flashover,necessitating a clear understanding of the electric field distribution on insulator surfaces with adsorbed particles.This is crucial for accurately assessing insulator safety and informing critical decision-making.Although previous research has demonstrated that particle arrangement significantly influences the electric field distribution around transmission lines,an in-depth analysis of its effects on insulator surfaces remains lacking.To address this gap,this study establishes a composite insulator model to examine how three types of spherical contamination layers affect the electric field distribution on insulator surfaces under varying environmental conditions.The results reveal that in dry environments,the electric field strength at the apex of single-particle contamination layers increases with the particle size and relative permittivity.For the double-particle contamination layers,the electric field intensity on the insulator surface decreases as the particle spacing increases,and larger particles are more likely to attract smaller charged particles.For triple-particle contamination layers arranged in a triangular pattern,the maximum surface field strength is nearly double that of the chain-arranged particles.Furthermore,within the chain-arranged triple-particle contamination layers,a large-small-large size arrangement has a more pronounced impact on the surface electric field than a small-large-small size arrangement.In humid environments,the surface electric field strength of insulators decreases with increasing contamination levels.These findings are of significant theoretical and practical importance for ensuring the safe operation of power systems.
基金supported by the Natural Science Foundation of Guangdong Province of China(Grant No.2025A1515011071)the National Natural Science Foundation of China(Grant Nos.92065110,11974048,and 12074334)the Beijing Municipal Natural Science Foundation Key Research Topics(Grant No.Z230006)。
文摘Recent studies have successfully demonstrated high-Tc superconductivity in bilayer nickelate La3Ni2O7.However,research on modulating the structural and transport characteristics of La3Ni2O7 films by applying“chemical”compressive pressure through cation substitution is still limited.Here,we address this issue in the La_(3−x)Nd_(x)Ni_(2)O_(7)(x=0,1.0,1.5,2.0,and 2.5)thin film samples.It was found that using Nd3+with a smaller radius instead of La3+can reduce the c-axis lattice constant and shift the metal-insulator transition(MIT)temperature TMIT.To probe the origin of the MIT at cryogenic temperatures,experimental measurements of magnetoresistance were conducted,and theoretical analysis was carried out using the Kondo model,Hikami-Larkin-Nagaoka equation,and other methods.The results indicate that as Nd doping rises,the contributions of the Kondo effect and two-dimensional weak localization(WL)first decrease and then increase.The total contribution of WL and the Kondo effect in the mid-doped La_(1.5)Nd_(1.5)Ni_(2)O_(7)sample was the smallest,which to some extent explains the changes in TMIT.The Kondo effect dominates in other La_(3−x)Nd_(x)Ni_(2)O_(7)(x=0,1.0,2.0,and 2.5)samples.This work demonstrates that cation doping has a significant impact on bilayer nickelates,providing experimental evidence for understanding the physical mechanism of the MIT in bilayer nickelates.
基金supported by the National Key Research and Development Program of China (Grant Nos. 2023YFA1406100 and 2021YFA1400200)the National Natural Science Foundation of China (Grant Nos. 12025408 and 12174424)+2 种基金the Youth Innovation Promotion Association of Chinese Academy of Scineces (Grant No. 2023007)support from the National Natural Science Foundation of China (Grant No. 11904272)the Open Fund of Hubei Provincial Key Laboratory of Metallurgical Industry Process Systems Science (Grant No. Z202202)。
文摘The cubic pyrochlore Tl_(2)Ru_(2)O_(7) undergoes concurrently a metal–insulator transition (MIT) and a first-order structural transition at T_(MIT)≈120 K,below which the system was found to form one-dimensional spin-one Haldane chains associated with an orbital ordering of Ru-4d electrons.With an aim to tune and access distinct ground states with strong entanglements of multiple degrees of freedom,i.e.,spin,orbital,charge,and lattice,we utilize a high-pressure approach to regulate the MIT of this system.Our detailed resistivityρ(T) measurements on the polycrystalline Tl_(2)Ru_(2)O_(7) samples under various hydrostatic pressures indeed reveal an unusual evolution of the electronic ground states.At first,the MIT is suppressed monotonically from 120 K at ambient to about 70 K at 1.5 GPa and then vanishes suddenly at about 1.8 GPa without achieving a metallic ground state.Meanwhile,the system evolves into a semiconducting ground state with magnitude ofρ(T) in the entire temperature range enhanced gradually by further increasing pressure.Prior to the abrupt disappearance of MIT,a new electronic order manifested as a kink-like anomaly inρ(T) emerges at T_(0)>T_(MIT) at 1.2 GPa and it continues to increase with pressure,producing a tricritical-point-like behavior in the T–P phase diagram of Tl_(2)Ru_(2)O_(7).The presence of two successive transitions at T_(0 )and T_(MIT )in the pressure range 1.2–1.5 GPa indicates an inhomogeneous electronic state nearby the tricritical point.At P≥3 GPa,another broad anomaly emerges inρ(T) at T_(1)>T_(0),and T_(1)continuously increases with pressure,dividing the semiconductingρ(T) into distinct thermally activated regions.These rich phenomena in the pressurized Tl_(2)Ru_(2)O_(7) should originate from the complex interplay of strongly entangled multiple quantum degrees of freedom in the system near the localized-to-itinerant crossover regime.
基金Project supported by the National Key Research and Development Program of China(Grant Nos.2023YFA1406400 and 2020YFA0308800)the National Natural Science Foundation of China(Grant No.12474064)。
文摘The excitonic insulator(EI)is a more than 60-year-old theoretical proposal that is still elusive.It is a purely quantum phenomenon involving the spontaneous generation of excitons in quantum mechanics and the spontaneous condensation of excitons in quantum statistics.At this point,the excitons represent the ground state rather than the conventional excited state.Thus,the scarcity of candidate materials is a key factor contributing to the lack of recognized EI to date.In this review,we begin with the birth of EI,presenting the current state of the field and the main challenges it faces.We then focus on recent advances in the discovery and design of EIs based on the first-principles Bethe-Salpeter scheme,in particular the dark-exciton rule guided screening of materials.It not only opens up new avenues for realizing excitonic instability in direct-gap and wide-gap semiconductors,but also leads to the discovery of novel quantum states of matter such as half-EIs and spin-triplet EIs.Finally,we will look ahead to possible research pathways leading to the first recognized EI,both theoretically and computationally.
基金supported by the Natural Science Basic Research Program of Shaanxi Province(No.2024JC-JCQN-06)the National Natural Science Foundation of China(Nos.12474337,12304370)Fundamental Research Funds for the Central Universities(No.xzy012024135).
文摘Topological insulators represent a new phase of matter,characterized by conductive surfaces,while their bulk remains insulating.When the dimension of the system exceeds that of the topological state by at least two,the insulators are classified as higher-order topological insulators(HOTI).The appearance of higher-order topological states,such as corner states,can be explained by the filling anomaly,which leads to the fractional spectral charges in the unit cell.Previously reported fractional charges have been quite limited in number and size.In this work,based on the two-dimensional(2D)Su-Schrieffer-Heeger model lattice,we demonstrated a new class of HOTIs with adjustable fractional charges that can take any value ranging from 0 to 1,achieved by utilizing the Lorentz transformation.Furthermore,this transformation generates novel bound-state-in-continuum-like corner states,even when the lattice is in a topological trivial phase,offering a new approach to light beam localization.This work paves the way for fabricating HOTIs with diverse corner states that offer promising applicative potential.
文摘In this investigation,we delve into the interplay between strong interactions and intricate topological configurations,leading to emergent quantum states such as magnetic topological insulators.The crux of our research centers on elucidating how lattice symmetry modulates antiferromagnetic quantum Hall phenomena.Utilizing the spinful Harper-Hofstadter model enriched with a next-nearest-neighbor(NNN)hopping term,we discern a half-filling bandgap,paving the way for the manifestation of a quantum Hall insulator characterized by a Chern number,C=2.Upon integrating a checkerboardpatterned staggered potential(△)and the Hubbard interaction(U),the system exhibits complex dynamical behaviors.Marginal NNN hopping culminates in a Ne′el antiferromagnetic Mott insulator.In contrast,intensified hopping results in stripe antiferromagnetic configurations.Moreover,in the regime of limited NNN hopping,a C=1 Ne′el antiferromagnetic quantum Hall insulator emerges.A salient observation pertains to the manifestation of a C=1 antiferromagnetic quantum Hall insulator when spin-flip mechanisms are not offset by space group symmetries.These findings chart a pathway for further explorations into antiferromagnetic Quantum Hall States.
基金supported by the National Natural Science Foundation of China(Grant Nos.62074036,61674038,and 11574302)the Foreign Cooperation Project of Fujian Province,China(Grant No.2023I0005)+2 种基金the Open Research Fund Program of the State Key Laboratory of Low Dimensional Quantum Physics(Grant No.KF202108)the National Key Research and Development Program of China(Grant No.2016YFB0402303)the Foundation of the Fujian Provincial Department of Industry and Information Technology of China(Grant No.82318075).
文摘The thickness dependence of linearly polarized light-induced momentum anisotropy and the inverse spin Hall effect(PISHE)in topological insulator(TI)Bi_(2)Te_(3)films has been investigated.A significant enhancement of the PISHE signal is observed in the 12-quintuple-layer(QL)Bi_(2)Te_(3)film compared with that of the 3-and 5-QL samples,whereas a minimal value of photoinduced momentum anisotropy is found in the 12-QL sample.The photoinduced momentum anisotropy and the PISHE in Bi_(2)Te_(3)films are more than three and two orders of magnitude larger than those in Bi2Se3 films grown on SrTiO_(3)substrates,respectively.The 3-QL sample exhibits a sinusoidal dependence of the PISHE current on the light spot position,while the 5-QL and 12-QL samples show aW-shaped dependence,which arises from the different angles between the coordinate axis x and the in-plane crystallographic axis of the Bi_(2)Te_(3)films.Our findings demonstrate the critical role of film thickness in modulating both the photoinduced momentum anisotropy and the PISHE current,thereby suggesting a thickness-engineering strategy for designing novel optoelectronic devices based on TIs.
基金Project supported by the National Key Research and Development Program of China (Grant Nos. 2021YFA1400900, 2021YFA0718300, and 2021YFA1402100)the National Natural Science Foundation of China (Grant Nos. 12174461, 12234012, 12334012, and 52327808)。
文摘We investigate the localization and topological properties of the Haldane model under the influence of random flux and Anderson disorder. Our localization analysis reveals that random flux induces a transition from insulating to metallic states, while Anderson localization only arises under the modulation of Anderson disorder. By employing real-space topological invariant methods, we demonstrates that the system undergoes topological phase transitions under different disorder manipulations, whereas random flux modulation uniquely induces topological Anderson insulator phases, with the potential to generate states with opposite Chern numbers. These findings highlight the distinct roles of disorder in shaping the interplay between topology and localization, providing insights into stabilizing topological states and designing robust topological quantum materials.
基金Project supported by the National Key R&D Program of China (Grant No. 2022YFA1403800)the National Natural Science Foundation of China (Grant Nos. 12250008 and 12188101)+1 种基金the Project for Young Scientists in Basic Research (Grant No. YSBR-059)performed in part at the Aspen Center for Physics, supported by the National Natural Science Foundation of China (Grant No. PHY2210452)。
文摘We theoretically study the effect of a uniform orbital magnetic field on spin waves in a triangular lattice tetrahedral antiferromagnetic insulator without spin–orbit coupling. Through symmetry analysis and microscopic calculation, we show that the optical spin wave mode at the Brillouin zone center can acquire a small orbital magnetic moment, although it exhibits no magnetic moment from the Zeeman coupling. Our results are potentially applicable to intercalated van der Waals materials and twisted double-bilayer graphene.
基金supported by the National Key Research&Development Program of China(Grant Nos.2022YFA1403500 and 2021YFA1400500)the National Science Foundation of China(Grant Nos.62321004,12234001,and 12474215)+1 种基金supported by New Cornerstone Science Foundationa fellowship and a CRF award from the Research Grants Council of the Hong Kong Special Administrative Region,China(Grant Nos.HKUST SRFS2324-6S01 and C7037-22GF)。
文摘Electron-hole interactions play a crucial role in determining the optoelectronic properties of materials,and in lowdimensional systems this is especially true due to the decrease of screening.In this review,we focus on one unique quantum phase induced by the electron-hole interaction in two-dimensional systems,known as“exciton insulators”(EIs).Although this phase of matter has been studied for more than half a century,suitable platforms for its stable realization remain scarce.We provide an overview of the strategies to realize EIs in accessible materials and structures,along with a discussion on some unique properties of EIs stemming from the band structures of these materials.Additionally,signatures in experiments to distinguish EIs are discussed.
基金financially supported by the National Natural Science Foundation of China(Nos.52201233,52371204,and 52031014).
文摘We report a systematic study on the transport properties of(Bi_(0.2)Sb_(0.8))_(2)Te_(3)and(Bi_(0.4)Sb_(0.6))_(2)Te_(3)nanoplates with a thickness of about 6 nm grown by chemical vapor deposition(CVD)on Si/SiO_(2)substrate.We achieve a significant ambipolar field effect in the two samples with different compositions by applying back-gate voltage,successfully tuning the Fermi level across the Dirac point of surface states.It is found that the Hall resistance exhibits strong non-linear behavior and magnetic field induced sign change of the slope when the Fermi level is near the Dirac point,indicating the coexistence of n-type and p-type carriers.Moreover,this coincides with the striking crossover from weak antilocalization(WAL)to linear magnetoresistance(LMR).These gate and temperature dependent magneto-transport studies provide a deeper insight into the nature of LMR and WAL in topological materials.
基金supported by the National Natural Science Foundation of China(Grant Nos.12104232 and 12074156).
文摘In recent years,the study of higher-order topological states and their material realizations has become a research frontier in topological condensed matter physics.We demonstrate that twisted bilayer graphene with small twist angles behaves as a second-order topological insulator possessing topological corner charges.Using a tight-binding model,we compute the topological band indices and corner states of finite-sized twisted bilayer graphene flakes.It is found that for any small twist angle,whether commensurate or incommensurate,the gaps both below and above the flat bands are associated with nontrivial topological indices.Our results not only extend the concept of second-order band topology to arbitrary small twist angles but also confirm the existence of corner states at acute-angle corners.
文摘The flashover of insulator strings occurring at normal working voltages undercontaminated/polluted conditions, obviously deserves serious consideration. Though much researchhas been gone into pollution-induced flashover phenomena but grey areas still exist in ourknowledge. In the present experimental study the breakdown (flashover) voltages across gaps oninsulator top surfaces and gaps between sheds (on the underside of an insulator), also the flashoverstudies on a single unit and a 3-unit insulator strings were carried out. An attempt has been madeto correlate the values obtained for all the cases. From the present investigation it was found thatresistance measurement of individual units of a polluted 3-unit string before and after flashoverindicates that strongly differing resistances could be the cause of flashover of ceramic discinsulator strings.
