Systems hosting flat bands offer a powerful platform for exploring strong correlation physics.Theoretically,topological degeneracy arising in systems with non-trivial topological orders on periodic manifolds of non-ze...Systems hosting flat bands offer a powerful platform for exploring strong correlation physics.Theoretically,topological degeneracy arising in systems with non-trivial topological orders on periodic manifolds of non-zero genus can generate ideal flat bands.However,experimental realization of such geometrically engineered systems is very difficult.In this work,we demonstrate that flat planes with strategically patterned hole defects can engineer ideal flat bands.We construct two families of models:singular flat band systems where degeneracy is stabilized by non-contractible loop excitations tied to hole defects and perfectly nested van Hove systems where degeneracy arises from line excitations in momentum space.These models circumvent the need for exotic manifolds while retaining the essential features of topological flat bands.By directly linking defect engineering to degeneracy mechanisms,our results establish a scalable framework for experimentally accessible flat band design.展开更多
Flat electronic bands in condensed matter provide a rich avenue for exploring novel quantum phenomena. Here, we report an optical spectroscopy study of a topological hourglass semimetal Nb_(3)SiTe_(6) with the electri...Flat electronic bands in condensed matter provide a rich avenue for exploring novel quantum phenomena. Here, we report an optical spectroscopy study of a topological hourglass semimetal Nb_(3)SiTe_(6) with the electric field of the incident light parallel to its crystalline ab-plane. The ab-plane optical conductivity spectra of Nb_(3)SiTe_(6) single crystals exhibit a remarkable peak-like feature around 1.20 eV, which is mainly contributed by the direct optical transitions between the two ab-initio-calculation-derived flat bands along the momentum direction Z–U. Our results pave the way for investigating exotic quantum phenomena based on the flat bands in topological hourglass semimetals.展开更多
The novel electronic properties of bilayer graphene can be fine-tuned via twisting,which may induce flat bands around the Fermi level with nontrivial topology.In general,the band structure of such twisted bilayer grap...The novel electronic properties of bilayer graphene can be fine-tuned via twisting,which may induce flat bands around the Fermi level with nontrivial topology.In general,the band structure of such twisted bilayer graphene(TBG)can be theoretically obtained by using first-principles calculations,tight-binding method,or continuum model,which are either computationally demanding or parameters dependent.In this work,by using the sure independence screening sparsifying operator method,we propose a physically interpretable three-dimensional(3D)descriptor which can be utilized to readily obtain theΓ-point gap of TBG at arbitrary twist angles and different interlayer spacings.The strong predictive power of the descriptor is demonstrated by a high Pearson coefficient of 99%for both the training and testing data.To go further,we adopt the neural network algorithm to accurately probe the flat bands of TBG at various twist angles,which can accelerate the study of strong correlation physics associated with such a fundamental characteristic,especially for those systems with a larger number of atoms in the unit cell.展开更多
The physics of flat band is novel and rich but difficult to access.In this regard,recently twisting of bilayer van der Waals(vd W)-bounded two-dimensional(2 D)materials has attracted much attention,because the reducti...The physics of flat band is novel and rich but difficult to access.In this regard,recently twisting of bilayer van der Waals(vd W)-bounded two-dimensional(2 D)materials has attracted much attention,because the reduction of Brillouin zone will eventually lead to a diminishing kinetic energy.Alternatively,one may start with a 2 D kagome lattice,which already possesses flat bands at the Fermi level,but unfortunately these bands connect quadratically to other(dispersive)bands,leading to undesirable effects.Here,we propose,by first-principles calculation and tight-binding modeling,that the same bilayer twisting approach can be used to isolate the kagome flat bands.As the starting kinetic energy is already vanishingly small,the interlayer vd W potential is always sufficiently large irrespective of the twisting angle.As such the electronic states in the(connected)flat bands become unstable against a spontaneous Wigner crystallization,which is expected to have interesting interplays with other flat-band phenomena such as novel superconductivity and anomalous quantum Hall effect.展开更多
Owing to the interaction between the layers,the twisted bilayer two-dimensional(2 D)materials exhibit numerous unique optical and electronic properties different from the monolayer counterpart,and have attracted treme...Owing to the interaction between the layers,the twisted bilayer two-dimensional(2 D)materials exhibit numerous unique optical and electronic properties different from the monolayer counterpart,and have attracted tremendous interests in current physical research community.By means of first-principles and tight-binding model calculations,the electronic properties of twisted bilayer biphenylene carbon(BPC)are systematically investigated in this paper.The results indicate that the effect of twist will not only leads to a phase transition from semiconductor to metal,but also an adjustable band gap in BPC(0 me V to 120 me V depending on the twist angle).Moreover,unlike the twisted bilayer graphene(TBG),the flat bands in twisted BPC are no longer restricted by"magic angles",i.e.,abnormal flat bands could be appeared as well at several specific large angles in addition to the small angles.The charge density of these flat bands possesses different local modes,indicating that they might be derived from different stacked modes and host different properties.The exotic physical properties presented in this work foreshow twisted BPC a promising material for the application of terahertz and infrared photodetectors and the exploration of strong correlation.展开更多
Recent experiments[Science Advances 4 eaao4513(2018)]have revealed the evidence of nodal-line superconductivity in half-Heusler superconductors,e.g.,YPt Bi.Theories have suggested the topological nature of such nodal-...Recent experiments[Science Advances 4 eaao4513(2018)]have revealed the evidence of nodal-line superconductivity in half-Heusler superconductors,e.g.,YPt Bi.Theories have suggested the topological nature of such nodal-line superconductivity and proposed the existence of surface Majorana flat bands on the(111)surface of half-Heusler superconductors.Due to the divergent density of states of the surface Majorana flat bands,the surface order parameter and the surface impurity play essential roles in determining the surface properties.We study the effect of the surface order parameter and the surface impurity on the surface Majorana flat bands of half-Heusler superconductors based on the Luttinger model.To be specific,we consider the topological nodal-line superconducting phase induced by the singlet-quintet pairing mixing,classify all the possible translationally invariant order parameters for the surface states according to irreducible representations of C3vpoint group,and demonstrate that any energetically favorable order parameter needs to break the time-reversal symmetry.We further discuss the energy splitting in the energy spectrum of surface Majorana flat bands induced by different order parameters and non-magnetic or magnetic impurities.We propose that the splitting in the energy spectrum can serve as the fingerprint of the pairing symmetry and mean-field order parameters.Our theoretical prediction can be examined in the future scanning tunneling microscopy experiments.展开更多
The discovery of each new allotropic manifestation of carbon has substantially propelled contemporary scientific research and applications,as vividly exemplified by the explosive progressions within the realms of full...The discovery of each new allotropic manifestation of carbon has substantially propelled contemporary scientific research and applications,as vividly exemplified by the explosive progressions within the realms of fullerene,carbon nanotube,graphene,and diamond sciences over recent decades.Here,using state-of-the-art first-principles calculations,we predict a new type of twodimensional carbon network,dubbed fullerphene,by replacing each C atom in graphene with a fullerene(C_(60)).Its high energetic stability is tied to the symmetric cycloaddition of the double bonds between each C_(60) superatom with each of its three neighbors.A kinetic pathway is also proposed for the selective fabrication of fullerphene on Cu(111)or Rh(111),upon exploiting its enhanced stability over other competing C_(60) assemblies and significantly lowered kinetic barrier in seed growing,as strategically supported by a recent experimental advance.Further investigations on fullerphene reveal an array of desirable characteristics,including a substantial band gap of∼2 eV,Dirac states for the conduction electrons,and flat bands for the valence electrons.These findings represent a distinctly new and significant advance in both fullerene and graphene sciences.展开更多
Paramagnetic LaCoSi,a ternary intermetallic electride,consists of CoSi blocks separated by two layers of La atoms.Its structure is similar to that of the widely studied 111 system of iron-based superconductors.Utilizi...Paramagnetic LaCoSi,a ternary intermetallic electride,consists of CoSi blocks separated by two layers of La atoms.Its structure is similar to that of the widely studied 111 system of iron-based superconductors.Utilizing angle-resolved photoemission spectroscopy and first-principles calculations,we demonstrate the existence of linear bands and flat bands mainly originating from the orbitals of Co 3d states near the Fermi energy.The anomalous scattering rate of the linear bands varies linearly with the binding energy.The flat band above the Fermi energy indicated by the calculations could be modulated by substitutions and pressure to induce new ordered quantum phases,such as magnetism and superconductivity.Our findings reveal flat-band physics in electrides.展开更多
Topological flat bands have attracted significant interest across various branches of physics,where synthetic gauge fields are typically considered an essential prerequisite.Numerous mechanisms have been proposed for ...Topological flat bands have attracted significant interest across various branches of physics,where synthetic gauge fields are typically considered an essential prerequisite.Numerous mechanisms have been proposed for implementing these fields,including magnetic fields on electrons,differential optical paths for photons,and strain-induced effective magnetic fields,among others.In this work,we introduce a novel approach to generating synthetic gauge fields through quantum statistics and demonstrate their effectiveness in realizing anyonic topological flat bands.Notably,we discover that a pair of strongly interacting anyons can induce square-root topological flat bands within a lattice model that remains dispersive and topologically trivial for a single particle.To validate our theoretical predictions,we experimentally simulate the quantum statistics-induced topological flat bands and square-root topological boundary states by mapping the eigenstates of two anyons onto modes in electric circuits.Our findings not only open a new pathway for creating topological flat bands but also deepen our understanding of anyonic physics and the underlying principles of flat-band topology.展开更多
Twisted graphene multilayers exhibit strongly correlated insulating states and superconductivity due to the presence of ultraflat bands near the charge neutral point.In this paper,the response of ultraflat bands to la...Twisted graphene multilayers exhibit strongly correlated insulating states and superconductivity due to the presence of ultraflat bands near the charge neutral point.In this paper,the response of ultraflat bands to lattice relaxation and a magnetic field in twisted trilayer graphene(tTLG)with different stacking arrangements is investigated by using a full tight-binding model.We show that lattice relaxations are indispensable for understanding the electronic properties of tTLG,in particular,of tTLG in the presence of mirror symmetry.Lattice relaxations renormalize the quasiparticle spectrum near the Fermi energy and change the localization of higher energy flat bands.Furthermore,different from the twisted bilayer graphene,the Hofstadter butterfly spectrum can be realized at laboratory accessible strengths of magnetic field.Our work verifies tTLG as a more tunable platform than the twisted bilayer graphene in strongly correlated phenomena.展开更多
At magic twisted angles,Dirac cones in twisted bilayer graphene(TBG)can evolve into flat bands,serving as a critical playground for the study of strongly correlated physics.When chiral symmetry is introduced,rigorous ...At magic twisted angles,Dirac cones in twisted bilayer graphene(TBG)can evolve into flat bands,serving as a critical playground for the study of strongly correlated physics.When chiral symmetry is introduced,rigorous mathematical proof confirms that the flat bands are locked at zero energy in the entire Moiré Brillouin zone(BZ).Yet,TBG is not the sole platform that exhibits this absolute band flatness.Central to this flatness phenomenon are topological nodes and their specific locations in the BZ.In this study,considering twisted bilayer systems that preserve chiral symmetry,we classify various ordered topological nodes in base layers and all possible node locations across different BZs.Specifically,we constrain the node locations to rotational centers,such as Γ and M points,to ensure the interlayer coupling retains equal strength in all directions.Using this classification as a foundation,we systematically identify the conditions under which Moiré flat bands emerge.Additionally,through the extension of holomorphic functions,we provide proof that flat bands are locked at zero energy,shedding light on the origin of the band flatness.Remarkably,beyond Dirac cones,numerous twisted bilayer nodal platforms can host flat bands with a degeneracy number of more than two,such as four-fold,six-fold,and eight-fold.This multiplicity of degeneracy in flat bands might unveil more complex and enriched correlation physics.展开更多
The Lieb lattice, characterized by its distinctive Dirac cone and flat-band electronic structures, hosts a variety of exotic physical phenomena. However, its realization remains largely confined to artificial lattices...The Lieb lattice, characterized by its distinctive Dirac cone and flat-band electronic structures, hosts a variety of exotic physical phenomena. However, its realization remains largely confined to artificial lattices. In this work, we propose the concept of a Lieb electride, where the non-bound electrons gather at the middle edges,behaving as the quasi-atoms of a Lieb lattice, enabling the emergence of flat bands. Using crystal structure prediction method MAGUS and first-principles calculations, we predict a stable candidate, Ca_(2)I, at ambient pressure. Distinct from conventional electrides with localized electrons at cavity centers, Ca_(2)I features interstitial electrons situated at cavity edges. The resultant flat bands lie close to the Fermi level, giving rise to a pronounced peak in the density of states and leading to Stoner-type ferromagnetism. With increasing pressures, we observe quantum phase transitions from ferromagnetic to non-magnetic and finally to antiferromagnetic orders in Ca_(2)I.Intriguingly, superconductivity emerges in the antiferromagnetic region, suggesting potential competition between these correlated states. Our study not only extends the concepts of electrides but also provides a novel strategy for realizing Lieb lattices through non-bound electrons. This work establishes Ca_(2)I as a promising platform for exploring flat-band physics and correlated electronic states, opening avenues for novel quantum phenomena in electride-based materials.展开更多
We report that the twisted few layer graphite(tFL-graphite)is a new family of moiréheterostructures(MHSs),which has richer and highly tunable moiréflat band structures entirely distinct from all the known MH...We report that the twisted few layer graphite(tFL-graphite)is a new family of moiréheterostructures(MHSs),which has richer and highly tunable moiréflat band structures entirely distinct from all the known MHSs.A tFL-graphite is composed of two few-layer graphite(Bernal stacked multilayer graphene),which are stacked on each other with a small twisted angle.The moiréband structure of the tFL-graphite strongly depends on the layer number of its composed two van der Waals layers.Near the magic angle,a tFL-graphite always has two nearly flat bands coexisting with a few pairs of narrowed dispersive(parabolic or linear)bands at the Fermi level,thus,enhances the DOS at EF.This coexistence property may also enhance the possible superconductivity as been demonstrated in other multiband superconductivity systems.Therefore,we expect strong multiband correlation effects in tFL-graphite.Meanwhile,a proper perpendicular electric field can induce several isolated nearly flat bands with nonzero valley Chern number in some simple tFL-graphites,indicating that tFL-graphite is also a novel topological flat band system.展开更多
Based on the effective continuum model,we study alternating-twist multilayer graphene system and emergence of magic angles and flat band topology.All the alternating-twist multilayer graphene system(from triple layers...Based on the effective continuum model,we study alternating-twist multilayer graphene system and emergence of magic angles and flat band topology.All the alternating-twist multilayer graphene system(from triple layers to few layers)are found to have flat bands at magic angles where the area of AA stacking equals n-fold(n is an integer)electron cyclotron area.From the pseudo-Landau-level representation,there is always an isolated Dirac band in the alternating-twist graphene system constructed by odd number of layers.Since each pair of flat bands can be perceived as the zeroth pseudo-Landau-levels in two dimensional Dirac fermions,electron in the flat band pair can feel a pseudo-magnetic field with the same magnitude but the opposite sign.Calculated Chern number for each flat band is+1(or-1)which can be tuned by twisting in the vicinity of magic angles or by gating.The concurrent appearance of strong correlation and band topology of flat bands in the alternating-twist multilayer graphene may pave an avenue for the new understanding of superconductivity observed in triple-layered graphene,and supply a new playground for realizing(quantum)anomalous Hall effect.展开更多
We show that a suitable combination of flat-band ferromagnetism,geometry and nontrivial electronic band topology can give rise to itinerant topological magnons.An SU(2) symmetric topological Hubbard model with nearly ...We show that a suitable combination of flat-band ferromagnetism,geometry and nontrivial electronic band topology can give rise to itinerant topological magnons.An SU(2) symmetric topological Hubbard model with nearly flat electronic bands,on a Kagome lattice,is considered as the prototype.This model exhibits ferromagnetic order when the lowest electronic band is half-filled.Using the numerical exact diagonalization method with a projection onto this nearly flat band,we can obtain the magnonic spectra.In the flat-band limit,the spectra exhibit distinct dispersions with Dirac points,similar to those of free electrons with isotropic hoppings,or a local spin magnet with pure ferromagnetic Heisenberg exchanges on the same geometry.Significantly,the non-flatness of the electronic band may induce a topological gap at the Dirac points,leading to a magnonic band with a nonzero Chern number.More intriguingly,this magnonic Chern number changes its sign when the topological index of the electronic band is reversed,suggesting that the nontrivial topology of the magnonic band is related to its underlying electronic band.Our work suggests interesting directions for the further exploration of,and searches for,itinerant topological magnons.展开更多
The quantum phase transition between Z_(2) plaquette valence bound solid(PVBS) and superfluid(SF) phases on the planar pyrochlore lattice(square ice) is under debate. To gain further insight, here, we focus on the dyn...The quantum phase transition between Z_(2) plaquette valence bound solid(PVBS) and superfluid(SF) phases on the planar pyrochlore lattice(square ice) is under debate. To gain further insight, here, we focus on the dynamical features of the hard-core Bose–Hubbard model on this lattice and study the excitation spectra by combining stochastic analytic continuation and quantum Monte Carlo simulation. In both PVBS and SF phases,a flat band with bow-tie structure is observed and can be explained by certain symmetries. At the transition point,the spectra turn to be continuous and gapless. A(2+1)-dimensional Abelian–Higgs model with mixed 't Hooft anomaly is proposed to describe the transition, where the anomaly matching predicts that the deconfinement can exist on the domain walls. From the snapshot of the spin configuration in real space, we found the existence of the domain wall. We also found that the spectrum along a specific path in momentum space from PVBS phase to the transition point can be well described by an XXZ spin chain, and the critical theory of XXZ spin chain matches the anomaly. The two-spinon continuum along this specific path implies additional domain walls(point defect) can emerge in the domain walls(line defect) and take the role of deconfinement at the transition point.展开更多
Kagome materials are a class of material with a lattice structure composed of corner-sharing triangles that produce various exotic electronic phenomena,such as Dirac fermions,van Hove singularities,and flat bands.Howe...Kagome materials are a class of material with a lattice structure composed of corner-sharing triangles that produce various exotic electronic phenomena,such as Dirac fermions,van Hove singularities,and flat bands.However,most of the known kagome materials have a flat band detached from the Fermi energy,which limits the investigation of the emergent flat band physics.In this work,by combining soft x-ray angle-resolved photoemission spectroscopy(ARPES)and the first-principles calculations,the electronic structure is investigated of a novel kagome metal CeNi_(5) with a clear dispersion along the kz direction and a Fermi level flat band in theΓ–K–M–Γplane.Besides,resonant ARPES experimental results indicate that the valence state of Ce ions is close to 4^(+),which is consistent with the transport measurement result.Our results demonstrate the unique electronic properties of CeNi_(5) as a new kagome metal and provide an ideal platform for exploring the flat band physics and the interactions between different types of flat bands by tuning the valence state of Ce ions.展开更多
We investigated the one-dimensional diamond ladder in the momentum lattice platform. By inducing multiple twoand four-photon Bragg scatterings among specific momentum states, we achieved a flat band system based on th...We investigated the one-dimensional diamond ladder in the momentum lattice platform. By inducing multiple twoand four-photon Bragg scatterings among specific momentum states, we achieved a flat band system based on the diamond model, precisely controlling the coupling strength and phase between individual lattice sites. Utilizing two lattice sites couplings, we generated a compact localized state associated with the flat band, which remained localized throughout the entire time evolution. We successfully realized the continuous shift of flat bands by adjusting the corresponding nearest neighbor hopping strength, enabling us to observe the complete localization process. This opens avenues for further exploration of more complex properties within flat-band systems, including investigating the robustness of flat-band localized states in disordered flat-band systems and exploring many-body localization in interacting flat-band systems.展开更多
We investigate the bound state problem in a one-dimensional flat band system with a Coulomb potential.It is found that,in the presence of a Coulomb potential of type I(with three equal diagonal elements),similarly to ...We investigate the bound state problem in a one-dimensional flat band system with a Coulomb potential.It is found that,in the presence of a Coulomb potential of type I(with three equal diagonal elements),similarly to that in the twodimensional case,the flat band could not survive.At the same time,the flat band states are transformed into localized states with a logarithmic singularity near the center position.In addition,the wave function near the origin would collapse for an arbitrarily weak Coulomb potential.Due to the wave function collapses,the eigen-energies for a shifted Coulomb potential depend sensitively on the cut-off parameter.For a Coulomb potential of type II,there exist infinite bound states that are generated from the flat band.Furthermore,when the bound state energy is very near the flat band,the energy is inversely proportional to the natural number,e.g.,E_(n)∝1/n,n=1,2,3,...It is expected that the 1/n energy spectrum could be observed experimentally in the near future.展开更多
We report structural and electronic properties of Na_(2)Ni_(3)S_(4),a quasi-two-dimensional compound composed of alternating layers of[Ni_(3)S_(4)]^(2-)and Na^(+).The compound features a remarkable Ni-based kagome lat...We report structural and electronic properties of Na_(2)Ni_(3)S_(4),a quasi-two-dimensional compound composed of alternating layers of[Ni_(3)S_(4)]^(2-)and Na^(+).The compound features a remarkable Ni-based kagome lattice with a square planar configuration of four surrounding S atoms for each Ni atom.Magnetization and electrical measurements reveal a weak paramagnetic insulator with a gap of about 0.5 eV.Our band structure calculation highlights a set of topological flat bands of the kagome lattice derived from the rotated dxz-orbital with C_(3)+T symmetry in the presence of crystal-field splitting.展开更多
基金supported by the Ministry of Science and Technology(Grant No.2022YFA1403901)the National Natural Science Foundation of China(Grant Nos.12494594,11888101,12174428,and 12504192)+2 种基金the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDB28000000)the New Cornerstone Investigator Program,the Chinese Academy of Sciences through the Youth Innovation Promotion Association(Grant No.2022YSBR-048)the Shanghai Science and Technology Innovation Action Plan(Grant No.24LZ1400800).
文摘Systems hosting flat bands offer a powerful platform for exploring strong correlation physics.Theoretically,topological degeneracy arising in systems with non-trivial topological orders on periodic manifolds of non-zero genus can generate ideal flat bands.However,experimental realization of such geometrically engineered systems is very difficult.In this work,we demonstrate that flat planes with strategically patterned hole defects can engineer ideal flat bands.We construct two families of models:singular flat band systems where degeneracy is stabilized by non-contractible loop excitations tied to hole defects and perfectly nested van Hove systems where degeneracy arises from line excitations in momentum space.These models circumvent the need for exotic manifolds while retaining the essential features of topological flat bands.By directly linking defect engineering to degeneracy mechanisms,our results establish a scalable framework for experimentally accessible flat band design.
基金Project supported by the Guangdong Basic and Applied Basic Research Foundation (Grant No. 2021B1515130007)the National Natural Science Foundation of China (Grant Nos. U21A20432 and 52273077)+1 种基金the National Key Research and Development Program of China (Grant No. 2022YFA1403800)the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB33000000)。
文摘Flat electronic bands in condensed matter provide a rich avenue for exploring novel quantum phenomena. Here, we report an optical spectroscopy study of a topological hourglass semimetal Nb_(3)SiTe_(6) with the electric field of the incident light parallel to its crystalline ab-plane. The ab-plane optical conductivity spectra of Nb_(3)SiTe_(6) single crystals exhibit a remarkable peak-like feature around 1.20 eV, which is mainly contributed by the direct optical transitions between the two ab-initio-calculation-derived flat bands along the momentum direction Z–U. Our results pave the way for investigating exotic quantum phenomena based on the flat bands in topological hourglass semimetals.
基金the National Natural Science Foundation of China(Grant No.62074114)。
文摘The novel electronic properties of bilayer graphene can be fine-tuned via twisting,which may induce flat bands around the Fermi level with nontrivial topology.In general,the band structure of such twisted bilayer graphene(TBG)can be theoretically obtained by using first-principles calculations,tight-binding method,or continuum model,which are either computationally demanding or parameters dependent.In this work,by using the sure independence screening sparsifying operator method,we propose a physically interpretable three-dimensional(3D)descriptor which can be utilized to readily obtain theΓ-point gap of TBG at arbitrary twist angles and different interlayer spacings.The strong predictive power of the descriptor is demonstrated by a high Pearson coefficient of 99%for both the training and testing data.To go further,we adopt the neural network algorithm to accurately probe the flat bands of TBG at various twist angles,which can accelerate the study of strong correlation physics associated with such a fundamental characteristic,especially for those systems with a larger number of atoms in the unit cell.
基金the National Natural Science Foundation of China(Grant No.11874314)supported by U.S.DOE under Grant No.DE-SC0002623。
文摘The physics of flat band is novel and rich but difficult to access.In this regard,recently twisting of bilayer van der Waals(vd W)-bounded two-dimensional(2 D)materials has attracted much attention,because the reduction of Brillouin zone will eventually lead to a diminishing kinetic energy.Alternatively,one may start with a 2 D kagome lattice,which already possesses flat bands at the Fermi level,but unfortunately these bands connect quadratically to other(dispersive)bands,leading to undesirable effects.Here,we propose,by first-principles calculation and tight-binding modeling,that the same bilayer twisting approach can be used to isolate the kagome flat bands.As the starting kinetic energy is already vanishingly small,the interlayer vd W potential is always sufficiently large irrespective of the twisting angle.As such the electronic states in the(connected)flat bands become unstable against a spontaneous Wigner crystallization,which is expected to have interesting interplays with other flat-band phenomena such as novel superconductivity and anomalous quantum Hall effect.
基金the National Natural Science Foundation of China(Grant No.11874314)the Natural Science Foundation of Hunan Province,China(Grant No.2018JJ2377)。
文摘Owing to the interaction between the layers,the twisted bilayer two-dimensional(2 D)materials exhibit numerous unique optical and electronic properties different from the monolayer counterpart,and have attracted tremendous interests in current physical research community.By means of first-principles and tight-binding model calculations,the electronic properties of twisted bilayer biphenylene carbon(BPC)are systematically investigated in this paper.The results indicate that the effect of twist will not only leads to a phase transition from semiconductor to metal,but also an adjustable band gap in BPC(0 me V to 120 me V depending on the twist angle).Moreover,unlike the twisted bilayer graphene(TBG),the flat bands in twisted BPC are no longer restricted by"magic angles",i.e.,abnormal flat bands could be appeared as well at several specific large angles in addition to the small angles.The charge density of these flat bands possesses different local modes,indicating that they might be derived from different stacked modes and host different properties.The exotic physical properties presented in this work foreshow twisted BPC a promising material for the application of terahertz and infrared photodetectors and the exploration of strong correlation.
基金support of the Office of Naval Research (Grant No. N0001418-1-2793)Kaufman New Initiative research grant KA201898553 of the Pittsburgh Foundationthe U.S. Department of Energy (Grant No. DESC0019064)
文摘Recent experiments[Science Advances 4 eaao4513(2018)]have revealed the evidence of nodal-line superconductivity in half-Heusler superconductors,e.g.,YPt Bi.Theories have suggested the topological nature of such nodal-line superconductivity and proposed the existence of surface Majorana flat bands on the(111)surface of half-Heusler superconductors.Due to the divergent density of states of the surface Majorana flat bands,the surface order parameter and the surface impurity play essential roles in determining the surface properties.We study the effect of the surface order parameter and the surface impurity on the surface Majorana flat bands of half-Heusler superconductors based on the Luttinger model.To be specific,we consider the topological nodal-line superconducting phase induced by the singlet-quintet pairing mixing,classify all the possible translationally invariant order parameters for the surface states according to irreducible representations of C3vpoint group,and demonstrate that any energetically favorable order parameter needs to break the time-reversal symmetry.We further discuss the energy splitting in the energy spectrum of surface Majorana flat bands induced by different order parameters and non-magnetic or magnetic impurities.We propose that the splitting in the energy spectrum can serve as the fingerprint of the pairing symmetry and mean-field order parameters.Our theoretical prediction can be examined in the future scanning tunneling microscopy experiments.
基金supported by the National Natural Science Foundation of China(Grant Nos.U23A2072,12204421,12074345,and 12174349).
文摘The discovery of each new allotropic manifestation of carbon has substantially propelled contemporary scientific research and applications,as vividly exemplified by the explosive progressions within the realms of fullerene,carbon nanotube,graphene,and diamond sciences over recent decades.Here,using state-of-the-art first-principles calculations,we predict a new type of twodimensional carbon network,dubbed fullerphene,by replacing each C atom in graphene with a fullerene(C_(60)).Its high energetic stability is tied to the symmetric cycloaddition of the double bonds between each C_(60) superatom with each of its three neighbors.A kinetic pathway is also proposed for the selective fabrication of fullerphene on Cu(111)or Rh(111),upon exploiting its enhanced stability over other competing C_(60) assemblies and significantly lowered kinetic barrier in seed growing,as strategically supported by a recent experimental advance.Further investigations on fullerphene reveal an array of desirable characteristics,including a substantial band gap of∼2 eV,Dirac states for the conduction electrons,and flat bands for the valence electrons.These findings represent a distinctly new and significant advance in both fullerene and graphene sciences.
基金supported by the National Key R&D Program of China(Grant No.2022YFB3608000)the National Natural Science Foundation of China(NSFC)(Grant Nos.12222413 and 12074041)+5 种基金the Natural Science Foundation of Shanghai(Grant Nos.23ZR1482200 and 22ZR1473300)the Funding of Ningbo Yongjiang Talent Program,the Natural Science Foundation of Ningbo,Ningbo University(No.LJ2024003)the Postdoctoral Fellowship Program of CPSF(Grant No.GZC20240951)the Natural Science Foundation of Shandong Province(Grant Nos.ZR2021QA031,ZR2023MA068,and ZR2024QA213)the Qingdao Postdoctoral Project Funding(No.QDBSH20240102115)the Fundamental Research Funds for the Central Universities(Grant No.2243300003).
文摘Paramagnetic LaCoSi,a ternary intermetallic electride,consists of CoSi blocks separated by two layers of La atoms.Its structure is similar to that of the widely studied 111 system of iron-based superconductors.Utilizing angle-resolved photoemission spectroscopy and first-principles calculations,we demonstrate the existence of linear bands and flat bands mainly originating from the orbitals of Co 3d states near the Fermi energy.The anomalous scattering rate of the linear bands varies linearly with the binding energy.The flat band above the Fermi energy indicated by the calculations could be modulated by substitutions and pressure to induce new ordered quantum phases,such as magnetism and superconductivity.Our findings reveal flat-band physics in electrides.
基金supported by the National Key R&D Program of China under Grant No.2022YFA1404900the National Natural Science Foundation of China under Grant No.12422411.
文摘Topological flat bands have attracted significant interest across various branches of physics,where synthetic gauge fields are typically considered an essential prerequisite.Numerous mechanisms have been proposed for implementing these fields,including magnetic fields on electrons,differential optical paths for photons,and strain-induced effective magnetic fields,among others.In this work,we introduce a novel approach to generating synthetic gauge fields through quantum statistics and demonstrate their effectiveness in realizing anyonic topological flat bands.Notably,we discover that a pair of strongly interacting anyons can induce square-root topological flat bands within a lattice model that remains dispersive and topologically trivial for a single particle.To validate our theoretical predictions,we experimentally simulate the quantum statistics-induced topological flat bands and square-root topological boundary states by mapping the eigenstates of two anyons onto modes in electric circuits.Our findings not only open a new pathway for creating topological flat bands but also deepen our understanding of anyonic physics and the underlying principles of flat-band topology.
基金supported by the National Natural Science Foundation of China(Grant Nos.11774269,and 12047543)the National Key R&D Program of China(Grant No.2018FYA0305800)+1 种基金the Natural Science Foundation of Hubei ProvinceChina(Grant No.2020CFA041)。
文摘Twisted graphene multilayers exhibit strongly correlated insulating states and superconductivity due to the presence of ultraflat bands near the charge neutral point.In this paper,the response of ultraflat bands to lattice relaxation and a magnetic field in twisted trilayer graphene(tTLG)with different stacking arrangements is investigated by using a full tight-binding model.We show that lattice relaxations are indispensable for understanding the electronic properties of tTLG,in particular,of tTLG in the presence of mirror symmetry.Lattice relaxations renormalize the quasiparticle spectrum near the Fermi energy and change the localization of higher energy flat bands.Furthermore,different from the twisted bilayer graphene,the Hofstadter butterfly spectrum can be realized at laboratory accessible strengths of magnetic field.Our work verifies tTLG as a more tunable platform than the twisted bilayer graphene in strongly correlated phenomena.
基金supported by Japan Science and Technology Agency(JST)as part of Adopting Sustainable Partnerships for Innovative Research Ecosystem(Grant No.JPMJAP2318)the JST Presto(Grant No.JPMJPR2357)+5 种基金supported by the National Key R&D Program of China(Grant No.2022YFA1403901)the National Natural Science Foundation of China(Grant No.11888101)the National Natural Science Foundation of China(Grant No.12047503)the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant Nos.XDB28000000 and XDB33000000)the New Cornerstone Investigator Programsupported by the National Key R&D Program of China(Grant No.2023YFA1407300)。
文摘At magic twisted angles,Dirac cones in twisted bilayer graphene(TBG)can evolve into flat bands,serving as a critical playground for the study of strongly correlated physics.When chiral symmetry is introduced,rigorous mathematical proof confirms that the flat bands are locked at zero energy in the entire Moiré Brillouin zone(BZ).Yet,TBG is not the sole platform that exhibits this absolute band flatness.Central to this flatness phenomenon are topological nodes and their specific locations in the BZ.In this study,considering twisted bilayer systems that preserve chiral symmetry,we classify various ordered topological nodes in base layers and all possible node locations across different BZs.Specifically,we constrain the node locations to rotational centers,such as Γ and M points,to ensure the interlayer coupling retains equal strength in all directions.Using this classification as a foundation,we systematically identify the conditions under which Moiré flat bands emerge.Additionally,through the extension of holomorphic functions,we provide proof that flat bands are locked at zero energy,shedding light on the origin of the band flatness.Remarkably,beyond Dirac cones,numerous twisted bilayer nodal platforms can host flat bands with a degeneracy number of more than two,such as four-fold,six-fold,and eight-fold.This multiplicity of degeneracy in flat bands might unveil more complex and enriched correlation physics.
基金supported by the National Natural Science Foundation of China(Grant Nos.12125404,T2495231,123B2049,and 12204138)the National Key R&D Program of China(Grant No.2022YFA1403201)+7 种基金the Advanced MaterialsNational Science and Technology Major Project (Grant No.2024ZD0607000)the Basic Research Program of Jiangsu (Grant Nos.BK20233001 and BK20241253)the Jiangsu Funding Program for Excellent Postdoctoral Talent (Grant Nos.2024ZB002,2024ZB075,2025ZB440 and2025ZB852)the China Postdoctoral Science Foundation (Grant No.2025M773331)the Postdoctoral Fellowship Program of CPSF (Grant No.GZC20240695 and GZC20252202)the AI&AI for Science Program of Nanjing UniversityArtificial Intelligence and Quantum physics (AIQ) program of Nanjing Universitythe Fundamental Research Funds for the Central Universities。
文摘The Lieb lattice, characterized by its distinctive Dirac cone and flat-band electronic structures, hosts a variety of exotic physical phenomena. However, its realization remains largely confined to artificial lattices. In this work, we propose the concept of a Lieb electride, where the non-bound electrons gather at the middle edges,behaving as the quasi-atoms of a Lieb lattice, enabling the emergence of flat bands. Using crystal structure prediction method MAGUS and first-principles calculations, we predict a stable candidate, Ca_(2)I, at ambient pressure. Distinct from conventional electrides with localized electrons at cavity centers, Ca_(2)I features interstitial electrons situated at cavity edges. The resultant flat bands lie close to the Fermi level, giving rise to a pronounced peak in the density of states and leading to Stoner-type ferromagnetism. With increasing pressures, we observe quantum phase transitions from ferromagnetic to non-magnetic and finally to antiferromagnetic orders in Ca_(2)I.Intriguingly, superconductivity emerges in the antiferromagnetic region, suggesting potential competition between these correlated states. Our study not only extends the concepts of electrides but also provides a novel strategy for realizing Lieb lattices through non-bound electrons. This work establishes Ca_(2)I as a promising platform for exploring flat-band physics and correlated electronic states, opening avenues for novel quantum phenomena in electride-based materials.
基金the National Natural Science Foundation of China(Grant Nos.11874160,12141401,and 11534001)the National Key Research and Development Program of China(Grant No.2017YFA0403501)the Fundamental Research Funds for the Central Universities(HUST:2017KFYXJJ027).
文摘We report that the twisted few layer graphite(tFL-graphite)is a new family of moiréheterostructures(MHSs),which has richer and highly tunable moiréflat band structures entirely distinct from all the known MHSs.A tFL-graphite is composed of two few-layer graphite(Bernal stacked multilayer graphene),which are stacked on each other with a small twisted angle.The moiréband structure of the tFL-graphite strongly depends on the layer number of its composed two van der Waals layers.Near the magic angle,a tFL-graphite always has two nearly flat bands coexisting with a few pairs of narrowed dispersive(parabolic or linear)bands at the Fermi level,thus,enhances the DOS at EF.This coexistence property may also enhance the possible superconductivity as been demonstrated in other multiband superconductivity systems.Therefore,we expect strong multiband correlation effects in tFL-graphite.Meanwhile,a proper perpendicular electric field can induce several isolated nearly flat bands with nonzero valley Chern number in some simple tFL-graphites,indicating that tFL-graphite is also a novel topological flat band system.
基金financially supported by the National Natural Science Foundation of China(No.52031014)the Ministry of Science and Technology of China(No.2017YFA0206301)Liaoning Provincial Natural Science Fund(No.2021-MS-006)。
文摘Based on the effective continuum model,we study alternating-twist multilayer graphene system and emergence of magic angles and flat band topology.All the alternating-twist multilayer graphene system(from triple layers to few layers)are found to have flat bands at magic angles where the area of AA stacking equals n-fold(n is an integer)electron cyclotron area.From the pseudo-Landau-level representation,there is always an isolated Dirac band in the alternating-twist graphene system constructed by odd number of layers.Since each pair of flat bands can be perceived as the zeroth pseudo-Landau-levels in two dimensional Dirac fermions,electron in the flat band pair can feel a pseudo-magnetic field with the same magnitude but the opposite sign.Calculated Chern number for each flat band is+1(or-1)which can be tuned by twisting in the vicinity of magic angles or by gating.The concurrent appearance of strong correlation and band topology of flat bands in the alternating-twist multilayer graphene may pave an avenue for the new understanding of superconductivity observed in triple-layered graphene,and supply a new playground for realizing(quantum)anomalous Hall effect.
基金Supported by the National Natural Science Foundation of China (Grant No.11774152)National Key R&D Program of China(Grant No.2016YFA0300401)。
文摘We show that a suitable combination of flat-band ferromagnetism,geometry and nontrivial electronic band topology can give rise to itinerant topological magnons.An SU(2) symmetric topological Hubbard model with nearly flat electronic bands,on a Kagome lattice,is considered as the prototype.This model exhibits ferromagnetic order when the lowest electronic band is half-filled.Using the numerical exact diagonalization method with a projection onto this nearly flat band,we can obtain the magnonic spectra.In the flat-band limit,the spectra exhibit distinct dispersions with Dirac points,similar to those of free electrons with isotropic hoppings,or a local spin magnet with pure ferromagnetic Heisenberg exchanges on the same geometry.Significantly,the non-flatness of the electronic band may induce a topological gap at the Dirac points,leading to a magnonic band with a nonzero Chern number.More intriguingly,this magnonic Chern number changes its sign when the topological index of the electronic band is reversed,suggesting that the nontrivial topology of the magnonic band is related to its underlying electronic band.Our work suggests interesting directions for the further exploration of,and searches for,itinerant topological magnons.
基金supported by the start-up funding of CQNU (Grant No. 24XLB010)supported by the Science and Technology Research Program of Chongqing Municipal Education Commission (Grant No. KJQN202100514)+3 种基金funding from Chongqing Natural Science Foundation under Grant No. CSTB2022NSCQ-JQX0018the Fundamental Research Funds for the Central Universities Grant No. 2021CDJZYJH-003Xiaomi Foundation/Xiaomi Young Talents Programfunding from the National Science Foundation of China under Grant Nos. 12404169, 12147172, 12274046, 11874094, 12147102, and 12347101。
文摘The quantum phase transition between Z_(2) plaquette valence bound solid(PVBS) and superfluid(SF) phases on the planar pyrochlore lattice(square ice) is under debate. To gain further insight, here, we focus on the dynamical features of the hard-core Bose–Hubbard model on this lattice and study the excitation spectra by combining stochastic analytic continuation and quantum Monte Carlo simulation. In both PVBS and SF phases,a flat band with bow-tie structure is observed and can be explained by certain symmetries. At the transition point,the spectra turn to be continuous and gapless. A(2+1)-dimensional Abelian–Higgs model with mixed 't Hooft anomaly is proposed to describe the transition, where the anomaly matching predicts that the deconfinement can exist on the domain walls. From the snapshot of the spin configuration in real space, we found the existence of the domain wall. We also found that the spectrum along a specific path in momentum space from PVBS phase to the transition point can be well described by an XXZ spin chain, and the critical theory of XXZ spin chain matches the anomaly. The two-spinon continuum along this specific path implies additional domain walls(point defect) can emerge in the domain walls(line defect) and take the role of deconfinement at the transition point.
基金Project support by the Science Fund from Shanghai Committee of Science and Technology,China (Grant No.23JC1403300)the Shanghai Municipal Science and Technology Major Project,China+3 种基金the TDLI Starting up Grant,the National Natural Science Foundation of China (Grant Nos.12374063,12204223,and 23Z990202580)the Fund from the Ministry of Science and Technology of China (Grant No.2023YFA1407400)the Shanghai Natural Science Fund for Original Exploration Program,China (Grant No.23ZR1479900)Shanghai Talent Program,China。
文摘Kagome materials are a class of material with a lattice structure composed of corner-sharing triangles that produce various exotic electronic phenomena,such as Dirac fermions,van Hove singularities,and flat bands.However,most of the known kagome materials have a flat band detached from the Fermi energy,which limits the investigation of the emergent flat band physics.In this work,by combining soft x-ray angle-resolved photoemission spectroscopy(ARPES)and the first-principles calculations,the electronic structure is investigated of a novel kagome metal CeNi_(5) with a clear dispersion along the kz direction and a Fermi level flat band in theΓ–K–M–Γplane.Besides,resonant ARPES experimental results indicate that the valence state of Ce ions is close to 4^(+),which is consistent with the transport measurement result.Our results demonstrate the unique electronic properties of CeNi_(5) as a new kagome metal and provide an ideal platform for exploring the flat band physics and the interactions between different types of flat bands by tuning the valence state of Ce ions.
基金Project supported by the National Natural Science Foundation of China (Grant No.12074367)Anhui Initiative in Quantum Information Technologies,the National Key Research and Development Program of China (Grant No.2020YFA0309804)+3 种基金Shanghai Municipal Science and Technology Major Project (Grant No.2019SHZDZX01)the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No.XDB35020200)Innovation Program for Quantum Science and Technology (Grant No.2021ZD0302002)New Cornerstone Science Foundation。
文摘We investigated the one-dimensional diamond ladder in the momentum lattice platform. By inducing multiple twoand four-photon Bragg scatterings among specific momentum states, we achieved a flat band system based on the diamond model, precisely controlling the coupling strength and phase between individual lattice sites. Utilizing two lattice sites couplings, we generated a compact localized state associated with the flat band, which remained localized throughout the entire time evolution. We successfully realized the continuous shift of flat bands by adjusting the corresponding nearest neighbor hopping strength, enabling us to observe the complete localization process. This opens avenues for further exploration of more complex properties within flat-band systems, including investigating the robustness of flat-band localized states in disordered flat-band systems and exploring many-body localization in interacting flat-band systems.
基金the supports of startup grant from Guangzhou Universitysupported by the National Natural Science Foundation of China(Grant No.11874127)。
文摘We investigate the bound state problem in a one-dimensional flat band system with a Coulomb potential.It is found that,in the presence of a Coulomb potential of type I(with three equal diagonal elements),similarly to that in the twodimensional case,the flat band could not survive.At the same time,the flat band states are transformed into localized states with a logarithmic singularity near the center position.In addition,the wave function near the origin would collapse for an arbitrarily weak Coulomb potential.Due to the wave function collapses,the eigen-energies for a shifted Coulomb potential depend sensitively on the cut-off parameter.For a Coulomb potential of type II,there exist infinite bound states that are generated from the flat band.Furthermore,when the bound state energy is very near the flat band,the energy is inversely proportional to the natural number,e.g.,E_(n)∝1/n,n=1,2,3,...It is expected that the 1/n energy spectrum could be observed experimentally in the near future.
基金supported by the National Natural Science Foundation of China(Grant Nos.12141002 and 12225401)the National Key Research and Development Program of China(Grant No.2021YFA1401902)+1 种基金the CAS Interdisciplinary Innovation Teamthe Strategic Priority Research Program of Chinese Academy of Sciences(Grant No.XDB28000000)。
文摘We report structural and electronic properties of Na_(2)Ni_(3)S_(4),a quasi-two-dimensional compound composed of alternating layers of[Ni_(3)S_(4)]^(2-)and Na^(+).The compound features a remarkable Ni-based kagome lattice with a square planar configuration of four surrounding S atoms for each Ni atom.Magnetization and electrical measurements reveal a weak paramagnetic insulator with a gap of about 0.5 eV.Our band structure calculation highlights a set of topological flat bands of the kagome lattice derived from the rotated dxz-orbital with C_(3)+T symmetry in the presence of crystal-field splitting.
