Epitaxial graphene grown on silicon carbide(Si C/graphene)is a promising solution for achieving a highprecision quantum Hall resistance standard.Previous research mainly focused on the quantum resistance metrology of ...Epitaxial graphene grown on silicon carbide(Si C/graphene)is a promising solution for achieving a highprecision quantum Hall resistance standard.Previous research mainly focused on the quantum resistance metrology of n-type Si C/graphene,while a comprehensive understanding of the quantum resistance metrology behavior of graphene with different doping types is lacking.Here,we fabricated both n-and p-type Si C/graphene devices via polymer-assisted molecular adsorption and conducted systematic magneto-transport measurements in a wide parameter space of carrier density and temperature.It is demonstrated that n-type devices show greater potential for development of quantum resistance metrology compared with p-type devices,as evidenced by their higher carrier mobility,lower critical magnetic field for entering quantized Hall plateaus,and higher robustness of the quantum Hall effect against thermal degeneration.These discrepancies can be reasonably attributed to the weaker scattering from molecular dopants for n-type devices,which is further supported by the analyses on the quantum interference effect in multiple devices.These results enrich our understanding of the charged impurity on electronic transport performance of graphene and,more importantly,provide a useful reference for future development of graphene-based quantum resistance metrology.展开更多
Monolayer transition metal dichalcogenides can normally exist in several structural polymorphs with distinct electrical,optical,and catalytic properties.Effective control of the relative stability and transformation o...Monolayer transition metal dichalcogenides can normally exist in several structural polymorphs with distinct electrical,optical,and catalytic properties.Effective control of the relative stability and transformation of different phases in these materials is thus of critical importance for applications.Using density functional theory calculations,we investigate the effects of low-work-function metal substrates including Ti,Zr,and Hf on the structural,electronic,and catalytic properties of monolayer MoS_(2) and WS_(2).The results indicate that such substrates not only convert the energetically stable structure from the 1H phase to the 1T'/1T phase,but also significantly reduce the kinetic barriers of the phase transformation.Furthermore,our calculations also indicate that the 1T' phase of MoS_(2) with Zr or Hf substrate is a potential catalyst for the hydrogen evolution reaction.展开更多
A machine learning(ML)potential for Au clusters is developed through training on a dataset including several different sized clusters.This ML potential accurately covers the whole configuration space of Au clusters in...A machine learning(ML)potential for Au clusters is developed through training on a dataset including several different sized clusters.This ML potential accurately covers the whole configuration space of Au clusters in a broad size range,thus expressing a good performance in search of their global minimum energy structures.Based on our potential,the low-lying structures of 17 different sized Au clusters are identified,which shows that small sized Au clusters tend to form planar structures while large ones are more likely to be stereo,revealing the critical size for the two-dimensional(2D)to three-dimensional(3D)structural transition.Our calculations demonstrate that ML is indeed powerful in describing the interaction of Au atoms and provides a new paradigm on accelerating the search of structures.展开更多
For conductors in the ballistic regime, electron-boundary scattering at the sample edge plays a dominant role in determining the transport performance, giving rise to many intriguing phenomena like low-field negative ...For conductors in the ballistic regime, electron-boundary scattering at the sample edge plays a dominant role in determining the transport performance, giving rise to many intriguing phenomena like low-field negative magnetoresistance effect. We systematically investigate the magneto-transport behaviors of BN-encapsulated graphene devices with narrow channel width W, wherein the bulk mean free path Lmfp can be very large and highly tunable. By comparing the magnetoresistance features and the amplitude of Lmfp in a large parameter space of temperature and carrier density, we reveal that the boundary-scattering-dominated negative magnetoresistance effect can still survive even when the ballistic ratio(Lmfp/W) is as low as 0.15. This striking value is much smaller than the expected value for achieving(quasi-) ballistic transport regime(Lmfp/W ≥ 1), and can be attributed to the ultra-low specularity of the sample edge of our graphene devices. These findings enrich our understanding of the effects of boundary scattering on channel transport, which is of vital importance for future designs of two-dimensional electronic devices with limited lateral sizes.展开更多
Based on first-principles calculations,a two-dimensional(2D)van der Waals(vd W)bilayer heterostructure consisting of two topologically trivial ferromagnetic(FM)monolayers CrI_(3)and ScCl_(2)is proposed to realize the ...Based on first-principles calculations,a two-dimensional(2D)van der Waals(vd W)bilayer heterostructure consisting of two topologically trivial ferromagnetic(FM)monolayers CrI_(3)and ScCl_(2)is proposed to realize the quantum anomalous Hall effect(QAHE)with a sizable topologically nontrivial band gap of 4.5 me V.Its topological nature is attributed to an interlayer band inversion between the monolayers and critically depends on the symmetry of the stacking configuration.We further demonstrate that the topologically nontrivial band gap can be increased nearly linearly by the application of a perpendicular external pressure and reaches 8.1 me V at 2.7 GPa,and the application of an external out-of-plane electric field can also modulate the band gap and convert the system back to topologically trivial via eliminating the band inversion.An effective model is developed to describe the topological phase evolution in this bilayer heterostructure.This work provides a new candidate system based on 2D vd W materials for realization of potential high-temperature QAHE with considerable controllability.展开更多
Novel two-dimensional thermoelectric materials have attracted significant attention in the field of thermoelectric due to their low lattice thermal conductivity.A comprehensive understanding of their microscopic struc...Novel two-dimensional thermoelectric materials have attracted significant attention in the field of thermoelectric due to their low lattice thermal conductivity.A comprehensive understanding of their microscopic structures is crucial for driving further the optimization of materials properties and developing novel functional materials.Here,by using in situ scanning tunneling microscopy,we report the atomic layer evolution and surface reconstruction on the cleaved thermoelectric material KCu_(4)Se_(3) for the first time.We clearly revealed each atomic layer,including the naturally cleaved K atomic layer,the intermediate Se^(2-)atomic layer,and the Se^(-)atomic layer that emerges in the thermodynamic-stable state.Departing from the maj ority of studies that predominantly concentrate on macroscopic measurements of the charge transport,our results reveal the coexistence of potassium disorder and complex reconstructed patterns of selenium,which potentially influences charge carrier and lattice dynamics.These results provide direct insight into the surface microstructures and evolution of KCu_(4)Se_(3),and shed useful light on designing functional materials with superior performance.展开更多
Hybrid structures of two distinct materials provide an excellent opportunity to optimize functionalities.We report the realization of wide quantum Hall plateaus in graphene field-effect devices on the LaAlO3/SrTiO3 he...Hybrid structures of two distinct materials provide an excellent opportunity to optimize functionalities.We report the realization of wide quantum Hall plateaus in graphene field-effect devices on the LaAlO3/SrTiO3 heterostructures.Well-defined quantized Hall resistance plateaus at filling factors ν=±2 can be obtained over wide ranges of the magnetic field and gate voltage,e.g.,extending from 2 T to a maximum available magnetic field of 9 T.By using a simple band diagram model,it is revealed that these wide plateaus arise from the ultralarge capacitance of the ultra-thin LAO layer acting as the dielectric layer.This is distinctly different from the case of epitaxial graphene on Si C substrates,where the realization of giant Hall plateaus relies on the charge transfer between the graphene layer and interface states in SiC.Our results offer an alternative route towards optimizing the quantum Hall performance of graphene,which may find its applications in the further development of quantum resistance metrology.展开更多
Incorporating valley as a degree of freedom into quantum anomalous Hall systems offers a novel approach to manipulating valleytronics in electronic transport.Using the Kane-Mele monolayer as a concrete model,we compre...Incorporating valley as a degree of freedom into quantum anomalous Hall systems offers a novel approach to manipulating valleytronics in electronic transport.Using the Kane-Mele monolayer as a concrete model,we comprehensively explore the various topological phases in the presence of inequivalent exchange fields and reveal the roles of the interfacial Rashba effect and external electric field in tuning topological valley-polarized states.We find that valley-polarized states can be realized by introducing Kane-Mele spin-orbit coupling and inequivalent exchange fields.Further introducing Rashba spin-orbit coupling and an electric field into the system can lead to diverse topological states,such as the valley-polarized quantum anomalous Hall effect with C=±1,±2 and valley-contrasting states with C=0.Remarkably,different valley-polarized topological states can be continuously tuned by varying the strength and direction of the external electric field in a fixed system.Our work demonstrates the tunability of topological states in valley-polarized quantum anomalous Hall systems and provides an ideal platform for applications in electronic transport devices in topological valleytronics.展开更多
It has been widely recognized that,based on standard density functional theory calculations of the electron-phonon coupling,the superconducting transition temperature(T_(c))in bulk FeSe is exceptionally low(almost 0 K...It has been widely recognized that,based on standard density functional theory calculations of the electron-phonon coupling,the superconducting transition temperature(T_(c))in bulk FeSe is exceptionally low(almost 0 K)within the Bardeen-Cooper-Schrieffer formalism.Yet the experimentally observed T_(c)is much higher(∼10 K),and the underlying physical origin remains to be fully explored,especially at the quantitative level.Here we present the first accurate determination of T_(c)in FeSe where the correlation-enhanced electron-phonon coupling is treated within first-principles dynamical mean-field theory.Our studies treat both the multiple electronic bands across the Fermi level and phononic bands,and reveal that all the optical phonon modes are effectively coupled with the conduction electrons,including the important contributions of a single breathing mode as established by previ-ous experiments.Accordingly,each of those phonon modes contributes pronouncedly to the electron pairing,and the resultant T_(c)is drastically enhanced to the experimentally observed range.The approach developed here should be broadly applicable to other superconducting systems where correlation-enhanced electron-phonon coupling plays an important role.展开更多
Room temperature ferroelectric thin films are the key element of high-density nonvolatile memories in modern electronics. However, with the further miniaturization of the electronic devices beyond the Moore’s law, co...Room temperature ferroelectric thin films are the key element of high-density nonvolatile memories in modern electronics. However, with the further miniaturization of the electronic devices beyond the Moore’s law, conventional ferroelectrics suffer great challenge arising from the critical thickness effect, where the ferroelectricity is unstable if the film thickness is reduced to nanometer or single atomic layer limit. Two-dimensional(2D) materials, thanks to their stable layered structure, saturate interfacial chemistry, weak interlayer couplings, and the benefit of preparing stable ultra-thin film at 2D limit, are promising for exploring 2D ferroelectricity and related device applications. Therefore, it provides an effective approach to overcome the limitation in conventional ferroelectrics with the study of 2D ferroelectricity in van der Waals(vdW) materials. In this review article,we briefly introduce recent progresses on 2D ferroelectricity in layered vdW materials. We will highlight the study on atomically thin α-In2Se3, which is an emergent ferroelectric semiconductor with the coupled in-plane and out-of-plane ferroelectricity. Furthermore, two prototype ferroelectric devices based on ferroelectric α-In2Se3 will also be reviewed.展开更多
Topological materials are a new and rapidly expanding class of quantum matter. To date, identification of the topological nature of a given compound material demands specific determination of the appropriate topologic...Topological materials are a new and rapidly expanding class of quantum matter. To date, identification of the topological nature of a given compound material demands specific determination of the appropriate topological invariant through detailed electronic structure calculations. Here we present an efficient criterion that allows ready screening of potential topological materials, using topological insulators as prototypical examples. The criterion is inherently tied to the band inversion induced by spin-orbit coupling,and is uniquely defined by a minimal number of two elemental physical properties of the constituent elements: the atomic number and Pauling electronegativity. The validity and predictive power of the criterion is demonstrated by rationalizing many known topological insulators and potential candidates in the tetradymite and half-Heusler families, and the underlying design principle is naturally also extendable to predictive discoveries of other classes of topological materials.展开更多
Nanoclusters consisting of a few atoms have attracted a lot of research interests due to their exotic size-dependent properties. Here, well-ordered two-dimensional Sb cluster superlattice was fabricated on Si substrat...Nanoclusters consisting of a few atoms have attracted a lot of research interests due to their exotic size-dependent properties. Here, well-ordered two-dimensional Sb cluster superlattice was fabricated on Si substrate by a two-step method and characterized by scanning tunneling microscopy. High resolution scanning tunneling microscope measurements revealed the fine structures of the Sb clusters, which consist of several Sb atoms ranging from 2 to 7. Furthermore, the electronic structure of the nanocluster displays the quantized energy-level which is due to the single-electron tunneling effects. We believe that the fabrication of Sb cluster superlattice broadens the species of the cluster superlattice and provides a promising candidate to further explore the novel physical and chemical properties of the semimetal nanocluster.展开更多
Topological insulators （Tls） are bulk insulators that possess robust helical conducting states along their interfaces with conventional insulators. A tremendous research effort has recently been devoted to TI-based ...Topological insulators （Tls） are bulk insulators that possess robust helical conducting states along their interfaces with conventional insulators. A tremendous research effort has recently been devoted to TI-based heterostructures, in which con- ventional proximity effects give rise to a series of exotic physical phenomena. This paper reviews our recent studies on the potential existence of topological proximity effects at the interface between a topological insulator and a normal insu- lator or other topologically trivial systems. Using first-principles approaches, we have realized the tunability of the vertical location of the topological helical state via intriguing dual-proximity effects. To further elucidate the control parameters of this effect, we have used the graphene-based heterostructures as prototypical systems to reveal a more complete phase diagram. On the application side of the topological helical states, we have presented a catalysis example, where the topo- logical helical state plays an essential role in facilitating surface reactions by serving as an effective electron bath, These discoveries lay the foundation for accurate manipulation of the real space properties of the topological helical state in TI- based heterostructures and pave the way for realization of the salient functionality of topological insulators in future device applications.展开更多
Based on first-principles density functional theory calculation,we discover a novel form of spin-orbit(SO)splitting in two-dimensional(2D)heterostructures composed of a single Bi(111)bilayer stacking with a 2D semicon...Based on first-principles density functional theory calculation,we discover a novel form of spin-orbit(SO)splitting in two-dimensional(2D)heterostructures composed of a single Bi(111)bilayer stacking with a 2D semiconducting In_(2)Se_(2) or a 2D ferroelectricα-In_(2)Se_(3) layer.Such SO splitting has a Rashba-like but distinct spin texture in the valence band around the maximum,where the chirality of the spin texture reverses within the upper spin-split branch,in contrast to the conventional Rashba systems where the upper branch and lower branch have opposite chirality solely in the region below the band crossing point.The ferroelectric nature ofα-In_(2)Se_(3) further enables the tuning of the spin texture upon the reversal of the electric polarization with the application of an external electric field.Detailed analysis based on a tight-binding model reveals that such SO splitting texture results from the interplay of complex orbital characters and substrate interaction.This finding enriches the diversity of SO splitting systems and is also expected to promise for spintronic applications.展开更多
Excitons in solid state are bosons generated by electron-hole pairs as the Coulomb screening is sufficiently reduced.The exciton condensation can result in exotic physics such as super-fluidity and insulating state.In...Excitons in solid state are bosons generated by electron-hole pairs as the Coulomb screening is sufficiently reduced.The exciton condensation can result in exotic physics such as super-fluidity and insulating state.In charge density wave(CDW)state,1T-TiSe_(2) is one of the candidates that may host the exciton condensation.However,to envision its excitonic effect is still challenging,particularly at the two-dimensional limit,which is applicable to future devices.Here,we realize the epitaxial 1T-TiSe_(2) bilayer,the two-dimensional limit for its 2×2×2 CDW order,to explore the exciton-associated effect.By means of high-resolution scanning tunneling spectroscopy and quasiparticle interference,we discover an unexpected state residing below the conduction band and right within the CDW gap region.As corroborated by our theoretical analysis,this mysterious phenomenon is in good agreement with the electron-exciton coupling.Our study provides a material platform to explore exciton-based electronics and opto-electronics.展开更多
Superconductors with reduced dimensionality have been widely explored for their exotic superconducting behaviors.Especially,at the two-dimensional limit,two-monolayer Pb films with two types of structures provide an i...Superconductors with reduced dimensionality have been widely explored for their exotic superconducting behaviors.Especially,at the two-dimensional limit,two-monolayer Pb films with two types of structures provide an ideal platform to unveil the underlying superconducting mechanism[Science 324,1314(2009)].Here,by combining scanning tunneling microscopy(STM)with the first-principle calculations,we successfully identify that these two types have different atomic lattice structures with varying stacking phases,which further enables us to calculate the phonon spectrum and electron phonon coupling strength of each type.The theoretical calculations are in good agreement with tunneling spectroscopy measurements of the superconducting transition temperatures(T_(c)),which established a correlation between atomic structures and superconductivity.Moreover,it was observed that the higher T_(c)of these two types also possess higher out-of-plane upper critical magnetic fields(Hc2).These findings will provide important new insights into two-dimensional superconductivity at the atomic level.展开更多
Two-dimensional systems that simultaneously harbor superconductivity and nontrivial band topology may serve as appealing platforms for realizing topological superconductivity with promising applications in fault-toler...Two-dimensional systems that simultaneously harbor superconductivity and nontrivial band topology may serve as appealing platforms for realizing topological superconductivity with promising applications in fault-tolerant quantum computing.Here,based on first-principles calculations,we show that monolayered Co N and Co P with the isovalent Fe Se-like structure are stable in freestanding form,even though their known bulk phases have no resemblance to layering.The two systems are further revealed to display intrinsic band inversions due to crystal field splitting,and such orderings are preserved with the inclusion of spin-orbit coupling(SOC),which otherwise is able to open a curved band gap,yielding a non-zero Z2 topological invariant in each case.Such a mechanism of topologicalization is distinctly contrasted with that identified recently for the closely related monolayers of CoX(X=As,Sb,Bi),where the SOC plays an indispensable role in causing a nontrivial band inversion.Next,we demonstrate that,by applying equi-biaxial tensile strain,the electron-phonon coupling strength in monolayered CoN can be significantly enhanced,yielding a superconducting transition temperature(Tc)up to 7-12 K for the Coulomb pseudopotential ofμ*=0.2-0.1,while the CoP monolayer shows very low Tc even under pronounced strain.Their different superconducting behaviors can be attributed to different variations in lattice softening and electronic density of states around the Fermi level upon pressuring.Our central findings enrich the understanding of different mechanisms of band inversions and topologicalization and offer platforms for achieving the coexistence of superconductivity and nontrivial band topology based on two-dimensional systems.展开更多
Cobalt pnictides have been theoretically proposed to be attractive candi-dates for high-temperature superconductors.Additionally,monolayered CoX(X=As,Sb,Bi)on SrTiO_(3) systems present a potential new platform for rea...Cobalt pnictides have been theoretically proposed to be attractive candi-dates for high-temperature superconductors.Additionally,monolayered CoX(X=As,Sb,Bi)on SrTiO_(3) systems present a potential new platform for realizing topological superconductors in the two-dimensional limit,due to their nontrivial band topology.To this end,we have successfully fabricated high-quality CoBi nanoislands on SrTiO_(3)(001)substrates by molecular beam epitaxy followed by an investigation of their atomic struc-ture and electronic properties via in situ scanning tunneling microscopyl spectroscopy.Beyond the previously predicted lattice with a=b=3.5 A,2×1 dimer row was observed in this study.Furthermore,our results reveal that the topography of CoBi islands is strongly influenced by various growth conditions,such as substrate temperature,the flux ratio between Co and Bi,and the annealing process.This study paves the way for further explorations of the superconductivity and topological properties of cobalt pnictidesystems.展开更多
Unconventional antiferromagnets(AFMs)with non-relativistic spin-splitting,such as the recently discovered altermagnet,have recently gained significant interest due to their potential for novel quantum phenomena and sp...Unconventional antiferromagnets(AFMs)with non-relativistic spin-splitting,such as the recently discovered altermagnet,have recently gained significant interest due to their potential for novel quantum phenomena and spintronic applications.The compensated magnetization in unconventional AFMs is protected by spin-space symmetries.In this work,we explore the symmetrybreaking effects and identify three distinct mechanisms for inducing net spin magnetizations in unconventional AFMs with collinear or non-collinear spins:(1)finite size effect,(2)extrinsic spin canting effect,and(3)irradiation with circularly polarized light.We show that the induced spin magnetizations are controllable and manifest as diverse intriguing phenomena.For the finite size system,the confined direction of a two-dimensional AM creates quantum-well-like subbands that determine the spin magnetization.This effect can be experimentally probed by measuring the spin density of states and the spin-polarization of Andreev-bound states within planar Josephson junctions.In the case of spin canting effect,it leads to peculiar anisotropic and non-monotonic behaviors in the superconducting proximity effect.Lastly,with circularly polarized light,spin magnetization is driven by the polarized light and the chirality of non-collinear magnetic order,thus offering a direct means of detecting the chirality of magnetic order in real materials.Our findings provide valuable insight into understanding and probing the spin magnetization in unconventional AFM materials.展开更多
A method based on the adsorption of ions on the surface of two-dimensional （2D） nanosheets has been developed for photocatalytic COz reduction. Isolated Bi ions, confined on the surface of TiO2 nanosheets using a si...A method based on the adsorption of ions on the surface of two-dimensional （2D） nanosheets has been developed for photocatalytic COz reduction. Isolated Bi ions, confined on the surface of TiO2 nanosheets using a simple ionic adsorption method facilitate the formation of a built-in electric field that effectively promotes charge carrier separation. This leads to an improved performance of the photocatalytic COa reduction process with the preferred conversion to CH4. The proposed surface ion-adsorption method is expected to provide an effective approach for the design of highly efficient photocatalytic systems. These findings could be very valuable in photocatalytic CO2 reduction applications.展开更多
基金supported by the CAS Project for Young Scientists in Basic Research(Grant No.YSBR-046)the National Natural Science Foundation of China(Grant Nos.92165201,11974324,12104435)+4 种基金the Innovation Program for Quantum Science and Technology(Grant No.2021ZD0302800)the Anhui Initiative in Quantum Information Technologies(Grant No.AHY170000)Hefei Science Center CAS(Grant No.2020HSC-UE014)the Fundamental Research Funds for the Central Universities(Grant Nos.WK3510000013 and WK2310000104)supported by USTC Tang Scholar。
文摘Epitaxial graphene grown on silicon carbide(Si C/graphene)is a promising solution for achieving a highprecision quantum Hall resistance standard.Previous research mainly focused on the quantum resistance metrology of n-type Si C/graphene,while a comprehensive understanding of the quantum resistance metrology behavior of graphene with different doping types is lacking.Here,we fabricated both n-and p-type Si C/graphene devices via polymer-assisted molecular adsorption and conducted systematic magneto-transport measurements in a wide parameter space of carrier density and temperature.It is demonstrated that n-type devices show greater potential for development of quantum resistance metrology compared with p-type devices,as evidenced by their higher carrier mobility,lower critical magnetic field for entering quantized Hall plateaus,and higher robustness of the quantum Hall effect against thermal degeneration.These discrepancies can be reasonably attributed to the weaker scattering from molecular dopants for n-type devices,which is further supported by the analyses on the quantum interference effect in multiple devices.These results enrich our understanding of the charged impurity on electronic transport performance of graphene and,more importantly,provide a useful reference for future development of graphene-based quantum resistance metrology.
基金Project supported by the National Key Research and Development Program of China(Grant Nos.2017YFA0204904 and 2019YFA0210004)the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDB30000000)the Fundamental Research Funds for the Central Universities,China(Grant No.WK3510000013).
文摘Monolayer transition metal dichalcogenides can normally exist in several structural polymorphs with distinct electrical,optical,and catalytic properties.Effective control of the relative stability and transformation of different phases in these materials is thus of critical importance for applications.Using density functional theory calculations,we investigate the effects of low-work-function metal substrates including Ti,Zr,and Hf on the structural,electronic,and catalytic properties of monolayer MoS_(2) and WS_(2).The results indicate that such substrates not only convert the energetically stable structure from the 1H phase to the 1T'/1T phase,but also significantly reduce the kinetic barriers of the phase transformation.Furthermore,our calculations also indicate that the 1T' phase of MoS_(2) with Zr or Hf substrate is a potential catalyst for the hydrogen evolution reaction.
基金Computational support was provided by Supercomputing Center in USTC and National Supercomputing Center in Tianjinthe National Key Research and Development Program of China(Grant Nos.2017YFA0204904 and 2019YFA0210004)。
文摘A machine learning(ML)potential for Au clusters is developed through training on a dataset including several different sized clusters.This ML potential accurately covers the whole configuration space of Au clusters in a broad size range,thus expressing a good performance in search of their global minimum energy structures.Based on our potential,the low-lying structures of 17 different sized Au clusters are identified,which shows that small sized Au clusters tend to form planar structures while large ones are more likely to be stereo,revealing the critical size for the two-dimensional(2D)to three-dimensional(3D)structural transition.Our calculations demonstrate that ML is indeed powerful in describing the interaction of Au atoms and provides a new paradigm on accelerating the search of structures.
基金supported by the National Natural Science Foundation of China(Grant Nos.92165201 and 11974324)the Innovation Program for Quantum Science and Technology(Grant No.2021ZD0302800)+3 种基金the Strategic Priority Research Program of Chinese Academy of Sciences(Grant No.XDC07010000)the Anhui Initiative in Quantum Information Technologies(Grant No.AHY170000)the Hefei Science Center CAS(Grant No.2020HSC-UE014)the Fundamental Research Funds for the Central Universities(Grant No.WK3510000013)。
文摘For conductors in the ballistic regime, electron-boundary scattering at the sample edge plays a dominant role in determining the transport performance, giving rise to many intriguing phenomena like low-field negative magnetoresistance effect. We systematically investigate the magneto-transport behaviors of BN-encapsulated graphene devices with narrow channel width W, wherein the bulk mean free path Lmfp can be very large and highly tunable. By comparing the magnetoresistance features and the amplitude of Lmfp in a large parameter space of temperature and carrier density, we reveal that the boundary-scattering-dominated negative magnetoresistance effect can still survive even when the ballistic ratio(Lmfp/W) is as low as 0.15. This striking value is much smaller than the expected value for achieving(quasi-) ballistic transport regime(Lmfp/W ≥ 1), and can be attributed to the ultra-low specularity of the sample edge of our graphene devices. These findings enrich our understanding of the effects of boundary scattering on channel transport, which is of vital importance for future designs of two-dimensional electronic devices with limited lateral sizes.
基金Project supported by the National Key Research and Development Program of China(Grant Nos.2017YFA0204904 and 2019YFA0210004)the National Natural Science Foundation of China(Grant No.11634011)+1 种基金the Strategic Priority Research Program of Chinese Academy of Sciences(Grant No.XDB30000000)the Fundamental Research Funds for the Central Universities(Grant No.WK2340000082)。
文摘Based on first-principles calculations,a two-dimensional(2D)van der Waals(vd W)bilayer heterostructure consisting of two topologically trivial ferromagnetic(FM)monolayers CrI_(3)and ScCl_(2)is proposed to realize the quantum anomalous Hall effect(QAHE)with a sizable topologically nontrivial band gap of 4.5 me V.Its topological nature is attributed to an interlayer band inversion between the monolayers and critically depends on the symmetry of the stacking configuration.We further demonstrate that the topologically nontrivial band gap can be increased nearly linearly by the application of a perpendicular external pressure and reaches 8.1 me V at 2.7 GPa,and the application of an external out-of-plane electric field can also modulate the band gap and convert the system back to topologically trivial via eliminating the band inversion.An effective model is developed to describe the topological phase evolution in this bilayer heterostructure.This work provides a new candidate system based on 2D vd W materials for realization of potential high-temperature QAHE with considerable controllability.
基金Project supported by the National Natural Science Foundation of China (Grant Nos.12374196,92165201,11634011,and 22109153)the Innovation Program for Quantum Science and Technology (Grant No.2021ZD0302800)+4 种基金the CAS Project for Young Scientists in Basic Research (Grant No.YSBR-046)the Fundamental Research Funds for the Central Universities (Grant Nos.WK3510000006 and WK3430000003)the Fund of Anhui Initiative in Quantum Information Technologies (Grant No.AHY170000)the University Synergy Innovation Program of Anhui Province,China (Grant No.GXXT-2022-008)the National Synchrotron Radiation Laboratory Joint Funds of University of Science and Technology of China (Grant No.KY2060000241)。
文摘Novel two-dimensional thermoelectric materials have attracted significant attention in the field of thermoelectric due to their low lattice thermal conductivity.A comprehensive understanding of their microscopic structures is crucial for driving further the optimization of materials properties and developing novel functional materials.Here,by using in situ scanning tunneling microscopy,we report the atomic layer evolution and surface reconstruction on the cleaved thermoelectric material KCu_(4)Se_(3) for the first time.We clearly revealed each atomic layer,including the naturally cleaved K atomic layer,the intermediate Se^(2-)atomic layer,and the Se^(-)atomic layer that emerges in the thermodynamic-stable state.Departing from the maj ority of studies that predominantly concentrate on macroscopic measurements of the charge transport,our results reveal the coexistence of potassium disorder and complex reconstructed patterns of selenium,which potentially influences charge carrier and lattice dynamics.These results provide direct insight into the surface microstructures and evolution of KCu_(4)Se_(3),and shed useful light on designing functional materials with superior performance.
基金Supported by the National Natural Science Foundation of China (Grant Nos.11974324,11804326 and U1832151)the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No.XDC07010000)+2 种基金the National Key Research and Development Program of China (Grant No.2017YFA0403600)Anhui Initiative in Quantum Information Technologies (Grant No.AHY170000)Hefei Science Center CAS (Grant No.2018HSC-UE014)。
文摘Hybrid structures of two distinct materials provide an excellent opportunity to optimize functionalities.We report the realization of wide quantum Hall plateaus in graphene field-effect devices on the LaAlO3/SrTiO3 heterostructures.Well-defined quantized Hall resistance plateaus at filling factors ν=±2 can be obtained over wide ranges of the magnetic field and gate voltage,e.g.,extending from 2 T to a maximum available magnetic field of 9 T.By using a simple band diagram model,it is revealed that these wide plateaus arise from the ultralarge capacitance of the ultra-thin LAO layer acting as the dielectric layer.This is distinctly different from the case of epitaxial graphene on Si C substrates,where the realization of giant Hall plateaus relies on the charge transfer between the graphene layer and interface states in SiC.Our results offer an alternative route towards optimizing the quantum Hall performance of graphene,which may find its applications in the further development of quantum resistance metrology.
基金supported by the National Natural Science Foundation of China(No.12004369)the Natural Science Foundation of Fujian Province(No.2022J05019)+1 种基金the China Postdoctoral Science Foundation(Nos.2023M733411 and 2023TQ0347)the Education and Research fund for Young Teachers of Fujian Province(No.JAT210014).
文摘Incorporating valley as a degree of freedom into quantum anomalous Hall systems offers a novel approach to manipulating valleytronics in electronic transport.Using the Kane-Mele monolayer as a concrete model,we comprehensively explore the various topological phases in the presence of inequivalent exchange fields and reveal the roles of the interfacial Rashba effect and external electric field in tuning topological valley-polarized states.We find that valley-polarized states can be realized by introducing Kane-Mele spin-orbit coupling and inequivalent exchange fields.Further introducing Rashba spin-orbit coupling and an electric field into the system can lead to diverse topological states,such as the valley-polarized quantum anomalous Hall effect with C=±1,±2 and valley-contrasting states with C=0.Remarkably,different valley-polarized topological states can be continuously tuned by varying the strength and direction of the external electric field in a fixed system.Our work demonstrates the tunability of topological states in valley-polarized quantum anomalous Hall systems and provides an ideal platform for applications in electronic transport devices in topological valleytronics.
基金supported by the National Key R&D Program of China(Grant No.2017YFA0303500)the National Natural Science Foundation of China(Grant Nos.11634011,11974323,and 12004364)+2 种基金the Anhui Initiative in Quantum Information Technologies(Grant No.AHY170000)the Strategic Priority Research Program of Chinese Academy of Sciences(Grant No.XDB30000000)the China Postdoctoral Science Foundation(Grant No.2019TQ0314)。
文摘It has been widely recognized that,based on standard density functional theory calculations of the electron-phonon coupling,the superconducting transition temperature(T_(c))in bulk FeSe is exceptionally low(almost 0 K)within the Bardeen-Cooper-Schrieffer formalism.Yet the experimentally observed T_(c)is much higher(∼10 K),and the underlying physical origin remains to be fully explored,especially at the quantitative level.Here we present the first accurate determination of T_(c)in FeSe where the correlation-enhanced electron-phonon coupling is treated within first-principles dynamical mean-field theory.Our studies treat both the multiple electronic bands across the Fermi level and phononic bands,and reveal that all the optical phonon modes are effectively coupled with the conduction electrons,including the important contributions of a single breathing mode as established by previ-ous experiments.Accordingly,each of those phonon modes contributes pronouncedly to the electron pairing,and the resultant T_(c)is drastically enhanced to the experimentally observed range.The approach developed here should be broadly applicable to other superconducting systems where correlation-enhanced electron-phonon coupling plays an important role.
基金supported by the National Key Research and Development Program of China (Grant Nos. 2017YFA0 205004, 2018YFA03066004, and 2016YFA0301700)the National Natural Science Foundation of China (Grant Nos. 11674295 and 11774328)+3 种基金the Fundamental Research Funds for the Central Universities (Grant No. WK2340000082)Anhui Initiative in Quantum Information Technologies (Grant No. AHY170000)the USTC start-up fundingthe China Government Youth 1000-Plan Talent Program
文摘Room temperature ferroelectric thin films are the key element of high-density nonvolatile memories in modern electronics. However, with the further miniaturization of the electronic devices beyond the Moore’s law, conventional ferroelectrics suffer great challenge arising from the critical thickness effect, where the ferroelectricity is unstable if the film thickness is reduced to nanometer or single atomic layer limit. Two-dimensional(2D) materials, thanks to their stable layered structure, saturate interfacial chemistry, weak interlayer couplings, and the benefit of preparing stable ultra-thin film at 2D limit, are promising for exploring 2D ferroelectricity and related device applications. Therefore, it provides an effective approach to overcome the limitation in conventional ferroelectrics with the study of 2D ferroelectricity in van der Waals(vdW) materials. In this review article,we briefly introduce recent progresses on 2D ferroelectricity in layered vdW materials. We will highlight the study on atomically thin α-In2Se3, which is an emergent ferroelectric semiconductor with the coupled in-plane and out-of-plane ferroelectricity. Furthermore, two prototype ferroelectric devices based on ferroelectric α-In2Se3 will also be reviewed.
基金financial support from the National Natural Science Foundation of China (11574236, 51671193, and 61434002)support from the National Science Fund for Distinguished Young Scholars (No. 51725103)
文摘Topological materials are a new and rapidly expanding class of quantum matter. To date, identification of the topological nature of a given compound material demands specific determination of the appropriate topological invariant through detailed electronic structure calculations. Here we present an efficient criterion that allows ready screening of potential topological materials, using topological insulators as prototypical examples. The criterion is inherently tied to the band inversion induced by spin-orbit coupling,and is uniquely defined by a minimal number of two elemental physical properties of the constituent elements: the atomic number and Pauling electronegativity. The validity and predictive power of the criterion is demonstrated by rationalizing many known topological insulators and potential candidates in the tetradymite and half-Heusler families, and the underlying design principle is naturally also extendable to predictive discoveries of other classes of topological materials.
基金Project supported by the National Key Basic Research Program of China(Grant No.2017YFA0205004)the National Natural Science Foundation of China(Grant Nos.92165201,11474261,and 11634011)+1 种基金the Fundamental Research Funds for the Central Universities(Grant Nos.WK3510000006,and WK3430000003)the Anhui Initiative in Quantum Information Technologies(Grant No.AHY170000)。
文摘Nanoclusters consisting of a few atoms have attracted a lot of research interests due to their exotic size-dependent properties. Here, well-ordered two-dimensional Sb cluster superlattice was fabricated on Si substrate by a two-step method and characterized by scanning tunneling microscopy. High resolution scanning tunneling microscope measurements revealed the fine structures of the Sb clusters, which consist of several Sb atoms ranging from 2 to 7. Furthermore, the electronic structure of the nanocluster displays the quantized energy-level which is due to the single-electron tunneling effects. We believe that the fabrication of Sb cluster superlattice broadens the species of the cluster superlattice and provides a promising candidate to further explore the novel physical and chemical properties of the semimetal nanocluster.
基金supported by the National Natural Science Foundation of China (Grant Nos. 91021019, 51074151, and 11034006)the National Basic Research Program of China (Grant Nos. 2010CB923401 and 2011CB921801)+2 种基金USDOE (Grant No. DE-FG03-02ER45958)US National Science Foundation (Grant No. 0906025)the BES Program of US Department of Energy (Grant No. ER45958)
文摘Topological insulators （Tls） are bulk insulators that possess robust helical conducting states along their interfaces with conventional insulators. A tremendous research effort has recently been devoted to TI-based heterostructures, in which con- ventional proximity effects give rise to a series of exotic physical phenomena. This paper reviews our recent studies on the potential existence of topological proximity effects at the interface between a topological insulator and a normal insu- lator or other topologically trivial systems. Using first-principles approaches, we have realized the tunability of the vertical location of the topological helical state via intriguing dual-proximity effects. To further elucidate the control parameters of this effect, we have used the graphene-based heterostructures as prototypical systems to reveal a more complete phase diagram. On the application side of the topological helical states, we have presented a catalysis example, where the topo- logical helical state plays an essential role in facilitating surface reactions by serving as an effective electron bath, These discoveries lay the foundation for accurate manipulation of the real space properties of the topological helical state in TI- based heterostructures and pave the way for realization of the salient functionality of topological insulators in future device applications.
基金Project supported by the Science Fund from the Ministry of Science and Technology of China(Grant Nos.2017YFA0204904 and 2019YFA0210004)the National Natural Science Foundation of China(Grant Nos.11674299 and 11634011)+2 种基金the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDB30000000)the Fund of Anhui Initiative Program in Quantum Information Technologies(Grant No.AHY170000)the Fundamental Research Funds for the Central Universities,China(Grant No.WK3510000013).
文摘Based on first-principles density functional theory calculation,we discover a novel form of spin-orbit(SO)splitting in two-dimensional(2D)heterostructures composed of a single Bi(111)bilayer stacking with a 2D semiconducting In_(2)Se_(2) or a 2D ferroelectricα-In_(2)Se_(3) layer.Such SO splitting has a Rashba-like but distinct spin texture in the valence band around the maximum,where the chirality of the spin texture reverses within the upper spin-split branch,in contrast to the conventional Rashba systems where the upper branch and lower branch have opposite chirality solely in the region below the band crossing point.The ferroelectric nature ofα-In_(2)Se_(3) further enables the tuning of the spin texture upon the reversal of the electric polarization with the application of an external electric field.Detailed analysis based on a tight-binding model reveals that such SO splitting texture results from the interplay of complex orbital characters and substrate interaction.This finding enriches the diversity of SO splitting systems and is also expected to promise for spintronic applications.
基金the National Key Research and Development Program of China(Grant Nos.2021YFA1400403,2018YFA0306800,2019YFA0210004,and 2016YFA0300401)the National Natural Science Foundation of China(Grant Nos.92165205,11774149,11790311,11774154,11674158,and 12074175)Innovation Program for Quantum Science and Technology(Grant No.2021ZD0302800)。
文摘Excitons in solid state are bosons generated by electron-hole pairs as the Coulomb screening is sufficiently reduced.The exciton condensation can result in exotic physics such as super-fluidity and insulating state.In charge density wave(CDW)state,1T-TiSe_(2) is one of the candidates that may host the exciton condensation.However,to envision its excitonic effect is still challenging,particularly at the two-dimensional limit,which is applicable to future devices.Here,we realize the epitaxial 1T-TiSe_(2) bilayer,the two-dimensional limit for its 2×2×2 CDW order,to explore the exciton-associated effect.By means of high-resolution scanning tunneling spectroscopy and quasiparticle interference,we discover an unexpected state residing below the conduction band and right within the CDW gap region.As corroborated by our theoretical analysis,this mysterious phenomenon is in good agreement with the electron-exciton coupling.Our study provides a material platform to explore exciton-based electronics and opto-electronics.
基金supported by the National Key Basic Research Program of China(Grant No.2017YFA0205004)the National Natural Science Foundation of China(Grant Nos.92165201,11474261,11634011,and 11974323)+2 种基金the Fundamental Research Funds for the Central Universities(Grant Nos.WK3510000006,and WK3430000003)the Innovation Program for Quantum Science and Technology(Grant No.2021ZD0302800)the Anhui Initiative in Quantum Information Technologies(Grant No.AHY170000)。
文摘Superconductors with reduced dimensionality have been widely explored for their exotic superconducting behaviors.Especially,at the two-dimensional limit,two-monolayer Pb films with two types of structures provide an ideal platform to unveil the underlying superconducting mechanism[Science 324,1314(2009)].Here,by combining scanning tunneling microscopy(STM)with the first-principle calculations,we successfully identify that these two types have different atomic lattice structures with varying stacking phases,which further enables us to calculate the phonon spectrum and electron phonon coupling strength of each type.The theoretical calculations are in good agreement with tunneling spectroscopy measurements of the superconducting transition temperatures(T_(c)),which established a correlation between atomic structures and superconductivity.Moreover,it was observed that the higher T_(c)of these two types also possess higher out-of-plane upper critical magnetic fields(Hc2).These findings will provide important new insights into two-dimensional superconductivity at the atomic level.
基金supported by the Innovation Program for Quantum Science and Technology(Grant No.2021ZD0302800)the National Natural Science Foundation of China(Grant Nos.11974323,and 12374458)+2 种基金the Anhui Initiative in Quantum Information Technologies(Grant No.AHY170000)the Strategic Priority Research Program of Chinese Academy of Sciences(Grant No.XDB0510200)the Anhui Provincial Key Research and Development Project(Grant No.2023z04020008)。
文摘Two-dimensional systems that simultaneously harbor superconductivity and nontrivial band topology may serve as appealing platforms for realizing topological superconductivity with promising applications in fault-tolerant quantum computing.Here,based on first-principles calculations,we show that monolayered Co N and Co P with the isovalent Fe Se-like structure are stable in freestanding form,even though their known bulk phases have no resemblance to layering.The two systems are further revealed to display intrinsic band inversions due to crystal field splitting,and such orderings are preserved with the inclusion of spin-orbit coupling(SOC),which otherwise is able to open a curved band gap,yielding a non-zero Z2 topological invariant in each case.Such a mechanism of topologicalization is distinctly contrasted with that identified recently for the closely related monolayers of CoX(X=As,Sb,Bi),where the SOC plays an indispensable role in causing a nontrivial band inversion.Next,we demonstrate that,by applying equi-biaxial tensile strain,the electron-phonon coupling strength in monolayered CoN can be significantly enhanced,yielding a superconducting transition temperature(Tc)up to 7-12 K for the Coulomb pseudopotential ofμ*=0.2-0.1,while the CoP monolayer shows very low Tc even under pronounced strain.Their different superconducting behaviors can be attributed to different variations in lattice softening and electronic density of states around the Fermi level upon pressuring.Our central findings enrich the understanding of different mechanisms of band inversions and topologicalization and offer platforms for achieving the coexistence of superconductivity and nontrivial band topology based on two-dimensional systems.
基金supported by the National Natural Science Foundation of China(Nos.12374196,92165201,and 11634011)the Innovation Program for Quantum Science and Technology(No.2021ZD0302800)+2 种基金the CAS Project for Young Scientists in Basic Research(Grant No.YSBR-046)the Fundamental Research Funds for the Central Universities(Grant Nos.WK3510000006and WK3430000003)Anhui Initiative in Quantum Information Technologies(No.AHY170000).
文摘Cobalt pnictides have been theoretically proposed to be attractive candi-dates for high-temperature superconductors.Additionally,monolayered CoX(X=As,Sb,Bi)on SrTiO_(3) systems present a potential new platform for realizing topological superconductors in the two-dimensional limit,due to their nontrivial band topology.To this end,we have successfully fabricated high-quality CoBi nanoislands on SrTiO_(3)(001)substrates by molecular beam epitaxy followed by an investigation of their atomic struc-ture and electronic properties via in situ scanning tunneling microscopyl spectroscopy.Beyond the previously predicted lattice with a=b=3.5 A,2×1 dimer row was observed in this study.Furthermore,our results reveal that the topography of CoBi islands is strongly influenced by various growth conditions,such as substrate temperature,the flux ratio between Co and Bi,and the annealing process.This study paves the way for further explorations of the superconductivity and topological properties of cobalt pnictidesystems.
基金the support of the startup funds at HFNLthe Innovation Program for Quantum Science and Technology(Grant No.2021ZD0302800)+3 种基金Anhui Initiative in Quantum Information Technologies(Grant No.AHY170000)supported by the start-up of Zhejiang Universitythe Fundamental Research Funds for the Central Universities(Grant No.226-2024-00068)funded by the Jane and Aatos Erkko Foundation and the Keele Foundation as part of the SuperC collaboration。
文摘Unconventional antiferromagnets(AFMs)with non-relativistic spin-splitting,such as the recently discovered altermagnet,have recently gained significant interest due to their potential for novel quantum phenomena and spintronic applications.The compensated magnetization in unconventional AFMs is protected by spin-space symmetries.In this work,we explore the symmetrybreaking effects and identify three distinct mechanisms for inducing net spin magnetizations in unconventional AFMs with collinear or non-collinear spins:(1)finite size effect,(2)extrinsic spin canting effect,and(3)irradiation with circularly polarized light.We show that the induced spin magnetizations are controllable and manifest as diverse intriguing phenomena.For the finite size system,the confined direction of a two-dimensional AM creates quantum-well-like subbands that determine the spin magnetization.This effect can be experimentally probed by measuring the spin density of states and the spin-polarization of Andreev-bound states within planar Josephson junctions.In the case of spin canting effect,it leads to peculiar anisotropic and non-monotonic behaviors in the superconducting proximity effect.Lastly,with circularly polarized light,spin magnetization is driven by the polarized light and the chirality of non-collinear magnetic order,thus offering a direct means of detecting the chirality of magnetic order in real materials.Our findings provide valuable insight into understanding and probing the spin magnetization in unconventional AFM materials.
文摘A method based on the adsorption of ions on the surface of two-dimensional （2D） nanosheets has been developed for photocatalytic COz reduction. Isolated Bi ions, confined on the surface of TiO2 nanosheets using a simple ionic adsorption method facilitate the formation of a built-in electric field that effectively promotes charge carrier separation. This leads to an improved performance of the photocatalytic COa reduction process with the preferred conversion to CH4. The proposed surface ion-adsorption method is expected to provide an effective approach for the design of highly efficient photocatalytic systems. These findings could be very valuable in photocatalytic CO2 reduction applications.
