Providing the strong spin-orbital interaction, Bismuth is the key element in the family of three-dimensional topological insulators. At the same time, Bismuth itself also has very unusual behavior, existing from the t...Providing the strong spin-orbital interaction, Bismuth is the key element in the family of three-dimensional topological insulators. At the same time, Bismuth itself also has very unusual behavior, existing from the thinnest unit to bulk crystals. Ultrathin Bi (111) bilayers have been theoretically proposed as a two-dimensional topological insulator. The related experimental realization achieved only recently, by growing Bi (111) ultrathin bilayers on topological insulator Bi2Te3 or Bi2Se3 substrates. In this review, we started from the growth mode of Bi (111) bilayers and reviewed our recent progress in the studies of the electronic structures and the one-dimensional topological edge states using scanning tunneling microscopy/spectroscopy (STM/STS), angle-resolved photoemission spectroscopy (ARPES), and first principles calculations.展开更多
The band structures of two-monolayer Bi(110) films on black phosphorus substrates are studied using angleresolved photoemission spectroscopy. Within the band gap of bulk black phosphorus, the electronic states near ...The band structures of two-monolayer Bi(110) films on black phosphorus substrates are studied using angleresolved photoemission spectroscopy. Within the band gap of bulk black phosphorus, the electronic states near the Fermi level are dominated by the Bi(110) film. The band dispersions revealed by our data suggest that the orientation of the Bi(110) film is aligned with the black phosphorus substrate. The electronic structures of the Bi(110) film strongly deviate from the band calculations of the free-standing Bi(110) film, suggesting that the substrate can significantly affect the electronic states in the Bi(110) film. Our data show that there are no non-trivial electronic states in Bi(110) films grown on black phosphorus substrates.展开更多
Here, large-scale and uniform hexagonal zinc oxide(ZnO) nanosheet films were deposited onto indium tin oxide(ITO)-coated transparent conducting glass substrates via a facile galvanic displacement deposition process. C...Here, large-scale and uniform hexagonal zinc oxide(ZnO) nanosheet films were deposited onto indium tin oxide(ITO)-coated transparent conducting glass substrates via a facile galvanic displacement deposition process. Compared with other commonly used solution methods, this process avoids high temperature and electric power as well as supporting agents to make it simple and cost-effective. The as-fabricated ZnO nanosheet films have uniform hexagonal wurtzite structure. The photoelectrochemical(PEC) cell based on ZnO nanosheet film/ITO photoelectrode was also fabricated and its performance was improved by optimizing the solution concentration. A higher photocurrent density of*500 l A cm^(-2)under AM 1.5 G simulated illumination of 100 m W cm^(-2)with zero bias potential(vs. Ag/AgCl electrode) was obtained, which may ascribe to the increased surface-to-volume ratio of disordered Zn O nanosheet arrays. Our developed method may be used to deposit other oxide semiconductors, and the Zn O nanosheet film/ITO PEC cell can be used to design low-cost optoelectronic and photoelectrochemical devices.展开更多
The graphene-based moiré superlattice has been demonstrated as an exciting system for investigating strong correlation phenomenon. However, the fabrication of such moiré superlattice mainly relies on transfe...The graphene-based moiré superlattice has been demonstrated as an exciting system for investigating strong correlation phenomenon. However, the fabrication of such moiré superlattice mainly relies on transfer technology. Here, we report the epitaxial growth of trilayer graphene(TLG) moiré superlattice on hexagonal boron nitride(h BN) by a remote plasma-enhanced chemical vapor deposition method. The as-grown TLG/h BN shows a uniform moiré pattern with a period of ~ 15 nm by atomic force microscopy(AFM) imaging, which agrees with the lattice mismatch between graphene and h BN. By fabricating the device with both top and bottom gates, we observed a gate-tunable bandgap at charge neutral point(CNP) and displacement field tunable satellite resistance peaks at half and full fillings. The resistance peak at half-filling indicates a strong electron–electron correlation in our grown TLG/h BN superlattice. In addition, we observed quantum Hall states at Landau level filling factors ν = 6, 10, 14,..., indicating that our grown trilayer graphene has the ABC stacking order. Our work suggests that epitaxy provides an easy way to fabricate stable and reproducible two-dimensional strongly correlated electronic materials.展开更多
Iron-based superconductor family FeX(X=S,Se,Te)has been one of the research foci in physics and material science due to their record-breaking superconducting temperature(FeSe film)and rich physical phenomena.Recently,...Iron-based superconductor family FeX(X=S,Se,Te)has been one of the research foci in physics and material science due to their record-breaking superconducting temperature(FeSe film)and rich physical phenomena.Recently,FeS,the least studied Fe X compound(due to the difficulty in synthesizing high quality macroscopic crystals)attracted much attention because of its puzzling superconducting pairing symmetry.In this work,combining scanning tunneling microscopy and angle resolved photoemission spectroscopy(ARPES)with sub-micron spatial resolution,we investigate the intrinsic electronic structures of superconducting FeS from individual single crystalline domains.Unlike FeTe or FeSe,FeS remains identical tetragonal structure from room temperature down to 5 K,and the band structures observed can be well reproduced by our ab-initio calculations.Remarkably,mixed with the 1×1 tetragonal metallic phase,we also observe the coexistence of √5×√5 reconstructed insulating phase in the crystal,which not only helps explain the unusual properties of FeS,but also demonstrates the importance of using spatially resolved experimental tools in the study of this compound.展开更多
Hg Te(111) surface is comprehensively studied by scanning tunneling microscopy/spectroscopy(STS).In addition to th√e prim√itive(1 × 1)√ hexagonal lattice,six reconstructed surface structures are observe...Hg Te(111) surface is comprehensively studied by scanning tunneling microscopy/spectroscopy(STS).In addition to th√e prim√itive(1 × 1)√ hexagonal lattice,six reconstructed surface structures are observed:(2 × 2),2 × 1,4 × 1,3 ×(1/2)3,2(1/2)2 × 2 and (1/2)11 × 2.The(2 × 2) reconstructed lattice maintains the primitive hexagonal symmetry,whi√le the lattices of the other five reconstructions are rectangular.Moreover,the topographic features of the3 ×(1/2)3 reconstruction are bias dependent,indicating that they have both topographic and electronic origins.The STSs obtained at different reconstructed surfaces show a universal dip feature with size ~100 mV,which may be attributed to the surface distortion.Our results reveal the atomic structure and complex reconstructions of the cleaved Hg Te(111) surfaces,which paves the way to understand the rich properties of Hg Te crystal.展开更多
Lattice superlattices constructed with different materials such as ferromagnets and insulators at atomic scale provide an ideal platform for exploring many emergent physical phenomena.In the present work,a new type of...Lattice superlattices constructed with different materials such as ferromagnets and insulators at atomic scale provide an ideal platform for exploring many emergent physical phenomena.In the present work,a new type of superlattices composed of ferromagnetic half-metal CrO_(2),with a thickness of two atomic layers,together with insulating MgH_(2) are constructed.Systematic theoretical studies on the(CrO_(2))_(2)/(MgH_(2))_(n) (n=2,3,4,5,6)superlattices are carried out based on first-principles density-functional theory calculations.These superlattices are ferromagnetic semiconductors with similar intra-layer magnetic exchange couplings between Cr ions.As the thickness of the MgH_(2) layer increases,the magnetic exchange interaction between inter-layer Cr ions shows oscillating decaying behavior,while the energy band gaps show a small increase.The understanding of magnetic couplings in these superlattices provides a pathway for constructing new ferromagnetic semiconductors.展开更多
Single-walled carbon nanotubes(SWCNTs),due to their outstanding electrical and optical properties,are expected to have extensive applications,such as in transparent conductive fims and ultra-small field-effect transis...Single-walled carbon nanotubes(SWCNTs),due to their outstanding electrical and optical properties,are expected to have extensive applications,such as in transparent conductive fims and ultra-small field-effect transistors(FETs).However,those applications can only be best realized with pure metallic or pure semiconducting SWCNTs.Hence,identifying and separating metallic from semiconducting SWCNTs in as-grown samples are crucial.In addition,knowledge of the type of an SWCNT is also important for further exploring its new properties in fundamental science.Here we report employing scanning near-field optical microscopy(SNOM)as a direct and simple method to identify metallic and semiconducting SWCNTs on SiO2/Si substrates.Metallic and semiconducting SWCNTs show distinct near-field optical responses because the metallic tubes support plasmons whereas the semiconducting tubes do not.The reliability of this method is verified using FET testing and Rayleigh scattering spectroscopy.Our result demonstrates that the SNOM technique provides a reliable,simple,noninvasive and in situ method to distinguish between metallic and semiconducting SWCNTs.展开更多
We investigate the surface structure and electronic properties ofβ-Sn islands deposited on a graphitized 6 H-SiC(0001)substrate via low temperature scanning tunneling microscopy and spectroscopy.Owing to the confinem...We investigate the surface structure and electronic properties ofβ-Sn islands deposited on a graphitized 6 H-SiC(0001)substrate via low temperature scanning tunneling microscopy and spectroscopy.Owing to the confinement of the island geometry,quantum well states(QWSs)are formed,manifesting as equidistant peaks in the tunneling spectra.Furthermore,a distinct strip feature appears on the surfaces of odd-layer Sn islands,ranging from 15-19 layers,which is not present on the surfaces of even-layer Sn islands.The spatial distribution of strips can be modified by applying a bias pulse,using an STM tip.Furthermore,the strip-like structure shows significant impacts on the QWS.An energy splitting of the lowest unoccupied QWSs is observed in strip regions;this may be ascribed to caused the phase shift of the wave functions of the QWSs on the top surface,due to surface distortions created by the aforementioned strips.展开更多
Surface structures and properties of Sn islands grown on superconducting substrate 2H-NbSe2(0001)are studied using low temperature scanning tunneling microscopy or spectroscopy.The pure face-centered cubic(fee)str...Surface structures and properties of Sn islands grown on superconducting substrate 2H-NbSe2(0001)are studied using low temperature scanning tunneling microscopy or spectroscopy.The pure face-centered cubic(fee)structure of Sn surface is obtained.Superconductivity is also detected on the fcc-Sn(111)surface,and the size of superconducting gap on the Sn surface is nearly the same as that on the superconducting substrate.Furthermore,phase transition occurs from fcc-Sn(111)toβ-Sn(001)by keeping the sample at room temperature for a certain time.Due to the strain relaxation on theβ-Sn islands,both the in-plane unit cell and out-of-plane structures distort,and the height of surface atoms varies periodically to form a universal ripple structure.展开更多
The spin-1/2 model system with antiferromagnetic(AF) couplings on a J1-J2checkerboard lattice, known as the planar pyrochlore model, is strongly frustrated and associated with a two-to-one dimensional crossover. Using...The spin-1/2 model system with antiferromagnetic(AF) couplings on a J1-J2checkerboard lattice, known as the planar pyrochlore model, is strongly frustrated and associated with a two-to-one dimensional crossover. Using the Projected Entangled Simplex States tensor network ansatz, we identify a large number of nearly degenerate states in the frustrated region(J_(1)<J_(2)).Specifically, we find the long-sought crossed-dimer valence bond solid(VBS) state to be the ground state at J_(1)≤J_(2), while various 1D AF correlated states take over the rest. We verify the stability of the VBS state against nematic perturbation. The corresponding bosonic picture provides an intuitive understanding of the low-energy physics. Particularly, it predicts weaker VBS states in the easy-plane limit, which we confirm numerically. Our results clarify the most essential ground state properties of this interesting system and demonstrate the usefulness of bosonic picture in dealing with frustrated magnetism.展开更多
YbMnBi2 is a recently discovered time-reversal-symmetry breaking type-Ⅱ Weyl semimetal.However, as a representation of the new category of topological matters, the scanning tunneling microcopy(STM) results on such im...YbMnBi2 is a recently discovered time-reversal-symmetry breaking type-Ⅱ Weyl semimetal.However, as a representation of the new category of topological matters, the scanning tunneling microcopy(STM) results on such important material are still absent.Here, we report the STM investigations on the morphology of vacuum cleaved single crystalline YbMnBi2 samples.A hill and valley type of topography is observed on the YbMnBi2 surface, which is consistent with the non-layer nature of its crystal structure.Analysis of STM images yields the information of the index of the vicinal surface.Our results here lay a playground of future atomic scale research on YbMnBi2.展开更多
Detection of local strain at the nanometer scale with high sensitivity remains challenging.Here we report near-field infrared nano-imaging of local strains in bilayer graphene by probing strain-induced shifts of phono...Detection of local strain at the nanometer scale with high sensitivity remains challenging.Here we report near-field infrared nano-imaging of local strains in bilayer graphene by probing strain-induced shifts of phonon frequency.As a non-polar crystal,intrinsic bilayer graphene possesses little infrared response at its transverse optical phonon frequency.The reported optical detection of local strain is enabled by applying a vertical electrical field that breaks the symmetry of the two graphene layers and introduces finite electrical dipole moment to graphene phonon.The activated phonon further interacts with continuum electronic transitions,and generates a strong Fano resonance.The resulted Fano resonance features a very sharp near-field infrared scattering peak,which leads to an extraordinary sensitivity of-0.002%for the strain detection.Our results demonstrate the first nano-scale near-field Fano resonance,provide a new way to probe local strains with high sensitivity in non-polar crystals,and open exciting possibilities for studying strain-induced rich phenomena.展开更多
We reveal the electronic structure in Yb Cd_(2)Sb_(2),a thermoelectric material,by angle-resolved photoemission spectroscopy(ARPES)and time-resolved ARPES(tr ARPES).Specifically,three bulk bands at the vicinity of the...We reveal the electronic structure in Yb Cd_(2)Sb_(2),a thermoelectric material,by angle-resolved photoemission spectroscopy(ARPES)and time-resolved ARPES(tr ARPES).Specifically,three bulk bands at the vicinity of the Fermi level are evidenced near the Brillouin zone center,consistent with the density functional theory(DFT)calculation.It is interesting that the spin-unpolarized bulk bands respond unexpectedly to right-and left-handed circularly polarized probe.In addition,a hole band of surface states,which is not sensitive to the polarization of the probe beam and is not expected from the DFT calculation,is identified.We find that the non-equilibrium quasiparticle recovery rate is much smaller in the surface states than that of the bulk states.Our results demonstrate that the surface states can be distinguished from the bulk ones from a view of time scale in the nonequilibrium physics.展开更多
Transferring high-quality exfoliated graphene flakes onto different substrates while keeping the graphene free of polymer residues is of great importance, but at the same time very challenging. Currently, the only fea...Transferring high-quality exfoliated graphene flakes onto different substrates while keeping the graphene free of polymer residues is of great importance, but at the same time very challenging. Currently, the only feasible way is the so-called all-dry "pick-and-lift" method, in which a hexagonal boron nitride(hBN) flake is employed to serve as a stamp to pick up graphene from one substrate and to lift it down onto another substrate. The transferred graphene samples, however,are always covered or encapsulated by hBN flakes, which leads to difficulties in further characterizations. Here, we report an improved "pick-and-lift" method, which allows ultra-clean graphene flakes to be transferred onto a variety of substrates without hBN coverage. Basically, by exploiting the superlubricity at the graphene/hBN stack interface, we are able to remove the top-layer hBN stamp by applying a tangential force and expose the underneath graphene.展开更多
We investigate the adsorption of organic molecular semiconductor perylene on(7×7)reconstructed Si(111)surface by ultraviolet photoemission spectroscopy.It is observed that seven features that derive from the orga...We investigate the adsorption of organic molecular semiconductor perylene on(7×7)reconstructed Si(111)surface by ultraviolet photoemission spectroscopy.It is observed that seven features that derive from the organic material are located at 0.71,2.24,4.0,5.9,7.46,8.65 and 9.95 eV in binding energy.The theoretical calculation results reveal the most stable adsorption geometry of organic molecule perylene on Si(111)(7×7)substrates is at the beginning of deposition.展开更多
We numerically study the thermodynamic properties of a hard ellipsoid fluid by mainly focusing on its phase transition from an isotropic phase into a nematic phase (i.e. isotropie-nematic phase transition). To impro...We numerically study the thermodynamic properties of a hard ellipsoid fluid by mainly focusing on its phase transition from an isotropic phase into a nematic phase (i.e. isotropie-nematic phase transition). To improve the accuracy, precision, and efficiency of our computations, we attempt to employ the Wang-Landau NPT Monte Carlo algorithm in our simulations to calculate the function p(V) that gives the probability of arriving at the threshold density of the isotropic-nematic transition. Our results directly reveal that the nematic fluid phase, which is characterized by an ordered direction rather than an ordered configuration, appears and coexists with the isotropic phase when the aspect ratio a of the ellipsoid is located in a relatively narrow range of α = 2.0-2.25, and it becomes dominant and is fully established when α≥αcut = 2.25. We find that our estimated αcut is significantly lower than previously reported values of around 2.75. This prediction is further confirmed by the calculations of both the fluid reduced density and pressure of coexistence which show that the pressure grows up as the density increases and the probability function p(V) exhibits double peaks when the pressure enters the coexistence region. Based on these consistent results we are able to conclude that when α≥2.25 an ellipsoid fluid can fully display the nematic behavior. This study will place a useful and tight theoretical constraint on investigations of the isotropic-nematic phase transition in the future.展开更多
Indacenodithiophene-co-benzothiadiazole(IDTBT) has emerged as one of the most exciting semiconducting polymers in recent years because of its high electronic mobility and charge transport along the polymer backbone....Indacenodithiophene-co-benzothiadiazole(IDTBT) has emerged as one of the most exciting semiconducting polymers in recent years because of its high electronic mobility and charge transport along the polymer backbone. By using the recently developed ion gel gating technique we studied the charge transport of IDTBT at carrier densities up to 10^21cm^-3.While the conductivity in IDTBT was found to be enhanced by nearly six orders of magnitude by ionic gating, the charge transport in IDTBT was found to remain 3D Mott variable range hopping even down to the lowest temperature of our measurements, 12 K. The maximum mobility was found to be around 0.2 cm^2·V^-1·s^-1, lower than that of Cytop gated field effect transistors reported previously. We attribute the lower mobility to the additional disorder induced by the ionic gating.展开更多
This article continues our highlights last two years[1,2]on the highest independently confirmed mainstream(silicon,perovskite,and organic)solar cell efficiencies in 2023.The world record power conversion efficiency(PC...This article continues our highlights last two years[1,2]on the highest independently confirmed mainstream(silicon,perovskite,and organic)solar cell efficiencies in 2023.The world record power conversion efficiency(PCE)of the single-junction silicon and perovskite/silicon tandem solar cells has reached over 27%and close to 34%,respectively,in 2023.We have also witnessed the rapid mass-production development of the silicon passivating contact and silicon back contact solar cells,as well as further progress with perovskite solar cells(PSCs).展开更多
Using high-resolution angle-resolved and time-resolved photoemission spectroscopy,we have studied the low-energy band structures in occupied and unoccupied states of three ternary compounds GeBi_(2)Te_(4),SnBi_(2)Te_(...Using high-resolution angle-resolved and time-resolved photoemission spectroscopy,we have studied the low-energy band structures in occupied and unoccupied states of three ternary compounds GeBi_(2)Te_(4),SnBi_(2)Te_(4) and Sn_(0.571)Bi_(2.286)Se_(4) near the Fermi level.In previously confirmed topological insulator GeBi_(2)Te_(4) compounds,we confirmed the existence of the Dirac surface state and found that the bulk energy gap is much larger than that in the first-principles calculations.In SnBi_(2)Te_(4) compounds,the Dirac surface state was observed,consistent with the first-principles calculations,indicating that it is a topological insulator.The experimental detected bulk gap is a little bit larger than that in calculations.In Sn_(0.571)Bi_(2.286)Se_(4) compounds,our measurements suggest that this nonstoichiometric compound is a topological insulator although the stoichiometric SnBi_(2)Se_(4) compound was proposed to be topological trivial.展开更多
基金supported by the National Basic Research Program of China (Grants Nos. 2012CB927401,2011CB921902,2013CB921902,and 2011CB922200)the National Natural Science Foundation of China (Grants Nos. 91021002,11174199,11134008,and 11274228)SCSTC (Grant Nos. 11JC1405000,11PJ1405200,and 12JC1405300)
文摘Providing the strong spin-orbital interaction, Bismuth is the key element in the family of three-dimensional topological insulators. At the same time, Bismuth itself also has very unusual behavior, existing from the thinnest unit to bulk crystals. Ultrathin Bi (111) bilayers have been theoretically proposed as a two-dimensional topological insulator. The related experimental realization achieved only recently, by growing Bi (111) ultrathin bilayers on topological insulator Bi2Te3 or Bi2Se3 substrates. In this review, we started from the growth mode of Bi (111) bilayers and reviewed our recent progress in the studies of the electronic structures and the one-dimensional topological edge states using scanning tunneling microscopy/spectroscopy (STM/STS), angle-resolved photoemission spectroscopy (ARPES), and first principles calculations.
基金Supported by National Key R&D Program of China under Grant Nos 2017YFA0402901,2016YFA0401004 and 2016YFB0901600the National Natural Science Foundation of China under Grant Nos 11534010,11404172,U1532136,U1632102,U1632272,11574201,11674296 and 11190022+3 种基金the National Basic Research Program of China under Grant No 2014CB921102the Key Research Program of the Chinese Academy of Sciences under Grant Nos QYZDY-SSW-SLH021 and XDPB01the Interdisciplinary Innovation Team of Chinese Academy of Sciencesthe Initiative Scientific Research Program of Shanghai Jiao Tong University
文摘The band structures of two-monolayer Bi(110) films on black phosphorus substrates are studied using angleresolved photoemission spectroscopy. Within the band gap of bulk black phosphorus, the electronic states near the Fermi level are dominated by the Bi(110) film. The band dispersions revealed by our data suggest that the orientation of the Bi(110) film is aligned with the black phosphorus substrate. The electronic structures of the Bi(110) film strongly deviate from the band calculations of the free-standing Bi(110) film, suggesting that the substrate can significantly affect the electronic states in the Bi(110) film. Our data show that there are no non-trivial electronic states in Bi(110) films grown on black phosphorus substrates.
基金supported by the National Major Basic Research Project of 2012CB934302the National 863 Program2011AA050518+1 种基金the Natural Science Foundation of China(Grant No.1117419711574203 and 61234005)
文摘Here, large-scale and uniform hexagonal zinc oxide(ZnO) nanosheet films were deposited onto indium tin oxide(ITO)-coated transparent conducting glass substrates via a facile galvanic displacement deposition process. Compared with other commonly used solution methods, this process avoids high temperature and electric power as well as supporting agents to make it simple and cost-effective. The as-fabricated ZnO nanosheet films have uniform hexagonal wurtzite structure. The photoelectrochemical(PEC) cell based on ZnO nanosheet film/ITO photoelectrode was also fabricated and its performance was improved by optimizing the solution concentration. A higher photocurrent density of*500 l A cm^(-2)under AM 1.5 G simulated illumination of 100 m W cm^(-2)with zero bias potential(vs. Ag/AgCl electrode) was obtained, which may ascribe to the increased surface-to-volume ratio of disordered Zn O nanosheet arrays. Our developed method may be used to deposit other oxide semiconductors, and the Zn O nanosheet film/ITO PEC cell can be used to design low-cost optoelectronic and photoelectrochemical devices.
基金Project supported by the National Key Research and Development Program of China (Grant No. 2020YFA0309600)the National Natural Science Foundation of China (Grant Nos. 61888102, 11834017, and 12074413)+3 种基金the Strategic Priority Research Program of CAS (Grant Nos. XDB30000000 and XDB33000000)the Key-Area Research and Development Program of Guangdong Province, China (Grant No. 2020B0101340001)support from the Elemental Strategy Initiative conducted by the MEXT, Japan (Grant No. JPMXP0112101001)JSPS KAKENHI (Grant Nos. 19H05790, 20H00354, and 21H05233), and A3 Foresight by JSPS
文摘The graphene-based moiré superlattice has been demonstrated as an exciting system for investigating strong correlation phenomenon. However, the fabrication of such moiré superlattice mainly relies on transfer technology. Here, we report the epitaxial growth of trilayer graphene(TLG) moiré superlattice on hexagonal boron nitride(h BN) by a remote plasma-enhanced chemical vapor deposition method. The as-grown TLG/h BN shows a uniform moiré pattern with a period of ~ 15 nm by atomic force microscopy(AFM) imaging, which agrees with the lattice mismatch between graphene and h BN. By fabricating the device with both top and bottom gates, we observed a gate-tunable bandgap at charge neutral point(CNP) and displacement field tunable satellite resistance peaks at half and full fillings. The resistance peak at half-filling indicates a strong electron–electron correlation in our grown TLG/h BN superlattice. In addition, we observed quantum Hall states at Landau level filling factors ν = 6, 10, 14,..., indicating that our grown trilayer graphene has the ABC stacking order. Our work suggests that epitaxy provides an easy way to fabricate stable and reproducible two-dimensional strongly correlated electronic materials.
基金Project supported by CAS-Shanghai Science Research Center,China(Grant No.CAS-SSRC-YH-2015-01)the National Key R&D Program of China(Grant No.2017YFA0305400)+4 种基金the National Natural Science Foundation of China(Grant Nos.11674229,11227902,and 11604207)the EPSRC Platform Grant(Grant No.EP/M020517/1)Hefei Science Center,Chinese Academy of Sciences(Grant No.2015HSC-UE013)Science and Technology Commission of Shanghai Municipality,China(Grant No.14520722100)the Strategic Priority Research Program(B)of the Chinese Academy of Sciences(Grant No.XDB04040200)。
文摘Iron-based superconductor family FeX(X=S,Se,Te)has been one of the research foci in physics and material science due to their record-breaking superconducting temperature(FeSe film)and rich physical phenomena.Recently,FeS,the least studied Fe X compound(due to the difficulty in synthesizing high quality macroscopic crystals)attracted much attention because of its puzzling superconducting pairing symmetry.In this work,combining scanning tunneling microscopy and angle resolved photoemission spectroscopy(ARPES)with sub-micron spatial resolution,we investigate the intrinsic electronic structures of superconducting FeS from individual single crystalline domains.Unlike FeTe or FeSe,FeS remains identical tetragonal structure from room temperature down to 5 K,and the band structures observed can be well reproduced by our ab-initio calculations.Remarkably,mixed with the 1×1 tetragonal metallic phase,we also observe the coexistence of √5×√5 reconstructed insulating phase in the crystal,which not only helps explain the unusual properties of FeS,but also demonstrates the importance of using spatially resolved experimental tools in the study of this compound.
基金Supported by the National Key Research and Development Program of China under Grant Nos 2016YFA0301003 and 2016YFA0300403the National Natural Science Foundation of China under Grant Nos 11521404,11634009,U1632102,11504230,11674222,11574202,11674226,11574201 and U1632272
文摘Hg Te(111) surface is comprehensively studied by scanning tunneling microscopy/spectroscopy(STS).In addition to th√e prim√itive(1 × 1)√ hexagonal lattice,six reconstructed surface structures are observed:(2 × 2),2 × 1,4 × 1,3 ×(1/2)3,2(1/2)2 × 2 and (1/2)11 × 2.The(2 × 2) reconstructed lattice maintains the primitive hexagonal symmetry,whi√le the lattices of the other five reconstructions are rectangular.Moreover,the topographic features of the3 ×(1/2)3 reconstruction are bias dependent,indicating that they have both topographic and electronic origins.The STSs obtained at different reconstructed surfaces show a universal dip feature with size ~100 mV,which may be attributed to the surface distortion.Our results reveal the atomic structure and complex reconstructions of the cleaved Hg Te(111) surfaces,which paves the way to understand the rich properties of Hg Te crystal.
基金supported by the National Natural Science Foundation of China (Grant No. 11521404)the Materials Genome Initiative Center of Shanghai Jiao Tong University (SJTU)。
文摘Lattice superlattices constructed with different materials such as ferromagnets and insulators at atomic scale provide an ideal platform for exploring many emergent physical phenomena.In the present work,a new type of superlattices composed of ferromagnetic half-metal CrO_(2),with a thickness of two atomic layers,together with insulating MgH_(2) are constructed.Systematic theoretical studies on the(CrO_(2))_(2)/(MgH_(2))_(n) (n=2,3,4,5,6)superlattices are carried out based on first-principles density-functional theory calculations.These superlattices are ferromagnetic semiconductors with similar intra-layer magnetic exchange couplings between Cr ions.As the thickness of the MgH_(2) layer increases,the magnetic exchange interaction between inter-layer Cr ions shows oscillating decaying behavior,while the energy band gaps show a small increase.The understanding of magnetic couplings in these superlattices provides a pathway for constructing new ferromagnetic semiconductors.
基金the National Natural Science Foundation of China under Grant Nos.11574204 and 11774224the National Key Research and Development Program of China(2016YFA0302001)+1 种基金Program for Professor of Special Appointment(Eastern Scholar)at Shanghai Institutions of Higher Learningadditional support from a Shanghai talent program。
文摘Single-walled carbon nanotubes(SWCNTs),due to their outstanding electrical and optical properties,are expected to have extensive applications,such as in transparent conductive fims and ultra-small field-effect transistors(FETs).However,those applications can only be best realized with pure metallic or pure semiconducting SWCNTs.Hence,identifying and separating metallic from semiconducting SWCNTs in as-grown samples are crucial.In addition,knowledge of the type of an SWCNT is also important for further exploring its new properties in fundamental science.Here we report employing scanning near-field optical microscopy(SNOM)as a direct and simple method to identify metallic and semiconducting SWCNTs on SiO2/Si substrates.Metallic and semiconducting SWCNTs show distinct near-field optical responses because the metallic tubes support plasmons whereas the semiconducting tubes do not.The reliability of this method is verified using FET testing and Rayleigh scattering spectroscopy.Our result demonstrates that the SNOM technique provides a reliable,simple,noninvasive and in situ method to distinguish between metallic and semiconducting SWCNTs.
基金Supported by the National Natural Science Foundation of China(Grant Nos.11521404,11634009,11674222,11674226,11790313,11574202,11874256,U1632102,11861161003 and 11874258)the National Key Research and Development Program of China(Grant Nos.2016YFA0300403 and 2016YFA0301003)+2 种基金the Key Research Program of the Chinese Academy of Sciences(Grant No.XDPB08-2)the Strategic Priority Research Program of Chinese Academy of Sciences(Grant No.XDB28000000)the Shanghai Municipal Science and Technology Major Project(Grant No.2019SHZDZX01)。
文摘We investigate the surface structure and electronic properties ofβ-Sn islands deposited on a graphitized 6 H-SiC(0001)substrate via low temperature scanning tunneling microscopy and spectroscopy.Owing to the confinement of the island geometry,quantum well states(QWSs)are formed,manifesting as equidistant peaks in the tunneling spectra.Furthermore,a distinct strip feature appears on the surfaces of odd-layer Sn islands,ranging from 15-19 layers,which is not present on the surfaces of even-layer Sn islands.The spatial distribution of strips can be modified by applying a bias pulse,using an STM tip.Furthermore,the strip-like structure shows significant impacts on the QWS.An energy splitting of the lowest unoccupied QWSs is observed in strip regions;this may be ascribed to caused the phase shift of the wave functions of the QWSs on the top surface,due to surface distortions created by the aforementioned strips.
基金Supported by the National Key Research and Development Program of China under Grant Nos 2016YFA0301003 and 2016YFA0300403the National Natural Science Foundation of China under Grant Nos 11521404,11634009,U1632102,11504230,11674222,11574202,11674226,11574201 and U1632272
文摘Surface structures and properties of Sn islands grown on superconducting substrate 2H-NbSe2(0001)are studied using low temperature scanning tunneling microscopy or spectroscopy.The pure face-centered cubic(fee)structure of Sn surface is obtained.Superconductivity is also detected on the fcc-Sn(111)surface,and the size of superconducting gap on the Sn surface is nearly the same as that on the superconducting substrate.Furthermore,phase transition occurs from fcc-Sn(111)toβ-Sn(001)by keeping the sample at room temperature for a certain time.Due to the strain relaxation on theβ-Sn islands,both the in-plane unit cell and out-of-plane structures distort,and the height of surface atoms varies periodically to form a universal ripple structure.
基金supported by the National Natural Science Foundation of China(Grant No.12274126)supports from the National Natural Science Foundation of China(Grant Nos.12074031,and 12234016)+1 种基金supports from the National Natural Science Foundation of China(Grant Nos.12274287,and 12042507)the Innovation Program for Quantum Science and Technology(Grant No.2021ZD0301900)。
文摘The spin-1/2 model system with antiferromagnetic(AF) couplings on a J1-J2checkerboard lattice, known as the planar pyrochlore model, is strongly frustrated and associated with a two-to-one dimensional crossover. Using the Projected Entangled Simplex States tensor network ansatz, we identify a large number of nearly degenerate states in the frustrated region(J_(1)<J_(2)).Specifically, we find the long-sought crossed-dimer valence bond solid(VBS) state to be the ground state at J_(1)≤J_(2), while various 1D AF correlated states take over the rest. We verify the stability of the VBS state against nematic perturbation. The corresponding bosonic picture provides an intuitive understanding of the low-energy physics. Particularly, it predicts weaker VBS states in the easy-plane limit, which we confirm numerically. Our results clarify the most essential ground state properties of this interesting system and demonstrate the usefulness of bosonic picture in dealing with frustrated magnetism.
基金Project supported by the “Shuguang Program” from Shanghai Education Development Foundation and Shanghai Municipal Education Commission,Ministry of Science and Technology of China(Grant Nos.2016YFA0301003 and 2016YFA0300403)the National Natural Science Foundation of China(Grant Nos.11521404,11634009,11874256,11574202,11874258,11790313,11674226,U1632102,11674222,and 11861161003)the Strategic Priority Research Program of Chinese Academy of Sciences(Grant No.XDB28000000)
文摘YbMnBi2 is a recently discovered time-reversal-symmetry breaking type-Ⅱ Weyl semimetal.However, as a representation of the new category of topological matters, the scanning tunneling microcopy(STM) results on such important material are still absent.Here, we report the STM investigations on the morphology of vacuum cleaved single crystalline YbMnBi2 samples.A hill and valley type of topography is observed on the YbMnBi2 surface, which is consistent with the non-layer nature of its crystal structure.Analysis of STM images yields the information of the index of the vicinal surface.Our results here lay a playground of future atomic scale research on YbMnBi2.
基金Supported by the National Key Research and Development Program of China (Grant No.2016YFA0302001)the National Natural Science Foundation of China (Grant Nos.11774224,12074244,11521404,and 61701394)+1 种基金support from the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learningadditional support from a Shanghai talent program。
文摘Detection of local strain at the nanometer scale with high sensitivity remains challenging.Here we report near-field infrared nano-imaging of local strains in bilayer graphene by probing strain-induced shifts of phonon frequency.As a non-polar crystal,intrinsic bilayer graphene possesses little infrared response at its transverse optical phonon frequency.The reported optical detection of local strain is enabled by applying a vertical electrical field that breaks the symmetry of the two graphene layers and introduces finite electrical dipole moment to graphene phonon.The activated phonon further interacts with continuum electronic transitions,and generates a strong Fano resonance.The resulted Fano resonance features a very sharp near-field infrared scattering peak,which leads to an extraordinary sensitivity of-0.002%for the strain detection.Our results demonstrate the first nano-scale near-field Fano resonance,provide a new way to probe local strains with high sensitivity in non-polar crystals,and open exciting possibilities for studying strain-induced rich phenomena.
基金support from the National Natural Science Foundation of China(Grant No.11974243)support from the National Natural Science Foundation of China(Grant No.11521404)+1 种基金additional support from a Shanghai talent programsupport by the Natural Science Foundation of Shanghai,China(Grant No.17ZR1443300)。
文摘We reveal the electronic structure in Yb Cd_(2)Sb_(2),a thermoelectric material,by angle-resolved photoemission spectroscopy(ARPES)and time-resolved ARPES(tr ARPES).Specifically,three bulk bands at the vicinity of the Fermi level are evidenced near the Brillouin zone center,consistent with the density functional theory(DFT)calculation.It is interesting that the spin-unpolarized bulk bands respond unexpectedly to right-and left-handed circularly polarized probe.In addition,a hole band of surface states,which is not sensitive to the polarization of the probe beam and is not expected from the DFT calculation,is identified.We find that the non-equilibrium quasiparticle recovery rate is much smaller in the surface states than that of the bulk states.Our results demonstrate that the surface states can be distinguished from the bulk ones from a view of time scale in the nonequilibrium physics.
基金Project supported by the National Key Research and Development Program of China(Grant No.2016YFA0302001)the National Natural Science Foundation of China(Grant Nos.11574204 and 11774224)
文摘Transferring high-quality exfoliated graphene flakes onto different substrates while keeping the graphene free of polymer residues is of great importance, but at the same time very challenging. Currently, the only feasible way is the so-called all-dry "pick-and-lift" method, in which a hexagonal boron nitride(hBN) flake is employed to serve as a stamp to pick up graphene from one substrate and to lift it down onto another substrate. The transferred graphene samples, however,are always covered or encapsulated by hBN flakes, which leads to difficulties in further characterizations. Here, we report an improved "pick-and-lift" method, which allows ultra-clean graphene flakes to be transferred onto a variety of substrates without hBN coverage. Basically, by exploiting the superlubricity at the graphene/hBN stack interface, we are able to remove the top-layer hBN stamp by applying a tangential force and expose the underneath graphene.
基金the National Natural Science Foundation of China under Grant Nos.U1632102,11521404,11634009,11674222,11674226,11790313,11574202,11874256,11861161003 and 11874258the National Key Research and Development Program of China(Grant Nos.2016YFA0300403 and 2016YFA0301003)+1 种基金in part by the Key Research Program of the Chinese Academy of Science(Grant No.XDPB082)the Strategic Priority Research Program of Chinese Academy of Sciences(Grant No.XDB28000000).
文摘We investigate the adsorption of organic molecular semiconductor perylene on(7×7)reconstructed Si(111)surface by ultraviolet photoemission spectroscopy.It is observed that seven features that derive from the organic material are located at 0.71,2.24,4.0,5.9,7.46,8.65 and 9.95 eV in binding energy.The theoretical calculation results reveal the most stable adsorption geometry of organic molecule perylene on Si(111)(7×7)substrates is at the beginning of deposition.
基金Supported by the National Natural Science Foundation of China under Grant Nos 10874111,11304169,and 11174196
文摘We numerically study the thermodynamic properties of a hard ellipsoid fluid by mainly focusing on its phase transition from an isotropic phase into a nematic phase (i.e. isotropie-nematic phase transition). To improve the accuracy, precision, and efficiency of our computations, we attempt to employ the Wang-Landau NPT Monte Carlo algorithm in our simulations to calculate the function p(V) that gives the probability of arriving at the threshold density of the isotropic-nematic transition. Our results directly reveal that the nematic fluid phase, which is characterized by an ordered direction rather than an ordered configuration, appears and coexists with the isotropic phase when the aspect ratio a of the ellipsoid is located in a relatively narrow range of α = 2.0-2.25, and it becomes dominant and is fully established when α≥αcut = 2.25. We find that our estimated αcut is significantly lower than previously reported values of around 2.75. This prediction is further confirmed by the calculations of both the fluid reduced density and pressure of coexistence which show that the pressure grows up as the density increases and the probability function p(V) exhibits double peaks when the pressure enters the coexistence region. Based on these consistent results we are able to conclude that when α≥2.25 an ellipsoid fluid can fully display the nematic behavior. This study will place a useful and tight theoretical constraint on investigations of the isotropic-nematic phase transition in the future.
基金Project supported by the Natural Science Foundation of Shanghai,China(Grant No.13ZR1456800)Ph.D. Programs Foundation of Ministry of Education of China(Grant No.20120073110093)+1 种基金the National Natural Science Foundation of China(Grant Nos.11274229,11474198,61274083,61334008,11274229,11474198,11204175)DOE under DE-FG02-04ER46159
文摘Indacenodithiophene-co-benzothiadiazole(IDTBT) has emerged as one of the most exciting semiconducting polymers in recent years because of its high electronic mobility and charge transport along the polymer backbone. By using the recently developed ion gel gating technique we studied the charge transport of IDTBT at carrier densities up to 10^21cm^-3.While the conductivity in IDTBT was found to be enhanced by nearly six orders of magnitude by ionic gating, the charge transport in IDTBT was found to remain 3D Mott variable range hopping even down to the lowest temperature of our measurements, 12 K. The maximum mobility was found to be around 0.2 cm^2·V^-1·s^-1, lower than that of Cytop gated field effect transistors reported previously. We attribute the lower mobility to the additional disorder induced by the ionic gating.
基金supported by the Major State Basic Research Development Program of China(Grant No.2022YFB4200101)the Inner Mongolia Science and Technology Project,China(No.2022JBGS0036)the National Natural Science Foundation of China(Grant Nos.52325306,11834011,11974242,and 22025505).
文摘This article continues our highlights last two years[1,2]on the highest independently confirmed mainstream(silicon,perovskite,and organic)solar cell efficiencies in 2023.The world record power conversion efficiency(PCE)of the single-junction silicon and perovskite/silicon tandem solar cells has reached over 27%and close to 34%,respectively,in 2023.We have also witnessed the rapid mass-production development of the silicon passivating contact and silicon back contact solar cells,as well as further progress with perovskite solar cells(PSCs).
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11521404,12074248,11974243,and 11804194)additional support from a Shanghai talent program。
文摘Using high-resolution angle-resolved and time-resolved photoemission spectroscopy,we have studied the low-energy band structures in occupied and unoccupied states of three ternary compounds GeBi_(2)Te_(4),SnBi_(2)Te_(4) and Sn_(0.571)Bi_(2.286)Se_(4) near the Fermi level.In previously confirmed topological insulator GeBi_(2)Te_(4) compounds,we confirmed the existence of the Dirac surface state and found that the bulk energy gap is much larger than that in the first-principles calculations.In SnBi_(2)Te_(4) compounds,the Dirac surface state was observed,consistent with the first-principles calculations,indicating that it is a topological insulator.The experimental detected bulk gap is a little bit larger than that in calculations.In Sn_(0.571)Bi_(2.286)Se_(4) compounds,our measurements suggest that this nonstoichiometric compound is a topological insulator although the stoichiometric SnBi_(2)Se_(4) compound was proposed to be topological trivial.