Charge-density wave mediated quasi-one-dimensional Kondo lattice in stripe-phase monolayer 1T-NbSe2.

The heavy fermion physics is dictated by subtle competing exchange interactions, posing a challenge to their understanding. One-dimensional (1D) Kondo lattice model has attracted special attention in theory, because of its exact solvability and expected unusual quantum criticality. However, such experimental material systems are extremely rare. Here, we demonstrate the realization of quasi-1D Kondo lattice behavior in a monolayer van der Waals crystal NbSe2, that is driven into a stripe phase via Se-deficient line defects. Spectroscopic imaging scanning tunneling microscopy measurements and first-principles calculations indicate that the stripe-phase NbSe2 undergoes a novel charge-density wave transition, creating a matrix of local magnetic moments. The Kondo lattice behavior is manifested as a Fano resonance at the Fermi energy that prevails the entire film with a high Kondo temperature. Importantly, coherent Kondo screening occurs only in the direction of the stripes. Upon approaching defects, the Fano resonance exhibits prominent spatial 1D oscillations along the stripe direction, reminiscent of Kondo holes in a quasi-1D Kondo lattice. Our findings provide a platform for exploring anisotropic Kondo lattice behavior in the monolayer limit.

Altering Spin Distribution of Tb2Pc3 via Molecular Chirality Manipulation.

Manipulating the chirality of the spin-polarized electronic state is pivotal for understanding many unusual quantum spin phenomena, but it has not been achieved at the single-molecule level. Here, using scanning tunneling microscopy and spectroscopy (STM/STS), we successfully manipulate the chirality of spin distribution in a triple-decker single-molecule magnet tris(phthalocyaninato)bis(terbium(III)) (Tb2Pc3), which is evaporated on a Pb(111) substrate via molecular beam epitaxy. The otherwise achiral Tb2Pc3 becomes chiral after being embedded into the self-assembled monolayer films of bis(phthalocyaninato)terbium(III) (TbPc2). The chirality of the spin distribution in Tb2Pc3 is manifested via the spatial mapping of its Kondo resonance state from its ligand orbital. Our first-principles calculations revealed that the spin and molecular chirality are associated with a small rotation followed by a structural distortion of the top Pc, consistent with the experimental observation. By constructing tailored molecular clusters with the STM tip, a single Tb2Pc3 molecule can be manipulated among achiral and differently handed chiral configurations of spin distributions reversibly. This paves the way for designing chiral spin enantiomers for fundamental studies and developing functional spintronic devices.

Moiré Enhanced Two-Band Superconductivity in a MnTe/NbSe2 Heterojunction.

Recent advances in creating moiré periods of two-dimensional heterostructures enable diverse and compatible tunability to modulate the conventional proximity effect involving superconductivity, magnetism, and topology. Here, by constructing a MnTe/NbSe2 heterojunction via molecular beam epitaxy growth, we report on a moiré-enhanced multiband superconductivity by low-temperature scanning tunneling microscopy/spectroscopy measurements. We observe a distinct double-gap superconducting spectrum on monolayer MnTe that is absent on the NbSe2 substrate. The subgap character exhibits a moiré-related oscillation in real space, which can be well described by an effective two-band model. The restored two-gap feature and its rapid suppression under a small magnetic field are speculated to be mediated by the moiré superlattice, which is closely related to the enhanced interband coupling strength of quasiparticle scattering. Our work paves the way for engineering proximitized properties of heterostructures by a moiré landscape with spatial modulations.

Manipulating single excess electrons in monolayer transition metal dihalide.

Polarons are entities of excess electrons dressed with local response of lattices, whose atomic-scale characterization is essential for understanding the many body physics arising from the electron-lattice entanglement, yet difficult to achieve. Here, using scanning tunneling microscopy and spectroscopy (STM/STS), we show the visualization and manipulation of single polarons in monolayer CoCl2, that are grown on HOPG substrate via molecular beam epitaxy. Two types of polarons are identified, both inducing upward local band bending, but exhibiting distinct appearances, lattice occupations and polaronic states. First principles calculations unveil origin of polarons that are stabilized by cooperative electron-electron and electron-phonon interactions. Both types of polarons can be created, moved, erased, and moreover interconverted individually by the STM tip, as driven by tip electric field and inelastic electron tunneling effect. This finding identifies the rich category of polarons in CoCl2 and their feasibility of precise control unprecedently, which can be generalized to other transition metal halides.

Manipulating Hubbard-type Coulomb blockade effect of metallic wires embedded in an insulator.

Correlated states have emerged in low-dimensional systems owing to enhanced Coulomb interactions. Elucidating these states requires atomic-scale characterization and delicate control capabilities. Herein, spectroscopic imaging-scanning tunneling microscopy was employed to investigate the correlated states residing in 1D electrons of the monolayer and bilayer MoSe2 mirror twin boundary (MTB). The Coulomb energies, determined by the wire length, drive the MTB into two types of ground states with distinct respective out-of-phase and in-phase charge orders. The two ground states can be reversibly converted through a metastable zero-energy state with in situ voltage pulses, which tune the electron filling of the MTB via a polaronic process, substantiated by first-principles calculations. Our Hubbard model calculation with an exact diagonalization method reveals the ground states as correlated insulators from an on-site U-originated Coulomb interaction, dubbed the Hubbard-type Coulomb blockade effect. Our study lays a foundation for understanding and tailoring correlated physics in complex systems.

Spin-Resolved Imaging of Antiferromagnetic Order in Fe4 Se5 Ultrathin Films on SrTiO3.

Unraveling the magnetic order in iron chalcogenides and pnictides at atomic scale is pivotal for understanding their unconventional superconducting pairing mechanism, but is experimentally challenging. Here, by utilizing spin-polarized scanning tunneling microscopy, real-space spin contrasts are successfully resolved to exhibit atomically unidirectional stripes in Fe4 Se5 ultrathin films, the plausible closely related compound of bulk FeSe with ordered Fe-vacancies, which are grown by molecular beam epitaxy. As is substantiated by the first-principles electronic structure calculations, the spin contrast originates from a pair-checkerboard antiferromagnetic ground state with in-plane magnetization, which is modulated by a spin-lattice coupling. These measurements further identify three types of nanoscale antiferromagnetic domains with distinguishable spin contrasts, which are subject to thermal fluctuations into short-ranged patches at elevated temperatures. This work provides promising opportunities in understanding the emergent magnetic order and the electronic phase diagram for FeSe-derived superconductors.

Self-Intercalated 1T-FeSe2 as an Effective Kagome Lattice.

In kagome lattice, with the emergence of Dirac cones and flat band in electronic structure, it provides a versatile ground for exploring intriguing interplay among frustrated geometry, topology and correlation. However, such engaging interest is strongly limited by available kagome materials in nature. Here we report on a synthetic strategy of constructing kagome systems via self-intercalation of Fe atoms into the van der Waals gap of FeSe2 via molecular beam epitaxy. Using low-temperature scanning tunneling microscopy, we unveil a kagome-like morphology upon intercalating a 2 × 2 ordered Fe atoms, resulting in a stoichiometry of Fe5Se8. Both the bias-dependent STM imaging and theoretical modeling calculations suggest that the kagome pattern mainly originates from slight but important reconstruction of topmost Se atoms, incurred by the nonequivalent subsurface Fe sites due to the intercalation. Our study demonstrates an alternative approach of constructing artificial kagome structures, which envisions to be tuned for exploring correlated quantum states.

Spin-orbital Yu-Shiba-Rusinov states in single Kondo molecular magnet.

Studies of single-spin objects are essential for designing emergent quantum states. We investigate a molecular magnet Tb2Pc3 interacting with a superconducting Pb(111) substrate, which hosts unprecedented Yu-Shiba-Rusinov (YSR) subgap states, dubbed spin-orbital YSR states. Upon adsorption of the molecule on Pb, the degeneracy of its lowest unoccupied molecular orbitals (LUMO) is lifted, and the lower LUMO forms a radical spin via charge transfer. This leads to Kondo screening and subgap states. Intriguingly, the YSR states display two pairs of resonances with clearly distinct behavior. The energy of the inner pair exhibits prominent inter and intra molecular variation, and it strongly depends on the tip height. The outer pair, however, shifts only slightly. As is unveiled through theoretical calculations, the two pairs of YSR states originate from the ligand spin and charge-fluctuating higher LUMO, coexisting in a single molecule, but only weakly coupled presumably due to different spatial distribution. Our work paves the way for understanding complex many-body excitations and constructing molecule-based topological superconductivity.

Planar Heterojunction of Ultrathin CrTe3 and CrTe2 van der Waals Magnet.

The fabrication of planar heterojunctions with magnetic van der Waals ultrathin crystals is essential for constructing miniaturized spintronic devices but is yet to be realized. Here, we report the growth of CrTe3 and CrTe2 ultrathin films with molecular beam epitaxy and characterize their morphological and electronic structure through low-temperature scanning tunneling microscopy/spectroscopy. The former is identified as a Mott insulator, and the latter has shown a robust magnetic order previously. Through vacuum annealing, CrTe3 can be transformed into CrTe2, whose relative ratio is controlled via the annealing time. This renders the feasibility of constructing CrTe3-CrTe2 planar heterojunctions, which express atomically sharp interfaces and smooth band bending. We also identified a superstructure conceivably formed via hybrid units of CrTe3 and CrTe2, whose electronic structure exhibits stunning tunability with the length of the superstructure. Our study sets a foundation for the development of magnetic tunneling junctions for building spintronic circuits and engineering electronic states in artificial superlattice structures.

Spin mapping of intralayer antiferromagnetism and field-induced spin reorientation in monolayer CrTe2.

Intrinsic antiferromagnetism in van der Waals (vdW) monolayer (ML) crystals enriches our understanding of two-dimensional (2D) magnetic orders and presents several advantages over ferromagnetism in spintronic applications. However, studies of 2D intrinsic antiferromagnetism are sparse, owing to the lack of net magnetisation. Here, by combining spin-polarised scanning tunnelling microscopy and first-principles calculations, we investigate the magnetism of vdW ML CrTe2, which has been successfully grown through molecular-beam epitaxy. We observe a stable antiferromagnetic (AFM) order at the atomic scale in the ML crystal, whose bulk is ferromagnetic, and correlate its imaged zigzag spin texture with the atomic lattice structure. The AFM order exhibits an intriguing noncollinear spin reorientation under magnetic fields, consistent with its calculated moderate magnetic anisotropy. The findings of this study demonstrate the intricacy of 2D vdW magnetic materials and pave the way for their in-depth analysis.

Atomic Visualization and Switching of Ferroelectric Order in ß-In2 Se3 Films at the Single Layer Limit.

2D ferroelectrics have received wide interest due to the remarkable quantum states of emerging physics at reduced dimensionality, associated with their exotic properties in high-performance and nonvolatile functional devices. Here, by combing molecular beam epitaxy synthesis and scanning tunneling microscopy characterization, two metastable phases of layered In2 Se3 films: ß'- and ß*-In2 Se3 are reported, which develop different types of in-plane spontaneous polarizations, thus resulting in different striped morphologies. The anti-ferroelectric order in ß'-In2 Se3 and ferroelectric order of ß*-In2 Se3 are identified, respectively, down to the 2D limit by comprehensive investigations of structural and spectroscopic signatures, including the lattice distortion, the spatial profile of images, the formation of domain structure, and the electronic band-bending by polarization charges at edges. The ferroelectric switching between those two phases are further controlled via applying an electric field generated from the scanning tunneling microscopy tip in a reversible manner. The intriguing tunability between the (anti-)ferroelectric orders in the 2D limit provides a promising platform for studying the interplay between electronic structure and ferroelectricity in van der Waals materials, and promotes potential development of miniaturized transistors and memory devices based on electric polarizations.

Charge Transfer Gap Tuning via Structural Distortion in Monolayer 1T-NbSe2.

The Mott state in 1T-TaS2 is predicted to host quantum spin liquids (QSLs). However, its insulating mechanism is controversial due to complications from interlayer coupling. Here, we study the charge transfer state in monolayer 1T-NbSe2, an electronic analogue to TaS2 exempt from interlayer coupling, using spectroscopic imaging scanning tunneling microscopy and first-principles calculations. Monolayer NbSe2 surprisingly displays two types of star of David (SD) motifs with different charge transfer gap sizes, which are interconvertible via temperature variation. In addition, bilayer 1T-NbSe2 shows a Mott collapse by interlayer coupling. Our calculation unveils that the two types of SDs possess distinct structural distortions, altering the effective Coulomb energies of the central Nb orbital. Our calculation suggests that the charge transfer gap, the same parameter for determining the QSL regime, is tunable with strain. This finding offers a general strategy for manipulating the charge transfer state in related systems, which may be tuned into the potential QSL regime.

Strain-Induced Bandgap Enhancement of InSe Ultrathin Films with Self-Formed Two-Dimensional Electron Gas.

Atomically thin indium selenide (InSe) is a representative two-dimensional (2D) family that have recently attracted extensive interest for their intriguing emerging physics and potential optoelectronic applications with high-performance. Here, by utilizing molecular beam epitaxy and scanning tunneling microscopy, we report a controlled synthesis of InSe thin films down to the monolayer limit and characterization of their electronic properties at atomic scale. Highly versatile growth conditions are developed to fabricate well crystalline InSe films, with a reversible and controllable phase transformation between InSe and In2Se3. The band gap size of InSe films, as enhanced by quantum confinement, increases with decreasing film thickness. Near various categories of lattice imperfections, the band gap becomes significantly enlarged, resulting in a type-I band alignments for lateral heterojunctions. Such band gap enhancement, as unveiled from our first-principles calculations, is ascribed to the local compressive strain imposed by the lattice imperfections. Moreover, InSe films host highly conductive 2D electron gas, manifesting prominent quasiparticle scattering signatures. The 2D electron gas is self-formed via substrate doping of electrons, which shift the Fermi level above the confinement-quantized conduction band. Our study identifies InSe ultrathin film as an appealing system for both fundamental research and potential applications in nanoelectrics and optoelectronics.

Realization of AlSb in the Double-Layer Honeycomb Structure: A Robust Class of Two-Dimensional Material.

Exploring two-dimensional (2D) van der Waals (vdW) systems is at the forefront of materials of physics. Here, through molecular beam epitaxy on graphene-covered SiC(0001), we report successful growth of AlSb in the double-layer honeycomb (DLHC) structure, a 2D vdW material which has no direct analogue to its 3D bulk and is predicted to be kinetically stable when freestanding. The structural morphology and electronic structure of the experimental 2D AlSb are characterized with spectroscopic imaging scanning tunneling microscopy and cross-sectional imaging scanning transmission electron microscopy, which compare well to the proposed DLHC structure. The 2D AlSb exhibits a band gap of 0.93 eV versus the predicted 1.06 eV, which is substantially smaller than the 1.6 eV of bulk. We also attempt the less-stable InSb DLHC structure; however, it grows into bulk islands instead. The successful growth of a DLHC material here demonstrates the feasibility for the realization of a large family of 2D DLHC traditional semiconductors with characteristic excitonic, topological, and electronic properties.

Observation of short-range Yu-Shiba-Rusinov states with threefold symmetry in layered superconductor 2H-NbSe2.

Yu-Shiba-Rusinov (YSR) states arise when magnetic impurities interact with superconductivity. The intricacy of coupling and the nature of the superconductivity determine the behavior of the YSR state, whose detailed correlations are not yet fully understood. Here, we study the YSR state of a single Fe adatom on the surface of 2H-NbSe2 with combined low temperature scanning tunneling microscopy/spectroscopy, density functional theory calculations and tight-binding modeling. It is found that the Fe adatom occupies the hollow site of the Se surface layer. A prominent YSR state close to the Fermi level is observed. The YSR state exhibits a threefold symmetry along the diagonal direction of the Se lattice. The spatial decay of the YSR state follows a behavior in three-dimensional superconductivity. This behavior contrasts with a previous study of imbedded Fe impurities, whose YSR state shows a six-fold symmetry and a two-dimensional long-range decay. According to our theoretical modeling, the coupling configurations affect the adatom-substrate hopping and the interlayer coupling of the substrate. Both factors are crucial for the consequent behavior of the YSR state.

Heterostructures of tellurium on NbSe2 from sub-monolayer to few-layer films.

As a single-elemental system, tellurium can exist stably in the form of layers with an intriguing multivalence character, which constructs a new member of the 2D family. However, the growth and electronic structure of tellurium films are still far from known at present. Here, combined with molecular beam epitaxy, scanning tunneling microscopy/spectroscopy measurements and density functional theory calculations, we report the geometric and electronic structures of tellurium grown on NbSe2 from sub-monolayer to few-layer films. At the sub-monolayer coverage, we obtain two types of adatom-induced ordered superstructures that are strongly coupled with NbSe2. With the increase in coverage, the few-layer tellurium films adopt the α-phase form, showing internal strain-induced ripple patterns in the few-layers and bulk-like in thick layers with distinct edge geometries. The band gap of α-tellurium films decreases with the increase in thickness, which is associated with notable in-gap states. These observations, corroborated with DFT calculations, emphasize the important role of the NbSe2 substrate in modulating the structural and electronic properties of tellurium films. Moreover, the interaction between tellurium adatoms and tellurium films leads to √2 × âˆš2 surface reconstruction prior to a new monolayer, conforming to our theoretical calculations. Our work clarifies the kinetic growth of tellurium films on NbSe2 and reveals the tunability of electronic properties via substrate modulation or surface decoration.

Dimensional Crossover and Topological Phase Transition in Dirac Semimetal Na3Bi Films.

Three-dimensional (3D) topological Dirac semimetal, when thinned down to 2D few layers, is expected to possess gapped Dirac nodes via quantum confinement effect and concomitantly display the intriguing quantum spin Hall (QSH) insulator phase. However, the 3D-to-2D crossover and the associated topological phase transition, which is valuable for understanding the topological quantum phases, remain unexplored. Here, we synthesize high-quality Na3Bi thin films with √3 × âˆš3 reconstruction on graphene and systematically characterize their thickness-dependent electronic and topological properties by scanning tunneling microscopy/spectroscopy in combination with first-principles calculations. We demonstrate that Dirac gaps emerge in Na3Bi films, providing spectroscopic evidence of dimensional crossover from a 3D semimetal to a 2D topological insulator. Importantly, the Dirac gaps are revealed to be of sizable magnitudes on three and four monolayers (72 and 65 meV, respectively) with topologically nontrivial edge states. Moreover, the Fermi energy of a Na3Bi film can be tuned via a certain growth process, thus offering a viable way for achieving charge neutrality in transport. The feasibility of controlling Dirac gap opening and charge neutrality enables realizing intrinsic high-temperature QSH effect in Na3Bi films and achieving potential applications in topological devices.

Unusual Electronic States and Superconducting Proximity Effect of Bi Films Modulated by a NbSe2 Substrate.

Heterostructures of two-dimensional layered materials can be functionalized with exotic phenomena that are unpresented with each constituting component. The interface effect plays a key role in determining the electronic properties of the heterostructure, whose characterization requires a correlation with the morphology with atomic-scale precision. Here, we report an investigation on the electronic properties of few-layer Bi(110) films mediated by a NbSe2 substrate. By utilizing scanning tunneling microscopy and spectroscopy, we show a significant variation of the density of states at different Bi film thicknesses, resulting in an unusual superconducting proximity effect that deviates from the conventional monotonous decay behavior. Moreover, the electronic states of the Bi films are also prominently modulated by the Moiré pattern spatially. With first-principles calculations, we illuminate these findings as the results of covalent-like quasi-bonds formed at the Bi/NbSe2 interface, which profoundly alter the charge distributions in the Bi films. Our study indicates a viable way of modulating the electronic properties of ultrathin films by quasi-covalent interfacial couplings beyond conventional van der Waals interactions.

Observation of topological states residing at step edges of WTe2.

Topological states emerge at the boundary of solids as a consequence of the nontrivial topology of the bulk. Recently, theory predicts a topological edge state on single layer transition metal dichalcogenides with 1T' structure. However, its existence still lacks experimental proof. Here, we report the direct observations of the topological states at the step edge of WTe2 by spectroscopic-imaging scanning tunneling microscopy. A one-dimensional electronic state residing at the step edge of WTe2 is observed, which exhibits remarkable robustness against edge imperfections. First principles calculations rigorously verify the edge state has a topological origin, and its topological nature is unaffected by the presence of the substrate. Our study supports the existence of topological edge states in 1T'-WTe2, which may envision in-depth study of its topological physics and device applications.Two-dimensional topological insulators support edge conduction electrons but its realization in real materials is rare. Here, Peng et al. report the direct observation of topological states at the step edge of WTe2.

Observation of Zeeman effect in topological surface state with distinct material dependence.

Manipulating the spins of the topological surface states represents an essential step towards exploring the exotic quantum states emerging from the time reversal symmetry breaking via magnetic doping or external magnetic fields. The latter case relies on the Zeeman effect and thereby we need to estimate the g-factor of the topological surface state precisely. Here, we report the direct observations of the Zeeman effect at the surfaces of Bi2Se3 and Sb2Te2Se by spectroscopic-imaging scanning tunnelling microscopy. The Zeeman shift of the zero mode Landau level is identified unambiguously by appropriately excluding the extrinsic effects arising from the nonlinearity in the band dispersion of the topological surface state and the spatially varying potential. Surprisingly, the g-factors of the topological surface states in Bi2Se3 and Sb2Te2Se are very different (+18 and -6, respectively). Such remarkable material dependence opens up a new route to control the spins of the topological surface states.