The kagome lattice of transition metal atoms provides an exciting platform to study electronic correlations in the presence of geometric frustration and nontrivial band topology1-18, which continues ...to bear surprises. Here, using spectroscopic imaging scanning tunnelling microscopy, we discover a temperature-dependent cascade of different symmetry-broken electronic states in a new kagome superconductor, CsV3Sb5. We reveal, at a temperature far above the superconducting transition temperature Tc ~ 2.5 K, a tri-directional charge order with a 2a0 period that breaks the translation symmetry of the lattice. As the system is cooled down towards Tc, we observe a prominent V-shaped spectral gap opening at the Fermi level and an additional breaking of the six-fold rotational symmetry, which persists through the superconducting transition. This rotational symmetry breaking is observed as the emergence of an additional 4a0 unidirectional charge order and strongly anisotropic scattering in differential conductance maps. The latter can be directly attributed to the orbital-selective renormalization of the vanadium kagome bands. Our experiments reveal a complex landscape of electronic states that can coexist on a kagome lattice, and highlight intriguing parallels to high-Tc superconductors and twisted bilayer graphene.
•The straight single oligomers formed on Au(111) surface.•The zipper-like feature of double straight chains.•Typical cross-coupling forms and structure models.
The dehalogenative homocoupling of DBTP ...molecules on Au(111) surface has been investigated at a single molecular level by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The length and morphology of the carbon-carbon coupling chain are adjusted by stepwise thermal excitation. Gradual annealing of the sample at 323 and 353 K promotes the dehalogenative homocoupling, resulting in the formation of oligomer and long straight chains that branch out or form combs. Such straight chains show an obvious zipper-like double-chain structure when annealed at 373 K. Our results show that Br-H bonds between straight chains play a stabilizing role in the zipper structure. During further annealing to a maximum of 453 K, both dehalogenative homocoupling and cross-coupling occur between the chains. The number of monomers in the molecular chains formed at 453 K is 16–20, accounting for the largest proportion.
The dehalogenative homocoupling of DBTP molecules on Au(111) surface investigated at a single molecular level by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations Display omitted
The electronic properties of heterostructures of atomically thin van der Waals crystals can be modified substantially by moiré superlattice potentials from an interlayer twist between crystals
. ...Moiré tuning of the band structure has led to the recent discovery of superconductivity
and correlated insulating phases
in twisted bilayer graphene (TBG) near the 'magic angle' of twist of about 1.1 degrees, with a phase diagram reminiscent of high-transition-temperature superconductors. Here we directly map the atomic-scale structural and electronic properties of TBG near the magic angle using scanning tunnelling microscopy and spectroscopy. We observe two distinct van Hove singularities (VHSs) in the local density of states around the magic angle, with an energy separation of 57 millielectronvolts that drops to 40 millielectronvolts with high electron/hole doping. Unexpectedly, the VHS energy separation continues to decrease with decreasing twist angle, with a lowest value of 7 to 13 millielectronvolts at a magic angle of 0.79 degrees. More crucial to the correlated behaviour of this material, we find that at the magic angle, the ratio of the Coulomb interaction to the bandwidth of each individual VHS (U/t) is maximized, which is optimal for electronic Cooper pairing mechanisms. When doped near the half-moiré-band filling, a correlation-induced gap splits the conduction VHS with a maximum size of 6.5 millielectronvolts at 1.15 degrees, dropping to 4 millielectronvolts at 0.79 degrees. We capture the doping-dependent and angle-dependent spectroscopy results using a Hartree-Fock model, which allows us to extract the on-site and nearest-neighbour Coulomb interactions. This analysis yields a U/t of order unity indicating that magic-angle TBG is moderately correlated. In addition, scanning tunnelling spectroscopy maps reveal an energy- and doping-dependent three-fold rotational-symmetry breaking of the local density of states in TBG, with the strongest symmetry breaking near the Fermi level and further enhanced when doped to the correlated gap regime. This indicates the presence of a strong electronic nematic susceptibility or even nematic order in TBG in regions of the phase diagram where superconductivity is observed.
Scanning tunneling microscope (STM) has presented a revolutionary methodology to nanoscience and nanotechnology. It enables imaging of the topography of surfaces, mapping the distribution of ...electronic density of states, and manipulating individual atoms and molecules, all at atomic resolutions. In particular, atom manipulation capability has evolved from fabricating individual nanostructures toward the scalable production of the atomic-sized devices bottom-up. The combination of precision synthesis and in situ characterization has enabled direct visualization of many quantum phenomena and fast proof-of-principle testing of quantum device functions with immediate feedback to guide improved synthesis. Several representative examples are reviewed to demonstrate the recent development of atomic-scale manipulation, focusing on progress that addresses quantum properties by design in several technologically relevant materials systems. Integration of several atomically precisely controlled probes in a multiprobe STM system vastly extends the capability of in situ characterization to a new dimension where the charge and spin transport behaviors can be examined from mesoscopic to atomic length scale. The automation of atomic-scale manipulation and the integration with well-established lithographic processes further push this bottom-up approach to a new level that combines reproducible fabrication, extraordinary programmability, and the ability to produce large-scale arrays of quantum structures.
We provide insight into surface-catalyzed dehalogenative polymerization, analyzing the organometallic intermediate and its evolution into planar polymeric structures. A combined study using scanning ...tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and first-principles calculations unveils the structural conformation of substrate-bound phenylene intermediates generated from 1,4-dibromobenzene precursors on Cu(110), showing the stabilizing role of the halogen. The appearance of covalently bonded conjugated structures is followed in real time by fast-XPS measurements (with an acquisition time of 2 s per spectrum and heating rate of 2 K/s), showing that the detaching of phenylene units from the copper substrate and subsequent polymerization occur upon annealing above 460 ± 10 K.
A minimalistic hydrocarbon nanocar was synthesized, deposited on a Au(111) surface, and driven by a STM tip thanks to electrostatic interactions. This nanomachine was able to win the Nanocar Race II ...championship, as reported by Jonas Björk, Emilio M. Pérez, David Écija et al. in their Research Article (e202212395).
Bilayer graphene can be modified by rotating (twisting) one layer with respect to the other. The interlayer twist gives rise to a moiré superlattice that affects the electronic motion and alters the ...band structure
. Near a 'magic angle' of twist
, where the emergence of a flat band causes the charge carriers to slow down
, correlated electronic phases including Mott-like insulators and superconductors were recently discovered
by using electronic transport. These measurements revealed an intriguing similarity between magic-angle twisted bilayer graphene and high-temperature superconductors, which spurred intensive research into the underlying physical mechanism
. Essential clues to this puzzle, such as the symmetry and spatial distribution of the spectral function, can be accessed through scanning tunnelling spectroscopy. Here we use scanning tunnelling microscopy and spectroscopy to visualize the local density of states and charge distribution in magic-angle twisted bilayer graphene. Doping the sample to partially fill the flat band, we observe a pseudogap phase accompanied by a global stripe charge order that breaks the rotational symmetry of the moiré superlattice. Both the pseudogap and the stripe charge order disappear when the band is either empty or full. The close resemblance to similar observations in high-temperature superconductors
provides new evidence of a deeper link underlying the phenomenology of these systems.
Kagome metals AV3Sb5 ( A = K , Rb, and Cs) exhibit intriguing superconductivity below 0.9 ∼ 2.5 K , a charge density wave (CDW) transition around 80 ∼ 100 K, and Z2 topological surface states. The ...nature of the CDW phase and its relation to superconductivity remains elusive. In this work, we investigate the electronic and structural properties of CDW by first-principles calculations. We reveal an inverse Star of David deformation as the 2 × 2 × 2 CDW ground state of the kagome lattice. The kagome lattice shows softening breathing-phonon modes, indicating the structural instability. However, electrons play an essential role in the CDW transition via Fermi surface nesting and van Hove singularity. The inverse Star of David structure agrees with recent experiments by scanning tunneling microscopy (STM). The CDW phase inherits the nontrivial Z2-type topological band structure. Further, we find that the electron-phonon coupling is too weak to account for the superconductivity Tc in all three materials. It implies the existence of unconventional pairing of these kagome metals. Our results provide essential knowledge toward understanding the superconductivity and topology in kagome metals.
•Electrochemical scanning tunnelling microscopy under oxygen reduction and evolution conditions on graphite surfaces.•In-situ identification of active sites.•ORR active sites are exclusively found at ...step and defective sites.•OER active sites are dependent on the applied potential.•The stability of the carbon surface under OER conditions is unaffected at lower potentials.
Due to their availability and electrochemical versatility, carbon-based electrodes are becoming an increasingly popular option as electrocatalysts for fuel cells and metal-air batteries. Additionally, they show great potential as bifunctional catalysts for the oxygen reduction and evolution reactions (ORR/OER) in an alkaline medium. However, to compete with state-of-the-art catalysts, the nature of the active sites and the surface stability under reaction conditions need to be understood in depth. Here, we present a principle study on highly oriented pyrolytic graphite (HOPG), evaluating the surface behavior under both ORR and OER conditions in 0.1 M KOH. We use noise analysis in electrochemical scanning tunneling microscopy (n-EC-STM) to monitor and compare ORR and OER active sites with resolution down to the nanoscale. Furthermore, surface degradation can be evaluated during the operation. We find that close to the respective reaction onset, step sites and defects are active for both ORR and OER. Terraces sites are largely inactive and only become involved in the OER at higher potentials. This could imply corrosion of the carbon. However, since the observed surface structures remain unaltered before and after applying the OER in our experiments, we find no clear evidence of surface destruction. These fundamental insights could inspire further research concerning the active sites and stability of carbon-based catalysts as well as carbon support structures, to discover ways to tune the surface activity and stability to the dedicated purpose.
Display omitted
The chemical inertness of the defect-free basal plane confers environmental stability to MoS2 single layers, but it also limits their chemical versatility and catalytic activity. The stability of ...pristine MoS2 basal plane against oxidation under ambient conditions is a widely accepted assumption however, here we report single-atom-level structural investigations that reveal that oxygen atoms spontaneously incorporate into the basal plane of MoS2 single layers during ambient exposure. The use of scanning tunnelling microscopy reveals a slow oxygen-substitution reaction, during which individual sulfur atoms are replaced one by one by oxygen, giving rise to solid-solution-type 2D MoS2−xOx crystals. Oxygen substitution sites present all over the basal plane act as single-atom reaction centres, substantially increasing the catalytic activity of the entire MoS2 basal plane for the electrochemical H2 evolution reaction.
PDF
