Dr Shuqiu Wang

Abstract:

Planar oxide atomic clusters are of considerable scientific interest because of their potential for enhanced catalytic activity versus their three-dimensional counterparts. This enhancement is the result of the substrate stabilizing novel planar configurations that have an extensive periphery where catalytic reactions can occur. A class of planar Nb_nO_m atomic clusters that are synthesized by the evaporation of metallic Nb onto an Au(111) substrate in an ultrahigh vacuum environment and subsequent oxidation at elevated temperatures is reported. The atomic structures of the clusters are determined using a combination of scanning tunneling microscopy and density functional theory. The clusters are composed of structural units with four-, five-, and sixfold rotational symmetry and these units can assemble to form larger planar clusters. The theoretical comparison of supported structures with their hypothetical freestanding counterparts shows that the atomic and electronic structures of the oxide clusters are significantly altered by the interaction with the Au substrate. The substrate effects include interfacial charge transfer and structural relaxation to relieve the strain in the Nb-O bonds. The substrate interactions also reduce the energy differences between clusters of different configurations and this enables the coexistence of a large variety of cluster configurations.

Abstract:

Grain boundaries (GBs) are ubiquitous in solids. Their description is critical for understanding polycrystalline materials and explaining their mechanical and electrical properties. A GB in a 2D material can be described as a line defect and its atomic structures have been intensively studied in materials such as graphene. These GBs accommodate the relative rotation of two neighboring grains by incorporating periodic units consisting of nonhexagonal rings along the boundary. Zero-degree GBs, called domain boundaries (DBs), where there is only a lattice offset between two grains without any rotation, are rare in 2D van-der-Waals (vdW) bonded materials where the grains can easily move. However, this movement is not possible in 2D materials that have a strong epitaxial relationship with their substrate such as the M2O3 (2 × 2) honeycomb monolayers on noble metal (111) supports. Involving experimental and theoretical investigations, four main DBs are observed here in a monolayer of Ti2O3 supported on Au(111) and their atomic structures are solved. The DB formation energies explain why some DBs are more frequently observed than others. The strong epitaxial constraint from the Au(111) substrate stabilizes some unique Ti2O3 monolayer DB structures that are not observed in vdW-bonded 2D materials.

Abstract:

An unidentified quantum fluid designated the pseudogap (PG) phase is produced by electron-density depletion in the CuO2 antiferromagnetic insulator. Current theories suggest that the PG phase may be a pair density wave (PDW) state characterized by a spatially modulating density of electron pairs. Such a state should exhibit a periodically modulating energy gap ΔP(r) in real-space, and a characteristic quasiparticle scattering interference (QPI) signature ΛP(q) in wavevector space. By studying strongly underdoped Bi2Sr2CaDyCu2O8 at hole-density ~0.08 in the superconductive phase, we detect the 8a0-periodic ΔP(r) modulations signifying a PDW coexisting with superconductivity. Then, by visualizing the temperature dependence of this electronic structure from the superconducting into the pseudogap phase, we find the evolution of the scattering interference signature Λ(q) that is predicted specifically for the temperature dependence of an 8a0-periodic PDW. These observations are consistent with theory for the transition from a PDW state coexisting with d-wave superconductivity to a pure PDW state in the Bi2Sr2CaDyCu2O8 pseudogap phase.