Julian Ceddia

Two‐dimensional (2D) materials with frustrated crystal geometries can host strongly correlated electrons, potentially leading to a range of exotic many‐body quantum phases, such as Mott insulators, quantum spin‐liquids, and Kondo lattices. The ability to control exchange‐coupling within these systems is therefore highly desirable. Here, we use an atomically sharp scanning tunneling microscope probe to vertically manipulate a 2D Mott insulating honeycomb‐kagome metal‐organic framework (MOF) featuring Kondo‐screened local magnetic moments on Ag(111). We show that by controlling the adsorption height of the MOF, we can also controllably and reversibly change the strength of Kondo coupling between the MOF's local spins and the substrate's conduction electrons. This mechanical control of Kondo coupling could be extended to other forms of interlayer exchange coupling, potentially allowing for atomic‐scale design or control of spintronics technologies.

X-ray Absorption Spectroscopy (XAS) is the mainstay for nanoscale structural studies of amorphous, crystalline, solid, and solution samples. However, X-ray Spectroscopy from phase lacks a spectroscopic measurement technique. We describe and implement novel holographic spectroscopy for the first simultaneous determination of both the amplitude (absorption) and phase spectra, which together we refer to as the Complex Atomic Fine Structure (CAFS). This pioneering work lays the foundation for X-ray Phase Spectroscopy and X-ray CAFS which enables the application of X-ray Spectroscopy to a wide range of low absorbing samples such as ultra-thin films, functional and bio-materials, and for studying material structure at the nano-scale. Our first results explicitly measure CAFS around the K-edges of copper and iron. The precision of the results is excellent, with a very well-defined oscillatory structure in both absorption and phase. Both phase and amplitude are in good agreement with published literature, yet their simultaneous measurement reveals new science and challenges. This raises important questions about new structural information in the phase of CAFS hitherto unseen and unknown. The new technique opens exciting opportunities for X-ray spectroscopy, promising a new tool for structural studies of low-absorbing samples, nanostructures, thin films, functional materials, and biological samples. It is well-suited as a diagnostic to give new insight for development and building of nanomaterials, and for advanced physics and chemistry for molecules and solids, crystalline or non-crystalline. It has impact on nanochemistry, biochemistry and nanoengineering. We discuss major consequences and opportunities for future growth.

Scanning Tunnelling Microscopes (STM) are capable of capturing images of surfaces with atomic-scale resolution. This is achieved by scanning an atomically sharp probe across the surface of the sample while monitoring an electric current. However, the quality of STM data relies heavily on the atomic-scale geometry and composition of the scanning probe apex, as well as the roughness and cleanliness of the scanned region. For instance, blunt tips result in blurry images while contaminated tips can lead to noisy images due to interactions with the sample. As a result, optimal STM data acquisition commonly requires time-consuming tasks such as probe conditioning-i.e., sharpening via "tip-shaping", where the apex of the probe can be refined by poking it into a clean metal surface-and identification of areas of interest of the sample. Moreover, the quality of the probe can vary during a scan, especially when scanning over debris or excessively rough areas, necessitating additional tip-shaping.

Electron-electron interactions in materials lead to exotic many-body quantum phenomena, including Mott metal-insulator transitions (MITs), magnetism, quantum spin liquids, and superconductivity. These phases depend on electronic band occupation and can be controlled via the chemical potential. Flat bands in two-dimensional (2D) and layered materials with a kagome lattice enhance electronic correlations. Although theoretically predicted, correlated-electron Mott insulating phases in monolayer 2D metal-organic frameworks (MOFs) with a kagome structure have not yet been realised experimentally. Here, we synthesise a 2D kagome MOF on a 2D insulator. Scanning tunnelling microscopy (STM) and spectroscopy reveal a MOF electronic energy gap of ∼200 meV, consistent with dynamical mean-field theory predictions of a Mott insulator. Combining template-induced (via work function variations of the substrate) and STM probe-induced gating, we locally tune the electron population of the MOF kagome bands and induce Mott MITs. These findings enable technologies based on electrostatic control of many-body quantum phases in 2D MOFs.