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Abstract: Magnetic two-dimensional materials are a promising platform for novel nano-electronic device architectures. One such layered crystal is the ferromagnetic semiconductor chromium germanium telluride (Cr2Ge2Te6) which recently attracted interest due to its potential for spintronics and memory applications. Here we investigate its properties from the structural standpoint using atomic resolution Scanning Transmission Electron Microscopy (STEM) and present the first atomic resolution images down to its monolayer limit. We develop a novel technique that allows one to map the local tilt with unprecedented spatial resolution using only high-resolution images, enabling mapping of the topography and morphological variation of atomically thin crystals. Using it, we show that the Cr2Ge2Te6 monolayer has an unusually large out-of-plane rippling, with local tilt variation reaching 20° over few nm length scales. We hypothesize that such a strongly buckled structure originates from both point and extended lattice defects which are more prevalent in monolayer crystals. In addition, we correlate the structural observations with the band structure measurements using Angle-Resolved Photoemission Spectroscopy (ARPES). We believe that both the atomic scale insights we have gained on Cr2Ge2Te6 and our novel approach to nanoscale topography mapping will benefit the development of van der Waals heterostructures in both fundamental and applied research.

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Abstract: The 4Hb polytype of TaS2 is a natural heterostructure of H and T-type layers. Intriguing recent evidence points towards a possibly chiral superconducting ground state, unlike the superconductivity found in other polytypes where the T layers are absent, requiring understanding of the possible contributions of electrons from the T layers. Here we use micro-focused angle resolved photoemission spectroscopy to reveal that the T termination of the 4Hb structure is metallic, but a subsurface T layer - seen below an H termination and thus more representative of the bulk case - is gapped. The results imply a complete charge transfer of 1 electron per 13 Ta from the T to adjacent H layers in the bulk, but an incomplete charge transfer at the T termination, yielding a metallic Fermi surface with a planar-chiral character. A similar metallic state is found in an anomalous region with likely T-H-H’ stacking at the surface. Our results exclude cluster Mott localisation in either the bulk or surface of 4Hb-TaS2 and point to a scenario of superconductivity arising from Josephson-like tunneling between the H layers.

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Abstract: This study uses angle-resolved photoemission spectroscopy to examine the low-temperature electronic structure of Cs⁢(V0.95⁢Nb0.05)3⁢Sb5, demonstrating that partially substituting V atoms with isoelectronic Nb atoms results in an increase of the bandwidth and enhanced gap opening at the Dirac-like crossings due to the resulting chemical pressure. This increases the magnetic circular dichroism signal in the angular distribution compared to CsV3⁢Sb5, enabling detailed analysis of magnetic circular dichroism in several bands near the Fermi level. These results substantiate the predicted coupling of orbital magnetic moments to three van Hove singularities near the Fermi level at 𝑀 points. Previous studies have observed that Nb doping lowers the charge density transition temperature and increases the critical temperature for superconductivity. This article demonstrates that Nb doping concomitantly increases the magnetic circular dichroism signal attributed to orbital moments.

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Abstract: Strong electron correlations drive Mott insulator transitions. Yet, there exists no framework to classify Mott insulators by their degree of correlation. Cuprate superconductors, with their tunable doping and rich phase diagrams, offer a unique platform to investigate the evolution of these interactions. However, spectroscopic access to a clean half-filled Mott-insulating state is lacking in compounds with the highest superconducting onset temperature. To fill this gap, we introduce a pristine, half-filled thallium-based cuprate system, Tl2Ba5Cu4Ox. Using high-resolution resonant inelastic x-ray scattering, we probe long-lived magnon excitations and uncover a pronounced kink in the magnon dispersion, marked by a simultaneous change in group velocity and lifetime broadening. Modeling the dispersion within a Hubbard-Heisenberg approach, we extract the interaction strength and compare it with other cuprate systems. Our results establish a cuprate universal relation between electron-electron interaction and magnon zone-boundary dispersion. Superconductivity seems to be optimal at intermediate correlation strength, suggesting an optimal balance between localization and itinerancy.

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Abstract: Advances in microelectronics have long relied on increasing integration density, but tran- sistor scaling is now hitting physical limits while the rise of artificial intelligence demands higher speed and energy e!ciency. This, combined with bottlenecks from separating mem- ory and computation, has driven interest in alternative architectures such as phase change memory (PCM) devices, which promise in-memory computing yet still face scalability and reliability challenges due to incomplete knowledge of their switching mechanisms. This thesis explores the layered van der Waals material 1T-TaS2, a compound known for its rich charge density wave (CDW) phases and ability to undergo electrical or optical switching into a metastable metallic "hidden" state. Due to its low dimensionality and strong electron correlations, 1T -TaS2 holds great promise for cryogenic memory devices that are fast, scalable, and energy e!cient, as it relies on charge reordering rather than charge movement. However, reliable switching requires a clear understanding of both in- and out- of-plane electronic and structural reordering. This thesis presents a set of experimental tools and methodologies aimed at visualizing and understanding phase transitions in 1T- TaS2 devices under operating conditions. To do so, we developed a novel fabrication strategy for in-situ cleaving of 1T-TaS2 devices in ultra-high vacuum environments. This allowed us to perform spatially resolved surface-sensitive angle-resolved photoemission spectroscopy (ARPES) while simultaneously measuring in-situ electrical transport. This approach led to the design of a new device geometry tailored for ARPES compatibility and the observation of a charge density wave gap down to 2 nm thin flakes. By tracking the density of states at the Fermi level across regions on and between electrodes, we uncovered a potential hint of increased metallicity of the device. To complement the surface-sensitive ARPES data, we performed temperature-dependent, bulk-sensitive X-ray di"raction (XRD) measurements, allowing us to monitor the evolution of CDW domain sizes across equilibrium phase transitions by means of Lorentzian peak fitting which we also complement using the Hendricks-Teller method applied to out-of- plane structure factor simulations. These measurements revealed sharp anomalies in both correlation lengths and peak intensities, providing a sensitive fingerprint of phase transi- tions. This sensitivity highlights the value of bulk-sensitive probes in understanding phase behavior beyond transport measurements. Finally, we combined spatially resolved XRD, X-ray fluorescence, and in-situ transport techniques to investigate the hidden metallic state non-destructively and in volume in op- erational devices. This unique combination enabled us to image and locate a reordering of the CDW stacking, including a breaking of dimerized out-of-plane layers, and observe local- ized lattice contractions in the switching region. These structural fingerprints, confined to specific regions within the device, represent a step toward non-destructive imaging of active switching channels in bulk phase-change materials that allows for control of van der Waals layer stacking and precise positioning of electrodes for next generation memory devices.