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Abstract: Using x-ray photoelectron diffraction (XPD) and angle-resolved photoemission spectroscopy, we study photoemission intensity changes related to changes in the geometric and electronic structure in the kagome metal CsV3⁢Sb5 upon transition to an unconventional charge density wave (CDW) state. The XPD patterns reveal the presence of a chiral atomic structure in the CDW phase. Furthermore, using circularly polarized x-rays, we have found a pronounced nontrivial circular dichroism in the angular distribution of the valence band photoemission in the CDW phase, indicating a chirality of the electronic structure. This observation is consistent with the proposed orbital loop current order. In view of a negligible spontaneous Kerr signal in recent magneto-optical studies, the results suggest an antiferromagnetic coupling of the orbital magnetic moments along the 𝑐 axis. While the inherent structural chirality may also induce circular dichroism, the observed asymmetry values seem to be too large in the case of the weak structural distortions caused by the CDW.

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Abstract: Strongly correlated electron systems display a complex interplay of structural, magnetic, and electronic degrees of freedom, leading to a variety of emergent phenomena. Utilising neutron and X-ray scattering techniques alongside a group-theoretical framework, I have developed and applied a unified approach to characterise this interplay in a selection of non-model 3d transition metal oxides, with a particular focus on magneto-structural coupling. I first investigate a spin reorientation transition in BiCrO₃, solving its magnetic structures and determining Dzyaloshinskii-Moriya interactions responsible for weak ferromagnetism in each phase. The interactions are demonstrated to couple spin to respective octahedral rotations and antiferroelectric distortions by a phenomenological model. I then present a study on PrMn₇O₁₂, where magneto-elastic coupling and a canted ground state are elucidated by neutron powder diffraction. A mean field model that captures the influence of competing exchange interactions on the complex ferrimagnetic ordering is developed that, together with the results, extends our understanding of magnetism in the A³⁺Mn₇O₁₂ quadruple perovskite manganites. Turning to BiMn₇O₁₂, I determine a highly canted magnetic phase composed of polar E-type antiferromagnetic order superposed on a ferrimagnetic order. Modelling reveals the coupling of the polar antiferromagnetism to ferroelectric distortions, exemplifying a novel category of type I multiferroics with inverse exchange-striction. I then explore the Cu-doped BiMn₇O₁₂ systems in the half-metallic limit, employing a host of complementary experimental techniques to characterise their structural, magnetic order, disorder and transport properties. The study reveals a diverse set of behaviours across the solid solutions, marked by coexisting short and long range magnetic correlations, spin freezing transitions, near zero thermal expansion and magneto-resistance. Lastly, in a resonant X-ray study on CoTi₂O₅, I demonstrate near-complete antiferromagnetic domain switching under uniaxial stress, exploiting the spin Jahn-Teller to relieve its perfectly frustrated exchange topology.

Open Access

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Abstract: The physical properties of magnetic topological materials are strongly influenced by their nontrivial band topology coupled with the magnetic structure. Co3Sn2S2 is a ferromagnetic kagome Weyl semimetal displaying giant intrinsic anomalous Hall effect which can be further tuned via elemental doping, such as Ni substitution for Co. Despite significant interest, the exact valency of Co and the magnetic order of the Ni dopants remained unclear. Here, we report a study of Ni-doped Co3Sn2S2 single crystals using synchrotron-based X-ray magnetic circular dichroism (XMCD), X-ray photoelectron emission microscopy (XPEEM), and hard/soft X-ray photoemission spectroscopy (XPS) techniques. We confirm the presence of spin-dominated magnetism from Co in the host material, and also the establishment of ferromagnetic order from the Ni dopant. The oxygen-free photoemission spectrum of the Co 2p core levels in the crystal well resembles that of a metallic Co film, indicating a Co0+ valency. Surprisingly, we find the electron filling in the Co 3d state can reach 8.7–9.0 electrons in these single crystals. Our results highlight the importance of element-specific X-ray spectroscopy in understanding the electronic and magnetic properties that are fundamental to a heavily studied Weyl semimetal, which could aid in developing future spintronic applications based on magnetic topological materials.

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Abstract: Phase transitions and competing orders in strongly correlated materials emerge from the delicate interplay of many interacting degrees of freedom (including charge, spin, and lattice). This intricate interplay makes these systems highly sensitive to external perturbations, making strongly correlated materials ideal for developing novel technologies and devices leveraging emergent phenomena. Their richness and technological potential, however, are counterbalanced by an inherent complexity originating from the strong intertwining of many degrees of freedom. In the first part of this thesis, we study non-equilibrium phases in prototypical Mott insulators (LaVO3 and V2O3) induced by means of ultrashort light pulses or application of current, with the aim of tackling open challenges in developing strategies for controlling quantum materials. When light excitations are employed, quantum coherence could be exploited to achieve enhanced functionalities and ultrafast and reversible manipulation of material properties. The light-induced excitonic population and decoherence dynamics is investigated in LaVO3, where long-range orders in the orbital and spin degrees of freedom strongly influence optical excitations and the evolution of excitonic states. By means of broadband pump-probe and two-dimensional electronic spectroscopy (2DES), we study how the interactions of the LaVO3 excitonic resonance with the ordered background influence the exciton spectral linewidth and decoherence time. When current is instead employed to control the phase of Mott materials, resistive switching - a sudden drop in resistance caused by a transition from an insulating to a metallic state - can take place. By combining transport measurements with Photo-Electron Emission Microscopy, we image the resistive switching process in V2O3 at the nanoscale. On this length scale, V2O3 displays spatial inhomogeneities resulting from the breaking of the crystal symmetry upon transitioning from the high-temperature metallic phase to the low-temperature insulating one. This experiment provides novel insights into the nature and mechanisms of resistive switching, as well as the role of the nanometric texture of the material, suggesting novel viable routes to control the current-induced insulator-metal transition. The second part of the thesis is dedicated to the quantum simulation of the physics of strongly correlated materials using artificial platforms. This approach aims to overcome the inherent complexity of quantum materials by employing systems where the phenomena typical of correlated systems can take place in a controlled way, with the relevant parameters that can be tuned on demand. We introduce synthetic lattices composed of lead halide perovskite nanocubes, which we propose as a suitable novel platform for quantum simulations. Pump-probe experiments on CsPbBr3 nanocube superlattices reveal the emergence of several phases relevant for strongly correlated materials (collective superradiant state, exciton gas and electron-hole liquid phases) that can be accessed upon controlling the excitation intensity, thus making the system a suitable platform for the investigation of long-range ordered phases in systems displaying insulator-metal Mott transitions. Nanocube superlattices of the hybrid organic-inorganic compound CH(NH2)2PbI3 are also investigated; 2DES is employed to trace the evolution of optical excitons in this artificial lattice, measure their decoherence time and address how the decoherence process is affected by the structural phase transition taking place in the system.

Open Access

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Abstract: The synthesis of large, freestanding, single-atom-thick two-dimensional (2D) metallic materials remains challenging due to the isotropic nature of metallic bonding. Here, we present a bottom-up approach for fabricating macroscopically large, nearly freestanding 2D gold (Au) monolayers, consisting of nanostructured patches. By forming Au monolayers on an Ir(111) substrate and embedding boron (B) atoms at the Au/Ir interface, we achieve suspended monoatomic Au sheets with hexagonal structures and triangular nanoscale patterns. Alternative patterns of periodic nanodots are observed in Au bilayers on the B/Ir(111) substrate. Using scanning tunneling microscopy, X-ray spectroscopies, and theoretical calculations, we reveal the role of buried B species in forming the nanostructured Au layers. Changes in the Au monolayer’s band structure upon substrate decoupling indicate a transition from 3D to 2D metal bonding. The resulting Au films exhibit remarkable thermal stability, making them practical for studying the catalytic activity of 2D gold.