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Abstract: Tetragonal CuMnAs is a room temperature antiferromagnet with an electrically reorientable Néel vector and a Dirac semimetal candidate. Direct measurements of the electronic structure of single-crystalline thin films of tetragonal CuMnAs using angle-resolved photoemission spectroscopy (ARPES) are reported, including Fermi surfaces (FS) and energy-wavevector dispersions. After correcting for a chemical potential shift of ≈− 390 meV (hole doping), there is excellent agreement of FS, orbital character of bands, and Fermi velocities between the experiment and density functional theory calculations. In addition, 2×1 surface reconstructions are found in the low energy electron diffraction (LEED) and ARPES. This work underscores the need to control the chemical potential in tetragonal CuMnAs to enable the exploration and exploitation of the Dirac fermions with tunable masses, which are predicted to be above the chemical potential in the present samples.

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Abstract: In this thesis, the static and dynamic properties of magnetic multilayer samples were studied using a variety of experimental techniques, based both at Exeter and the Diamond Light Source (DLS) and Advanced Light Source (ALS) synchrotron facilities. The exchange interaction, which acts to align spins, is a fundamental part in these magnetic multilayer samples. First, the glancing angle deposition (GLAD) technique was investigated as a tool for creating magnetic multilayers with exciting new exchange interactions. For this, Co thin films were grown by DC magnetron sputtering to tailor the magnetic anisotropy of the samples. These Co samples were structurally characterized using x-ray diffraction (XRD) and transmission electron microscopy (TEM). Vibrating sample magnetometry (VSM) was then performed to investigate the magnetic properties of the thin films as a result of the GLAD technique. From this, the necessary conditions for effective anisotropy control using the GLAD technique were identified. Synchrotron x-ray measurements, such as x-ray magnetic circular / linear dichroism (XMCD/XMLD) for static measurements, are vital for investigating magnetic multilayer samples with elemental resolution. To add depth-sensitivity to the synchrotron measurements, the idea of an ultra-thin Mn “spy layer” was investigated by inserting different thicknesses (tMn) of Mn into the NiFe layer in a FePt / NiFe bilayer. The effect on the static magnetic properties was studied using VSM and XMCD hysteresis loops before structural information was obtained using scanning transmission electron microscopy (STEM). The magnetization dynamics were probed using vector network analyzer ferromagnetic resonance (VNA-FMR) and element resolved x-ray ferromagnetic resonance (XFMR) measurements. From this, the ideal “spy layer” thickness of Mn was found to lie in the region 0Å < tMn < 5Å . Spin currents are a dynamic process found in magnetic multilayers and are driven by the exchange interaction. The measurement of a transverse charge current generated via the inverse spin Hall effect (ISHE) has become the principal technique for observing spin currents. During ISHE measurements, parasitic microwave effects were observed and a method to separate out the inverse spin Hall effect was identified. This method was then tested for a reference YIG / Pt bilayer. A more complex NiFe / NiO / Pd / FeCo sample was then studied using this procedure and the ISHE voltage was identified, despite the presence of additional parasitic effects. In addition to the DC spin current component, there is an AC spin current contribution. The AC spin current component was also investigated for the NiFe / NiO / Pd / FeCo sample series using XMCD, XMLD and XFMR measurements. The XMCD and XMLD data revealed the Ni and (Fe)Co spins possess perpendicular in-plane coupling relative to the magnetic moments within the NiO layer. To understand the magnetization dynamics in these samples, an evanescent spin wave model was invoked. This provides crucial insights for interpreting spin current propagation through NiO. Through the combination of work described above, new avenues for the fabrication of magnetic multilayers and the measurement of the magnetization dynamics in such systems are presented to yield a more complete understanding of the crucial role of the exchange interaction in the magnetization dynamics of magnetic multilayers.

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Abstract: Macroscopic scale hollow microcrystals are a promising group of materials for gas and liquid uptake as well as sensing. In this contribution we describe the structure of hollow hexagonal cross-section crystals formulated as salts of a silicon catecholate anion and a tetramethylenediamine (TEMED) cation. Using a combination of X-ray single crystal diffraction, Raman spectroscopy and quantum chemistry we explore the structural properties of the hollow microcrystals. With the X-ray structural data as a starting point and assisted with quantum chemistry we compute Raman tensors to fit polarisation sensitive spectral responses and predict the orientation and packing of unit cells in respect to the long and short axis of the synthesised microcrystals. Using these newly developed methods for predicting molecular Raman responses in space with dependence on local orientation, we present the quantitative analysis of experimental Raman images of both hexagonal and tetragonal cross section hollow microcrystals formed from silicon catecholate anions using different amines as counterions. We describe the distributions of chemical components at the surfaces and edges of microcrystals, address the effect of catcholate hydrophobicity on water uptake and discuss possible strategies in chemical and post-assembly modifications to widen the functional properties of this group of environmentally friendly silicon organic framework (SOF) materials.

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Abstract: Virulence gene expression in the human pathogen, S. aureus is regulated by the agr (accessory gene regulator) quorum sensing (QS) system which is conserved in diverse Gram-positive bacteria. The agr QS signal molecule is an autoinducing peptide (AIP) generated via the initial processing of the AgrD pro-peptide by the transmembrane peptidase AgrB. Since structural information for AgrB and AgrBD interactions are lacking, we used homology modelling and molecular dynamics (MD) annealing to characterise the conformations of AgrB and AgrD in model membranes and in solution. These revealed a six helical transmembrane domain (6TMD) topology for AgrB. In solution, AgrD behaves as a disordered peptide, which binds N-terminally to membranes in the absence and in the presence of AgrB. In silico, membrane complexes of AgrD and dimeric AgrB show non-equivalent AgrB monomers responsible for initial binding and for processing, respectively. By exploiting split luciferase assays in Staphylococcus aureus, we provide experimental evidence that AgrB interacts directly with itself and with AgrD. We confirmed the in vitro formation of an AgrBD complex and AIP production after Western blotting using either membranes from Escherichia coli expressing AgrB or with purified AgrB and T7-tagged AgrD. AgrB and AgrD formed stable complexes in detergent micelles revealed using synchrotron radiation CD (SRCD) and Landau analysis consistent with the enhanced thermal stability of AgrB in the presence of AgrD. Conformational alteration of AgrB following provision of AgrD was observed by small angle X-ray scattering from proteodetergent micelles. An atomistic description of AgrB and AgrD has been obtained together with confirmation of the AgrB 6TMD membrane topology and existence of AgrBD molecular complexes in vitro and in vivo.

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Abstract: The ability to control the structural properties of molecular layers is a key for the design and preparation of organic electronic devices. While microscopic growth studies of planar, rigid and symmetric π-conjugated molecules have been performed to a larger extent, this is less the case for elongated donor-acceptor molecules with flexible functional groups, which are particularly interesting due to their high dipole moments. Prototypical molecules of this type are merocyanines (MCs), which have been widely studied for the use as efficient absorbers in organic photodetectors. For maximized light absorption and optimized electronic properties the molecular arrangement which is affected by the initial assembly of the films at the supporting substrate interface is decisive. The situation deserves special attention, when the surface nucleation leads to so far not known and bulk-unlike aggregates. Here, we report on the growth of a typical MC (HB238) on the Ag(100) surface, serving as the substrate. In the energetically preferred phase, the molecules adsorb in a face-on geometry and organize in tetramers with a circular dipole arrangement. The tetramers further self-order in large, enantiopure domains with a periodicity that is commensurate to the Ag(100) surface, likely due to a specific bonding of the thiophene and thiazol rings to the Ag surface. Using scanning tunneling microscopy (STM) in combination with low energy electron diffraction we derive the detailed structure of the tetramers. The center of the tetramer, which is most prominent in STM images, consists of four upward pointing tert-butyl groups from four molecules. It is encircled by a ring of four hydrogen bonds between terminal CN-groups and thiophene rings on neighboring molecules. In parallel, the surface interaction modifies the intramolecular dipole, which is revealed from photoemission spectroscopy. Hence, this example shows how the surface template effect leads to an unforeseen molecular organization which is considerably more complex to that in the bulk phases of HB238, which feature paired dipoles.