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Abstract: In the search for high energy density cathodes for next-generation lithium-ion batteries, the disordered rocksalt oxyfluorides are receiving significant attention due to their high capacity and lower voltage hysteresis compared with ordered Li-rich layered compounds. However, a deep understanding of these phenomena and their redox chemistry remains incomplete. Using the archetypal oxyfluoride, Li2MnO2F, we show that the oxygen redox process in such materials involves the formation of molecular O2 trapped in the bulk structure of the charged cathode, which is reduced on discharge. The molecular O2 is trapped rigidly within vacancy clusters and exhibits minimal mobility unlike free gaseous O2, making it more characteristic of a solid-like environment. The Mn redox process occurs between octahedral Mn3+ and Mn4+ with no evidence of tetrahedral Mn5+ or Mn7+. We furthermore derive the relationship between local coordination environment and redox potential; this gives rise to the observed overlap in Mn and O redox couples and reveals that the onset potential of oxide ion oxidation is determined by the degree of ionicity around oxygen, which extends models based on linear Li–O–Li configurations. This study advances our fundamental understanding of redox mechanisms in disordered rocksalt oxyfluorides, highlighting their promise as high capacity cathodes.

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Abstract: Motivated by the recent synthesis of Ba CuO (BCO), a high temperature superconducting cuprate with putative ground state symmetry, we investigated its electronic structure by means of Cu x-ray absorption (XAS) and resonant inelastic x-ray scattering (RIXS) at the Cu edge on a polycrystalline sample. We show that the XAS profile of BCO is characterised by two peaks associated to inequivalent Cu sites, and that its RIXS response features a single, sharp peak associated to crystal-field excitations. We argue that these observations are only partially compatible with the previously proposed crystal structure of BCO. Based on our spectroscopic results and on previously published powder diffraction measurements, we propose a crystalline structure characterized by two inequivalent Cu sites located at alternated planes along the axis: nominally trivalent Cu(1) belonging to very short Cu-O chains, and divalent Cu(2) in the oxygen deficient CuO planes. We also analyze the low-energy region of the RIXS spectra to estimate the magnitude of the magnetic interactions in BCO and find that in-plane nearest neighbor superexchange exceeds 120 meV, similarly to that of other layered cuprates. Although these results do not support the pure ground state scenario, they hint at a significant departure from the common quasi-2D electronic structure of superconducting cuprates of pure symmetry.

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Abstract: Upon progressive refinement of energy resolution, the conventional resonant inelastic X-ray scattering (RIXS) instrumentation reaches the limit where the bandwidth of incident photons becomes insufficient to deliver an acceptable photon-count rate. Here it is shown that RIXS spectra as a function of energy loss are essentially invariant to their integration over incident energies within the core-hole lifetime. This fact permits RIXS instrumentation based on the hv2-concept to utilize incident synchrotron radiation over the whole core-hole lifetime window without any compromise on the much finer energy-loss resolution, thereby breaking the photon-count limit.

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Abstract: Copper oxide high-TC superconductors possess a number of exotic orders that coexist with or are proximal to superconductivity. Quantum fluctuations associated with these orders may account for the unusual characteristics of the normal state, and possibly affect the superconductivity1,2,3,4. Yet, spectroscopic evidence for such quantum fluctuations remains elusive. Here, we use resonant inelastic X-ray scattering to reveal spectroscopic evidence of fluctuations associated with a charge order5,6,7,8,9,10,11,12,13,14 in nearly optimally doped Bi2Sr2CaCu2O8+δ. In the superconducting state, while the quasielastic charge order signal decreases with temperature, the interplay between charge order fluctuations and bond-stretching phonons in the form of a Fano-like interference increases, an observation that is incompatible with expectations for competing orders. Invoking general principles, we argue that this behaviour reflects the properties of a dissipative system near an order–disorder quantum critical point, where the dissipation varies with the opening of the pseudogap and superconducting gap at low temperatures, leading to the proliferation of quantum critical fluctuations, which melt charge order.

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Abstract: This thesis presents new high-resolution resonant inelastic x-ray scattering (RIXS) studies in high-Tc cuprate superconductors (HTSs). RIXS measures elementary excitations of the type which characterise the ordered phases known to exist alongside superconductivity in the cuprates. The role that these phases play in the pairing mechanism which gives rise to superconductivity remains unclear. The work in this thesis therefore presents new measurements of the excitations with the aim of helping to constrain theoretical models of pairing. Spin fluctuations are characteristic of the antiferromagnetic phase of cuprates but are also seen to persist into superconducting phases. As a result, theories based on the Hubbard model often consider spin fluctuations to be important to pairing. The role of charge fluctuations, either competing or contributing to superconductivity, is also important to understand. The experimental work was performed at two new RIXS spectrometers, I21 at Diamond Light Source and ID32 at the European Synchrotron Radiation Facility, both optimised to achieve high intensity and energy resolution in the soft x-ray RIXS regime. Measurements are presented on single layer cuprate La2CuO4 where consideration of the prefactors to the RIXS intensity reveals the wavevector dependence of the dynamical susceptibility in new regions of the Brillouin zone. The measurements are extended to two doped compositions of La2-xSrxCuO4, x = 0.12 and 0.16. The spin fluctuations are characterised within a damped harmonic oscillator model which reveals the wavevector dependent anisotropy of the damping in doped compositions. New RIXS measurements in two-layer YBa2Cu3O6-x (YBCO) show similar effects. The charge order is also examined in YBCO revealing a broad wavevector dependent peak in the elastic intensity. This work is also a test of RIXS as a probe of low-energy excitations in well studied cuprate materials and provides insights into the RIXS intensity by comparison with a similar probe: inelastic neutron scattering.