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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.

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Abstract: The discovery of charge-density-wave-related effects in the resonant inelastic x-ray scattering spectra of cuprates holds the tantalizing promise of clarifying the interactions that stabilize the electronic order. Here, we report a comprehensive resonant inelastic x-ray scattering study of La 2 − x Sr x CuO 4 finding that charge-density wave effects persist up to a remarkably high doping level of x = 0.21 before disappearing at x = 0.25 . The inelastic excitation spectra remain essentially unchanged with doping despite crossing a topological transition in the Fermi surface. This indicates that the spectra contain little or no direct coupling to electronic excitations near the Fermi surface, rather they are dominated by the resonant cross section for phonons and charge-density-wave-induced phonon softening. We interpret our results in terms of a charge-density wave that is generated by strong correlations and a phonon response that is driven by the charge-density-wave-induced modification of the lattice.

Open Access

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Abstract: Understanding many-body physics of elementary excitations has advanced our control over material properties. Here, we study spin-flip excitations in NiO using Ni L3-edge resonant inelastic x-ray scattering (RIXS) and present a strikingly different resonant energy behavior between single and double spin-flip excitations. Comparing our results with single-site full-multiplet ligand field theory calculations we find that the spectral weight of the double-magnon excitations originates primarily from the double spin-flip transition of the quadrupolar RIXS process within a single magnetic site. Quadrupolar spin-flip processes are among the least studied excitations, despite being important for multiferroic or spin-nematic materials due to their difficult detection. We identify intermediate state multiplets and intra-atomic core-valence exchange interactions as the key many-body factors determining the fate of such excitations. RIXS resonant energy dependence can act as a convincing proof of existence of nondipolar higher-ranked magnetic orders in systems for which, only theoretical predictions are available.

Open Access

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Abstract: Electron correlations play a dominant role in the charge dynamics of the cuprates. We use resonant inelastic X-ray scattering (RIXS) to track the doping dependence of the collective charge excitations in electron doped La 2−x 2−x Ce x x CuO 4 4 (LCCO). From the resonant energy dependence and the out-of-plane momentum dependence, the charge excitations are identified as three-dimensional (3D) plasmons, which reflect the nature of the electronic structure and Coulomb repulsion on both short and long lengthscales. With increasing electron doping, the plasmon excitations increase monotonically in energy, a consequence of the electron correlation effect on electron structure near the Fermi surface (FS). Importantly, the plasmon excitations evolve from a broad feature into a well-defined peak with much increased life time, revealing the evolution of the electrons from incoherent states to coherent quasi-particles near the FS. Such evolution marks the reduction of the short-range electronic correlation, and thus the softening of the Mottness of the system with increasing electron doping.