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Abstract: Optimization of unconventional superconductivity involves a balance of interaction strengths. Precise determination of correlation strength across different material families is therefore important. Here, we present a combined X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) study of infinite-layer PrNiO2 and SrCuO2 that enables fair comparison of their interaction strengths. For both compounds, we study the orbital and magnetic excitations and extract their dispersions along high-symmetry directions. Using a single-band Hubbard model and including physically plausible assumptions about higher-order exchange interactions, we estimate the correlation factor U/t for both compounds. A key finding is that despite the prediction of a smaller Coulomb repulsion U, PrNiO2 exhibits a correlation strength U/t that is 20% stronger than that of its isostructural cuprate counterpart SrCuO2. This indicates that moderation of the correlation strength may further optimize superconductivity in nickelates.

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Abstract: Understanding the interplay between electronic, magnetic, and lattice degrees of freedom is essential for advancing two-dimensional (2D) van der Waals magnetic ma- terials toward next-generation spintronic and quantum technologies. The magnetic ordering of 2D magnetic compounds is often related to the electronic structure param- eters, such as spin orbit coupling, Hund’s coupling (JH ), p − d covalency, and inter- orbital Coulomb interactions. Accurately determining such parameters is paramount for understanding the physics of these compounds. This thesis presents a compre- hensive study of the electronic structure and low-energy excitations in 2D magnetic material families CrX3 (X = Cl, Br, and I), VX3 (X = Br and I), and CrSBr, using a combination of synchrotron based x-ray absorption spectroscopy, resonant inelas- tic x-ray scattering (RIXS), and state-of-the-art theoretical modeling, together with atomic multiplet simulations. High-resolution Cr L3−edge RIXS measurements on CrX3 enabled the first clear experimental energy separation between spin-allowed quartet states and spin-forbidden doublet states, which increases upon going from CrCl3 to CrI3. The refined crystal- field and Racah parameters revealed systematic trends in ligand-field strength and covalency from Cr to I. The RIXS measurements on VX3 revealed a sign opposition relation between the trigonal distortion parameters ∆ , hence an e′2 ground state in D3d g VBr and an e′1a1 ground state in VI , consistent with trigonal elongation and com- 3g1g 3 pression, respectively. Momentum-resolved RIXS measurements on CrSBr revealed dispersing multi-phonons along the crystallographic a axis, and only one phonon mode along the b axis, consistent with its quasi-1D behavior. These findings significantly advance fundamental understanding of vdW magnets and provide a strong foundation for future exploration of their potential in spintronics, optoelectronics, and quantum information technologies.

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Abstract: Geometrically frustrated lattices can display a range of correlated phenomena, ranging from spin frustration and charge order to dispersionless flat bands due to quantum interference. One particularly compelling family of such materials is the half-valence spinel LiB2O4 materials. On the B-site frustrated pyrochlore sublattice, the interplay of correlated metallic behavior and charge frustration leads to a superconducting state in LiTi2O4 and heavy fermion behavior in LiV2O4. To date, however, LiTi2O4 has primarily been understood as a conventional BCS superconductor despite a lattice structure that could host more exotic ground states. Here, we present a multimodal investigation of LiTi2O4, combining ARPES, RIXS, proximate magnetic probes, and ab-initio many-body theoretical calculations. Our data reveals a novel mobile polaronic ground state with spectroscopic signatures that underlie co-dominant electron-phonon coupling and electron-electron correlations also found in the lightly doped cuprates. The cooperation between the two interaction scales distinguishes LiTi2O4 from other superconducting titanates, suggesting an unconventional origin to superconductivity in LiTi2O4. Our work deepens our understanding of the rare interplay of electron-electron correlations and electron-phonon coupling in unconventional superconducting systems. In particular, our work identifies the geometrically frustrated, mixed-valence spinel family as an under-explored platform for discovering unconventional, correlated ground states.

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Abstract: Oxygen redox (OR) in Li1.2Ni0.13Co0.13Mn0.54O2 (LRNMC) and Na0.67Mg0.28Mn0.72O2 (NMMO) has been associated with the formation of embedded molecular O2 due to the appearance of their distinctive features in RIXS, while it is unclear whether OR also affects the oxygen left in the lattice. Here we use high-resolution oxygen K-edge resonant inelastic X-ray scattering (RIXS) at threshold excitation (527.5 – 529.5 eV) revealing lattice responses due to OR. We find that both cathodes show pronounced multiphonon progressions, which are either altered or activated upon charging. The first progression, with a fundamental energy loss ∼67-74 meV, matches the A1g lattice-oxygen mode observed by Raman spectroscopy, confirming its bulk origin. The second progression that exhibits a fundamental loss of ∼98 meV appears only at the highest state of charge and is resonant with the new pre-edge states at 527.5 eV. The latter mode emerges concurrently with the characteristic trapped-O2 RIXS signal and is strongly coupled to an occupied electronic band near the Fermi level, indicating that OR may not only contribute to the formation of molecular O2, but also perturbs lattice oxygen states, likely via polaron-forming oxidized lattice oxygen.

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Abstract: Mott-insulating 5d double perovskites (DPs) have recently emerged as strongly correlated electron systems of interest due to the novel multipolar physics they display. Key to understanding these properties is proper identification of the energy scales in the electronic Hamiltonian, for which resonant inelastic X-ray scattering (RIXS) is an ideal tool. In the A2MgReO6 (A = Ca, Sr, Ba) 5d1 DPs, Re L3 edge RIXS reveals that the 5d shell is split at a high level by the crystalline electric field (CEF) and at a low level by strong spin-orbit coupling (SOC). Unexpectedly, the SOC excitations are dressed by Jahn-Teller (JT) active phonon modes, implying the presence of a dynamic JT effect. This dynamic JT effect couples the electronic and lattice degrees of freedom, resulting in a spin-orbit-lattice entangled ground state. Vibronic RIXScalculations support these findings and estimate the dynamic JT effect to be one order of magnitude weaker than the SOC. Higher resolution O K edge RIXS measurements of the A2MgReO6 DPs are also presented, wherein the 5d1 ground state is probed via Re 5d - O 2p orbital hybridization. This higher resolution resolves the individual vibronic modes dressing the SOC excitations, and expanded temperature dependence measurements reveal systematic evolution of the dynamic JT effect. Significant enhancement of low energy phononic features is observed at the O K edge as predicted theoretically.Intermediate state effects are believed to imprint the spectra in O K edge RIXS due to the longer core-hole lifetime. In the 5d2 DPs Ba2YReO6 and Sr2CrReO6, Re L2 and L3 edge RIXS reveals that the 5d levels are split at a high level by the CEF as in the 5d1 case. The 5d2 and 5d1 cases diverge at lower energy where the 5d2 levels are split into several multiplets by the interplay of SOC and Hund’s coupling introduced by the presence of a second electron. Computational methods are used to successfully untangle the individual energy scales of the Hund’s and spin-orbit couplings. Differences between the L2 and L3 spectra reflective of dipole selection rules prove key to being able to unambiguously separate these energy scales.