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Abstract: In doped Hund's metals, such as the iron-based superconductors, effects like charge doping and chemical pressure are often considered the dominant factors. Partial chemical substitution, however, inevitably introduces disorder. Here, we investigate spin excitations in Ba(Fe1−𝑥⁢Cr𝑥)2As2 (CrBFA) by high-resolution resonant inelastic x-ray scattering for samples with 𝑥=0,0.035, and 0.085. In CrBFA, Cr acts as a hole dopant, but also introduces localized spins that compete with Fe-derived magnetic excitations. We found that the Fe-derived magnetic excitations are softened and damped, becoming overdamped for 𝑥=0.085. At this doping level, complementary angle-resolved photoemission spectroscopy measurements show increased electronic localization and a suppression of the nematic 𝑑𝑥⁢𝑧/𝑑𝑦⁢𝑧 band splitting present in the parent compound. We thus propose a localized spin model that explicitly incorporates substitutional disorder and Cr local moments, successfully reproducing our key observations. Our findings reveal a case where disorder dominates over charge doping in the case of a Hund's metal.

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Abstract: Epitaxial YBa 2 Cu3O7−δ/MgO thin films fabricated with pulsed laser deposition are grown as idealised epitaxial systems with a minimal number of different elements for X-ray absorption spectroscopy studies of irradiation damage. These films are characterised in terms of their superconducting performance, crystallinity, surface morphology, and Cu local environment. This reveals a structural heterogeneity of [100] oriented material, referred to as “a-axis grains”, decorating the desired [001] oriented phase of the thin film, coinciding with suppressed superconducting performance.

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Abstract: This study uses angle-resolved photoemission spectroscopy to examine the low-temperature electronic structure of Cs⁢(V0.95⁢Nb0.05)3⁢Sb5, demonstrating that partially substituting V atoms with isoelectronic Nb atoms results in an increase of the bandwidth and enhanced gap opening at the Dirac-like crossings due to the resulting chemical pressure. This increases the magnetic circular dichroism signal in the angular distribution compared to CsV3⁢Sb5, enabling detailed analysis of magnetic circular dichroism in several bands near the Fermi level. These results substantiate the predicted coupling of orbital magnetic moments to three van Hove singularities near the Fermi level at 𝑀 points. Previous studies have observed that Nb doping lowers the charge density transition temperature and increases the critical temperature for superconductivity. This article demonstrates that Nb doping concomitantly increases the magnetic circular dichroism signal attributed to orbital moments.

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Abstract: This thesis reports the synthesis and characterisation of various layered mixed transition metal chalcogenide and pnictide materials. The commonality between the studied systems is that they exhibit exotic magnetic phenomena, such as high- and low-Tc superconductivity and long-range magnetic order of the 3d transition metal moments. Chapter 3 details the effect of Ni substitution in ACo2Ch2 phases, where A = K, Rb, Cs, and Ch = S, Se. All of the Co-based phases show ferromagnetism, with the sole exception of antiferromagnetic CsCo2Se2, suggesting that larger interlayer separation favours antiferromagnetic order. On the other hand, all ANi2Ch2 phases show paramagnetism with a low-Tc superconducting transition below 2 K. Upon substitution of Ni for Co, a solid solution forms, which is evident from Synchrotron PXRD, and magnetometry measurements reveal a region of antiferromagnetic ordering, before the onset of paramagnetism at high substitution levels. These drastic changes in magnetism arise from the addition of an electron to the 3d band of the metallic ACo2Ch2 phases. The antiferromagnetic members of the KCo2−xNixCh2 (Ch = S, Se) series were studied by neutron powder diffraction, revealing A-type antiferromagnetism in x = 0.5 and 1 members, while the long-range magnetic order was not detected in x = 1.5 member. The refined magnetic model is highly anisotropic and is characterised by ferromagnetic coupling between Co/Ni ions within the (Co/Ni)2Se2 sheets and an antiferromagnetic interaction between the adjacent (Co/Ni)2Se2 sheets along the stacking axis. The mechanisms responsible for the different types of magnetic coupling are direct ferromagnetic interactions between transition metal ions within the plane, while the RKKY mechanism is responsible for the between-the-plane antiferromagnetic coupling, as the distance between the planes is too large (~5 Å) for any kind of direct interaction. PND of the CsCo2−xNixSe2 solid solution also showed A-type antiferromagnetism in its x = 1 member, and an intriguing structural distortion was found from Synchrotron PXRD for a small region of compositions between 1.1 ≤ x ≤ 1.4. The tetragonal symmetry of the parent phase is lowered to orthorhombic in these compositions. Chapters 4 and 5 detail the effect of chemical disorder and pressure, respectively, on the 1144 family of iron-based superconductors. CaKFe4As4 is subject to chemical doping with small amounts of Ca to introduce chemical disorder on one of its sublattices, however, the phase behaves as a line phase and its structural features do not allow any disorder on any of the sublattices. On the other hand, SrRbFe4As4 is subject to elevated pressures in a high-pressure Synchrotron PXRD experiment. It is found that no lattice collapse, driven by formation of As-As bonds, occurs between 0-26 GPa, as was reported in the case in related compounds. Chapter 6 details the high-pressure synthesis of LaNixBi2, a low-Tc superconductor with a very small superconducting volume fraction. It is found that superconductivity likely has an extrinsic origin, as even a highly pure LaNixBi2 shows minimal superconducting volume fraction and its superconducting features, including Tc and Hc1 are coincidental with that of Ni-Bi secondary phases.

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Abstract: Layered oxide materials, with their two-dimensional crystalline architectures and tunable interlayer interaction, serve as a fertile field for harnessing emergent quantum phenomena. Among these materials, metallic delafossites (e.g., PdCoO2) have emerged as a prominent system with extraordinary two-dimensional electronic properties, though their intrinsic lack of ferromagnetism has remained a fundamental constraint. Here, we report the creation of robust, bulk high-temperature ferromagnetism (𝑇𝑐>420  K) in inherently nonmagnetic PdCoO2 through controlled hydrogenation while preserving the delafossite structure. This process induces layer-selective electron doping into CoO2 layers, stabilizing Ising-type ferromagnetism with pronounced perpendicular magnetic anisotropy while preserving the material’s exceptional metallicity. Remarkably, the system self-assembles into a superlattice of alternating metallic Pd and insulating ferromagnetic hydrogenated CoO2 layers, enabling an unconventional anomalous Hall effect mediated by interlayer spin-charge coupling. These findings demonstrate that bulk ferromagnetism can be achieved in delafossite oxides while preserving their structural integrity, positioning hydrogenated delafossites as a versatile platform for exploring correlated quantum effects and designing multifunctional devices.