Study of charge ordered materials using polarization dependent x-ray absorption and emission spectroscopy

Transition metal compounds often display a wide range of properties, which are typically caused by strong electron correlations. With magnetic, atomic and electronic transitions leading to novel quantum states, these materials have become the focus of many studies. High temperature superconductivity (SC), Mott states and metal-insulator transitions have all been intensely researched,

especially since it is also the case that these states can be tuned by chemical doping, allowing an enhancement of certain properties. The strong interplay between the atomic and electronic structures in many of these materials can led to difficulties in separating the different degrees of freedom. The understanding of how the different states can manifest and the mechanisms which may drive their emergence has become an important research field since many of these properties have important applications.

X-ray absorption and emission spectroscopy (XAS/XES) are element selective techniques which can probe both the electronic state and local structure around the absorbing species. In this thesis conventional XAS, high energy resolution fluorescence detected XAS (HERFD-XAS) and valence-to-core XES (vtc-XES) measurements are the techniques employed to study the transitions in 3 compounds. In all cases, polarization dependent measurements have been used to track changes along different crystallographic planes, since these systems are typically highly anisotropic. Combined with additional simulations of the electronic structure, these techniques are shown to be successful in providing further information regarding the nature of the transitions investigated.

The aim of this thesis is to investigate the atomic and electronic transitions in 3 materials to provide insight into the critical factors that drive the states that can emerge. Measurements have also been conducted once their properties were tuned by chemical doping, to further understand the factors that drive the transitions. Additionally this thesis aims to further develop these techniques. An investigation using the Lβ emission lines with vtc-XES to probe the occupied states is presented. While in recent years the vtc technique has been used in the study of materials with 3d elements with light ligands, very little literature is available on the heavier metals/ligands where the Lβ emission is most applicable. A procedure for providing a correction for the self-absorption effect is also presented. Due to the highly localized nature of the d orbitals in transitions metals, numerical

instabilities often affect the standard correction procedures. This issue is successfully addressed by additionally taking into account the background fluorescence intensity. The role of iridium dimers and the electronic state of this species in the 5d transition metal IrTe2 is investigated in relation to a metal to insulator transition. Robust evidence shows that the

Ir dimers are present above and below this transition and that the electronic structure of Ir does not undergo a significant change. The presence of high temperature disordered dimers is inconsistent with the majority of the current literature and these results suggest that changes in the Te species electronic and atomic structure is the most critical factor in the metal to partial insulator transition. Additionally it is known that when the system is doped with platinum, a low temperature SC phase can emerge. This research shows that above this phase, the nature of the material is the same with respect to the parent compound. Two competing scenarios have been presented for the metal to insulator transition in the 4d transition metal Ca2RuO4.

While it is clear that a Mott-type mechanism makes the 2/3 filled t2g band insulating, the nature of this transition and the role played by the orbital degree of freedom is unclear. One scenario suggests that the transition is orbital selective and that it only affects the xy band, which becomes metallic. The second scenario suggests that it is not orbital selective and the transition is assisted by the crystal field splitting. The investigation presented in this thesis provides evidence that there is a charge transfer between the non-degenerate t2g bands, where below this transition the xy band become full. While recent studies have suggested that in the metallic phase all 3 bands are conducting, the current investigation shows how the interplay between the atomic and electronic structures is an important factor. These results are further assisted by the study of this material once doped with lanthanum, where the metal to insulator transition is suppressed. The doping has the affect of increasing the t2g bandwidth, and shows how the atomic and electronic transitions are closely linked. The iron pnictide (NaFeAs) system shows both a structural and magnetic transition along with a orbital/spin nematic state, which once doped can display bulk SC. The original aim of this research was to investigate the nematic state, where the origin of this transition is unclear but theoretically linked to both spin and orbital degrees of freedom. Due to degradation of the samples used, this investigation was not successful in providing information regarding the nematic state. However the results do suggest that the FeAs4 structure is critical for the emergence of the different phases and small distortions are detrimental to the transitions.