HAXPES on Transition Metal Oxides:: new insights into an effective use of the photoionization cross-sections

Materials containing transition metals and rare earth elements have continued to attract attention due to many fascinating properties that emerge from the intricate interplay between the electron correlation effects, which arise from the strong Coulomb interactions often present in d and f orbitals, with the band formations in the periodic structure of the solids. The mathematical description of such systems, however, is highly complex and results in unsolvable sets of equations. Thus, an appropriate model must be chosen on each case and tested. Experimental input is thus needed as a verification, and also as guidance to make better models. Amongst the wide range of experimental techniques available to determine the electronic structure, photoelectron spectroscopy is special due to the close relation between the spectra that is measured with these techniques and the one particle Green's functions, providing very direct information content. Photoelectron spectroscopy is a very well established experimental technique, but when used on bulk materials, it can have one major issue: the surface sensitivity. The electronic structure of the surface is not the same as in the bulk, and in correlated systems, these differences can lead into a major alteration of the electronic structure due to the delicate balance of different interaction strengths, and so it is crucial that spectra representative of the bulk can be obtained. The most reliable way in which the surface contributions can be minimised for photoelectron spectroscopy is by using its high photon energy variant also known as Hard X-ray Photoelectron Spectroscopy (HAXPES). HAXPES is more bulk sensitive and also has several further advantages with respect to its lower energy counterparts such as the possibility to measure buried interfaces, reach deeper core levels, or greater polarisation dependence effects, to name a few examples. Despite all these advantages, HAXPES is still not very widely used for the study of valence bands due to several challenges in the interpretation of the spectra. In this thesis, we show that while often considered to be a minor detail or even neglected, understanding the photo-ionization cross-sections is crucial for the correct interpretation of its spectra. We study many of the different ways in which the cross-sections can affect the HAXPES experimental spectra, in order to learn how to make an effective use of them in our favour to get the information that is most relevant to us in each case. In order to achieve this, we study several carefully selected transition metal oxides with HAXPES as well as with other PES variants. We start by solving an apparent contradiction which prevented a proper understanding of the HAXPES valence band spectra of compounds containing transition metals and rare earths, and could have been one of the reasons why there are not so many studies with HAXPES on the valence band in the literature. We will show that due to the cross-section relations, we cannot neglect contributions from e.g. the La 5p, which are typically considered almost core-like and irrelevant for the physics studied in transition metal oxides. We will focus on LaCoO3 as a representative example, but also provide a few more examples and provide a guide to see in which cases it may be necessary to make such considerations. We make use of the very high cross-sections of 5d materials in HAXPES to study the class of double perovskite iridates, which have recently attracted interest as candidates for Kitaev physics. Due to the high Ir 5d cross-sections, the valence band spectra is completely dominated by the iridium spectral weight, making it possible to directly compare the spectra of many different iridates despite containing a wide range of ions in their composition. By fitting our calculations parameters to the experimental spectra, we conclude that the double perovskite iridates are highly covalent systems with essentially zero charge transfer, which would result in long-ranged interactions limiting the extent to which the Kitaev model can materialise. Two different compounds are studied by complementing the information from the element-specific core level spectra and the valence band spectra, which has other contributions mixed but is more sensitive to parameters such as the hybridization strength: First, we present a temperature dependence study of the LaCoO3 HAXPES combined with O-K X-ray absorption spectroscopy to investigate the gradual low-to-high spin transition observed with increased temperature. Our results suggest a scenario in which paramagnetic LaCoO3 should be considered as an inhomogeneous mixed spin-state system. Then, we present a comprehensive photoemission study of CaCu3Ru4O12, a very rare system with 3d transition metal ions that according to some claims display Kondo behaviour. The HAXPES spectra is used to tune the parameters for LDA+DMFT calculations. Additional photoemission measurements are also performed with a wide range of energies to change the ratio of the multiple contributions, allowing us to find a small resonance peak in the Cu 3d as predicted by the calculations. From the calculations and their good agreement with the experimental studies, we conclude that CaCu3Ru4O12 is a Kondo material, but with a very high Kondo temperature, finding a compromise between both sides of the literature and concluding a long standing debate. Finally, we experimentally show that the initial state dependence, as predicted by Fadley et al. from the cross-section formulas, can be observed by measuring a single crystal sample with HAXPES on different orientations. We show, using the example of ReO3, that a clear orientation dependence is present in many of the valence band feature. By applying the appropriate corrections, the shape of the atomic orbitals is recovered in the angular intensity plot. Other samples are also measured to prove that this technique can be used in a wide range of compounds

On the Octahydrate of Magnesium Sulphate

"Reprinted from the Memoirs of the College of Science, Kyoto Imperial University, Vol. V, No. 4, 1922."--cover.Mode of access: Internet