Study of the anisotropic electronic, and atomic structures of novel layered materials using polarised x-ray absorption spectroscopy

Charge ordering in materials is often paired with interesting electronic properties, such as charge/spin density waves (C/SDW) or Mott-insulator transitions. It also leads to many interesting ground states, such as spin-Peierls, antiferromagnetism, and superconductivity. This has lead to an increased interest in the physics field to study materials that exhibit these properties. The materials presented in this thesis are interesting due to their charge ordering properties. Such as the predicted CDW in \(M_2Mo_6Se_6\) (M=Rb,Tl), this has been investigated with diffraction but due to suspected breaking in the long range order it has not been experimentally observed. Or the elusive nature of the Mott insulator transition in \(Ca_2RuO_4\), where it is not clear whether the driving force of the transition is orbital selective or not. The work presented in this thesis will focus on x-ray absorption experiments. One of the main properties of synchrotron radiation is that the x-rays are produced linearly polarised in the plane of the orbit of the electron. By taking advantage of this, more information can be obtained from an x-ray absorption spectroscopy (XAS) experiment than by the standard application used to measure powder samples. In this work single crystals have been measured, allowing for more direct comparisons to be made with other techniques, such as diffraction. By measuring the x-ray absorption at different angles between the polarisation of the beam and a specific crystallographic axis, it is possible to directly measure anisotropic changes in the structure of a sample. This is very useful when investigating transitions which are paired with an angular change, such as a Jahn-Teller distortion. This technique takes advantage of the adaptability of XAS beamlines, where the only requirements are a linearly polarised beam and the ability to rotate the sample. When using a bending magnet beamline with a highly focused beam it is possible to measure small crystals (~500μm x 500μm). This is an advantage as some single crystals are difficult to grow to a large size. This work will present the measurements and analysis of three different materials using Polarised \(XAS (P-XAS), Sr_2MnCu_{1.5-δ}S_2O_2, M_2Mo_6Se_6 (M=Rb, Tl), Ca_2RuO_4\). The advantages and difficulties of this technique and analysis method will be discussed, along with the information gained about these materials using this technique. A general weighting factor is introduced which increases the usefulness of P-XAS measurements, allowing for more freedom in the fitting process. The \(Sr_2MnCu_{1.5-δ}S_2O_2\) chapter shows that P-XAS provides a greater sensitivity than powder XAS. The analysis of the \(M_2Mo_6Se_6\) data provides evidence of a structural transition that may be due to the presence of a CDW. The \(Ca_2RuO_4\) chapter shows how P-XAS can be used to directly observe the nature of a Jahn-Teller distortion.