The nature of the metal-insulator transition in 5d transition metal oxides

• Main Author: Vale, J. G
• Format: Dissertation
• Language: English
• Published: UCL (University College London) 2017

A number of 5d transition metal oxides (TMOs) either undergo, or lie proximate to, a metal-insulator transition (MIT). However these MITs frequently depart from a Mott-Hubbard picture, in which the interactions are dominated by the interplay between the on-site Coulomb repulsion and electronic bandwidth. In 5d TMOs the sizeable intrinsic spin-orbit coupling plays an important role, and gives rise to electronic and magnetic ground states -- at both sides of the MIT -- that cannot be adequately described within a purely L-S coupling scenario. In this thesis the aim is to understand the role of spin-orbit coupling in determining the electronic and magnetic properties of 5d TMOs. There has been a large amount of recent interest within this field (both experimentally and theoretically), however thus far has mostly been limited to the 5d5, j =1/2 limit. The perovskite iridates Sr2IrO4 and Sr3Ir2O7 lie within this limit. Theoretical predictions suggest a significant easy-plane anisotropy is present for the single layer Sr2IrO4. I show that this anisotropy can be observed and quantified, using magnetic critical scattering and previously published resonant inelastic X-ray scattering (RIXS) data. This differs from previous results that suggest purely 2D Heisenberg behaviour. Meanwhile the critical fluctuations in bilayer Sr3Ir2O7 have a three-dimensional nature, which can be directly related to the intra-bilayer coupling and significant anisotropy previously probed by RIXS. I also demonstrate that resonant X-ray scattering techniques can be successfully applied to other 5d systems, especially the d3 osmates. Both NaOsO3 and Cd2Os2O7 undergo MITs directly linked to the onset of antiferromagnetic order (Slater or Lifshitz mechanisms). The first ever high-resolution RIXS measurements at the Os L3 absorption edge reveal that there is a correlated evolution of the electronic and magnetic excitations through the respective MITs. The behaviour is consistent with a scenario in which the effect of spin-orbit coupling and electron correlations are reduced with respect to the iridates, yet still manifests through a strong spin wave anisotropy. Finally I show that the study of 5d TMOs can be extended into the time domain. Through the development of new instrumentation, the transient dynamics of photo-doped Sr2IrO4 were probed by time-resolved resonant (in)elastic X-ray scattering. The relevant time scales can be directly compared to the interaction strengths and anisotropies in the undoped state. Moreover, there seems to be an effective mapping of the transient behaviour in the photo-doped state to an equivalent level of bulk electron doping in (Sr_{1-x}La_x)2IrO4.