X-Ray Scattering Studies of Charge and Orbital Ordering in Transition Metal Oxides

Abstract

This thesis focusses on the phenomena of charge ordering and orbital ordering in transition metal oxides, using the technique of resonant x-ray diffraction (RXD). The technique is suited to such studies, as it provides an elemental and band specific probe of long range electronic correlations. Here, we present hard and soft x-ray diffraction results on iron and manganese compounds, and demonstrate the utility of resonant x-ray diffraction in revealing their electronic properties.

By performing comprehensive hard x-ray RXD studies on the complex charge ordered structures of magnetite and iron oxyborate, we demonstrate that both systems possess non-integer charge order by using the FDMNES simulation code. We find certain reflections have anisotropic polarisation dependencies, due entirely to the crystal structure, with no need to invoke orbital order to explain the observations. In both materials we demonstrate that the anomalous scattering components can result in a surprising conversion from linearly to non-linearly polarised light, through two subtly different interference processes. This is the first report of such mechanisms.

By performing soft x-ray scattering on Pr(SrCa)MnO, we have provided the first direct observation of the orbital rotation within this compound. We have found that there is no apparent change in the orbital occupation during the stripe rotation, but an increase in the charge disproportionation upon cooling. We have used soft x-ray RXD on lutetium ferrate to discover an anisotropic contribution to the scattering at the charge order wavevector. This is not apparent when probing the crystal structure directly using hard x-rays. We attribute such a signal to charge-orbital order within the system, in contrast to previous claims that the system is an orbital glass.