Experimental Electronic Structure of Co Oxides

Experimental electronic structure of LaCoO ₃ and other RCoO ₃ (R = rare earth) in connection with the spin crossover phenomena in LaCoO ₃ is reviewed. As the experimental probes, we mostly deal with electron spectroscopic methods, namely photoemission spectroscopy and x-ray absorption spectroscopy. Experimental results are compared with ab initio band-structure calculations and configuration-interaction cluster-model calculations. First, we discuss how electronic-structure measurements can contribute to studying spin crossover phenomena, particularly of LaCoO ₃. Then in Sect. 2.2, we give a brief historical review of electron spectroscopic studies on LaCoO ₃ in order to highlight the conflict between the two major models, namely the low spin-high spin model and the low spin-intermediate spin-high spin model. Sections 2.3 to 2.5 are devoted to discussing this conflict and how we may be able to resolve it beyond this binarism. In Sect. 2.6, we remark the probing depth of the methods and the surface magnetism of LaCoO ₃. In Sect. 2.7, we briefly discuss a part of recent advances both in experiment and theory in this field. We run through the electronic structure of RCoO ₃ and other related Co oxides in Sect. 2.8, and finally we give the concluding remarks in Sect. 2.9.