Using soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co-$L_{2,3}$ edge, we reveal that the spin state transition in ${\mathrm{LaCoO}}_3$ can be well described by a low-spin ground state and a triply degenerate high-spin first excited state. From the temperature dependence of the spectral line shapes, we find that ${\mathrm{LaCoO}}_3$ at finite temperatures is an inhomogeneous mixed-spin state system. It is crucial that the magnetic circular dichroism signal in the paramagnetic state carries a large orbital momentum. This directly shows that the currently accepted low- or intermediate-spin picture is at variance. Parameters derived from these spectroscopies fully explain existing magnetic susceptibility, electron spin resonance, and inelastic neutron data.

In 2004, Ohtomo and Hwang discovered that an electron gas is created at the interface between insulating ${\mathrm{LaAlO}}_3$ and ${\mathrm{SrTiO}}_3$ compounds. Here we show that the generation of a conducting electron gas is related to an orbital reconstruction occurring at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface. Our results are based on extensive investigations of the electronic properties and of the orbital structure of the interface using x-ray absorption spectroscopy. In particular, we find that the degeneracy of the Ti $3d$ states is fully removed and that the Ti $3d_{xy}$ levels become the first available states for conducting electrons.

Motivated by the search for design principles of rare-earth-free strong magnets, we present a study of electronic structure and magnetic properties of the ferromagnetic metal ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ within the local-density approximation (LDA) of the density-functional theory, and its combination with dynamical mean-field theory (DMFT). To compare these calculations, we measure magnetic and thermodynamic properties as well as x-ray magnetic circular dichroism and the photoemission spectrum of single-crystal ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$. We find that the experimentally determined Sommerfeld coefficient is enhanced by an order of magnitude with respect to the LDA value. This enhancement can be partially explained by LDA+DMFT. In addition, the inclusion of dynamical electronic correlation effects provides the experimentally observed magnetic moments, and the spectral density is in better agreement with photoemission data. These results establish the importance of electronic correlations in this ferromagnet.