Electronic Structure of CoO Nanocrystals and a Single Crystal Probed by Resonant X-ray Emission Spectroscopy

We have performed 2p X-ray absorption (XAS) and 2p3d resonant X-ray emission (RXES) experiments on a CoO bulk single crystal as well as on 4.2 nm CoO nanocrystals. The single crystal data were measured with linearly polarized incident X-rays in the scattering and perpendicular to the scattering plane. An unprecedented total experimental resolution of 100 meV allowed the first Xray observation of the CoO T-4(lg)(F-4) manifold that occurs 120 meV above ground state. Detailed theoretical modeling was performed to assess the tetragonal crystal field splitting, spin - orbit, and superexchange parameters for both the single crystal and the nanocrystals. We show that 2p XAS is mainly sensitive to the octahedral field and 3d spin-orbit coupling, while the T-4(1g)(F-4) manifold in 2p3d RXES probes the tetragonal distortion and the superexchange interactions with high sensitivity. We observe that the nanocrystals have a reduced cubic crystal field splitting and a broadened T-4(1g)(F-4) RXES manifold that we ascribe to a larger average Co-O distance and an increased magnetic exchange in the nanocrystals. We demonstrate that further improvement of the RXES experimental resolution would not only allow for better disentanglement of the tetragonal distortion, spin orbit, and superexchange interactions, but it would also allow observing anti-Stokes features in the RXES spectrum.