Mn 3d bands and Y-O hybridization of hexagonal and orthorhombic YMnO3 thin films.

We report here the O K-edge x-ray absorption spectra of hexagonal and orthorhombic YMnO3 thin films, aiming at comparing the changes in the Mn 3d bands as well as the role of Y 4d-O 2p hybridization. The experimental results were analyzed using first principles (GGA) band structure calculations. The spectra present clear differences in the Mn 3d bands, which are attributed to changes in the Mn-O coordination and symmetry. A strong Y 4d-O 2p hybridization is observed in both the hexagonal and orthorhombic films, and its possible role on the occurrence of the observed ferroelectricity is discussed.

Effects of Ni vacancies and crystallite size on the O 1s and Ni 2p x-ray absorption spectra of nanocrystalline NiO.

We have studied the electronic structure of nanocrystalline NiO thin films, grown by radio-frequency magnetron sputtering under different experimental conditions, using x-ray absorption spectroscopy. The O 1s and Ni 2p spectra showed distinct changes as a function of O2 content in the plasma, which were reproduced with cluster model calculations. These changes are attributed to the incrementing of the surface contribution due to a decrease of the crystallite size as the O2 content in the plasma increases, and to the presence of induced nickel vacancies. Thus, the changes in the electronic structure can be related to the modification of structural and transport properties of these nanocrystalline films.

Electronic structure of the band-filling-controlled CaVO3 and LaVO3 compounds.

We studied the electronic structure of the band-filling CaVO(3) and LaVO(3) compounds. The experimental techniques were photoemission (PES) and x-ray absorption (XAS) spectroscopy. The experimental results were analyzed using an extended cluster model. The ground states of CaVO(3) and LaVO(3) are highly covalent and contain a considerable 3d(n + 1)L contribution. The CaVO(3) compound is in the charge transfer regime (Δ < U), whereas the LaVO(3) material is in the intermediate regime (Δ âˆ¼ U). The spectral weight distributions reveal that CaVO(3) is a coherent metal and that LaVO(3) is a p-d insulator. The photoemission of CaVO(3) shows the coherent peak (3d(1)C) and the incoherent feature (3d(1)L). The spectrum of insulating LaVO(3) presents only the incoherent structure (3d(2)L), whereas the coherent peak is replaced by the Mott-Hubbard screening (3d(2)D). This transfer of spectral weight is responsible for the opening of the experimental bandgap. The incoherent feature contains a considerable O 2p character and cannot be attributed to the lower Hubbard band. Further, the relative V 3d-O 2p cross section helps to explain the photon energy dependence of the PES spectra. The addition spectra of both CaVO(3) and LaVO(3) are dominated by the 3d(n + 1) final state configuration. The distribution of spectral weight is mainly dictated by intra-atomic exchange and crystal field splittings. The coherent contribution is less important than in photoemission, and is greatly diminished in the O 1s x-ray absorption spectra.

Similarities in the screening effects of the core level and valence band spectra of VO2.

We calculated the core level and valence band spectra of VO(2) using an extended cluster model. The distribution of spectral weight is similar in both core level and valence band spectra and is related to the different screening mechanisms. Namely, the low energy structures in the spectra correspond to well screened final states, whereas the poorly screened final states appear at higher energies. The latter produce the charge transfer satellite observed in the core level spectra, and are related to the resonance in the constant initial state spectra of the valence band. The changes in the spectral weight across the metal-insulator transition are due to the different non-local screening channels present in each phase, and are also similar in both core level and valence band spectra. The calculation also shows a relatively large O 2p contribution in both spectra, which, in turn, helps to explain the enhancement of the coherent feature at higher photon energies. This suggests that the incoherent peak cannot be solely attributed to the lower Hubbard band.