R. J. O. Mossanek

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.

Optical response of metallic and insulating VO2 calculated with the LDA approach

We calculated the optical response of metallic and insulating VO2 using the local density approximation (LDA) approach. The band structure calculation was based on the full-potential linear-muffin-tin method. The imaginary part of the dielectric function e2(ω) is related to the different optical transitions. The Drude tail in the calculation of the metallic phase corresponds to intraband d–d transitions. The calculation in the insulating phase is characterized by the transitions to the band. The low-frequency features, 0.0–5.0 eV, correspond to V 3d–V 3d transitions, whereas the high-frequency structures, 5.0–12 eV, are related to O 2p–V 3d transitions. The calculation helps to explain the imaginary part of the dielectric function e2(ω), as well as the electron-energy-loss and reflectance spectra. The results reproduce not only the energy position and relative intensity of the features in the spectra, but also the main changes across the metal–insulator transition and the polarization dependence. The main difference is a shift of about 0.6 eV in the calculation of the insulating phase. This discrepancy arises because the LDA calculation underestimates the value of the band gap.

Cluster model calculations with nonlocal screening channels of metallic and insulating V O 2

We studied the changes in the electronic structure of

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

\mathrm{V}{\mathrm{O}}_{2}

Importance of the V 3 d – O 2 p hybridization in the Mott-Hubbard material V 2 O 3

across the metal-insulator transition. The main technique was cluster model calculations with nonlocal screening channels. The calculation included a screening from a coherent state at the Fermi level in the metallic phase, and a screening from a Hubbard charge fluctuation within the V-V dimmer in the insulating phase. The calculation results are compared to previous photoemission and x-ray absorption spectra. The coherent feature at the Fermi level in the metallic phase is due to the coherent screening. But the Hubbard screened state in the insulating phase appears at higher energies opening the band gap. The changes in the electronic structure of

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

\mathrm{V}{\mathrm{O}}_{2}

Similarities in the screening effects of the core level and valence band spectra of V O2

are thus related to the nonlocal screening channels.

Evolution of the spectral weight in the Mott-Hubbard series SrVO 3 -CaVO 3 -LaVO 3 -YVO 3

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 ∼ 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.

Optical conductivity and x-ray absorption spectra of the Mott-Hubbard compounds RVO3 (R=Sr, Ca, La, and Y)

We studied the changes in the electronic structure of

The growth of cobalt oxides on HOPG and SiO2 surfaces: A comparative study

{\mathrm{V}}_{2}{\mathrm{O}}_{3}