Charge transfer and dd excitations in transition metal oxides

Abstract

Abstract. Electronic excitations in transition-metal oxides MnO, FeO, CoO and NiO are investigated by inelastic X-ray scattering and optical reflectivity measurements. The dielectric functions are derived from the experimental data as a function of the momentum transfer, q. Based on the derived q-dependent dielectric functions, two types of the charge transfer excitations, i.e., dipolar and non-dipolar charge transfer, are clearly identified. We show that the Mott gaps around 5 eV are defined by the former whereas the latter occurs at higher energies of 8-12 eV. Based on a molecular orbital analysis, we associate the dipolar and the non-dipolar excitations with non-local charge transfer and conventional charge transfer, respectively. These types of excitations are shown to be common for the 3d metal monoxides. On the other hand, the dd excitations observed in NiO and CoO at energy <4 eV do not appear in FeO and MnO. The reasons are addressed in this report.PACS. 71.20.-b Electron density of states and band structure of crystalline solids -78.70.Ck X-ray scattering

Low-energy charge transfer excitations in NiO

“…5. A rather strong CT band peaked at 4.5 eV is clearly visible in the absorption of thin NiO films [19], RIXS spectra [20,21], the reflectance spectra (4.3 eV) [15], the nonlinear absorption spectra [5]. All this points to dipole-allowed CT transition.…”

“…Strong dipole-allowed Franck-Condon d(e g )-d(e g ) CT transition in NiO manifests itself as a strong spectral feature near 4.5 eV clearly visible in the absorption of thin NiO films [19], the RIXS spectra [20,21], the reflectance spectra (4.3 eV) [15], the nonlinear absorption spectra (4.1-4.3-4.5 eV) [5]. Such a strong absorption near 4 eV is beyond the predictions of the p-d CT model and indeed is lacking in absorption spectra of MgO:Ni [16].…”

“…Thus, if the most intensive CT band with a maximum around 7 eV observed in RIXS spectra [20,21] to attribute to the strongest dipole-allowed O 2pt 1u (σ) → Ni 3de g CT transition then one should expect the low-energy p-d CT counterparts with maximuma around 4, 5, and 6 eV respectively, which are related to dipoleforbidden t 1g (π) → e g , weak dipole-allowed t 2u (π) → e g , and relatively strong dipole-allowed t 1u (π) → e g CT transitions, respectively. It is worth noting that the π → σ p-d CT t 1u (π) − e g transition borrows a portion of intensity from the strongest dipole-allowed σ → σ t 1u (σ) − e g CT transition because the t 1u (π) and t 1u (σ) states of the same symmetry are partly hybridized due to p-p covalency and overlap.…”

Exciton dispersion from first principles

“…In fact, IXS and EELS are unique tools to measure electronic excitations that are dipole forbidden and thus not visible in optics. Typical examples are intra-atomic d-d excitations in strongly correlated transition-metal oxides [17][18][19] or longitudinal p-like excitons in sp cubic crystals. 2,3,9 In addition, the new electron microscopes are able today to reach very high spatial resolution (well under 1 nm) for the loss spectroscopy, [20][21][22] which, in order to be investigated from the theoretical point of view, requires the knowledge of the dielectric function not at just q = 0, but at many q's, in order to Fourier transform back in real space.…”

Inelastic X-ray Scattering Studies of Electronic Excitations

“…Larson et al 38) succeeded in observing d-d excitations, which are dipole-forbidden, in NiO and CoO via large-q NIXS measurements. Hiraoka et al 63) also obtained a similar result in their systematic measurement of transition-metal monoxides. The intensity of these d-d excitations was found to be highly anisotropic in momentum space, as expected from the matrix element in Eq.…”