Electronic signature of the vacancy ordering in NbO (Nb3O3)

We investigated the electronic structure of the vacancy-ordered 4d-transition metal monoxide NbO (Nb3O3) using angle-integrated soft- and hard-x-ray photoelectron spectroscopy as well as ultra-violet angle-resolved photoelectron spectroscopy. We found that density-functional-based band structure calculations can describe the spectral features accurately provided that self-interaction effects are taken into account. In the angle-resolved spectra we were able to identify the so-called vacancy band that characterizes the ordering of the vacancies. This together with the band structure results indicates the important role of the very large inter-Nb-4d hybridization for the formation of the ordered vacancies and the high thermal stability of the ordered structure of niobium monoxide

Optimizing polarization dependent hard X-ray photoemission experiments for solids

Polarization dependent hard X-ray photoemission (HAXPES) experiments are a very powerful tool to identify the nature of the orbitals contributing to the valence band. To optimize this type of experiments we have set up a photoelectron spectroscopy system consisting of two electron energy analyzers mounted such that one detects the photoelectrons propagating parallel to the polarization vector (E) of the light and the other perpendicular. This method has the advantage over using phase retarders (to rotate the E-vector of the light) that the full intensity and full polarization of the light is available for the experiments. Using NiO as an example, we are able to identify reliably the Ni 3d spectral weight of the valence band and at the same time demonstrate the importance of the Ni 4s for the chemical stability of the compound. We have also discovered the limitations of this type of polarization dependent experiments: the polarization dependence is less than expected on the basis of calculations for free atoms and we can ascribe this incompleteness of the polarization dependence to the presence of appreciable side-scattering effects of the outgoing electrons, even at these high kinetic energies in the 6–8 keV range. © Springer International Publishing Switzerland 2016

Challenges from experiment: electronic structure of NiO

We report on a detailed experimental and theoretical study of the electronic structure of NiO. The charge-transfer nature of the band gap as well as the intricate interplay between local electronic correlations and band formation makes NiO to be a challenging case for a quantitative ab-initio modeling of its electronic structure. To reproduce the compensated-spin character of the first ionization state and the state created by hole doping requires a reliable determination of the charge transfer energy Δ relative to the Hubbard U. Furthermore, the presence of non-local screening processes makes it necessary to go beyond single-site many body approaches to explain the valence band spectrum

Spin-orbit coupling and crystal-field distortions for a low-spin 3d(5) state in BaCoO3

We have studied the electronic structure of BaCoO3 using soft x-ray absorption spectroscopy at the Co L-2,L-3 and O K edges, magnetic circular dichroism at the Co L-2,L-3 edges, and valence band hard x-ray photoelectron spectroscopy. The quantitative analysis of the spectra established that the Co ions are in the formal low-spin tetravalent 3d(5) state and that the system is a negative charge transfer Mott insulator. The spin-orbit coupling also plays an important role for the magnetism of the system. At the same time, a trigonal crystal field is present with sufficient strength to bring the 3d(5) ion away from the J(eff )= 1/2 state. The sign of this crystal field is such that the a(1g) orbital is doubly occupied, explaining the absence of a Peierls transition in this system, which consists of chains of face-sharing CoO6 octahedra. Moreover, with one hole residing in e(g)(pi), the presence of an orbital moment and strong magnetocrystalline anisotropy can be understood. Yet we also infer that crystal fields with lower symmetry must be present to reproduce the measured orbital moment quantitatively, thereby suggesting the possibility for orbital ordering to occur in BaCoO3

Polarization dependent hard X-ray photoemission experiments for solids: Efficiency and limits for unraveling the orbital character of the valence band

We have investigated the efficiency and limits of polarization dependent hard X-ray photoelectron spectroscopy (HAXPES) in order to establish how well this method can be used to unravel quantitatively the contributions of the orbitals forming the valence band of solids. By rotating the energy analyzer rather than the polarization vector of the light using a phase retarder, we obtained the advantage that the full polarization of the light is available for the investigation. Using NiO, ZnO, and Cu2O as examples for solid state materials, we established that the polarization dependence is much larger than in photoemission experiments utilizing ultra-violet or soft X-ray light. Yet we also have discovered that the polarization dependence is less than complete on the basis of atomic calculations, strongly suggesting that the trajectories of the outgoing electrons are affected by appreciable side-scattering processes even at these high kinetic energies. We have found in our experiment that these can be effectively described as a directional spread of +/- 18 degrees of the photoelectrons. This knowledge allows us to identify, for example, reliably the Ni 3d spectral weight of the NiO valence band and at the same time to demonstrate the importance of the Ni 4s for the chemical stability of the compound. (C) 2014 Elsevier B.V. All rights reserved

Polarization dependent hard X-ray photoemission experiments for solids: Efficiency and limits for unraveling the orbital character of the valence band

We have investigated the efficiency and limits of polarization dependent hard X-ray photoelectron spectroscopy (HAXPES) in order to establish how well this method can be used to unravel quantitatively the contributions of the orbitals forming the valence band of solids. By rotating the energy analyzer rather than the polarization vector of the light using a phase retarder, we obtained the advantage that the full polarization of the light is available for the investigation. Using NiO, ZnO, and Cu2O as examples for solid state materials, we established that the polarization dependence is much larger than in photoemission experiments utilizing ultra-violet or soft X-ray light. Yet we also have discovered that the polarization dependence is less than complete on the basis of atomic calculations, strongly suggesting that the trajectories of the outgoing electrons are affected by appreciable side-scattering processes even at these high kinetic energies. We have found in our experiment that these can be effectively described as a directional spread of +/- 18 degrees of the photoelectrons. This knowledge allows us to identify, for example, reliably the Ni 3d spectral weight of the NiO valence band and at the same time to demonstrate the importance of the Ni 4s for the chemical stability of the compound. (C) 2014 Elsevier B.V. All rights reserved

Quantitative study of the f occupation in CeMIn5 and other cerium compounds with hard X-rays

We present bulk-sensitive hard X-ray photoelectron spectroscopy (HAXPES) data of the Ce3d core levels and lifetime-reduced L-edge X-ray absorption spectroscopy (XAS) in the partial fluorescence yield (PFY) mode of the CeMIn5 family with M=Co, Rh, and Ir. The HAXPES data are analyzed quantitatively with a combination of full multiplet and configuration interaction model which allows correcting for the strong plasmons in the CeMIn5 HAXPES data, and reliable weights w(n) of the different f(n) contributions in the ground state are determined. The CeMIn5 results are compared to HAXPES data of other heavy fermion compounds and a systematic decrease of the hybridization strength V-eff from CePd3 to CeRh3B2 to CeRu2Si2 is observed, while it is smallest for the three CeMIn5 compounds. The f-occupation, however, increases in the same sequence and is close to one for the CeMIn5 family. The PFY-XAS data confirm an identical f-occupation in the three CeMIn5 compounds and a phenomenological fit to these PFY-XAS data combined with a configuration interaction model yields consistent results. (C) 2016 The Authors. Published by Elsevier B.V