[en] The electronic and magnetic properties of the perovskite system La_1-xSr_xCoO₃ has been investigated for many decades now and still is subject of active research. In the undoped compound LaCoO₃, the correlated nature of the cobalt 3+ ions leads to a temperature driven spin state transition from a S=0 low spin (LS) ground state to a higher spin state, the nature of which has been discussed controversially as either S=1 intermediate or S=2 high spin (HS) state. Upon introducing Co²⁺ by doping with Sr, a metal insulator transition and long range ferromagnetic ordering have been observed. Very recently, the related system La_2-xSr_xCoO₄ has been synthesized in order to investigate properties of Co^2+/3+ ions in a environment, which leads in the case of cuprates and nickelates to interesting charge and spin ordering phenomena. While the magnetism of the cobaltates appears to be dominated by Co²⁺ ions in HS, exhibiting short range magnetic order at low temperatures, the spin state of the Co³⁺ is still under debate. Using bulk sensitive soft-x-ray photoelectron spectroscopy, we investigated the valence band electronic structure of La_2-xSr_xCoO₄, 0.3≤x ≤0.8, and its temperature dependence. Our results demonstrate a strong Co³⁺ LS contribution

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[en] The layered Ca_2-xSr_xRuO₄ system shows a very rich phase diagram as a function of the x-composition, temperature and magnetic field. Particularly fascinating is the electronic structure in the range of 0< x<0.2 where the metal-insulator transition (MIT) takes place at a continuously decreasing temperature with x getting closer to 0.2. This transition is accompanied by a drop in the resistivity of several orders of magnitude, comparable to that in V₂O₃ and VO₂. In order to reveal the mechanism of the MIT in Ca_2-xSr_xRuO₄, we have carried out a photoemission study with hν = 700 eV to obtain reasonable sensitivity to the Ru 4d spectral weight. We have found that there is a dramatic transfer of the spectral weight across the MIT. Since X-ray absorption spectroscopy has shown that the orbital occupation of the three t_2g bands changes significantly across the MIT, as is the case also in the MITs of V₂O₃ and VO₂, our results suggest the crucial role of the orbital switching for the MIT of Ca_2-xSr_xRuO₄.

[en] The novel layered Perovskite RBaCo₂O_5.5 (R=rare earth) has attracted considerable interest in the last decade. It shows an intriguing mix of properties, including giant magneto-resistance, metal-insulator and antiferro-ferromagnetic transitions, and a sign change of the thermoelectric power across these transitions. Explanation of these properties is subject of on-going debate. The so-called spin-blockade mechanism together with the occurrence of a spin-state transition of the octahedral Co³⁺ ions was proposed. Other scenarios invoke, for example, order-disorder effects, involving essentially all possible spin state configurations for the octahedral and pyramidal Co³⁺ ions. Using high quality single crystals and bulk sensitive photoelectron and x-ray absorption spectroscopy, we were able to identify the spin-state of the Co ions, thereby arriving at a very different scenario than proposed so far in the literature. We also find that the transfer of spectral weight near the Fermi level across the metal-insulator transition is very modest, in contrast to existing assumptions but in agreement with our observation on the evolution of the spin-state as a function of temperature.

[en] We have utilized bulk-sensitive photoelectron spectroscopy to study the valence band spectral weight distribution of d¹ Mott insulators LaTiO₃ and YTiO₃. We observed appreciable differences in the spectra, reflecting the difference in the one-electron band width. We also found that the Ti 3d spectra of both materials are much broader than the occupied 3d bands calculated by band theories. The mean-field inclusion of the Hubbard U explains the band gap but produces even narrower bands, indicating the complete breakdown of standard mean-field theories in describing excitation spectra. We associate the observed spectra with the propagation of a hole in a system with surprisingly well suppressed charge fluctuations thereby showing characteristics of a t-J model

[en] Highlights: • Efficiency and limits of polarization dependent HAXPES for solid state systems. • The polarization dependence is less than expected from atomic cross-sections. • Still high contrast (∼20–25) for s orbitals. • Quantitative determination of contributions to the valence band. - Abstract: 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° 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