The experimental development of the Resonant Inelastic X-ray Scattering (RIXS) technique in the soft X-ray energy range and has been tremendous during the last years. Several RIXS instruments at synchrotron radiation sources world-wide have recently boosted the scientific capabilities with soft X-ray RIXS. The ADRESS beamline of the Swiss Light Source at the Paul Scherrer Institut in Switzerland and its RIXS spectrometer SAXES have increased the resolving power for the incident and the outgoing X-ray beam to above 10’000 [1]. Such an extremely high spectral resolution and the possibility to rotate the spectrometer to different scattering geometries allows now analyzing the collective behavior of charge, orbital, lattice and spin excitations by assessing their momentum dependence.

For iron-based superconductors and their parent compounds we sho, w that RIXS at the Fe L₃ edge can be used to measure collective magnetic excitations even for materials with significant electronic itinerancy [2]. Our experiments on hole doped Ba_1-xKxFe₂As₂ and electron doped Ba(Fe_1-xCo_x)₂As₂ single crystals with under-, nearly optimal- and over-doping show well-defined spin-excitations dispersing up to 200 meV and persisting into the superconducting phase, thereby manifesting that an universal correlated spin state is responsible for the spin fluctuations in these materials.

High-energy spin-excitations in Ba(Fe_1-xCo_x)₂As₂are independent on electron doping, in contrast toBa_1-xK_xFe₂As₂[2] samples that are clearly softening relative to parent BaFe₂As₂. This indicates that electron doping BaFe₂As₂ actually does not significantly affect the high energy spin-excitations, which is consistent with the lower degree of electron correlations of electron doped iron pnictide compounds [3].

Perovskite rare-earth (Re) nickelates ReNiO₃ continue to attract a lot of interest owing to their intriguing properties like a sharp metal to insulator transition (MIT), unusual magnetic order [4] and expected superconductivity in specifically tuned super-lattices [5]. Full understanding of these materials, however, is hampered by the difficulties in describing their electronic ground state (GS). From X-ray absorption (XAS) at the Ni 2p_3/2 edge of thin films of ReNiO₃ and corresponding RIXS maps vs. incident and transferred photon energies we reveal that the electronic GS configuration is composed of delocalized andlocalized components. Our study conveys that a Ni3d8L-like configuration takes on the leading role in the GS of ReNiO₃ as proposed by recent model theories [6].

References
[1] V. N. Strocov, T. Schmitt et al., J. Synchrotron Rad. 17, 631–643 (2010); G. Ghiringhelli et al., Rev. Sci. Instrum. 77, 113108 (2006).
[2] Ke-Jin Zhou, Thorsten Schmitt et al., Nature Communications 4, 1470 (2013), DOI: 10.1038/ncomms2428.
[3] M. S. Liu et.al., Nature Physics 8, 376 (2012).
[4] M. L. Medarde, J. Phys. Cond. Matt. 9, 1679 (1997).
[5] J. Chaloupka et al., Phys. Rev. Lett. 100, 016404 (2008).
[6] H. Park et al., Phys. Rev. Lett. 109, 156402 (2012); S. Johnston et al., arXiv:1310.2674.