Unconventional superconductivity in complex materials, such as copper oxides and iron pnictides, remains a defining problem of condensed-matter research. Recently, superconductivity was discovered in thin-films of hole-doped nickelates with the infinite-layer (IL) crystal structure . These first nickel oxide-based superconductors share formal similarities with the isostructural and nominally isoelectronic copper oxide superconductors, which points towards the existence of general design principles for unconventional superconductors. Nevertheless, in spite of formal similarities, the relationship between the two material classes is currently under intense scrutiny since ab-initio calculations and first spectroscopic studies indicate a distinct electronic structure of IL nickelates.
In this talk, I will discuss the electronic structure of the parent compounds of IL nickelates, revealed by resonant inelastic x-ray scattering (RIXS) and x-ray absorption spectroscopy (XAS) . Whereas hybridized states between copper and oxygen dominate the low-energy electronic structure in copper oxides, we find that rare-earth 5d electrons also contribute to the low-energy physics in nickelates. This weakly interacting metallic band hybridizes with a quasi two-dimensional band with strongly correlated electrons in the nickel-oxygen planes, giving rise to an exceptional electronic structure that has not been observed in other transition-metal oxides. Furthermore, we recently synthesized the first bulk crystals of hole-doped IL nickelates, whereas previous studies were carried out on thin-films or non-superconducting powders. I will discuss how spectroscopy on these bulk single-crystalline nickelates can advance the understanding of the material class.
 D. Li et al., Nature 572, 624 (2019).
 M. Hepting et al., Nat. Mater. 19, 381 (2020).
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