Speaker
Description
The discovery of superconductivity in infinite-layer nickelates without alkaline-earth doping, as well as in Ruddlesden-Popper nickelates, raises the question of whether these cases are related and whether they are based on a common mechanism. For the infinite-layer nickelates a critical step is the topotactic reduction. We investigate the electronic configurations arising in the reduction process by employing soft x-ray absorption spectroscopy (XAS) on PrNiO$_x$ thin films with $x$=2-3 at various stages of topotactic reduction. A comparison of the Ni-L edge spectra, calculated using single and double cluster ligand-field models, shows that none of the measured samples can be described by the expected pure d9 configuration. Even in the most reduced samples, the concomitant changes at the O-K edge upon reduction exhibit signs of remaining 2p holes. To quantify these findings, we analysed the Ni-L edge spectra using the charge sum rule and found that, in the maximally reduced films, the average number of 3d holes remained at 1.35. The superconducting samples have even higher average hole numbers, which calls into question hole doping levels assumed for cation doped infinite-layer nickelates and their similarity to the undoped superconducting compounds. Overall, XAS signatures for different reduction stages and configurations will be presented and different possible sources of the hole doping discussed.