Orbital and spin character of doped carriers in infinite-layer nickelates*

The recent discovery of superconductivity in Sr-doped infinite-layer NdNiO₂ [1] has sparked significant interest in condensed matter physics. The search of superconductivity in nickel oxides was inspired by mimicking some of the essential characteristics of cuprates, such as the layered crystal structure and nominal 3d⁹ valence configuration [2]. Yet, some distinctions emerge including large charge-transfer energy and presence of small electron pockets from rare-earth 5d orbitals [3,4]. At this early stage, a key open question is the evolution of the electronic structure with doping. The competition between electron correlation, charge-transfer energy, crystal field splitting and Hund’s exchange may give rise to different scenarios, where doped holes could be introduced into Ni (in a high- or low-spin state), O or Nd bands. Here, we use a combination of x-ray absorption spectroscopy (XAS), resonant inelastic x-ray scattering (RIXS) and multiplet calculations to determine the dominant configuration of doped holes in Nd_1-xSr_xNiO₂.

References
[1] D. Li et al. Nature 572, 624 (2019)
[2] V. I. Anisimov et al. Phys. Rev. B 59, 7901 (1999)
[3] K. W. Lee and W. E. Pickett, Phys. Rev. B 70, 165109 (2004)
[4] M. Hepting et al. Nat. Mater. 19, 381 (2020)