Stability of Zhang–Rice Singlets in Infinite-Layer Nickelates

Establish whether Zhang–Rice Singlet (ZRS) bound states form and are energetically stable in infinite-layer nickelates such as NdNiO2, given that the charge transfer energy in these materials is inherently larger than in cuprates.

Background

The paper compares the Ni 2p3/2 core-level XPS spectra of infinite-layer nickelates with the well-studied Cu core-level XPS in cuprates. In cuprates, the low-energy feature (A) is associated with non-local screened final states involving Zhang–Rice Singlets (ZRS), and it typically dominates over the local screened feature (B) due to the low charge transfer energy that favors ZRS formation.

For infinite-layer nickelates, the authors note that the charge transfer energy is inherently larger, which brings into question the stability and energetic favorability of ZRS bound states in these materials. Their capped NdNiO2 data display an A/B spectral weight ratio opposite to that of undoped cuprates, consistent with a Hubbard (rather than charge-transfer) picture, but the specific issue of whether ZRS bound states themselves are stable remains explicitly unclear.