Where do the hole carriers reside in the new superconducting nickelates?

The families of high-temperature superconductors recently welcome a new member: hole doped nickelate Nd_0.8Sr_0.2NiO₂ with a ~15K transition temperature.
To understand its emergent low-energy behaviors and experimental properties, an immediate key question is whether the superconducting hole carriers reside in oxygen as in the cuprates, or in nickel as in most nickelates.
We answer this crucial question via a "(LDA+U)+ED'' scheme: deriving an effective interacting Hamiltonian of the hole carriers from density functional LDA+Ucalculation, and studying its local many-body states via exact diagonalization.
Surprisingly, distinct from the expected Ni²⁺ spin-triplet state found in most nickelates, the local ground state of two holes is actually a Ni-O spin-singlet state with second hole residing greatly in oxygen.
The emerged eV-scale model therefore resembles that of the cuprates, advocating further systematic experimental comparisons.
Tracing the microscopic origin of this unexpected result to the lack of apical oxygen in this material, we proposed a route to increase superconducting temperature, and a possible new quantum phase transition absent in the cuprates.