Orbital and Spin Character of Doped Carriers in Infinite-Layer Nickelates

TL;DR

This study uses XAS and RIXS to investigate how doping affects the electronic structure of Nd$_{1-x}$Sr$_{x}$NiO$_2$, revealing that doped holes occupy $d_{x^2-y^2}$ orbitals and induce a $d^8$ spin singlet state.

Contribution

It provides detailed spectroscopic evidence for the orbital and spin character of doped carriers in infinite-layer nickelates, linking doping to changes in electronic structure.

Findings

Doped holes reside in $d_{x^2-y^2}$ orbitals.

Doping induces a $d^8$ spin singlet state.

Orbital excitations soften with doping, indicating Fermi level shifts.

Abstract

The recent discovery of superconductivity in Nd$_{1-x}$Sr$_{x}$NiO$_2$ has drawn significant attention in the field. A key open question regards the evolution of the electronic structure with respect to hole doping. Here, we exploit x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) to probe the doping dependent electronic structure of the NiO$_2$ planes. Upon doping, a higher energy feature in Ni $L_3$ edge XAS develops in addition to the main absorption peak. By comparing our data to atomic multiplet calculations including $D_{4h}$ crystal field, the doping induced feature is consistent with a $d^8$ spin singlet state, in which doped holes reside in the $d_{x^2-y^2}$ orbitals, similar to doped single band Hubbard models. This is further supported by orbital excitations observed in RIXS spectra, which soften upon doping, corroborating with Fermi level shift associated with increasing holes in the $d_{x^2-y^2}$ orbital.