From perovskite to infinite-layer nickelates: hole concentration from x-ray absorption

TL;DR

This study uses soft x-ray absorption spectroscopy to analyze the electronic structure and hole doping mechanisms in infinite-layer nickelate thin films, revealing complex interactions affecting superconductivity.

Contribution

It provides the first detailed spectroscopic analysis of nickel valence states and oxygen holes in reduced nickelate films, challenging previous assumptions about hole doping limits.

Findings

Nickel in the films does not exhibit a pure d^9 configuration.

Even maximally reduced films have an average of 1.35 holes in Ni 3d orbitals.

Superconducting samples show higher hole counts, indicating complex doping mechanisms.

Abstract

The difficulty of determining cation concentrations and oxygen stoichiometry in infinite-layer nickelate thin films has so far prevented clear experimental identification of the nickel electron configuration in the superconducting phase. We used soft x-ray absorption spectroscopy to study the successive changes in PrNiO$_x$ thin films at various intermediate stages of topotactic reduction with $x=2-3$. By comparing the Ni-$L$ edge spectra to single and double cluster ligand-field calculations, we find that none of our samples exhibit a pure $d^9$ configuration. Our quantitative analysis using the charge sum rule shows that even when films are maximally reduced, the averaged number of nickel $3d$ holes is 1.35. Superconducting samples have even higher values, calling into question the previously assumed limit of hole doping. Concomitant changes in the oxygen $K$-edge absorption spectra upon reduction indicate the presence of oxygen $2p$ holes, even in the most reduced films. Overall, our results suggest a complex interplay of hole doping mechanisms resulting from self-doping effects and oxygen non-stoichiometry.