Soft X-ray Spectroscopy of Low-Valence Nickelates

TL;DR

This review discusses how soft x-ray spectroscopy reveals the electronic and magnetic properties of low-valence nickelates, highlighting their similarities and differences with cuprates in the context of high-temperature superconductivity.

Contribution

It provides a comprehensive overview of the electronic structure and magnetic excitations of low-valence nickelates as revealed by XAS and RIXS, clarifying their relation to cuprates.

Findings

Nickelates show electronic structures similar to cuprates in XAS and RIXS.

Magnetic excitations in nickelates partly resemble those in cuprates.

Spectroscopic evidence informs the debate on nickelates' potential for high-Tc superconductivity.

Abstract

Low-valence nickelates $-$ including infinite-layer (IL) and trilayer (TL) compounds $-$ are longstanding candidates for mimicking the high-temperature superconductivity of cuprates. A recent breakthrough in the field came with the discovery of superconductivity in hole-doped IL nickelates. Yet, the degree of similarity between low-valence nickelates and cuprates is the subject of a profound debate for which soft x-ray spectroscopy experiments at the Ni $L$- and O $K$-edge provided critical input. In this review, we will discuss the essential elements of the electronic structure of low-valance nickelates revealed by x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS). Furthermore, we will review magnetic excitations observed in the RIXS spectra of IL and TL nickelates, which exhibit characteristics that are partly reminiscent of those of cuprates.