Introduction

High critical-temperature (T \({ }_{c})\) superconductivity in the cuprates has been a defining challenge of condensed matter physics for the 35 years since their discovery. One strategy to address this challenge has been to look for “cuprate analog” materials: alternative transition metal oxides that exhibit ingredients that are considered proxies for cuprate physics. These key ingredients include a quasi-2D structure based on the CuO \({ }_2\) planes, a nominal oxidation state for Cu \({ }^{2+}\) : 3 \(d^9\) with a single hole in the uppermost \(d_{x^2-y^2}\) orbital, and a strong O(2p) and Cu(3d) hybridization. Nickelates have been an obvious choice of study in this context due to the proximity of Ni to Cu on the periodic table. After a 30 year wait, superconductivity in nickelates was realized for the first time in 2019 in hole-doped NdNiO \({ }_{2}\) (Li et al., Nature 572:624–627). This material contains NiO \({ }_2\) planes (analog to the CuO \({ }_2\) planes of the cuprates), and realizes a Ni \({ }^{1+}\) oxidation state (analog to Cu \({ }^{2+}\) ). NdNiO \({ }_{2}\) is simply the infinite-layer member of a larger family of materials represented by the chemical formula \(R_{n+1}\) Ni \({ }_{n}\) O \({ }_{2n+2}\) ( \(R=\) La, Pr, Nd; \(n \geq 2\) ), where n refers to the number of NiO \({ }_{2}\) planes along the c axis.