Giant orbital polarization of Ni$^{2+}$ in square planar environment

TL;DR

This study reveals a giant orbital polarization of Ni$^{2+}$ in a square planar environment, highlighting key electronic differences from cuprates and implications for nickelate superconductivity.

Contribution

It provides the first detailed polarization-dependent X-ray absorption spectroscopy analysis of Ni in a square planar environment, showing a low spin state contrary to expectations.

Findings

Ni$^{2+}$ exhibits very large orbital polarization.

Ni$^{2+}$ is in a low spin $S$=0 state, unlike the high spin state expected.

The electronic phase space of nickelates differs from high $T_c$ cuprates.

Abstract

Understanding the electronic behavior of Ni$^{2+}$ in a square planar environment of oxygen is the key to unravel the origin of the recently discovered superconductivity in the hole doped nickelate Nd$_{0.8}$Sr$_{0.2}$NiO$_2$. To identify the major similarities/dissimilarities between nickelate and cuprate superconductivity, the study of the electronic structure of Ni$^{2+}$ and Cu$^{2+}$ in an identical square planar environment is essential. In order to address these questions, we investigate the electronic structure of Sr$_2$CuO$_3$ and Ni doped Sr$_2$CuO$_3$ single crystals containing (Cu/Ni)O$_4$ square planar units. Our polarization dependent X-ray absorption spectroscopy experiments for Ni in Sr$_2$Cu$_{0.9}$Ni$_{0.1}$O$_3$ have revealed very large orbital polarization, which is a characteristic feature of high $T_c$ cuprate. This arises due to the low spin $S$=0 configuration with two holes in Ni 3$d_{x^2-y^2}$ orbitals - in contrast to the expected high spin $S$=1 state from Hund's first rule. The presence of such $S$=0 Ni$^{2+}$ in hole doped nickelate would be analogous to the Zhang Rice singlet. However, the Mott Hubbard insulating nature of the NiO$_4$ unit would point towards a different electronic phase space of nickelates, compared to high $T_c$ cuprates.