Giant Orbital Polarization of Ni2+ in a Square Planar Environment

Finding large orbital polarization in Ni-based oxides has become a topic of paramount interest in recent years due to the prospect of finding superconductivity. In this work, we investigate the electronic structure of single-crystalline samples of Sr2CuO3 and Ni-doped Sr2CuO3 containing Cu/NiO4 square planar units. Our polarization-dependent x-ray absorption spectroscopy experiments reveal extremely large orbital polarization (similar to 63%) for Ni2+. Cluster calculations and ab initio calculations find that the giant orbital polarization arises due to the low-spin (S = 0) configuration with two holes in the Ni 3d(x2-y2) orbital, contrary to the expected high-spin (S = 1) state from Hund's first rule.