Giant orbital polarization of Ni 2 + 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 ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}$ and Ni-doped ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}$ containing $\mathrm{Cu}/{\mathrm{NiO}}_{4}$ square planar units. Our polarization-dependent x-ray absorption spectroscopy experiments reveal extremely large orbital polarization $(\ensuremath{\sim}63%)$ for ${\mathrm{Ni}}^{2+}$. 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 $3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbital, contrary to the expected high-spin $(S=1)$ state from Hund's first rule.