Low spin configuration and giant orbital polarization of Ni²⁺ in square planar environment

The high T_c superconductivity in nickelates has been finally discovered in Sr_0.2Nd_0.8NiO₂, in which Ni is in square planar (D_4h) coordination of oxygen. Despite being iso-electronic (3d⁹), the parent compounds of nickelate and cuprate superconductors have markedly different electronic and magnetic behavior. To understand this, we have investigated the electronic and magnetic structure of Sr₂CuO₃ and Sr₂Cu_0.9Ni_0.1O₃. These compounds provide a unique opportunity to compare the electronic structure of both Ni and Cu in square planar (D_4h) environment. Polarization-dependent X-ray absorption spectroscopy (XAS) at Ni L_3,2 edge revealed extremely large (∼ 63%) orbital polarization (OP) for Ni. Our simulations based on multiplet ligand field theory for polarization dependent XAS of Ni²⁺ and the results from ab initio density functional theory have revealed that such high orbital polarization arises due to two holes in Ni 3d_x²-y² that give rise to nonmagnetic S = 0 state of Ni²⁺ - in contrast to S = 1 state from Hund’s first rule. Such nonmagnetic S = 0 Ni²⁺ in hole dope nickelate would be analogous to the Zhang Rice singlet.