Accessing Experimentally Inaccessible States in Infinite Layer Nickelate

In 2019 H. Hwang and coworkers [Nature 572, 624] discovered superconductivity up to T_c= 15 K in thin film NdNiO₂ when hole-doped. This phenomenon is the long sought cuprate-like nickelate superconductity. NdNiO₂ (also LaNiO₂) with formally Ni¹⁺ ions is isostructural and isovalent with infinite layer CaCuO₂ (Cu²⁺, d⁹) that superconducts up to 110 K when doped. Two fundamental aspects of the difference between this nickelate and the undoped cuprate give the grand comparison: NdNiO₂ is conducting and not magnetically ordered, while undoped cuprate CaCuO₂ is insulating and antiferromagnetic (AFM).
In the (experimentally inaccessible) AFM ordered phase, a Ni d(z²) flat band exactly lying at the Fermi energy appears on the entire k_z=π/c zone faces, leading to a 1D-like van Hove singularity (vHs). This vHs supports spin, charge, and lattice instabilities. However, these symmetry-breaking orders are not observed, since the magnetic ordering and structural instabilities may be overcome by thermal or quantum zero-point motions of oxygen and non-adiabatic electron-lattice coupling due to the narrowness of the vHs. Our results from first principles approach indicate that this strongly AFM correlated and conducting spin-liquid phase with significantly involved Ni d(z²) orbital establishes the platform for superconductivity in the hole-doped NdNiO₂.