Electronic structure of the parent compound of superconducting infinite-layer nickelates

TL;DR

This paper investigates the electronic structure of infinite-layer nickelates, revealing a hybridized state with metallic and strongly correlated components, offering insights into their superconductivity mechanism similar to heavy fermion systems.

Contribution

It provides a detailed analysis of the electronic structure of RNiO2 nickelates, highlighting their hybrid metallic and correlated states, which differ from cuprates and suggest a new perspective on their superconductivity.

Findings

Presence of a weakly-interacting 5d metallic state in the spacer layer.

Hybridization of metallic and strongly correlated states in NiO2 layers.

Infinite-layer nickelates resemble heavy Fermion intermetallics in electronic structure.

Abstract

The search for oxide materials with physical properties similar to the cuprate high Tc superconductors, but based on alternative transition metals such as nickel, has grown and evolved over time. The recent discovery of superconductivity in doped infinite-layer nickelates RNiO2 (R = rare-earth element) further strengthens these efforts.With a crystal structure similar to the infinite-layer cuprates - transition metal oxide layers separated by a rare-earth spacer layer - formal valence counting suggests that these materials have monovalent Ni1+ cations with the same 3d electron count as Cu2+ in the cuprates. Here, we use x-ray spectroscopy in concert with density functional theory to show that the electronic structure of RNiO2 (R = La, Nd), while similar to the cuprates, includes significant distinctions. Unlike cuprates with insulating spacer layers between the CuO2 planes, the rare-earth spacer layer in the infinite-layer nickelate supports a weakly-interacting three-dimensional 5d metallic state. This three-dimensional metallic state hybridizes with a quasi-two-dimensional, strongly correlated state with 3dx2-y2 symmetry in the NiO2 layers. Thus, the infinite-layer nickelate can be regarded as a sibling of the rare earth intermetallics, well-known for heavy Fermion behavior, where the NiO2 correlated layers play an analogous role to the 4f states in rare-earth heavy Fermion compounds. This unique Kondo- or Anderson-lattice-like "oxide-intermetallic" replaces the Mott insulator as the reference state from which superconductivity emerges upon doping.