Infinite Layer LaNiO(2): Ni(1+)is not Cu(2+)

TL;DR

This study uses ab initio calculations to explore the electronic structure of LaNiO$_2$, revealing reduced $3d-2p$ mixing, La $5d$ contributions, and complex correlation effects leading to a Ni$^{2+}$-like state with unique magnetic properties.

Contribution

It provides a detailed theoretical analysis of LaNiO$_2$'s electronic and magnetic properties, highlighting the effects of site energy separation and correlation effects not previously understood.

Findings

Reduced $3d-2p$ mixing due to increased site energy separation.

La $5d$ states contribute to Fermi surface pockets and hole doping.

Correlation effects induce Mott transitions and complex magnetic states.

Abstract

The Ni ion in LaNiO$_2$ has the same formal ionic configuration $3d^9$ as does Cu in isostructural CaCuO$_2$, but it is reported to be nonmagnetic and probably metallic whereas CaCuO$_2$ is a magnetic insulator. From ab initio calculations we trace its individualistic behavior to (1) reduced $3d-2p$ mixing due to an increase of the separation of site energies ($\epsilon_d - \epsilon_p$) of at least 2 eV, and (2) important Ni $3d(3z^2-r^2)$ mixing with La $5d(3z^2-r^2)$ states that leads to Fermi surface pockets of La $5d$ character that hole-dope the Ni 3d band.Correlation effects do not appear to be large in LaNiO$_2$. However, ad hoc increase of the intraatomic repulsion on the Ni site (using the LDA+U method) is found to lead to a novel correlated state: (i) the transition metal $d(x^2-y^2)$ and $d(3z^2-r^2)$ states undergo consecutive Mott transitions, (ii) their moments are antialigned leading (ideally) to a "singlet" ion in which there are two polarized orbitals, and (iii) mixing of the upper Hubbard $3d(3z^2-r^2)$ band with the La $5d(xy)$ states leaves considerable transition metal 3d character in a band pinned to the Fermi level. The magnetic configuration is more indicative of a Ni$^{2+}$ ion in this limit, although the actual charge changes little with U.