A $d^8$ anti-Hund's Singlet Insulator in an Infinite-layer Nickelate

TL;DR

This paper reports the discovery of an unusual $d^8$ anti-Hund's singlet insulator in a new infinite-layer nickelate, revealing novel electronic physics that could influence understanding of nickelate superconductivity.

Contribution

It introduces a new $d^8$ anti-Hund's singlet ground state in an infinite-layer nickelate, expanding the understanding of electronic phases in nickelates.

Findings

Ground state is an $e_g$ singlet with anti-Hund's spin alignment.

Large Mott gaps in both $e_g$ orbitals.

Comparison with LaNiO$_2$ highlights unique electronic structure.

Abstract

The status of nickelate superconductors in relation to cuprate high temperature superconductors is one of the concepts being discussed in high temperature superconductivity in correlated transition metal oxides. New additions to the class of infinite layer nickelates can provide essential input relating to connections or distinctions. A recently synthesized compound \bnoas, which contains isolated `infinite layer' NiO$_2$ planes, may lead to new insights. Our investigations have discovered that, at density functional theory mean field level, the ground state consists of an unusual $e_g$ singlet on the Ni$^{2+}$ ion arising from large but separate Mott insulating gaps in both $e_g$ orbitals, but with different, anti-Hund's, spin directions of their moments. This textured singlet incorporates at the least new physics, and potentially a new platform for nickelate superconductivity, which might be of an unconventional form for transition metal oxides due to the unconventional undoped state. We include in this paper a comparison of electronic structure parameters of Ba$_2$NiO$_2$(AgSe)$_2$ with a better characterized infinite layer nickelate LaNiO$_2$. We provide more analysis of the $d^8$ anti-Hund's singlet that emerges in this compound, and consider a minimally correlated wavefunction for this singlet in an itinerant background, and begin discussion of excitations -- real or virtual -- that may figure into new electronic phases.