Quantum Confinement Induced Molecular Mott Insulating State in La$_4$Ni$_3$O$_8$

TL;DR

This paper demonstrates a novel quantum confinement mechanism leading to a Mott insulating state in La$_4$Ni$_3$O$_8$, without charge order, driven by spin-aligned Ni $d_{z^2}$ states across layers.

Contribution

It introduces a new mechanism for Mott insulation in layered nickelates, emphasizing quantum coupled states over charge order, supported by density functional calculations.

Findings

Mott insulating state arises without charge order.

Quantum coupled Ni $d_{z^2}$ states drive insulation.

Weak magnetic coupling explains magnetic transition features.

Abstract

The recently synthesized layered nickelate La$_4$Ni$_3$O$_8$, with its cuprate-like NiO$_2$ layers, seemingly requires a Ni1 ($d^8$)+2Ni2 ($d^9$) charge order, together with strong correlation effects, to account for its insulating behavior. Using density functional methods including strong intra-atomic repulsion (Hubbard U), we obtain an insulating state via a new mechanism: {\it without charge order}, Mott insulating behavior arises based on quantum coupled, spin-aligned Ni2-Ni1-Ni2 $d_{z^2}$ states across the trilayer (rather than based on atomic states), with antiferromagnetic ordering within layers. The weak and frustrated magnetic coupling between cells may account for the small spin entropy that is removed at the N\'eel transition at 105 K and the lack of any diffraction peak at the N\'eel point.