Confinement-induced metal-to-insulator transition in strained LaNiO$_3$/LaAlO$_3$ superlattices

TL;DR

This study uses density functional theory to show how strain and confinement induce a metal-insulator transition in LaNiO3/LaAlO3 superlattices, revealing asymmetric responses to tensile and compressive strain.

Contribution

It demonstrates the strain-dependent electronic phase transition and the role of confinement in LaNiO3/LaAlO3 superlattices, highlighting the asymmetric effects of tensile and compressive strain.

Findings

Tensile strain opens a band gap via charge disproportionation.

Compressive strain collapses the band gap, leading to a semimetallic state.

Increasing superlattice thickness restores metallicity.

Abstract

Using density functional theory calculations including a Hubbard $U$ term we explore the effect of strain and confinement on the electronic ground state of superlattices containing the band insulator LaAlO$_3$ and the correlated metal LaNiO$_3$. Besides a suppression of holes at the apical oxygen, a central feature is the asymmetric response to strain in single unit cell superlattices: For tensile strain a band gap opens due to charge disproportionation at the Ni sites with two distinct magnetic moments of 1.45$\mu_{\rm B}$ and 0.71$\mu_{\rm B}$. Under compressive stain, charge disproportionation is nearly quenched and the band gap collapses due to overlap of $d_{3z^2-r^2}$ bands through a semimetallic state. This asymmetry in the electronic behavior is associated with the difference in octahedral distortions and rotations under tensile and compressive strain. The ligand hole density and the metallic state are quickly restored with increasing thickness of the (LaAlO$_3$)$_n$/(LaNiO$_3$)$_n$ superlattice from $n=1$ to $n=3$.