Nature of the metal-insulator transition in few-unit-cell-thick LaNiO3 films

TL;DR

This study investigates the metal-insulator transition in ultrathin LaNiO3 films, revealing that oxygen vacancies and film thickness critically influence the transition, with metallic behavior persisting down to 1.5 unit cells.

Contribution

It provides detailed experimental and theoretical insights into how oxygen vacancies and film thickness affect the metal-insulator transition in LaNiO3 ultrathin films.

Findings

Metallic behavior persists down to 1.5 unit cells thickness.

Oxygen vacancies contribute to the transition.

High-quality films were grown using atomic layer-by-layer laser MBE.

Abstract

The nature of the metal insulator transition in thin films and superlattices of LaNiO3 with only few unit cells in thickness remains elusive despite tremendous effort. Quantum confinement and epitaxial strain have been evoked as the mechanisms, although other factors such as growth-induced disorder, cation non-stoichiometry, oxygen vacancies, and substrate-film interface quality may also affect the observable properties in the ultrathin films. Here we report results obtained for near-ideal LaNiO3 films with different thicknesses and terminations grown by atomic layer-by-layer laser molecular beam epitaxy on LaAlO3 substrates. We find that the room-temperature metallic behavior persists until the film thickness is reduced to an unprecedentedly small 1.5 unit cells (NiO2 termination). Electronic structure measurements using x-ray absorption spectroscopy and first-principles calculation suggest that oxygen vacancies existing in the films also contribute to the metal insulator transition.