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

Abstract

The nature of the metal-insulator transition in thin films and superlattices of LaNiO3 only a 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 of 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.

Depolarizing-Field Effects in Epitaxial Capacitor Heterostructures

“…Because most of the commonly used metal-oxide electrodes exhibit a thickness-dependent conductivity [64][65][66][72][73][74], our conclusions are independent of the choice of the oxide material and the epitaxial growth technique. The ability to tune the depolarizing field during the deposition goes beyond the capacity to engineer the domain formation during growth.…”

Depth-resolved charge reconstruction at the LaNiO3/CaMnO3 interface

“…Both of the above mentioned mechanisms require an interfacial charge reconstruction, which results in the presence of the transition-metal cations with the valence states other than the formal valence state of the stoichiometric compounds (LaNi 3+ O 3 and CaMn 4+ O 3 ). Such charge reconstruction, resulting in the formation of off-stoichiometric cations (Ni 2+ and Mn 3+ ) at the interface, can be explained in terms of the charge leakage from LaNiO 3 to CaMnO 3 [13][14][15], as well as the presence of oxygen vacancies driven to the interface by polar compensation [21][22][23][24].…”

“…In superlattices with thinner LaNiO 3 , which is closer to the metal-insulator transition and hence more resistive transport properties, Ni 2+ -O-Mn 4+ superexchange interaction (made possible by oxygen vacancies driven to the interface) has been identified as the source of the interfacial ferromagnetism [14,15]. On the other hand, oxygen vacancies have also been observed in coherently strained epitaxial CaMnO 3 [21] and LaNiO 3 [22][23][24] and, in some cases, have been both predicted and observed to segregate at the interfaces due to polar energetics at the interface [22,23].…”

Interface-engineered hole doping in Sr2IrO4/LaNiO3 heterostructure

“…In the Sr 2 IrO 4 , two atomic planes of SrO are observed between each layer of IrO 2 in accord with the expected layered-perovskite structure. To avoid extra conducting channels, the electron acceptor LaNiO 3 layers were designed thin enough (∼2 unit cells) to be insulating [23]. Sharp interfaces between Sr 2 IrO 4 and LaNiO 3 resulting from the layer-by-layer structure of the heterostructure are also evident from the STEM images.…”

“…In the following, we propose a possible scenario to interpret the electronic reconstruction at Sr 2 IrO 4 /LaNiO 3 interface. As seen in figure 3(d), bulk LaNiO 3 is a negative charge-transfer metal, whose Fermi surface lies in hybridized O 2p and Ni 3d state [35,36]. Due to the strong O-Ni hybridization, the electronic structure of bulk LaNiO 3 is best represented as a mixture of 3d 7 , 3d 8 L and 3d 9 LL(L is a hole on oxygen 2p orbital) [37].…”