Programme

O.10 -- Oral, NCTP-43

Date: Monday, 30 July 2018

Time: 16:20 - 16:40

Electronic Structures and Metal-Insulator Transition in LaNiO$_{3}$ thin films

Huy Duy Nguyen (1), Bach Thanh Cong (1), Yoshitada Morikawa (2)

(1) VNU University of Science, 334 Nguyen Trai, Thanh Xuan, Ha Noi, Vietnam; Osaka University 2-1,Yamada-oka, Suita, Osaka 565-0871, Japan

Thin films and heterostructures based on transition-metal oxides are known to host a variety of fascinating phenomena such as superconductivity, ferroelectricity, and quantum magnetism. Recent observations of LaNiO$_{3}$ thin films on a LaAlO$_{3}$ or SrTiO$_{3}$ substrate have reported the abrupt changes in the electronic structures across the metal-insulator transition, as the film thickness is confined to a few unit cells. Various proposals have been made to clarify the onset of the insulating ground state but the precise mechanism is not yet fully understood. Furthermore, we find very limited theoretical works which explicitly take into account the effects of substrates and the film thickness. This paper reports a first-principles study on the electronic structures of LaNiO$_{3}$ thin films on a SrTiO$_{3}$ substrate as a function of the surface terminating layer and the film thickness. The first-principles calculations are carried out within the framework of density-functional theory as implemented in STATE-Senri code. The electron-ion interaction is described using ultrasoft pseudopotential and the exchange-correlation interaction is treated within the generalized gradient approximation plus Hubbard \textit{U} correction with \textit{U$_{eff}$} = 6 eV. Our results show that the electronic structure evolves from semimetallic to insulating, and finally to metallic state as the film thickness is increased. In detail, the NiO$_{2}$-terminated film with one unit cell thickness shows a pseudogap at the Fermi level owing to the negative charge transfer energy. On the other hand, the 1.5-unit-cell-thick LaO-terminated film exhibits an insulating gap of 1.0 eV as a result of the large exchange splitting energy. Such a large energy gap of 1.0 eV has not been obtained by other DFT calculations, and previous proposals such as octahedral breathing distortion only account for an energy gap of approximately 0.2 eV. For thicker films with either NiO$_{2}$ or LaO termination, the metallic state is quickly restored resembling that in bulk nickelate. In other words, the insulator to metal transition is observed as the film thickness is increased. As opposed to the compensation of polar interface in the LaAlO$_{3}$/ SrTiO$_{3}$ systems, we find no charge transfer across the LaNiO$_{3}$/ SrTiO$_{3}$ interface. Our results contribute to the understanding of the interplay between dimensionality and electron correlations towards the atomic-scale control of electronic phases in transition-metal oxides.

Presenter: Nguyễn Duy Huy