Programme

P.69 -- Poster, VCTP-46

Date: Wednesday, 6 October 2021

Time: 08:30 - 10:00

Oxygen vacancy induced insulator-metal transition in LaNiO3 ultrathin films

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

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

The field of nickelates is a vivid one and recent debate about the intriguing insulator-metal transition in LaNiO3 thin films has triggered much attention [1]. Despite tremendous effort in describing the inherent mechanism, the physics of the insulating phase remains elusive. In addition, the lack of theoretical and explicit works on LaNiO3 thin films, instead of LaNiO3-based heterostructures as in the majority of publications, makes the study of the transition difficult. Recently, x-ray absorption spectroscopy results have revealed that, when the LaNiO3 films are thin and insulating, a large amount of Ni2+ is observed instead of the formal Ni3+ in thick and metallic films, and the emergence of Ni2+ is closely related to the insulator-metal transition. Indeed, our previous study has pointed out that the addition of one LaO layer on top of the 1.0-unit-cell-thick LaNiO3 film results in the formation of Ni2+ and the insulating ground state [2]. In this study, we employ first-principles study of LaNiO3 thin films on SrTiO3 substrate to demonstrate that Ni2+ is also promoted by the oxygen vacancy and the oxygen vacancy is likely an important factor in driving the insulator-metal transition. In detail, an insulating ground state, as a result of the large exchange and Coulomb interactions, is found for the 1.0-unit-cell-thick LaNiO3 film with an oxygen vacancy. The obtained energy gap of 1.2 eV is much larger than that predicted by previous publications, and is in good agreement with the results of photoemission spectra. On the other hand, the metallic state is quickly recovered for oxygen deficient films with 2.0 or larger unit cell thickness because of the negative charge transfer energy. The thermodynamic study reveals that the oxygen vacancy may stabilize the LaNiO3 film so as the oxygen vacancy-induced insulating 1.0-unit-cell-thick LaNiO3 film is the preferable structure when the film is ultrathin. In addition, the small calculated formation energy of 0.26 eV for the oxygen vacancy indicates that the insulating 1.0-unit-cell-thick LaNiO3 film may survive even the annealing process [3]. Finally, work functions of films with various surfaces are discussed. References [1] P. D. C. King, H. I. Wei, Y. F. Nie, M. Uchida, C. Adamo, S. Zhu, X. He, I. Božović, D. G. Schlom, and K. M. Shen, Nat. Nanotechnol. 59, 443 (2014). [2] H. D. Nguyen, B. T. Cong, and Y. Morikawa, J. Phys.Soc. Jpn. 87, 114704 (2018). [3] H. D. Nguyen, C. T. Bach, and Y. Morikawa, Phys. Rev. B 102, 165411 (2020).

Presenter: Nguyen Duy Huy