Electronic properties of the interface between perovskite and brownmillerite phases in La_1-xSr_xCo₃O_3-δ

The family of La_1-xSr_xCoO_3-δ(LSCO) oxides are promising materials to realize low-power neuromorphic devices due to a metal to insulator transition (MIT) between different phases that can be efficiently induced by, e.g. varying the oxygen vacancy content (δ). We previously carried out a series of studies^1–3 to unravel the MIT in LSCO as a function of δ, and we investigated the transformation from a metallic perovskite (P) to an insulating brownmillerite (BM) phase. In this study, we examined the electronic properties of several LCO-P/L(S)CO_3-δ heterostructures using DFT+U method implemented in the Quantum Espresso Code. We identified values of δ, x and strain conditions under which a local closing of the gap occurs at the interface between two insulating phases. We find that when the position of the valence band edge in the L(S)CO_3-δ phase is higher than that of the perovskite phase, and concomitantly the out-of-plane Co-O-Co bond angle is larger than a threshold angle 163º, then a charge transfer to the P phase can occur. These conditions are in turn favored by higher δ values, in-plane tensile strain and Sr doping. A necessary condition for the closing of the local gap is δ>=0.375, leading to the transfer of an extra electron to the e_g orbital of the P phase, localized in proximity of the interfacial layer. The presence of metallic interfacial layers presents an opportunity to engineer atomic resistive switching states in La_1-xSr_xCoO_3-δ similar to those that one would utilize in a Mott field-effect transistor.

^1-3 S. Zhang et al., npj Comput. Mater 2020; Chem. Mat. 2021; Chem. Mat. 2022.