Studying Charge Transfer Phenomena in the Interface of SrCoO3 / SrIrO3 Superlattices and Sr2CoIrO6 Double Perovskites Grown by Molecular Beam Epitaxy

Abstract

When two different transition-metal oxide thin films are stacked together, charge transfer can occur across the interface in certain cases. Interfacial charge transfer has been experimentally established as a promising mechanism to induce emergent electronic and magnetic phenomena. However, there is no established theoretical framework to predict or explain charge transfer in Transition metal oxides and experimental verification of existing theories is critical towards bridging this gap.

Motivated by the prediction of interfacial charge transfer in 3d-5d oxide heterostructures by Phys. Rev. X 7, 011023 (2017), high quality epitaxial (SrIrO3)n/SrCoO3)m (SIO/SCO) superlattices were grown using Molecular Beam Epitaxy (MBE). Scanning transmission electron microscopy and X-ray diffraction data confirmed excellent structural and crystalline quality of the films. X-ray absorption study (XAS) of Co L and O K edges confirms the perovskite nature of the SCO films. While SrCoO3 underwent significant degradation when exposed to air for over 24 hours, it remained intact even after 6 months, when incorporated into a superlattice with SrIrO3. This is likely because that Ir donates electron to Co and makes the structure stable.

Charge transfer was confirmed using synchrotron-based polarization-dependent hard X-ray absorption spectroscopy on Co K and Ir L2,3 edges. The findings were complemented by In-vacuo X-ray photoelectron spectroscopy (XPS) and ex-situ Hard X-ray photoelectron spectroscopy (HAXPES) of Co 2p spectrum. An anisotropy between in-plane and out-of-plane charge transfers was observed indicating a corresponding anisotropy in the electronic structure. This suggests that the interface added another degree of symmetry-breaking, in consistent with previous studies. Polarization-dependent Ir L2 edge data indicated a strain-induced orbital polarization in SrIrO3 layers arising due to charge transfer or polar distortions due to interfacial effects. Angle-dependent pre-edge data on Co K edges indicated minimal Co 3d-4p mixing suggesting that SrCoO3 layers in the superlattice remained just as distorted Octahedral coordination. A stronger hybridization effect was observed towards out-of-plane with the decreasing SIO : SCO layers ratio. Charge transfer was observed in Sr2CoIrO6 double perovskite films with a higher ratio of Co2+ than the superlattices. These findings provide new insights into charge transfer mechanisms in metallic transition metal oxides, offering pathways to improve existing theoretical frameworks and explore novel interfacial physics.