Complex oxide thin films exhibit a wide range of unique phenomena that have been investigated for use in technologies ranging from renewable energy, data storage, and quantum information processing. Perovskite oxides with the formula ABO₃ represent a particularly interesting class of complex oxides because of the wide array of transition metal ions that can occupy the B site of the material. To date, most research has focused on 3d transition metals on the B site, such as cobalt. However, emerging work focused on 5d transition metal B site ions has shown that these materials exhibit stronger spin-orbit coupling and a wide array of correlated electronic phenomena. Many of these properties are modulated through charge transfer across epitaxial interfaces or between cations in ordered phases.

We will employ a hybrid molecular beam epitaxy technique for the delivery of refractory tantalum and iridium cations and examine charge transfer to cobalt. Through systematic studies of superlattices and double perovskites that combine the three B site cations, we will validate models governing charge transfer in oxide systems. Additionally, in situ experiments at the Advanced Photon Source will measure charge transfer and cation ordering in films during the growth process using X-ray absorption spectroscopy and X-ray diffraction. Ultrafast optical spectroscopy will be employed to examine the electronic structure and induced polarization in these materials that results from interfacial charge transfer. Films and heterostructures synthesized in this work will also be examined to probe emergent electronic and magnetic topological phenomena.

In this EPSCoR-State/National Laboratory Partnership, we will use in situ techniques to examine the role of charge transfer in epitaxial oxide superlattices and ordered double perovskite thin films. Partners include Argonne National Laboratory, Brookhaven National Laboratory, and Pacific Northwest National Laboratory. Collectively, this work will advance our synthesis capabilities and provide a fundamental understanding of interfacial charge transfer in complex oxides heterostructures, critical for the development and control of new materials for quantum information technologies and renewable energy as highlighted in the Department of Energy Basic Energy Sciences roadmap.