Atomic-Scale Heterostructures of Iridates and Ruthenates

This proposal aims to investigate atomic-scale heterostructures consisting of 5d iridates and 4d ruthenates. Both systems have received great attentions due to the comparable energy scale of electron-correlation (U) and spin-orbit interaction (SOI) inherent in 5d and 4d orbitals. Whereas tuning the parameters of U and SOI in condensed matter is thought to provide a fascinating platform for creating nontrivial electronic and magnetic states, experimental realization of the tunable U and SOI is a formidable task. The idea of this proposal is to investigate nanoscale heterostructures, where 5d iridates and 4d ruthenates can interact strongly to each other across the ideal two-dimensional interfaces. According to a few recent studies and our own preliminary data, the iridate/ruthenate heterointerfaces show long-range coupling of U and SOI, resulting in strong renormalization of exchange interactions and magnetic order. These observations cannot be understood by simple proximity or screening effects. One can ask, “What microscopic states are realized in the iridate/ruthenate heterostructures?” To answer this question, we will first improve the sample quality of thin-film heterostructures consisting of iridates and ruthenates using an advanced pulsed laser deposition system equipped with dual in-situ, real-time monitoring techniques. We will investigate their microscopic structures using resonant X-ray scattering and high-resolution transmission electron microscopy. Our main effort is to find experimental evidence of novel electronic and magnetic states by characterizing our samples using advanced spectroscopic tools such as resonant inelastic X-ray scattering, Raman spectroscopy, and infrared spectroscopic ellipsometry. We will also develop comprehensive pictures for understanding the strong interfacial interactions between iridates and ruthenates. Intellectual Merit: The confluence of strong spin-orbit interaction (SOI) and electron-correlation (U) in iridates and ruthenates can result in exotic ground states, which can lead to a breakthrough in our understanding of fundamental condensed matter physics. Current experimental approaches for exploring the novel properties of ruthenates and iridates requires a new tool capable of tuning the physical parameters. The atomic-scale epitaxial heterostructure approach of this proposal is considered one to make a model system, in which exotic ground states of strongly-correlated, spin-orbit coupled electrons can emerge. This protocol offers tunability in controlling the dimensionality and the lattice-symmetry via strong interactions at two-dimensional interfaces. Tuning the spin-orbit interaction and electron-correlation will compel the extended orbitals of 5d and 4d electrons to exhibit unprecedented electromagnetic states. Since these artificial heterostructures will stabilize meta-stable quantum phases as these parameters are varied, this approach offers a way to investigate the phase diagrams of exotic ground states. The outcomes of this project will fill the existing gap between condensed matter experiments and theories, and lead us to a better understanding of strongly-correlated, spin-orbit coupled electrons in condensed matter. The preliminary results of the PI's group have shown that they can create high-quality epitaxial heterostructures of 4d ruthenates and 5d iridates, which is a crucial step to initiate this project. Broader Impact: The proposed research activities are intertwined with educating both graduate and undergraduate students. We are in dire need of experimental condensed matter physicists with strong expertise in advanced materials synthesis and characterization in the United States, for both academia and industry. The PI will provide his students with the education and training in state-of-the-art materials synthesis and opportunities of collaborating with scientists at National Labs. These opportunities will help them to be successful in their careers. The PI also values the education of younger generations in science and technology areas. He will improve the awareness of STEM careers in under-educated and under-represented areas by developing a research summer program for pre-service teachers and implementing outreach visits to local high school classrooms in Kentucky. These outreach programs will provide not only a unique research experience to share but also a heightened appreciation of the development of cutting-edge science in general.