****NONTECHNICAL ABSTRACT**** This award supports experimental condensed-matter physics research on the unusual physical properties near the interface of films and multilayers of complex oxide compounds. These artificially structured materials, which exhibit exotic functionalities, are relevant to future advanced electronic devices and energy applications. Especially, the interfaces of these materials generally show new properties different from the corresponding bulk compounds. On the other hand, almost all electronic devices began with an understanding of interface barrier formation, electronic/magnetic structure, and control: the interface is the device. Incorporating the diversity of physical properties of complex oxides into devices needs to begin with a basic understanding. What is very exciting is that modern oxide thin-film growth and probe techniques give the requisite atomic-scale precision to construct interface structure and manifest its electronic properties such as metal to insulator transition, which is exactly the research in this project. The goal of this research is to understand and ultimately learn how to manipulate the transport properties at the interfaces of oxide materials and thus to design new functionalities for next generation of electronic devices. Moreover, a significant product of this endeavor will be to integrate materials fabrication, characterization, analysis and design into a unique research and educational program for the basic understanding of complex oxide interfaces and the training of the future science and technology (S&T) workforce. Both graduate and undergraduate students will be involved in both material fabrication and characterization efforts and they will develop the essential ability to think "chemically" and "physically".
This research project is designed to study how the broken symmetry at a surface or interface and spatial confinements can be used to manipulate the emergent properties in a thin film of transition metal oxide (TMO) materials, especially the metal-insulator transition (MIT). The objective will be achieved by utilizing a comprehensive set of in-situ materials growth and atomically-resolved characterization capabilities. It is becoming increasingly clear that surfaces, interfaces, and thin films of TMOs display a rich diversity of fascinating properties that are related to, but not identical to, the bulk phenomena. MIT is one of these emerging properties. While the mechanisms for these phenomena are still hotly contested. Possible explanations for these phenomena include doping with electrons and oxygen vacancies, interdiffusion and lattice distortion. This project is going to take advantage of atomic characterization combined with in-situ growth capability to explore the role of strain, defects and oxygen vacancies, chemical composition and interface structural distortion in MIT emerging at interfaces and thin films. Specifically, it will focus on thin films of (Sr,Ca)RuO3 and (Sr,Ca)VO3, two well-known pseudo-cubic metallic perovskites, but at the verge of MIT. By varying strain induced by substrate, film thickness, cation-site composition, the project aims to gain insight into the interplay of Jahn-Teller distortion and electron-electron correlation driving the MIT.
This project focused on research of the unusual physical properties near the surfaces, interfaces and films of complex oxide compounds, especially the metal-insulator transition (MIT). By exploiting our unique combination of in-situ materials growth and atomically-resolved characterization capabilities, we have studied the surface properties and ultrathin films of several compounds such as SrTiO3, La1-xSrxMnO3, doped Sr3Ru2O7, and Fe-based superconductors. We have found some interesting correlations between electronic/transport properties and structural relaxation as well as chemical composition. These findings include: 1) The observation of strain-mediated defect superstructure on SrTiO3(110); 2) Unusual surface structure and electronic properties as well a quasi-two dimensional magnetic structure of Mn-doped Sr3Ru2O7; 3) The observation for the conduction quantization in oxide-based resistive switching memory, thus providing experimental evidence for nanoscale filament transport character in oxide materials; 4) The role of oxygen vacancies in the thickness-driven MIT in the ultrathin La1-xSrxMnO3 films, thus identifying the disorder-induced localization effects as the driving force for the non-metallic behavior in the ultrathin metallic oxide films; and 5) the observation of enhanced spin-lattice coupling on the surface of Fe-based superconductors and its correlation with the enhanced surface orthorhombicity. These findings enhance our understanding of the surfaces and thin films of this class of correlated electron materials and also provide some important fundamental knowledge to manipulate the transport properties at the interfaces and thin films, and thus to design new functionalities for next generation of electronic devices. Moreover, a significant product of this endeavor is to integrate materials fabrication, characterization, analysis and design into a unique research and educational program for the basic understanding of complex oxide interfaces and the training of the future science and technology (S&T) workforce. Both graduate and undergraduate students have been involved in both material fabrication and characterization efforts and they will develop the essential ability to think "chemically" and "physically". The project has supported or partially supported few graduate students, including three of them who received Ph. D. degrees (one of them is a minority student and two of them are female). Two undergraduates have also been involved in the project and one of them is in the national top-ranking materials science graduate program. Furthermore, this project has enhanced our international collaboration on research, research training and educational outreach, including the dual-degree program between LSU and Institute of Physics, Chinese Academy of Sciences.
