Exotic Quantum Responses in Complex Oxide Thin Films

Nontechnical description. The project explores the intrinsic electronic and optical properties in strongly correlated electron systems. Unlike in semiconductors, which are currently utilized in information technology, electrons in these material systems are strongly coupled and collectively respond to external stimuli, thus offering responses that are beyond conventional semiconductor materials. Enabled by the improvement to synthesize vanadate and titanate thin films with exceptionally low defect concentration by hybrid molecular beam epitaxy the electronic properties of these ultrapure materials are explored. Growth experiments are performed to synthesize films of the newly proposed insulator BiVO3 and ultimately the quaternary compound Bi1-xSrxVO3. The goal is to demonstrate that electric field control over electronic properties can be realized in these materials. The doping of the strained thin films is explored to experimentally confirm that the superconducting transition temperature can be induced and enhanced by utilizing epitaxial strain. The research is expected to have significant scientific and technological impact, by providing fundamentally new insights into electronic phase transition materials that can be used in sensors, logic devices for high performance power efficient computing, and quantum computation. The program provides multidisciplinary training opportunities to graduate and undergraduate students in the area of thin film synthesis and characterization of electronic phase transition materials. Outreach activities target K-12 audiences to nurture their curiosity in science and technology.

Technical description. The project focuses on fundamental studies of growth kinetics relevant to the synthesis of complex oxide thin films with perovskite structure. Growth experiments aim to map conditions to access a self-regulated growth mode for the ternary oxide compounds BiVO3 using a combinatorial growth approach, i.e. conventional molecular beam epitaxy and chemical beam epitaxy. A growth strategy is developed for the self-regulated growth of BiVO3, which enables the thin film synthesis of the quaternary compound Bi1-xSrxVO3. A collaborative research approach is taken to analyze the quaternary system using angle resolved photoemission spectroscopy, transmission electron microscopy, scanning tunneling microscopy, and temperature dependent magnetotransport and nonlinear optical spectroscopy. The in-depth analysis of electronic and optical properties of the solid solution Bi1-xSrxVO3 opens up an entirely new design space in the strongly correlated vanadate material system, in which electric field control over electronic properties near the quantum critical point of the band-filled metal-to-insulator transition is inherently built-in. Detailed donor doping studies of the strained quantum paraelectric SrTiO3 and CaTiO3 are conducted to explore whether a superconducting phase can be induced in CaTiO3, and if the superconducting transition temperatures in SrTiO3 can be enhanced if the quantum critical behavior is stabilized at higher temperature using epitaxial strain. The overarching goal of the project is to explore new quantum phases in complex oxides exhibiting strong electron correlation effects and to understand their exotic responses.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.