Transport and Carrier Dynamics Near the Metal-Insulator Transition in VO2

University Of Alabama Tuscaloosa, Tuscaloosa AL

Investigators

Abstract

Non-technical Description: Vanadium dioxide is a material that undergoes a transition from an electrical insulator to an electrical conductor just above room temperature (about 68 degree C). This behavior allows it to act as an electrical switch, which is an essential functionality required for transistors in computers. However, a complete microscopic understanding of this transition is still lacking, hindering future applications of this material. This project combines theoretical and experimental expertise of the team to develop an in-depth understanding of the microscopic mechanisms for the transition and discover new approaches to control it. Experimental characterization acts as a feedback for the further development of theoretical models, and these models also make further theory predictions to guide experimental efforts. Various outreach and education activities include: (1) providing the theoretical and experimental hands-on research experience in highly topical research areas for students; (2) participating in an international internship program to bring in undergraduates during the academic year; (3) disseminating research findings to industry through long-standing industrial collaborations; and (4) broadening the participation of under-represented groups specifically through an existing collaboration with Historically Black Colleges and Universities. Technical Description: Vanadium dioxide transforms from a low-temperature insulating phase to a high-temperature metallic phase, during which the electrical resistivity can change by a factor of as much as 100,000, accompanied by a large change in infrared reflectivity. The fundamental mechanism of the temperature-driven metal-insulator transition of bulk vanadium dioxide (VO2) at 341 K is still under debate, over fifty years after its discovery. Specifically, there is a need to understand the microscopic physics responsible for the anomalous transport properties, in particular the presence of metallic clusters in the insulating state. The mixed phase cannot be explained in terms of a simple phase separation model; nor can a simple semiconductor or percolation model explain the transport. This project combines experimental and theoretical efforts, with each providing a crucial feedback for the other. The approach addresses the relevant physical processes involved, namely electron motion correlated with ionic displacements, at a microscopic level. Detailed experimental transport studies of the mixed phase are a key part of the project, particularly electrical noise and tunneling spectroscopy. The team is also investigating novel transistor-like structures and piezoelectric-induced strain effects with a goal of an electrically controlled transition. The theoretical model contains the essential physics of VO2, by treating it as a strongly interacting electron-ion system. Non-perturbative many-body techniques are applied to study how the transition can occur and metallic clusters appear, beyond the reach of density functional theory,