[PAST EVENT] Tyler James Huffman: Physics Dissertation Defense
Abstract: The pairing of all of the vanadium ions in the M?1 insulating phase is a salient feature of the structural transition that accompanies the metal-insulator transition in bulk vanadium dioxide (VO2). The pairing and unit-cell doubling have long been thought critical to the emergence of insulating behavior. Interestingly, in addition to the more common M1 phase, there exist two additional insulating phases, M2 and T, which have significantly different V-V pairing. With infrared and optical photon spectroscopy, we find that the inter-band transitions and the energy gap are insensitive to the different lattice structures. This result conclusively establishes that intra-atomic Coulomb repulsion, not V-V pairing, provides the dominant contribution to the energy gap in all insulating phases of VO2. VO2 is a candidate material for novel technologies, including ultrafast data storage, memristors, photonic switches, smart windows, and transistors which move beyond the limitations of silicon. The attractiveness of correlated electron systems for applications lies in the sensitivity of their novel properties to external parameters. For advances in fundamental physics and applications, it is imperative that these properties be measured over a wide range of regimes. We have studied the effects of compressive strain in a VO2 thin film, and the intrinsic anisotropy of the infrared properties in a single-domain VO2 crystal. Moreover, we report that the patterns of phase coexistence in a polycrystalline VO2 film are reproducible on repeated heating and cooling cycles. This reproducibility promises that reliable nanoscale VO2 devices can be realized.
Bio: My love of learning was established early on by my mother and grandmother, who between them combine for almost 80 years in elementary education. Mom is still going strong. She?ll make it to 45 years. My fascination with the way things work was inspired by my grandfather. Despite an 8th grade education, he was something of a legend when it came to building and fixing things. He remains the most brilliant technical mind I?ve ever encountered. Any talents I may have are a shadow of his own. I was born in Kittanning, Pennsylvania in 1987. I received my B.S. degree from Muhlenberg College in 2010. I was lucky enough to be the first graduate student through the door at Mumtaz Qazilbash?s lab. There, I gained the invaluable experience and the satisfaction that comes from building a lab from the beginning. It was rarely straightforward, but it all (mostly) worked in the end. I was fortunate for the opportunity to work on vanadium dioxide for my Ph.D., one of the great and classic questions of condensed matter physics.