Open Access Dissertation
Date of Award
Physics and Astronomy
Among strongly correlated systems, vanadium dioxide (VO2) shows a metal insulator transition (MIT) near room temperature (340K). Both Mott and structural transitions contribute to the MIT in VO2. To gain a better understanding of the changing electronic structure, we perform quantum capacitance measurement. Quantum capacitance measurement has already yielded insight into a variety of systems, including the negative compressibility for strongly interacting charges in GaAs two-dimensional charges. Our work demonstrates a unique method to accurately distinguish the quantum capacitance from large resistance changes at the MIT by using a home-made capacitance bridge. We observe a steep increase in the density of states (DOS) near Fermi energy as the sample approaches a metallic state upon heating. Our work is the first experimental study directly to probe the DOS during the MIT in VO2 and is important for unraveling the long-standing mystery behind the driving mechanism for this phase change. Additionally, the bridge method for measuring the quantum capacitance in a highly resistive sample can be readily applied to other systems that exhibit a MIT, which is universal to many systems.
The consequences of electron-electron interactions are far-reaching beyond the transition metal oxides. Present research seeks to understand the role of e-e interactions and whether these can drive a strongly correlated ground state such as Wigner crystal or Wigner glass. One of the major barriers to experimental progress in this area is the difficulty of fabricating high purity samples with dilute charges. Furthermore, making Ohmic contact to dilute charge systems represents a significant challenge. A significant portion of this work is to fabricate ultra-high purity devices using both doped p-type GaAs/AlGaAs quantum square wells and un-doped heterojunction gated field effect transistors (HIGFETS). These samples demonstrate excellent mobility at low charge densities, allowing us to identify pinning behavior in a reentrant insulating phase near in the fractional quantum Hall regime and also to probe the transport between two edges of a topological insulator. These studies are critical to understanding the physics of strongly correlated charges and their relation to topological phases, which is a fascinating area of intense current research.
Wu, Zhe, "Quantum Capacitance Study Of Novel Two- And One-Dimensional Systems" (2017). Wayne State University Dissertations. 1756.