The Generation and Understanding of a Quantum Structure
At the heart of this research program is the concept that oxide epitaxy is the growth of a quantum structure. This quantum structure is defined by the interplay of the substrate, substrate-film interface, the actual film, and the film-vacuum interface. The effects of the substrate-film and film-vacuum interfaces can very strongly influence the overall physical properties of an epitaxial ultra-thin Transition Metal Oxide (TMO) film, primarily because the rich and varied properties of TMOs are highly sensitive to small changes in lattice parameter and symmetry. In order to manipulate properties of TMO thin films effectively via epitaxy -- and, ultimately, to develop a complete theory of oxide epitaxy -- the full quantum structure formed by the growth of a thin film must be taken into account.
The theoretical side of the program (in collaboration with Dr. Ilya Elfimov) concentrates on realistic calculations of the full quantum structure. This is done using ab-initio methods of Density Functional Theory (DFT) supplemented with some aspects of electron correlation. Rapid advances in basic theory and new algorithms have made it possible to study larger systems and obtain unique, non-empirical information about structural, electronic, vibrational and transport properties. Also accessible are dynamical processes, such as diffusion on the surface or through an interface, which can be used to study the basic mechanisms in material preparation and growth.
The theoretical work acts as an inspiration and guide for the experimental side of the program. The epitaxial growth of the TMOs on oxide substrates is done in an ultra-high vacuum Molecular Beam Epitaxy chamber, which is equipped with some of the most powerful tools to study the resulting quantum structure. These tools include Low Energy Electron Diffraction, Scanning Tunneling Microscopy and Spectroscopy, Core level X-ray Photoemission, and Angle Resolved Photoemission Spectroscopy. The combination of these tools gives direct experimental access to the surface chemistry, crystallographic structure, and electronic structure.
The feedback from the close integration of the theoretical and experimental portions of this program should help to isolate materials that exhibit unique properties within this type of quantum structure, and also clarify the fundamental underpinnings of epitaxy in the oxides.
The growth of ultra-thin oxide films for spectroscopic studies
Transition metal oxides (TMO) make up one of the most fascinating classes of inorganic materials. They show an extremely wide range of phenomena and do not fit easily into a simple theoretical framework. For these reasons, they have been one of the primary playgrounds for the condensed matter community to decipher the underlying principles in solids. The wide array of properties seen in TMOs is due to the close proximity of their various energy scales. Small changes in experimentally accessible parameters -- such as temperature, pressure, and doping -- are often enough to subtly rearrange the order of the energy scales, thereby radically changing the properties of these materials. One parameter which has yet to be fully explored, and which can also strongly alter the properties of TMOs, is the presence, absence, or proximity of neighboring atoms.
Surfaces, interfaces, and vacancies in ultra-thin films encompass a very powerful means of exploring this parameter. By controlling and manipulating the presence, absence, or proximity of neighboring atoms in the TMOs in this context, one can potentially expose new electronic properties, and thereby effectively create new materials. For example, the systematic absence of Ca in CaO has been theoretically predicted to transform CaO from a nonmagnetic insulator to a half-metallic ferromagnet. Similarly, the change in atomic spacing, achieved via the presence of an interface, has been shown to drive V2O3 through a metal-insulator transition.
From an experimental point of view, ultra-thin films have a physical form that is particularly well suited to some of the most recent high resolution techniques for studying electronic structure and electron dynamics, namely: Angle Resolved Photoemission Spectroscopy (ARPES) and Scanning Tunneling Spectroscopy (STS). Both of these techniques are surface sensitive, and therefore avoid any complications from the substrate used to grow the ultra-thin film. They also both require a vacuum clean and smooth surface, which is by default generated during high quality oxide film growth. In many ways the full instrumental integration of ultra-thin film growth with these momentum and real space electronic structure probes (ARPES and STS, respectively) presents an exciting new direction for the field of condensed matter physics.
The main focus of this research program is twofold. Primarily, it focuses on the molecular beam epitaxial growth and characterization of TMOs in ultra-thin film form, -- concentrating on those that are conducive to manipulating the presence, absence, or distance of neighboring atoms. Additionally, it includes a strongly collaborative effort to explore the electronic structure of these novel materials by using, in an integrated fashion, the expanding facilities present in the AMPEL and the Canadian Light Source. ARPES and STS are the principal tools used to study the electronic structure and electron dynamics of these ultra-thin films, however, the research will ideally incorporate many other exciting new techniques that are still coming on-line, such as inelastic x-ray scattering.