Thomas Zac Ward

Configurationally Complex Materials: Entropy stabilization can be used during synthesis to create ionically and covalently bonded materials in the form of high entropy oxides (HEO) where 5 or more elements can reside on a single sublattice. Unlike metal-metal bonded high entropy alloys, the presence of an anion sublattice in HEOs opens a wide range of functionalities resulting from electron localizations and complex bonding between resident cations. Our interest lies in how these complex interactions might be used in strongly correlated systems where emergent phenomena are known to result from variant competing local energy scales. We use Pulsed Laser Deposition to drive non-equilibrium growth conditions that have enabled us to produce the world's first examples of single crystal perovskite, spinel, and Ruddlesden-Popper configurationally complex phases. Careful control over how the individual sublattices are populated is providing new opportunities for magnetic, structural, and thermal manipulation. This exceptional tunability over element-specific parameters on a structurally ordered lattice, open many exciting possibilities in both the Quantum Materials and Green Energy Materials arenas.

Metastable Quantum Materials: Electronic phase separation in complex materials has been linked to diverse exotic behaviors, such as colossal magnetoresistance, the metal–insulator transition, and high-temperature superconductivity. In these materials, competing regions with drastically different electronic properties can form nanoscopic and mesoscopic phase domains within the macroscopic crystal matrix. We are working to understand how these coexisting self-organized phases form and drive functionality. We are also interested in utilizing these coexisiting phases to access novel electronic behaviors that may be critical to the development of new logic states accessible when lowering device dimensionality to the scale of the inherent competing phases. Recent work has shown that these domains can be moved to form rewritable circuits and that new types of multimodal properties can be controlled through multiple stimuli. The huge catalog of phase-separated materials, each offering unique combinations of competing phases, may aid design of specialized architectures for targeted multifunctional applications not possible with current single-chip, single-function approaches--a critical step in moving beyond Moore.

Strain Doping: Helium ions can be used to control the length of a single axis in an epitaxial crystal lattice. Our recent work has demonstrated that insertion of non-binding He atoms into functionally important materials allows never before possible control over electronic orbital populations, perovskite octahedra tilts, optical bandgaps, morphotropic phases composition, and magnetic anisotropy.  The control of lattice distortions also provides a novel means of designing Berry Phase and resulting magnetotransport behaviors. This method offers a critical advancement in our ability to minutely design structural properties which drive many intriquing behaviors, such as superconductivity, multiferroicity, and magnetic textures.

Ionic Control of Strongly Correlated Solids: This project blends ORNL expertise in materials physics, chemistry, microscopy, and neutron sciences to advance a new method of designing materials properties and function through manipulation of solid crystals using interfaced ionic liquids. Ionic liquids have been well studied for use in catalytic and electrochemical applications. However, a new use for these liquid salts has recently emerged as a novel method for investigating electrostatic and electrochemical effects on fundamental physics problems in strongly correlated materials. Here, ionic liquids are interfaced with films or bulk crystals to create field effect heterostructures that allow small applied biases (< 2V) to control charge densities at the interface which are orders of magnitude higher than traditional solid-solid field effect transistors. Our efforts have focused on understanding the solid switching mechanisms, solid-liquid interface structure, field geometries, and accessing new functional properties using these interfaces.