****Technical Abstract**** This collaborative project will combine analytic and predictive computational theory with experimental molecular beam epitaxy (MBE) growth and characterization to investigate the physical phenomena of well-characterized epitaxial materials with dimensions on the order of one to a few unit cells. These materials are expected to have enhanced functionalities such as superconductivity emerging from the interplay of strain, proximity to the substrate, and correlations and spin-orbit interactions. Through the use of different substrates, the doping, spin-orbit strength, correlations, and magnetic coupling can be varied, thus mapping out large areas of phase space appropriate to different models of superconductivity. The material systems to be studied include the three-dimensional topological insulators (TI) and Fe-based superconducting thin films grown by MBE on various substrates. The research will address issues related to 1) the proximity-induced superconductivity in 3D TIs; 2) the effect of interfacial bonding on superconducting properties of ultrathin FeSeTe films; 3) the polarization doping of Fe-based superconductors at interfaces; and 4) the strain-induced normal to topological insulator transitions. In situ scanning tunneling microscopy and spectroscopy will be used to study the electronic and superconducting properties, coupled with ex situ scanning transmission electron microscopy and transport measurements. Density functional theory calculations will be used to determine the normal state properties of the bulk and surface materials and, with the ability to vary parameters such as spin-orbit coupling strengths, these calculations will constrain and provide realistic input to different models of superconductivity. Together, the various calculations will help optimize the experiments in an iterative manner, as well as providing a deeper understanding of the physics.
Advances in predictive computational electronic structure theory, models of correlated systems, and the ability to engineer the structure of materials at the atomic scale provide the opportunity of synthesizing materials with specific properties. In this project, experiment and theory will work closely and iteratively to produce "designer materials" that can further our understanding of the fundamental materials physics as well as being of potential technological use. Specifically, well-characterized epitaxial films of topological insulators and iron-based superconductors a few atomic layers thick will be grown on different substrates. At these spatial dimensions, properties can vary significantly compared to the bulk, possibly generating novel physics and emergent functionalities through the interplay of strain, proximity to the substrate, correlations, and spin-orbit interactions. Participating graduate and undergraduate students benefit from a highly collaborative and multidisciplinary research training.
