Hunter Sims completed a research fellowship funded by the NSF EPSCoR Research Infrastructure Initiative Track 4 Award. During the two funded summers, he collaborated with the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory, including both virtual and in-person activities. He was accompanied in this endeavor by two undergraduate researchers from Francis Marion University (FMU): Alexander Kellerhouse and Eli Hellmig. Together with their collaborator at CNMS, they developed and implemented computational tools and work flows for building realistic nanoscale models out of quantum building blocks.

Sims has a passion for using computational theory to dig into the complex and often surprising behavior that emerges out of the atomic-scale structure of materials, especially those with possible energy applications. He spent this award period studying the electronic properties of superconducting iron selenide (FeSe) - in a layer only only three atoms thick - grown on the common substrate strontium titanate (SrTiO₃). In its superconducting state, persisting at temperatures 5 - 10 times higher than in normal FeSe, the electrons in this material have properties that are unlike other iron-based superconductors. Although FeSe / SrTiO₃ loses superconductivity well below room temperature, careful study of the system's electronic, atomic, and magnetic structure should provide insight into how to push the superconducting temperatures even higher in this or other materials.

Sims and coworkers' investigations started with quantum mechanical calculations, upon which they successfully built tools that allowed them to simulate structures hundreds of times broader or deeper than would be possible in the initial method. Using these computational approaches, they explored how small atomic variations, a missing oxygen or an extra titanium, can affect the electrons in FeSe beyond simple doping (providing "extra" electrons). Depending on the physical alignment of the top layer with respect to the substrate - and particularly with respect to these atomic variations - electrons in the substrate can mix with electrons in FeSe. When this happens, the electronic structure of the system begins to resemble what is observed in the superconducting state. They used random arrangements of the missing or extra atoms to simulate the messiness of real-world materials and showed how this messiness manifests in experimental data. More generally, this project lays out a set of procedures for connecting measurable properties like electronic spectra to atomic scale that can be difficult to directly observe experimentally.

In keeping with the values of the NSF, Sims has pursued full transparency in the research procedures that led to these results and plans to make the software his group developed freely and publicly available. He hopes that researchers studying this system and others in the ever-growing world of two-dimensional materials will find the work funded by this project useful, and he plans to continue to maintain and update the code in response to the needs of its users.

In addition to the scientific results and new research tools, the fellowship funds allowed Sims to purchase research software that will allow researchers at his home institution - a primarily undergraduate institution serving a largely rural region - to continue to carry out state-of-the-art quantum mechanical calculations on nanomaterials. The undergraduate researchers gained work experience and built technical skills, and each was able to present his work at regional and national conferences. One of the students recently received an undergraduate degree in physics and is now employed at a national laboratory. This fellowship aided Sims in establishing his research program while maintaining a strong commitment to teaching during the academic year. He plans to continue collaboration with CNMS and ORNL and to continue involving undergraduate students in his work.

Last Modified: 05/29/2023
Modified by: Hunter R Sims