2024-2025 Kavli Heising-Simons Junior Fellow
Faculty Advisor: Professor Jeff Neaton
At the moment, Stephen is a 5th year physics Ph.D. student in Jeffrey Neaton’s research group. Prior to Berkeley, he graduated from Ohio State University in 2019 with a B.S. in physics and two minors in mathematics and computational science. His current work focuses on the use of condensed matter theory to calculate the properties of materials from first principles. Specifically, he uses a combination density functional theory (DFT) and many body perturbation theory (MBPT) to compute quantities such as the ground state electron density, the band structures of quasiparticles (e.g. electrons, phonons or excitons), and optical absorption spectra.
Stephen's current research focus is using novel algorithms to compute exciton and exciton-phonon interactions in solids. Recently, he has been a part of efforts to use wavefunction interpolation to obtain a highly converged non-uniform sampling of exciton wavefunctions in the Bethe Salpeter Equation, dramatically speeding up exciton binding energy convergence and allowing for the renormalizing effects of lattice vibrations to be accounted for. As a part of these efforts, he has sped up the interpolation process by a factor of ~40x, enabling the study of more complex materials. He intends to apply these advancements in two ways.
First, he would like to benchmark the use of a dense non-uniform patch sampling of the exciton wavefunction in conjunction with a system-specific non-empirical optimally tuned hybrid functional as there are very few studies in the literature which combine both fine non-uniform sampling with highly accurate DFT starting points. The focus of this work will be to investigate photovoltaic candidate materials like halide perovskites possibly including novel high-entropy variants.
Second, he will investigate the effects of phonons on optical properties in complex systems. Of particular interest is BiVO4, a system that is a popular photocatalytic candidate and where, in prior calculations, they have found a pronounced exciton peak which is not observed in experiment. Recent studies suggest lattice vibrations significantly renormalize electronic states, so he would like to investigate if the same is true for excitonic states. Specifically, he aims to understand the effects of phonon scattering on the standard BSE exciton wavefunctions and binding energy. Special attention will be paid to the imaginary part of the phonon screening Kernel to estimate exciton lifetimes.