CAREER: Correlated excited states of point defects in insulators

NONTECHNICAL SUMMARY This award supports computational research and education activities that aim to understand the properties of localized imperfections in materials, called "point defects", such as atoms missing from their usual locations or impurities within the material. Such point defects are ubiquitous in all materials and can have profound effects on their properties even if present in minute quantities. In the context of electronic devices, point defects may be detrimental, e.g., lowering the efficiency of solar cells; or functional, e.g., allowing the properties of materials to be tuned. Defects themselves can even be used as tiny quantum bits for next generation quantum computers. The small and dilute nature of point defects makes them a challenge for experimental characterization, thus computational simulations are vital. However, conventional computational methods have limited accuracy for key defect properties including their response to external stimuli like light or electrical pulses. More advanced theories with significantly better accuracy exist but require too much computational power to be applied to point defects. This project seeks to develop and utilize computational tools that overcome these issues via “embedding,” i.e., by combining the conventional methods with the advanced theories to obtain both computational efficiency and accuracy. These new techniques will allow the PI and his team to develop unprecedented understanding of complex defects excited by external stimuli. The PI will apply these embedding methods to explore a variety of defects and host materials that are promising for the next generation electrical devices including quantum computers. This award also supports the development of computational physics education at all levels. At the graduate level, a course specifically aimed at teaching state-of-the-art methods in computational condensed-matter physics will be developed; at the undergrad level, the computational physics class required for physics majors will be altered to make it more interactive and project-based; and at the high-school level, outreach will be conducted to improve computational literacy. TECHNICAL SUMMARY This award supports research and educational activities that aim to understand the physics of excited electronic states of point defects. The PI will develop quantum embedding techniques for combining density-functional theory and many-body methods to accurately capture electron correlations and excitations in defects relevant for electronic devices and quantum technologies. Projects aimed at methodological improvements will develop more accurate and robust treatments of Coulomb interactions between defect orbitals, hybridization between the defect and bulk states, and the double-counting intrinsic to combining density-functional theory with many-body methods. Comparisons with other many-body methods based on quantum Monte Carlo will be performed to benchmark the various approximations involved in the embedding procedure. In addition, the PI will focus on problems involving interplay between the defect and the crystal lattice to determine the role of correlated excited states in optical and nonradiative processes at defects. The specific defect/host systems targeted will include carbon-based defects in hexagonal BN, transition metals in group-III nitrides, and rare earths in transition-metal dichalcogenides, all of which are of fundamental as well as technological interest for conventional and quantum electronic devices. This award also supports the development of computational physics education at all levels. At the graduate level, a course specifically aimed at teaching state-of-the-art methods in computational condensed-matter physics will be developed; at the undergrad level, the computational physics class required for physics majors will be altered to make it more interactive and project-based; and at the high-school level, outreach will be conducted to improve computational literacy. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.