First Principles Simulation of Color-Center Defects in Lithium Fluoride for Sensing Low-Energy Nuclear Recoils

It has recently been proposed that rare particle interactions, such as those of neutrinos and dark matter, might be detected by observing color center defects generated by low-energy nuclear recoils. To realize such detectors, first-principles calculations can be used to identify optimal materials in which the vacancies and self-interstitials generated from nuclear recoils result in optically bright color centers emitting in the visible spectrum. In order to gauge the predictive power of these first-principles methods, we consider color centers in lithium fluoride (LiF) which have been extensively characterized due to its widespread use as a commercial radiation dosimetric material. Using Heyd-Scuseria-Ernzerhof (HSE) hybrid functional theory, we compute the absorption, emission, energetic, and spin properties of lithium and fluorine self-interstitials and vacancies in LiF, as well as larger optically active fluorine vacancy clusters. The HSE parameters are iteratively tuned to obtain a solution which simultaneously reproduces the bandgap of LiF and satisfies the generalized Koopmans' condition for defect orbitals. We show that while electrostatic energy corrections are sufficient to obtain a solution for the single fluorine vacancy, for multisite centers the procedure is complicated by a greater delocalization of defect orbitals which leads to a significant error in their Kohn-Sham eigenvalues.