Quantum vibronic effects on the electronic properties of solid-state spin defects*

We present a study of the electronic properties of solid-state spin defects to unravel the impact of quantum nuclear vibrations on vertical excitation energies. Our analysis focused on the negatively charged nitrogen vacancy center in diamond as a prototypical example. Electronic properties were computed using DFT, and nuclear vibrations were determined using stochastic methods,¹ which were validated against first-principles molecular dynamics with a quantum thermostat (QT-FPMD).² We found a significant dynamic Jahn-Teller splitting of the doubly degenerate single-particle levels within the diamond's band gap, even at 0 K, with a magnitude exceeding 180 meV.  This pronounced splitting leads to substantial renormalizations of the defect levels and consequently, of the vertical excitation energies of the doubly degenerate singlet and triplet excited states. Our study underscores the pressing need to incorporate quantum vibronic effects in first-principles calculations of spin defects, especially when comparing computed vertical excitation energies with experimental data. We utilized several computational tools, including PyEPFD (https://pyepfd.readthedocs.io/) and i-PI (http://ipi-code.org/) for stochastic and QT-FPMD simulations, respectively, Qbox (http://qboxcode.org/) to compute DFT forces and the WEST code (https://west-code.org/) to compute  TDDFT vertical excitation energies.

 

A. Kundu et al, ¹JCTC 2023, ² PRM. 2021 & PNAS 2022.