Ab initio study of the internal conversion processes in solid-state spin defects

Optically active spin defects in solids represent a promising platform for quantum technologies, and prototypical systems such as the nitrogen-vacancy (NV) center in diamond and the divacancy (VV⁰) center in silicon carbide (SiC) have been studied with a variety of experimental and first principles, theoretical methods. However, while the roles of radiative and intersystem-crossing (ISC) transitions in the spin defects’ optical cycles have been extensively investigated using ab initio approaches, non-radiative internal-conversion (IC) transitions have remained less explored. The description of these processes is challenging as the calculation of non-adiabatic couplings (NACs) between nuclear and electronic states is required. We address this challenge by computing NACs in extended systems within the framework of linear-response time-dependent density functional theory (LR-TDDFT) [1,2]. We present results for the IC transitions between electronic excited states in the negatively charged NV center in diamond and the VV⁰ center in SiC. The framework developed here and implemented in the WEST code [3] is general and applicable to a broad variety of spin-defective systems, allowing for the complete characterization of their optical spin-polarization cycle from first principles.

[1] S.P. Villani et al., in preparation

[2] Y. Jin, V. W.Z. Yu, M. Govoni, A. C. Xu, and G. Galli, J. Chem. Theory Comput. 19, 8689 (2023).

[3] https://west-code.org/