Fluxional Doping-Si Manipulates Spin Coupling Characteristics in Silicon-Vacancy Defect Nanodiamond

Open-shell carbon-based diradical magnets have received substantial attention, owing to promising applications in molecular spintronics. However, as is known for most reported π-type diradical systems, their intrinsic high activity is a great challenge in practical applications. Here, we explore a stable σ-type silicon-vacancy (SiV⁰) nanodiamond and for the first time reveal its spin coupling characteristics, as well as its response to temperature and applied electric field, using DFT calculations combined with ab initio molecular dynamics simulation. The results indicate that the static SiV⁰ nanodiamond presents triplet diradical character with extremely strong ferromagnetic J-coupling (1913.8 cm^–1), and intriguing anisotropic response to temperature and applied electric field. Temperature-manipulated J-coupling presents persistent oscillation due to the mobility of the doping Si modifying the multiradical distribution character. Statistics indicates that the ferromagnetic J-coupling constants of SiV⁰ mainly oscillate in the range of 1400–1900 cm^–1 at 25 K, but its distribution is considerably widened and downshifted at 300 K. Under applied electric field, the ferromagnetic coupling strength of the SiV⁰ center exhibits noticeably anisotropic response to the electric field direction. Further, the zero-field splitting (D) of SiV⁰ presents a sensitive anisotropic response to an applied electric field, featuring a considerable increase of the D value to 1.43 GHz when the electric field is along the C_3v axis direction, signifying that the SiV⁰ system has excellent potential as a qubit. This work provides insights into the dynamic spin coupling characteristics in such multiradical centers of Si-doped defect nanodiamonds and facilitates the development of SiV⁰ for applications in quantum information science.