Fluxional Doping-Si\nManipulates Spin Coupling Characteristics\nin Silicon-Vacancy Defect Nanodiamond

Open-shell carbon-based diradical magnets have received\nsubstantial\nattention, owing to promising applications in molecular spintronics.\nHowever, as is known for most reported π-type diradical systems,\ntheir intrinsic high activity is a great challenge in practical applications.\nHere, we explore a stable σ-type silicon-vacancy (SiV<sup>0</sup>) nanodiamond and for the first time reveal its spin coupling characteristics,\nas well as its response to temperature and applied electric field,\nusing DFT calculations combined with ab initio molecular dynamics\nsimulation. The results indicate that the static SiV<sup>0</sup> nanodiamond\npresents triplet diradical character with extremely strong ferromagnetic <i>J</i>-coupling (1913.8 cm<sup>–1</sup>), and intriguing\nanisotropic response to temperature and applied electric field. Temperature-manipulated <i>J</i>-coupling presents persistent oscillation due to the mobility\nof the doping Si modifying the multiradical distribution character.\nStatistics indicates that the ferromagnetic <i>J</i>-coupling\nconstants of SiV<sup>0</sup> mainly oscillate in the range of 1400–1900\ncm<sup>–1</sup> at 25 K, but its distribution is considerably\nwidened and downshifted at 300 K. Under applied electric field, the\nferromagnetic coupling strength of the SiV<sup>0</sup> center exhibits\nnoticeably anisotropic response to the electric field direction. Further,\nthe zero-field splitting (<i>D</i>) of SiV<sup>0</sup> presents\na sensitive anisotropic response to an applied electric field, featuring\na considerable increase of the <i>D</i> value to 1.43 GHz\nwhen the electric field is along the C<sub>3v</sub> axis direction,\nsignifying that the SiV<sup>0</sup> system has excellent potential\nas a qubit. This work provides insights into the dynamic spin coupling\ncharacteristics in such multiradical centers of Si-doped defect nanodiamonds\nand facilitates the development of SiV<sup>0</sup> for applications\nin quantum information science.