Ligand hyperfine interactions at silicon vacancies in 4H-SiC.

The negative silicon vacancy (V_Si^-) in SiC has recently emerged as a promising defect for quantum bits and room-temperature quantum sensing. However, its electronic structure is still not well characterized. While the isolated Si vacancy is expected to give rise to only two paramagnetic centers corresponding to two inequivalent lattice sites in 4H-SiC, there have been five electron paramagnetic resonance (EPR) centers assigned to V_Si^- in the past: the so-called isolated no-zero-field splitting (ZFS) V_Si^- center and another four axial configurations with small ZFS: TV1a, TV2a, TV1b, and TV2b. Due to overlapping with 29Si hyperfine (hf) structures in EPR spectra of natural 4H-SiC, hf parameters of TV1a have not been determined. Using isotopically enriched 4H-28SiC, we overcome the problems of signal overlapping and observe hf parameters of nearest C neighbors for all three components of the S=3/2 TV1a and TV2a centers. The obtained EPR data support the conclusion that only TV1a and TV2a are related to V_Si^- and the two configurations of the so-called isolated no-ZFS V_Si^- center, V_Si^-(I) and V_Si^-(II), are actually the central lines corresponding to the transition |-1/2> ↔ |+1/2> of the T_V2a and T_V1a centers, respectively.