Symmetry of the Hyperfine and Quadrupole Interactions of Boron Vacancies in a Hexagonal Boron Nitride

The concept of optically addressable spin states of deep-level defects in wide band gap materials is successfully applied for the development of quantum technologies. Recently discovered negatively charged boron vacancy defects (VB-) in hexagonal boron nitride (hBN) potentially allow a transfer of this concept onto atomic-thin layers due to the van der Waals nature of the defect host. Here, we experimentally explore all terms of the VB- spin Hamiltonian reflecting interactions with the three nearest nitrogen atoms by means of conventional electron spin resonance and high frequency (94 GHz) electron-nuclear double resonance. We establish symmetry, aniso-tropy, and principal values of the corresponding hyperfine interaction (HFI) and nuclear quadrupole interaction (NQI). The HFI can be expressed in the axially symmetric form as A perpendicular to = 45.5 +/- 0.9 MHz and A parallel to = 87 +/- 0.5 MHz, while the NQI is characterized by quadrupole coupling constant Cq = 1.96 +/- 0.05 MHz with slight rhombisity parameter eta = (Pxx - Pyy)/Pzz = -0.070 +/- 0.005. Utilizing a conventional approach based on a linear combination of atomic orbitals and HFI values measured here, we reveal that almost all spin density (approximate to 84%) of the VB- electron spin is localized on the three nearest nitrogen atoms. Our findings serve as valuable spectroscopic data and direct experimental demonstration of the VB- spin localization in a single two-dimensional BN layer.

Automated summary

We have experimentally studied the spin Hamiltonian of the negatively charged boron vacancy defects (VB-) in hexagonal boron nitride (hBN) and determined the symmetry, anisotropy, and principal values of the hyperfine interaction (HFI) and nuclear quadrupole interaction (NQI). We have found that approximately 84% of the spin density of the VB- electron spin is localized on the three nearest nitrogen atoms, providing valuable spectroscopic data and experimental evidence for the VB- spin localization in a single two-dimensional BN layer.