High concentration of negatively charged nitrogen-vacancy (NV−) centers was created in diamond single crystals containing approximately 100 ppm nitrogen using electron and neutron irradiation and subsequent thermal annealing in a stepwise manner. Continuous wave electron paramagnetic resonance (EPR) was used to determine the transformation efficiency from isolated N atoms to NV− centers in each production step and its highest value was as high as 17.5 %. Charged vacancies are formed after electron irradiation as shown by EPR spectra, but the thermal annealing restores the sample quality as the defect signal diminishes. We find that about 25 % of the vacancies form NVs during the annealing process. The large NV− concentration allows to observe orientation dependent spin-relaxation times and also the determination of the hyperfine and quadrupole coupling constants with high precision using electron spin echo (ESE) and electron-nuclear double resonance (ENDOR). We also observed the EPR signal associated with the so-called W16 centers, whose spectroscopic properties might imply a nitrogen dimer-vacancy center for its origin.
@article{arxiv.2110.02126,
title = {Ultrahigh nitrogen-vacancy center concentration in diamond},
author = {S. Kollarics and F. Simon and A. Bojtor and K. Koltai and G. Klujber and M. Szieberth and B. G. Márkus and D. Beke and K. Kamarás and A. Gali and D. Amirari and R. Berry and S. Boucher and D. Gavryushkin and G. Jeschke and J. P. Cleveland and S. Takahashi and P. Szirmai and L. Forró and E. Emmanouilidou and R. Singh and K. Holczer},
journal= {arXiv preprint arXiv:2110.02126},
year = {2022}
}