Ultrahigh nitrogen-vacancy center concentration in diamond

TL;DR

This study demonstrates a method to produce ultrahigh concentrations of negatively charged nitrogen-vacancy centers in diamond, enabling detailed spin and spectroscopic analysis with potential quantum applications.

Contribution

The paper reports a novel process combining electron/neutron irradiation and thermal annealing to achieve record NV center concentrations in diamond.

Findings

Achieved up to 17.5% transformation efficiency from N to NV centers.

High NV concentration enables precise measurement of spin-relaxation and hyperfine interactions.

Identified the EPR signal of W16 centers, possibly nitrogen dimer-vacancy centers.

Abstract

High concentration of negatively charged nitrogen-vacancy ($\text{NV}^{-}$) 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 $\text{NV}^{-}$ 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 $\text{NV}^{-}$ 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.