UC Berkeley

Pulsed ESR experiments are reported for ensembles of negatively-charged nitrogen-vacancy centers (NV −) in diamonds at X-band magnetic fields (280–400 mT) and low temperatures (2–70 K). The NV − centers in synthetic type IIa diamonds (nitrogen impurity concentration  <1 ppm) are prepared with bulk concentrations of 2 ⋅ 1013 cm −3 to 4 ⋅ 1014 cm −3 by high-energy electron irradiation and subsequent annealing. We find that a proper post-radiation anneal (1000°C for 60 min) is critically important to repair the radiation damage and to recover long electron spin coherence times for NV −s. After the annealing, spin coherence times of T2 = 0.74ms at 5 K are achieved, being only limited by 13C nuclear spectral diffusion in natural abundance diamonds. By measuring the temperature dependence of T2 in the under-annealed diamonds (900°C) we directly extract the density (1014 −16 cm −3) and activation energy (2.5 meV) of unannealed defects responsible for the faster NV − decoherence. At X-band magnetic fields, strong electron spin echo envelope modulation (ESEEM) is observed originating from the central 14N nucleus, and we extract accurate 14N nuclear hypefine and quadrupole tensors. In addition, the ESEEM effects from two proximal 13C sites (second-nearest neighbor and fourth-nearest neighbor) are resolved and the respective 13C hyperfine coupling constants are extracted.