Spin Coherence and ${}^{14}$N ESEEM Effects of Nitrogen-Vacancy Centers in Diamond with X-band Pulsed ESR

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 IIb diamonds (nitrogen impurity concentration $<1$~ppm) are prepared with bulk concentrations of $2\cdot {10}^{13}$ cm${}^{-3}$ to $4\cdot {10}^{14}$ cm${}^{-3}$ by high-energy electron irradiation and subsequent annealing. We find that a proper post-radiation anneal (1000${}^{\circ }$C for 60 mins) 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 T$_2 = 0.74$msat5Kareachieved,beingonlylimitedby$^{13}$C nuclear spectral diffusion in natural abundance diamonds. At X-band magnetic fields, strong electron spin echo envelope modulation (ESEEM) is observed originating from the central ${}^{14}$N nucleus. The ESEEM spectral analysis allows for accurate determination of the ${}^{14}$N nuclear hypefine and quadrupole tensors. In addition, the ESEEM effects from two proximal ${}^{13}$C sites (second-nearest neighbor and fourth-nearest neighbor) are resolved and the respective ${}^{13}$C hyperfine coupling constants are extracted.