  Nuclear spin ensembles in diamond are promising candidates for quantum
sensing applications, including rotation sensing. Here we perform a
characterization of the optically detected nuclear-spin transitions associated
with the 14N nuclear spin within diamond nitrogen vacancy (NV) centers. We
observe nuclear-spin-dependent fluorescence with the contrast of optically
detected 14N nuclear Rabi oscillations comparable to that of the NV electron
spin. Using Ramsey spectroscopy, we investigate the temperature and
magnetic-field dependence of the nuclear spin transitions in the 77.5-420 K and
350-675 G range, respectively. The nuclear quadrupole coupling constant Q was
found to vary with temperature T yielding d|Q|/dT=-35.0(2) Hz/K at T=297 K. The
temperature and magnetic field dependencies reported here are important for
quantum sensing applications such as rotation sensing and potentially for
applications in quantum information processing.
