Optically Detected Magnetic Resonances of Nitrogen-Vacancy Ensembles in   13C Enriched Diamond
  We present an experimental and theoretical study of the optically detected magnetic resonance signals for ensembles of negatively charged nitrogen-vacancy (NV) centers in 13C isotopically enriched single-crystal diamond. We observe four broad transition peaks with superimposed sharp features at zero magnetic field and study their dependence on applied magnetic field. A theoretical model that reproduces all qualitative features of these spectra is developed. Understanding the magnetic-resonance spectra of NV centers in isotopically enriched diamond is important for emerging applications in nuclear magnetic resonance. 
A. Jarmola, Z. Bodrog, P. Kehayias, M. Markham, J. Hall, D. J.   Twitchen, V. M. Acosta, A. Gali, D. Budker
quant-ph cond-mat.mes-hall