  One of the most remarkable properties of the nitrogen-vacancy (NV) center in
diamond is that optical illumination initializes its electronic spin almost
completely, a feature that can be exploited to polarize other spin species in
their proximity. Here we use field-cycled nuclear magnetic resonance (NMR) to
investigate the mechanisms of spin polarization transfer from NVs to 13C spins
in diamond at room temperature. We focus on the dynamics near 51 mT, where a
fortuitous combination of energy matching conditions between electron and
nuclear spin levels gives rise to alternative polarization transfer channels.
By monitoring the 13C spin polarization as a function of the applied magnetic
field, we show 13C spin pumping takes place via a multi-spin cross relaxation
process involving the NV- spin and the electronic and nuclear spins of
neighboring P1 centers. Further, we find that this mechanism is insensitive to
the crystal orientation relative to the magnetic field, although the absolute
level of 13C polarization - reaching up to ~3% under optimal conditions - can
vary substantially depending on the interplay between optical pumping
efficiency, photo-generated carriers, and laser-induced heating.
