Achieving 5% C nuclear spin hyperpolarization in high-purity diamond at room temperature and low magnetic field.

Optically polarizable nitrogen-vacancy (NV) centers in diamond enable hyperpolarization of C nuclear spins at a low magnetic field and room temperature. However, it remains a challenge to achieve a high level of polarization, comparable to that of conventional dynamic nuclear polarization. In this paper, we demonstrate that a C polarization of 5%, equivalent to an enhancement ratio of over [Formula: see text], can be attained at less than 10 mT. We used a high-purity diamond with an initial nitrogen concentration below 1 ppm, which resulted in a storage time exceeding 100 min. Aligning the magnetic field along [100] increased the number of NV spins involved in polarization transfer by a factor of four. For this orientation, a comprehensive optimization of the magnetic field intensity and microwave (MW) sweep parameters has been performed. The optimum MW sweep width suggests that polarization transfer occurs primarily to the bulk C spins through the integrated solid effect, followed by nuclear spin diffusion.