  Color-center-hosting semiconductors are emerging as promising source
materials for low-field dynamic nuclear polarization (DNP) at or near room
temperature, but hyperfine broadening, susceptibility to magnetic field
heterogeneity, and nuclear spin relaxation induced by other paramagnetic
defects set practical constraints difficult to circumvent. Here, we explore an
alternate route to color-center-assisted DNP using nitrogen-vacancy (NV)
centers in diamond coupled to substitutional nitrogen impurities, the so-called
P1 centers. Working near the level anti-crossing condition - where the P1
Zeeman splitting matches one of the NV spin transitions - we demonstrate
efficient microwave-free 13C DNP through the use of consecutive magnetic field
sweeps and continuous optical excitation. The amplitude and sign of the
polarization can be controlled by adjusting the low-to-high and high-to-low
magnetic field sweep rates in each cycle so that one is much faster than the
other. By comparing the 13C DNP response for different crystal orientations, we
show that the process is robust to magnetic field/NV misalignment, a feature
that makes the present technique suitable to diamond powders and settings where
the field is heterogeneous. Applications to shallow NVs could capitalize on the
greater physical proximity between surface paramagnetic defects and outer
nuclei to efficiently polarize target samples in contact with the diamond
crystal.
