Temperature-Dependent Dynamic Nuclear Polarization of Diamond

Dynamic nuclear polarization (DNP) can increase nuclear magnetic resonance signals by several orders of magnitude. We report on ¹³C DNP experiments in diamond at 3.4 and 7 T static magnetic fields in a temperature range of 300 to 1.7 K. Nuclear polarization enhancements between 100 and 600 were measured for all temperatures, corresponding to polarizations between 0.1% (300 K) and 38% (1.7 K) at 7 T. A strong temperature dependence of the DNP profiles was observed with broad lines at low temperatures and more structured features at room temperature. Longitudinal-detected electron paramagnetic resonance (EPR) experiments revealed an additional broad temperature-dependent electron line centered around the m _I = 0 line of the P1 triplet transitions. This additional electron line leads to an asymmetry of the low-temperature EPR spectrum and might arise from clustered P1 centers or other nitrogen defects in diamond, e.g., N2 or N3 centers, which are known to shorten P1 electronic relaxation times. Our results suggest that nuclei are preferentially polarized via a direct hyperfine mediated polarization transfer, while nuclear spin diffusion in the sample plays a minor role.