Electron and nuclear spins in diamond have long coherence and relaxation times at room temperature, making them a promising platform for applications such as biomedical and molecular imaging and nanoscale magnetic field sensing. While the optically-active nitrogen-vacancy (NV) defect has received a great deal of attention, the substitutional nitrogen (or P1) center also exhibits long coherence and relaxation times. These P1 centers are typically present at significantly larger concentrations (about an order magnitude larger) than NVs, allowing us to explore the role of P1-P1 interactions in mediating DNP. The system can, in principle, show DNP via the solid effect (SE), cross effect (CE) and Overhauser effect (OE) depending on the P1 concentration and the field.
Here, we show enhancement of natural abundance 13C nuclei found within the diamond, using the unpaired electron of the P1 center (concentration 110-130 ppm) in particles with a 15-25 μm diameter, under static conditions at room temperature and 3.4 T. From the DNP spectrum we conclude that both the SE-DNP and OE-DNP mechanisms are active. The OE, in our case, results in negative enhancement, in contrast to previous results reporting positive OE enhancements. A negative OE implies that zero-quantum relaxation is more effective than double-quantum relaxation, likely due to strong anisotropic hyperfine interactions. We also explore the effect of frequency modulation (FM) of the DNP mechanism. Preliminary results suggest that the OE benefits from faster FM (>100 kHz) whereas the SE does not. This suggests that we can control which DNP mechanism is effective using FM parameters such as frequency, amplitude and shape.