Exchange coupling-assisted ¹³C dynamic nuclear polarization in microdiamonds at 14 T

In this study, we investigated nitrogen substitution defects, also known as P1 centers, in Type 1b diamonds generated under high pressure and high temperature (HPHT) as a source of electron spin polarization. The open question was how readily electron spin polarization in this diamond transfers to ¹³C nuclear spins within the diamond matrix at 14 T by dynamic nuclear polarization (DNP). The goal was to refine the model for clustered P1 centers in HPHT diamonds and evaluate their potential as a source for DNP hyperpolarization or contrast. The study relied on frequency-stepped measurements of DNP profiles under magic angle spinning (MAS) using the mm-wave output of a frequency-tunable gyrotron and a regular superconducting NMR magnet at a single field. We observe up to 700-fold ¹³C on/off signal enhancements in both MAS and static mode at room temperature, and 130-fold between 35 and 100 K. Modelling the experimental results reveal the dominant role of P1 clusters harboring inter-P1 dipolar and exchange couplings exceeding 100 MHz in achieving ¹³C DNP at 14.1 T. Our results exemplify the importance of exchange coupling for high-field DNP and provide a refined model for DNP via P1 centers of HPHT diamonds.