Radio-Frequency manipulation of strongly interacting electron-nuclear spin systems.

We use optical spin pumping of NV-hosting diamond to investigate the impact of a continuous radio-frequency (RF) drive on the generation of 13C spin polarization. Since this DNP mechanism is based on electron spin cross relaxation, we particularly address the manipulation of hybrid electron-nuclear (1) spin states. We focus on strongly hyperfine-coupled carbons and experimentally show that sufficiently strong RF excitation during optical spin pumping can invert the sign of the observed nuclear polarization. With the aid of numerical modeling, we interpret this as a consequence of electron-nuclear spin mixing, allowing us to drive RF forbidden transitions between different electron spin manifolds. We interprete this as a form of competition between ‘cross effect’ and ‘solid effect’ in an effective four-level system where initialization derives from NV optical pumping, not temperature. The system response to variable magnetic fields allows us to discuss how this process emerges from a subtle interplay between the number of nuclei featuring a given hyperfine coupling and the type and relative concentrations of paramagnetic defects present in the sample.
(1) D. Pagliero et al, “Magnetic-field-induced delocalization in hybrid electron-nuclear spin ensembles”, arXiv: 2006.0773