Dynamic nuclear polarization of ²⁹Si via spin S=1 centers in isotopically controlled silicon

We report the results of the experiments on dynamic nuclear polarization (DNP) of ²⁹Si nuclei under saturation the electron paramagnetic resonance (EPR) transitions of the photoexcited spin S=1 states of the oxygen+vacancy complexes in irradiated ²⁹Si isotopically controlled silicon. The effect of isotope ²⁹Si abundance on the line width and hyperfine structure of the Si-SL1 EPR spectra was observed. It was shown clearly that the decrease of the ²⁹Si abundance leads to the transformation the DNP mechanism from the "differential" to "resolved" solid-effect accompanied with the increase of DNP degree. High steady-state ²⁹Si nuclear polarization of 6% due to "resolved" solid-effect was achieved in silicon crystals with the ²⁹Si isotope abundance below 4.7%. It was found that the DNP induced by saturation the hyperfine structure EPR lines of triplet centers does not follow the symmetric first derivative EPR line shape, showing the additional contribution to the DNP process.