Dynamic nuclear polarization at ultralow temperatures

摘要: We report on electron spin resonance study of dynamic nuclear polarization (DNP) and relaxation of phosphorus donors in silicon in a magnetic field of 4.56 T and at temperatures below 1 K. The DNP occurs due to the Overhauser effect following a cross relaxation via the forbidden flip-flop or flip-flip transitions. Nuclear polarization values P> 0. 99 were reached after 15 min of pumping with 0.4 µW of RF power. An inverse sign DNP has been created by pumping the low field ESR line of P followed by the flip-flip cross relaxation. We observed non-exponential behavior of DNP and the subsequent relaxation, which may be explained by a polarization and spin diffusion of 29Si nuclei surrounding 31P donors.