Overhauser effect of 67Zn nuclear spins in ZnO via cross relaxation induced by the zero-point fluctuations of the phonon field

Hole burning in and displacements of the magnetic-resonance absorption line of the electron spin of the shallow hydrogen-related donor in ZnO are observed upon resonant irradiation with microwaves at 275 GHz and at 4.5 K in a magnetic field of 10 T. These effects arise from an almost complete polarization of the many 67Zn (I=5/2) nuclear spins that have an isotropic hyperfine interaction with the electron spin of the shallow donor. It is proposed that this huge dynamic nuclear polarization is caused by a spontaneous-emission-type cross relaxation in the coupled electron-spin nuclear-spin system induced by the zero-point fluctuations of the phonon field.