Solid-State Nuclear Spin-Flip Maser Pumped by Dynamic Nuclear Polarization

Abstract

We report the observation of a coherent, maser-type rf radiation of the ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ nuclear-spin system in ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$: ${\mathrm{Cr}}^{3+}$ pumped by dynamic nuclear polarization to negative temperature. A theoretical description is presented for the sharp, superradiant bursts which follow the fast tuning of a resonant $\mathrm{LC}$ circuit to a selected NMR line of ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$. Experimental results are discussed which demonstrate the interplay between dynamic nuclear polarization, relaxation, and the radiation field.