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

We report the observation of a coherent, maser-type rf radiation of the ²⁷Al nuclear-spin system in Al₂{O}₃$: ${Cr}³⁺$ 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 $LC$ circuit to a selected NMR line of $²⁷Al$. Experimental results are discussed which demonstrate the interplay between dynamic nuclear polarization, relaxation, and the radiation field.