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 Al₂O₃: 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.