  We study superradiant masing in a hybrid system composed of nitrogen-vacancy
center spins in diamond coupled to a superconducting microwave cavity. After
the first fast superradiant decay we observe transient pulsed and then
quasi-continuous masing. This emission dynamics can be described by a
phenomenological model incorporating the transfer of inverted spin excitations
into the superradiant window of spins resonant with the cavity. After
experimentally excluding cQED effects associated with the pumping of the masing
transition we conjecture that direct higher-order spin-spin interactions are
responsible for creating the dynamics and the transition to the sustained
masing. Our experiment thus opens up a novel way to explore many-body physics
in disordered systems through cQED and superradiance.
