  We study the superradiant phase transition of an array of Rydberg atoms in a
dissipative microwave cavity. Under the interplay of the cavity field and the
long-range Rydberg interaction, the steady state of the system exhibits an
interaction-enhanced superradiance, with vanishing critical atom-cavity
coupling rates at a discrete set of interaction strengths. We find that, while
the phenomenon can be analytically understood in the case of a constant
all-to-all interaction, the enhanced superradiance persists under typical
experimental parameters with spatially dependent interactions, but at modified
critical interaction strengths. The diverging susceptibility at these critical
points is captured by emergent quantum Rabi models, each of which comprises a
pair of collective atomic states with different numbers of atomic excitations.
These collective states become degenerate at the critical interaction
strengths, resulting in a superradiant phase for an arbitrarily small
atom-cavity coupling.
