  We investigate the interplay of superradiant phase transition (SPT) and
energy band physics in an extended Dicke-Hubbard lattice whose unit cell
consists of a Dicke model coupled to an atomless cavity. We found in such a
periodic lattice the critical point that occurs in a single Dicke model becomes
a critical region that is periodically changing with the wavenumber $k$. In the
weak-coupling normal phase of the system we observed a flat band and its
corresponding localization that can be controlled by the ground-state SPT. Our
work builds the connection between flat band physics and SPT, which may
fundamentally broaden the regimes of many-body theory and quantum optics.
