  We study a simple model describing superradiance in a system of two-level
atoms interacting with a single-mode bosonic field. The model permits a
continuous crossover between integrable and partially chaotic regimes and shows
a complex thermodynamic and quantum phase structure. Several types of
excited-state quantum phase transitions separate quantum phases that are
characterized by specific energy dependences of various observables and by
different atom-field and atom-atom entanglement properties. We observe an
approximate revival of some states from the weak atom-field coupling limit in
the strong coupling regime.
