Quantum phases and entanglement properties of an extended Dicke model

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. (C) 2017 Elsevier Inc. All rights reserved.