Comparative study of indicators of chaos in the closed and open Dicke model

The Dicke model, renowned for its superradiant quantum phase transition (QPT), also exhibits a transition from regular to chaotic dynamics. In this work, we provide a systematic, comparative study of static and dynamical indicators of chaos for the closed and open Dicke model. In the closed Dicke model, we find that indicators of chaos sensitive to long-range correlations in the energy spectrum such as the the spectral form factor can deviate from the Poissonian random matrix theory (RMT) predictions and show a dip-ramp-plateau feature even in the normal region of the Dicke model unless very large values of the spin size are chosen. Thus, care is needed in using such indicators of chaos. In the open Dicke model with cavity damping, we find that the dissipative spectral form factor emerges as a robust diagnostic displaying a quadratic dip-ramp-plateau behavior in agreement with the Ginebre Unitary Ensemble (GinUE) RMT in the superradiant regime. Moreover, by examining the spectral properties of the Liouvillian, we provide indirect evidence for the concurrence of the dissipative superradiant quantum phase transition and the change in Liouvillian eigenvalue statistics from 2-D Poissonian to GinUE RMT behavior.