Quantum criticality in the tricritical Dicke model.

The Dicke model, which describes a quantized light field interacting with an ensemble of two-level atoms, is a cornerstone model of quantum optics. It illustrates the collective phenomena of superradiance in a non-transient way through the second-order superradiant phase transition observed when the light-atom interaction strength is varied. Here we present a generalization of this model, the tricritical Dicke model (TDM), where the transition between the normal and superradiant phases can be tuned from second- to first-order, across a tricritical point. This is achieved by replacing the two-level atoms with three-level atoms. A full characterization of all different critical manifolds is done through the determination of the scaling behavior of the different observables. Additionally, we consider the robustness of these rich phase diagram regions when losses are incorporated into the model, leading to multiple stable phases and a modification of the phase boundary geometries. The richness of the phase diagram of the TDM and other associated generalized Dicke models makes them attractive candidates to explore quantum criticality both in and out of equilibrium.