We propose a generalized Dicke model that supports a quantum tricritical point. We map out the phase diagram and investigate the critical behavior of the model through an exact low-energy effective Hamiltonian in the thermodynamic limit. As predicted by the Landau theory of phase transition, the order parameter shows nonuniversality at the tricritical point. Nevertheless, as a result of the separation of the classical and the quantum degrees of freedom, we find a universal relation between the excitation gap and the entanglement entropy for the entire critical line including the tricritical point. Here the universality is carried by the emergent quantum modes, whereas the order parameter is determined classically.

• Received 21 February 2019

DOI:https://doi.org/10.1103/PhysRevLett.122.193201