Phase transitions of the anisotropic Dicke model

We systematically analyze the various phase transitions of the anisotropic Dicke model that is endowed with both rotating and counterrotating light-matter couplings. In addition to the ground-state quantum phase transition (QPT) from the normal to the superradiant phase, the anisotropic Dicke model also exhibits other transitions, namely, the excited state quantum phase transition (ESQPT), ergodic to nonergodic transition (ENET), and the temperature-dependent phase transition. We show that these phase transitions are profitably studied not only with the standard consecutive level spacing ratio, but also with the aid of various eigenvector quantities such as von Neumann entanglement entropy, the participation ratio, multifractal dimension, and mutual information. For ENET, both the statics and dynamics of the participation ratio offer a consistent and useful picture. An exciting finding from our work is that the ESQPT and the ENET are closely related to each other. We show this with the aid of two characteristic energies in the spectrum corresponding to jumps in von Neumann entropy.