Quantum phase transition of a two-dimensional Rydberg atom array in an optical cavity

We study the two-dimensional Rydberg atom array in an optical cavity with the help of a variational method and large-scale quantum Monte Carlo simulations. The strong dipole-dipole interactions between Rydberg atoms can make the system exhibit a crystal structure, and the coupling between a two-level atom and a cavity photon mode can result in the formation of a polariton. The interplay between them provides a rich quantum phase diagram including the Mott, solid-1/2, superradiant, and superradiant solid (SRS) phases. As a two-order coexisted phase, the superradiant solid breaks both translational and U(1) symmetries. Different from the fragile SRS phase in a one-dimensional system [Zhang et al., Phys. Rev. Lett. 110, 090402 (2013)], the SRS phase stays in a larger parameter region. Thus, it is more feasible to detect a SRS phase and corresponding quantum criticality in the real system involving dissipations. Our work not only extends the understanding of the light-atom interacting system, but also provides the guidelines and benchmark for the future experiments.