Collective phenomena originating from interactions between light and matter have become a major focus of interest spanning different fields of research. Quantum materials represent ideal test beds for the investigation of novel phenomena induced by the light-matter interaction. By allowing the creation of entangled quantum states of light and collective matter excitations, cavity quantum electrodynamics offers a fascinating platform in this context.
In this talk I will discuss how a cooperative effect between long-range correlations induced by the light-matter interaction and strong electron interactions intrinsic to quantum materials can lead to the stabilization of coherent phases of light and matter. By studying a simple model of interacting electrons coupled to a single mode cavity field I will show that a phase characterized by the simultaneous condensation of excitons and photon superradiance, the “superradiant excitonic insulator" (SXI) can be realized. Superradiance cannot be reached in the absence of electronic interactions. At the same time coupling with the cavity field promotes excitonic condensation in regimes of temperature and interaction where it cannot be stabilized in the absence of light-matter coupling. Therefore the SXI intertwines excitonic condensation and superradiance in conditions where the two phases cannot be individually stabilized in the absence of such cooperative effect. I will discuss probes as photon spectra and optical conductivity of the hybrid light-matter system.