Superradiant phase transition in two-dimensional materials*

Light-matter interaction is at the core of many fascinating and recent developments at the crossroad of quantum optics and condensed matter physics. In this talk we will present for the first time a general theory for the superradiant quantum phase transition in materials with arbitrary electron-electron interaction. We find that the no-go theorem presented in [1] is circumvented by taking into account the finite wave vector exchanged between light and matter [2]. The superradiant ground state is characterized by the spontaneous emergence of a magnetic flux state, which gives rise to a spatially modulated orbital ferromagnet [2]. Interestingly, we discover that the spontaneous breaking of time-reversal is responsible for novel topilogical properties. Our results open new perspectives on the interpretation of the superradiant phase transition and envision twisted bilayer graphene as a natural material hosting this hybrid phase of light and matter.

[1] G.M.Andolina et al, Phys. Rev. B 100, 121109(R) (2019)
[2] D.Guerci, P.Simon, and C.Mora arXiv:2005.08994 (2020)