Exciton insulators in two-dimensional systems

Electron–hole interactions play a crucial role in determining the optoelectronic properties of materials, and in low-dimensional systems this is especially true due to the decrease of screening. In this review, we focus on one unique quantum phase induced by the electron–hole interaction in two-dimensional systems, known as “exciton insulators” (EIs). Although this phase of matter has been studied for more than half a century, suitable platforms for its stable realization remain scarce. We provide an overview of the strategies to realize EIs in accessible materials and structures, along with a discussion on some unique properties of EIs stemming from the band structures of these materials. Additionally, signatures in experiments to distinguish EIs are discussed.