Excitons and excitonic materials

An exciton is a bound pair of negatively charged electron and positively charged hole (electron vacancy within a solid), both of which are held together by their mutual Coulomb attraction to form a bound state. One hundred years after their discovery, excitons act as the backbone of a large class of low-dimensional and quantum materials showing a truly “exotic” set of physical, chemical as well as biophysical properties. In this issue of MRS Bulletin, we designate all such materials whose properties are crucially dependent on the presence of these excitons as “Excitonic materials.” Current studies of these materials are progressing rapidly in newer directions, including those in novel materials and next-generation technologies. Therefore, the main focus of this issue is to catch recent progresses in the physics of “excitons” and “excitonic materials,” encompassing both fundamental understandings of the nature of these quasiparticles and their emerging device applications in various fields. This article is an overview of this issue, recalling the basics of exciton physics, the historical contexts, and recent progresses without claiming to be exhaustive.