High-Temperature Bose-Einstein Condensation of Dark Excitons in Holey Graphyne

Abstract

We investigated the optical and excitonic properties of holey graphyne (HGY), which is a recently synthesized two-dimensional (2D) carbon allotrope, using first-principles calculations. The potential of HGY for and band-edge wave-function symmetry of HGY lead to strong Coulomb interactions and symmetry-forbidden optical transition, resulting in the formation of long-lived dark excitons. The lowest-energy dark exciton in HGY has a large binding energy of 0.63 eV and can be well described by the screened hydrogenic model. By analyzing the constraints on exciton density and temperature necessary for BEC, a phase diagram for the electron-hole system in HGY is constructed, and a maximum BEC transition temperature of 503 K is predicted. Our findings thus reveal the great possibility of achieving above-room-temperature excitonic BEC in 2D carbon materials.