Electric-field-driven excitonic instability in an organometallic manganese-cyclopentadienyl wire

Excitonic insulator is a macroscopic quantum system whose ground state consists of spontaneously formed and condensed excitons. While various excitonic insulators have been explored, they are almost exclusively characterized by the condensation of dark (optically inactive) excitons. Our first-principles GW-BSE calculations show that a transverse electric field will drive an organometallic manganese-cyclopentadienyl wire into a unique "bright" excitonic insulator phase, as the combined consequence of giant Stark effect and electric-field-induced symmetry breaking. Such bright excitonic insulators can be directly identified by experiment due to their coherent radiation. Our results are not only of scientific interest in electric-field control of exciton condensation, but may also open an opportunity to create a coherent light source based on the excitonic insulator.