Linear response theory for electron-hole pair kinetics: exciton formation

A linear response theory for electron-hole pair density is developed, which constitutes a new theoretical method, and a definition of exciton density in a first-principles context is derived by considering both the electron-hole attractive interaction and the screening effect. This allows the exciton time evolution to be examined. This formulation is applied to a jellium model in order to prove the existence of a transient exciton, and to observe crossover from a transient to a stable exciton in response to decreased electron density. The exciton formation mechanism is also revealed.