Nonlinear photocurrent with a threshold of excitation density induced by the long-range electron-electron interaction in the charge-ordered molecular conductor (BEDT-TTF)3(ClO4)2.

We investigated a photoexcited state in the molecular conductor (BEDT-TTF)3(ClO4)2 (BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene) with charge localization due to the electron-electron Coulomb interaction. Photocurrent induced by intramolecular excitation was observed in a charge-ordered insulating state. As a result, nonlinear photocurrent with a threshold of excitation light density was experimentally obtained. The threshold decreased as the temperature increased. This nonlinear photocurrent indicates a transition from an excitonic state to a free excited electronic state. The excitonic state below the threshold is formed by the long-range electron-electron Coulomb interaction. In the free excited electronic state above the threshold, high-density photoexcitation induces Coulomb screening, which results in exciton dissociation and a free electronic state.