Optical modulation of effective on-site coulomb energy for the Mott transition in an organic dimer insulator

We report an optical modulation of the effective on-site Coulomb energy U on a dimer (Udimer) for achieving the Mott insulator-to-metal transition in κ-(BEDT-TTF)2Cu[N(CN)2]Br, as investigated by pump-probe spectroscopy. A reduction of Udimer is optically induced by molecule displacement in the dimer under intradimer excitation. The mechanism of this metallization differs greatly from the photodoping-type mechanism reported previously. In contrast, a faster transition via the photodoping mechanism is detected for interdimer excitation. A metallic-domain-wall oscillation originating from the modulation of Udimer was also observed near the critical end point of the Mott transition line.