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 (U-dimer) for achieving the Mott insulator-to-metal transition in kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br, as investigated by pump-probe spectroscopy. A reduction of U-dimer 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 U-dimer was also observed near the critical end point of the Mott transition line.