Femtosecond Carrier Relaxation Dynamics and Photoinduced Phase Separation in κ-(BEDT-TTF)2Cu[N(CN)2]X (X = Br; Cl)

We investigate the relaxation dynamics of nonequilibrium carriers in organic conductors kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]X (X = Br and Cl) using ultrafast time-resolved optical spectroscopy. The dynamics for both salts show similar temperature dependences, which is well characterized by the carrier relaxation across the pseudogap (PG) of the magnitude Delta(PG) approximate to 16 meV for Br salt and 7.0 meV for Cl salt. On the other hand, only the Br salt shows an abrupt increase of the decay time at low temperature, indicating an additional decay component associated with the superconducting (SC) gap below T-c. The fluence dependent dynamics at low temperature evidences the superposition of the SC component onto the PG component. These results indicate a metallic-insulating phase separation in the Br salt triggered by photoexcited nonequilibrium carriers.