Dynamics of Photoinduced Phase Transition from a Dimer Mott Insulator to a Metal

We theoretically investigate the electron-phonon dynamics induced by photoexcitation in dimer Mott insulators of the form K-(BEDT-TTF)2X [BEDT-TTF: bis(ethylenedithio)-tetrathiafulvalene, X: a counteranion]. We adopt the quarter-filled extended Hubbard Hamiltonian coupled with the dimer-bond stretching phonon modes. Utilizing the wave function where electron wave functions in the ground-state and photoexcited-state manifolds are coupled with different phonon wave functions, we solve the time-dependent Schrodinger equation numerically with a small initial disorder in the lattice. In the case where each of the dominant peaks of the interdimer charge transfer excited states is excited, the electron wave function is almost unchanged when the electron-phonon coupling constant s is smaller than the threshold value sTH. When s > similar to s sub(TH) holds, there is an incubation time, and the decoherence of dimer-bond oscillations and the transitions to metallic states proceed simultaneously after the incubation time. This characteristic time dependence comes from the feedback effects between the lattice disorder and the transitions without momentum conservation. An electron-lattice-coupled collective mode appears in the photogenerated metallic phase. In the case where the largest peak of the intradimer excited state is excited, no threshold behavior is observed. Furthermore, there is no incubation time, and the transitions to interdimer charge transfer excited states dominate the change in the electron wave function. In all the cases, the early stage of photoinduced transition cannot be described by the configurational coordinate model with the configurational coordinate of the dimer-bond length. These results are consistent with recent observations by femtosecond pump-probe spectroscopy.