Ultrafast dynamics of Mott-state quench and formation in strongly correlated BEDT-TTF molecular conductors observed by three-pulse pump probe spectroscopy

We investigate the quench and real-time formation of the Mott state and photoexcited carrier relaxation dynamics in the Mott insulator kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl(kappa-Cl) and the superconductor kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br(kappa -Br) using three-pulse femtosecond optical spectroscopy, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene. In both salts, we find that transient reflectivity amplitude gradually recovers with time after strong near-infrared pulse quench, but its relaxation time is nearly constant throughout. This indicates that in K-Cl, the energy gap for charge excitations is filled rather than closed by photoinduced carriers of only similar to 0.5% per dimer. The Mott state is re-formed on a few-picosecond timescale with the disappearance of the in-gap photodoping-induced states near the Fermi energy. In kappa-Br, a behavior similar to that in kappa-Cl is observed and attributed to the disorder-induced phase-separated Mott insulating regions