We investigate the quench and real-time formation of the Mott state and photoexcited carrier relaxation dynamics in the Mott insulator $\kappa \text{−}{\left(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\text{−}\mathrm{T}\mathrm{T}\mathrm{F}\right)}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{C}\mathrm{N}\right)}_2\right]\mathrm{Cl}\left(\kappa \text{−}\mathrm{Cl}\right)$ and the superconductor $\kappa \text{−}{\left(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\text{−}\mathrm{T}\mathrm{T}\mathrm{F}\right)}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{C}\mathrm{N}\right)}_2\right]\mathrm{Br}\left(\kappa \text{−}\mathrm{Br}\right)$ 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 $\kappa$-Cl, the energy gap for charge excitations is filled rather than closed by photoinduced carriers of only $\sim 0.5$% 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.

• Received 1 July 2021
• Revised 15 September 2021
• Accepted 16 September 2021

DOI:https://doi.org/10.1103/PhysRevB.104.115152