Photoinduced phase transitions in α-, θ-, and κ-type ET Salts: Ultrafast melting of the electronic ordering

Photoinduced phase transitions in organic compounds with strong electron correlation ET [bis(ethylenedithio)-tetrathiafulvalene)-based salts α-(ET)₂I₃, θ-(ET)₂RbZn(SCN)₄, κ-(d-ET)₂Cu[N(CN)₂Br] were discussed based, on time resolved optical pump-probe spectroscopy using ~150 fs mid-infrared pulse, 12 fs near infrared pulse, and sub-picosecond terahertz pulse. (i) In charge-ordered insulators α-(ET)₂I₃ and θ-(ET)₂RbZn(SCN)₄, we captured ultrafast snapshots of charge dynamics i.e., immediate (ca. 15 fs) generation of a microscopic metallic state (or equivalently the microscopic melting of the charge order) which is driven by the coherent oscillation (period; 18 fs) of correlated electrons. Subsequently, condensation of the microscopic metallic state to the macroscopic scale occurs in α-(ET)₂I₃. However, in θ-(ET)₂RbZn(SCN)₄, such condensation is prevented by the large potential barrier reflecting the structural difference between the insulator and metal; (ii) In a Dimer-Mott insulator κ-(d-ET)₂Cu[N(CN)₂Br], photogeneration of the metallic state rises during ca. 1 ps that is much slower than the melting of charge order, because the photoinduced insulator to metal transition is driven by the intradimer molecular displacement in the dimer Mott insulator. The ultrafast dynamics of photoinduced insulator-metal transitions depend strongly on the molecular arrangement, reflecting various competing phases in the ET sheets.