Abstract
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.
Highlights
The insulator to metal (I–M) transition is the most well known electronic phase transition in correlated electron systems, as described in the text book [1]
Many examples of I–M transitions induced by the filling control (FC) and the bandwidth control (BC) have been reported in 3d transition metal oxides [2] and/or low dimensional organic conductors [3,4] (Figure 2a)
In this review, photoinduced I–M transitions in CO organic insulators α-(ET)2I3, θ-(ET)2RbZn(SCN)4, DM insulator κ-(d-ET)2Cu[N(CN)2Br] are discussed, based on results of pump-probe spectroscopy in near-IR, mid-IR, and THz regions in the timescale of 10 fs–100 ps
Summary
The insulator to metal (I–M) transition is the most well known electronic phase transition in correlated electron systems, as described in the text book [1]. In such salts, a CO insulator, in which the electrons (or holes) are sometimes localized on the molecular site in a pattern as shown by a red circle, is stabilized at low temperature (Figure 5a,b). The I–M transition in another CO compound, -(ET)2RbZn(SCN) (Figure 5b, Tco = 195 K) [23,24,25], shows the large symmetry breaking of the molecular arrangement upon the I–M transition (C2(CO), I222(metal)), which is induced by changes of the intermolecular dihedral angle. The differences in molecular re-arrangement in the thermal I–M transition for respective polytypes are evident Such an approach is effective for clarifying the roles of the electron correlation and the electron-phonon interaction in the PIMT.
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