Optical freezing of charge motion in an organic conductor

Dynamical localization, that is, reduction of the intersite electronic transfer integral t by an alternating electric field, E ( ω ), is a promising strategy for controlling strongly correlated systems with a competing energy balance between t and the Coulomb repulsion energy. Here we describe a charge localization induced by the 9.3 MV cm −1 instantaneous electric field of a 1.5 cycle (7 fs) infrared pulse in an organic conductor α-(bis[ethylenedithio]-tetrathiafulvalene) 2 I 3 . A large reflectivity change of >25% and a coherent charge oscillation along the time axis reflect the opening of the charge ordering gap in the metallic phase. This optical freezing of charges, which is the reverse of the photoinduced melting of electronic orders, is attributed to the ~10% reduction of t driven by the strong, high-frequency ( ω ≧ t / ħ ) electric field. In strongly correlated systems, the material properties can be drastically altered through subtle external perturbations. Here, the authors show that photoexcitation of the organic conductor α-(ET) 2 I 3 with ultrashort pulses leads to a counter-intuitive freezing of the electron motion.