The quasistable state in the photoinduced phase transition for the quasi-one-dimensional quarter-filled organic conductor (EDO-TTF)2PF6 has been examined by ultrafast reflective measurements and time-dependent model calculations incorporating both electron-electron and electron-phonon interactions. The transient optical conductivity spectrum over a wide probe photon-energy range revealed that photoexcitation induced a new type of charge-disproportionate state. Additionally, coherent and incoherent oscillations dependent on probe photon energies were found, as predicted by the calculation.

The photoinduced phase formation in a strongly correlated crystal (EDO-TTF) 2 PF 6 (EDO-TTF: 4,5ethylenedioxytetrathiafulvalene) is investigated using a 12 fs laser pulse. The formation time is determined as 40 fs with observation of coherence of electron-phonon coupled excited states prior to formation. The temperature-independent dephasing time is determined as ß22 fs up to 180 K and the frequency of phonon oscillation is ß38 THz, corresponding to the intramolecular vibrations in EDO-TTF. The phase formation is coherently controlled by relative-phase-controlled two-pulse excitation.Strongly correlated materials exhibit intriguing phases such as superconductor, Mott insulator, spin liquid, and charge density wave [1]. Competitions between electrons, phonons, and spins in the materials play a key role in creating these phases. Photoexcitation controls this competition and initiates cooperative response resulting in a macroscopic ordering, called photoinduced phase transition (PIPT). In PIPT, electronphonon interactions lead to significant changes in dynamics. Thus, not only pure electronic but also electron-phonon dynamics have attracted considerable attention. Time-resolved optical spectroscopy has been widely used to study the electronic and vibrational dynamics and subsequent PIPT by monitoring the transient changes in optical constants on a femtosecond time scale [2][3][4][5][6]. However, the interactions dominating electronic phases are often inferred indirectly from these measurements.Coherent nonlinear spectroscopy utilizing third-order nonlinear response is a powerful tool to directly investigate interactions and has been applied to reveal ultrafast dynamics in semiconductors [7,8], light-harvesting protein complexes [9][10][11][12], and other functional materials [13,14]. However, this spectroscopy has been applied only to a few strongly correlated materials [15] because their electronic coherence is considered to be too short owing to their strong electron-electron interaction. In this Rapid Communication we have investigated 10-fs time scale dynamics of the strongly correlated organic chargetransfer (CT) complex (EDO-TTF) 2 PF 6 (EDO-TTF: 4,5ethylenedioxytetrathiafulvalene) [16], which exhibits unique PIPT due to strong electron-phonon interactions. We also revealed the formation process of the photoinduced phase * onda.k.aa@m.titech.ac.jp from the phonon-coupled excited state as well as the role of coherences in the excited state.(EDO-TTF) 2 PF 6 undergoes metal-insulator phase transition at 280 K because of its strong electron-electron and electron-phonon interactions and shows (0110)-type charge ordering below the transition temperature [16], where the number in parentheses represent the order of charge for EDO-TTF molecules, as shown in the upper panel of Fig. 1(a). This material exhibits a two-step phase transition triggered by photoexcitation in the low-temperature phase, that is, "insulator"-"photoinduced phase"-"metal" transition. The first step takes place within 100 fs and the photoinduced ph...

We have investigated the nature of the photo-induced state and coherent phonon in the conducting charge transfer complex (EDO-TTF) 2 PF 6 by measuring the ultrafast reflectivity change over a wide photon energy range from 0.069 eV (18 μm) to 2.1 eV (580 nm). The photo-induced spectra just after photo-excitation indicate that the photo-induced phase is similar to but clearly different from that in a thermally induced metal phase though they are the same for general photo-induced phase transition. The temporal profiles at each probe photon energy are accompanied by extraordinarily large amplitude oscillation originating from coherent phonon generation. The period of the coherent phonon depends discretely on the probe photon energy, while the generation efficiency of both the photo-induced phase and coherent photon reaches a maximum when the peak top of a charge transfer band is excited. These results are probably attributable to the strong electron-vibration coupling in this complex.

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