Coherent dynamics of photoinduced phase formation in a strongly correlated organic crystal

Abstract

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...

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“…This is not the case in our study and this is an indication of the local nature of the structural trapping of the electronic excited state. This local trapping was theoretically introduced to explain the two-step dynamics [6][7][8][9], which we will discuss in the last part of this paper.…”

“…It exhibits first-order insulator-to-metal (I-M) phase transition at thermal equilibrium and a gigantic ultrafast change of reflectivity under irradiation by a laser pulse in the I phase [5]. * It was shown by time-resolved optical [6,7] and vibrational spectroscopy [8] as well as ultrafast electron diffraction [9] that the photoinduced transformation proceeds stepwise. The initial electronic excited state is trapped at the molecular level within 40 fs following the electron-phonon coupled precursor state [7], while the crystal transformation is driven by slower propagation at the 100 ps time scale.…”

“…* It was shown by time-resolved optical [6,7] and vibrational spectroscopy [8] as well as ultrafast electron diffraction [9] that the photoinduced transformation proceeds stepwise. The initial electronic excited state is trapped at the molecular level within 40 fs following the electron-phonon coupled precursor state [7], while the crystal transformation is driven by slower propagation at the 100 ps time scale. Actually, different isostructural compounds of the same (EDO-TTF) 2 XF 6 family, where X stands for P, As, or Sb, have already been synthesized.…”