Photoinduced Change in the Charge Order Pattern in the Quarter-Filled Organic Conductor $(\mathrm{EDO}\mathrm{\text{−}}\mathrm{TTF}{)}_2{\mathrm{PF}}_6$ with a Strong Electron-Phonon Interaction

The quasistable state in the photoinduced phase transition for the quasi-one-dimensional quarter-filled organic conductor $(\mathrm{EDO}\mathrm{\text{−}}\mathrm{TTF}{)}_2{\mathrm{PF}}_6$ 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.

Figure 1

(a) Schematic illustration of charge patterns in $(\mathrm{EDO}\mathrm{\text{−}}\mathrm{TTF}{)}_2{\mathrm{PF}}_6$ for thermally induced LT and HT phases along with the nonequilibrium photoinduced state proposed in this Letter. (b) Spectrum of the photoinduced reflectivity change ($\Delta R/R$) at 0.1 ps after photoexcitation (red or gray line). The data around 1.58 eV could not be obtained due to the stray light of the pump pulse. Black and green (or light gray) dashed lines represent, respectively, the thermally induced $\Delta R/R$ spectrum from 180 to 290 K and the simulated $\Delta R/R$ by the multilayer model described in Refs. 14, 15. (c) Static and photoinduced optical conductivity spectra derived by the Kramers-Kronig transformation with interpolation, smoothing, and extrapolation.

Figure 2

Phase diagram in the $\gamma \mathrm{\text{−}}V$ plane. The insets show typical spectra in the neighborhood of the phase separation line. The red (or gray) circle marks the point, $\gamma =0.2\mathrm{eV}$ and $V=0.28\mathrm{eV}$, which we used in our time-dependent calculation, while the blue (or dark gray) triangle marks the point, $\gamma =0.2\mathrm{eV}$ and $V=0.29\mathrm{eV}$.

Figure 3

(a) Time dependence of charge densities $⟨n_j⟩$. Red (or gray), green (or light gray), blue (or dark gray), and purple (or lightest gray) correspond to sites 1, 2, 3, and 4, respectively, as shown in the legend. (b) Transient spectra $\omega {\sigma }^{\prime }(\omega ,t)$ at $t=0$, 300, and 900 under the same parameters in (a).

Figure 4

Time dependence of the spectra ${\sigma }^{\prime }(\omega ,t)$ for (a) $\omega >0.34$ and (b) $\omega <0.1$. Experimentally observed temporal profiles of reflectivity change ($\Delta R/R$) with probe photon energies (c) above 0.5 eV and (d) below 0.4 eV. Clear periodic oscillation is observed in (a) and (c), while no regular oscillation is seen in (b) and (d).