Direct observations of the photoinduced change in dimerization in K-TCNQ

The photoinduced dynamics of a potassium-tetracyanoquinodimethane (K-TCNQ) single crystal in the generalized Peierls phase are evaluated via time-resolved vibrational spectroscopy. The transient reflectivity spectrum of the photoinduced state in the mid-IR range shows a decrease in the height and width of the reflectivity band because of the electron-molecular-vibration-coupled CN stretching mode at approximately 2180 ${\mathrm{cm}}^{-1}$. This spectral change suggests that the photoexcitation of the charge transfer in TCNQ molecules induces melting of the dimerization of the molecules. From detailed analysis of the spectral evolution, the relaxation time constant from the photoinduced state to the dimerized state is estimated to be approximately 0.6 ps. Even after the recovery of the dimerization, a fluctuation is still observed, probably because of a domain-wall soliton. The fluctuation gradually dissipates with a time constant of approximately 2.3 ps. Direct observation of the dimerization process reveals the true dynamics of the photoinduced cooperative phenomenon within this system.

Figure 1

(a): Chemical formula of TCNQ. (b): The crystal structure of K-TCNQ at $25{}^{\circ }\mathrm{C}$ [9]. The purple (online) balls represent the K atom. The thin lines represent the unit cell. (c) and (d): Schematic view of the TCNQ chain at a temperature($T$) above the transition temperature ($T_c$), and $T<T_c$. The rectangles and arrows represent the TCNQ molecules and electrons with spin, respectively. The rounded and dashed rectangles represent the dimerization pairings.

Figure 2

(a) Schematic view of the ${\mathrm{a}}_g{\nu }_2$ mode. (b) Temperature dependence of the reflectivity spectra with $E\left|\right|a$ in the frequency range of the ${\mathrm{a}}_g{\nu }_2$ mode. (c) and (d) Temporal profile of $\mathrm{\Delta }R/R$ at 2189.4 ${\mathrm{cm}}^{-1}$ and the photoinduced transient reflectivity spectra at certain delay times with an excitation intensity of 0.29 photon/TCNQ. The red thin lines in (d) are fitting curves. (e) The excitation intensity dependence of the photoinduced transient reflectivity spectra at time zero.

Figure 3

Temperature dependence of (a) the value of reflectivity at 2180 ${\mathrm{cm}}^{-1}$, (b) oscillator strength ($\beta$), and (c) damping constant ($\mathrm{\Gamma }$). The vertical dashed line represents $T_c$. The excitation intensity dependence of (d) reflectivity at 2180 ${\mathrm{cm}}^{-1}$, (e) $\beta$, and (f) $\mathrm{\Gamma }$.

Figure 4

Time dependence of the fitting parameters. (a) The relative change in $\beta$, (b) ${\omega }_0$, and (c) $\mathrm{\Gamma }$. (d) Time dependence of the relative reflectivity change at 2016 ${\mathrm{cm}}^{-1}$ (closed circles) with the results of the fitting. The dashed lines are the components of the fast and slow relaxation (see text). Inset of (d): Reflectivity spectrum of K-TCNQ at RT with $E\left|\right|a$.

Figure 5

Schematic photoinduced dynamics. In each image, the rectangles are TCNQ and the arrows are electrons. Rounded and dashed rectangles surrounding the molecules represent the pairing of the dimerization of TCNQ.

Figure 6

Time dependence of the pump-probe signal in the negative delay time.

Figure 7

Probe photon energy dependence of the oscillation frequencies in the negative delay time. The thin solid lines are the expected photon energy dependencies of the frequency, assuming a perturbed FPD.

Figure 8

Fluence dependency of the temporal profile of the relative reflectivity change in the negative delay time. Inset: Fluence dependency of the relative reflectivity change at a certain delay time.