Optical selection of a multiple phase order in the charge density wave condensate $o{\text{-TaS}}_3$ using a spectrally resolved nonequilibrium measurement

We investigate the spectrally resolved transient reflectivity changes $\Delta R\left(T,\tau ,\lambda \right)$ in the charge density wave (CDW) conductor $o{\text{-TaS}}_3$. A distinct near-infrared resonance in this compound emphasizes the characteristic $\Delta R\left(\lambda \right)$ resonances, and allows a selection of coexisting CDW phases with different nonequilibrium carrier dynamics. Furthermore, the spectrally resolved $\Delta R\left(\tau \right)$ characterizes the collective oscillations associated with the individual states. We believe that this demonstration paves the way for the optical selection of the multiphase order that plays an important role in various macroscopic quantum systems.

Figure 1

(Color online) (a) Optical reflectivity $R\left(\lambda \right)$ around an interband resonance of $o{\text{-TaS}}_3$ at various temperatures. The incident laser polarization was set parallel to the chain axis (conducting axis). (b) A typical series of transient reflectivity changes, $\Delta R\left(\tau \right)$, at various temperatures. The $\Delta R$ obtained at ${\lambda }_{pr}=880\text{nm}$ showing a decrease in $R$ from its equilibrium value is selected to make it easy to compare results obtained below and above the CDW transition temperature $\left(T_{CDW}=220\text{K}\right)$. The pump and probe polarizations were parallel to the conducting axis.

Figure 2

(Color online) (a)–(e) 3D view of transient reflectivity changes, $\Delta R\left(\tau ,\lambda \right)$, at various temperatures. (f)–(j) Transient spectral changes $\Delta R\left(\lambda \right)$ at $\tau =0$ (gray, solid circle), 1 (red, solid circle), and 20 ps (blue, open circle). The arrow indicates a negative peak whose shape and $T$ dependence roughly match the changes of stationary reflectivity spectra.

Figure 3

(Color online) Temperature dependences of $\Delta R$ [$\tau =1$ (red, solid circle) and 20 ps (blue, open circle)] at various ${\lambda }_{pr}$.

Figure 4

(Color online) (a) Typical transient oscillation signals $\left(\Delta R_{osc}\right)$ obtained at various $\lambda \text{s}$ at 10 K, where $\Delta R_{osc}$ is evaluated by subtracting the exponential contributions from $\Delta R$. (b) A Fourier-transform (FT) spectrum obtained at $\lambda =830\text{nm}$ at $T=10\text{K}$. Note that some of the high-frequency modes can also be identified in optical Raman spectra (Ref. 20). (c) Plots of magnitudes of the coherent oscillation modes at $T=10\text{K}$ as a function of $\lambda$, where the magnitudes of individual modes are evaluated by the amplitudes of the FT spectral peaks. The symbols (colors) are defined in (b). For comparison, DR spectrum is also plotted on top. (d) $T$ dependences of the typical three oscillation amplitudes. The lowest frequency mode disappears around $T=100\text{K}$ due to a damped-overdamped transition of the oscillation. (e) $\Delta R\left(\tau ,760\text{nm}\right)$ in the low-temperature range. A large oscillation corresponds to the lowest frequency mode. (f) $T$ dependences of the frequency $\left(\nu \right)$ and damping constant $\left(\Gamma \right)$ of the lowest mode.