Selective optical probing of the charge-density-wave phases in ${\mathrm{NbSe}}_3$

We report a study of the ultrafast single particle dynamics in quasi-one-dimensional ${\mathrm{NbSe}}_3$ whisker crystals using a two-color pump-probe measurement. A selective enhancement of the relaxation dynamics connected with one of the two charge-density-wave (CDW) phases was realized by tuning the probe energy to around $1.5\mathrm{eV}$, in which the temperature dependence across the corresponding phase transition shows a pronounced anomaly. A similar selective excitation was possible in the coherent CDW oscillation.

Figure 1

(Color online) The time-resolved reflectivity change $\mathrm{\Delta }R$ at various temperatures observed in the two-color pump-probe measurement. The pump and probe energies are, respectively, $1.07\mathrm{eV}$ and $1.56\mathrm{eV}$ in (a) and the reverse in (b).

Figure 2

(Color online) Temperature dependence of ${\tau }_{\mathrm{eff}}$ (upper) and $S_{\mathrm{eff}}$ (lower) obtained at probe energies of (a) $1.56\mathrm{eV}$ and (b) $1.07\mathrm{eV}$. The solid line in ${\tau }_{\mathrm{eff}}$ shows a theoretical fit to the data, and the lines in $S_{\mathrm{eff}}$ is a guide for the eye. The inset represents the expanded view of ${\tau }_{\mathrm{eff}}$ around $T_{C1}$.

Figure 3

(Color online) (a) The transient $\mathrm{\Delta }R$ at various probe energies measured at $58\mathrm{K}$. The fundamental and OPO excitation limits are between 1.16 and $1.39\mathrm{eV}$. The dotted line shows the longest fast decay component observed at a probe energy of $1.56\mathrm{eV}$. (b) Semilogarithmic plots of ${\tau }_{\mathrm{eff}}$ as a function of probe energy. The solid line is a guide for the eye.

Figure 4

(Color online) (a) The oscillation transient signal $\mathrm{\Delta }R$ after subtraction of the decay components at various probe energies obtained at $8.5\mathrm{K}$. (b) The FFT spectra of their signals. (c) Semilogarithmic plots of the FFT amplitude of AM (the lower peak at $1.13\mathrm{THz}$) as a function of probe energy. The fundamental and OPO excitation limits are between 1.16 and $1.39\mathrm{eV}$.

Figure 5

Schematic diagram of the resonant probe transitions measured in the vicinity of $T_{C2}$ (a) and well below $T_{C2}$ (b). For simplicity, only the transitions from occupied states ($E_0$ and $E_1$) to an unoccupied state $\left(E_2\right)$ are shown. These two transitions are overlapped in (a).