Stability of the light-induced hidden charge density wave state within the phase diagram of $1\mathit{T}\text{−}{\mathrm{TaS}}_{2-x}{\mathrm{Se}}_x$

The femtosecond transient optical spectroscopy is employed to study the relaxation dynamics of the equilibrium and hidden metastable charge-density-wave states in single crystals of $1T\text{−}{\mathrm{TaS}}_{2-x}{\mathrm{Se}}_x$ as a function of the Se doping $x$. Similar to pristine $1T\text{−}{\mathrm{TaS}}_2$, the transition to a hidden phase is observed at low temperature after a quench with a single 50 fs laser pulse, in the commensurate Mott phase up to $x=0.6$. The photo-induced hidden-phase formation is accompanied by a notable change in the coherent phonon spectra, and particularly the collective amplitude mode. While the temperature stability of the hidden phase is only slightly dependent on the Se content, the creation-threshold fluence strongly increases with Se content from 1 to $\sim 4$ mJ/${\mathrm{cm}}^2$, which is attributed predominantly to the change in optical absorption coefficient at the laser excitation wavelength, and not an increased barrier of the hidden state with increasing Se.

Figure 1

(a) The crystal structure of $1T\text{−}{\mathrm{TaS}}_{2-x}{\mathrm{Se}}_x$. Transient reflectivity in different phases (b) with the corresponding FTA spectra (c). The thick lines are DCE fits. The thin lines in (c) correspond to the fit residua.

Figure 2

Temperature dependence of the normalized transient reflectivity amplitude for different $x$. The data for different $x$ are shifted verticaly while the arrows indicate cooling and warming scans.

Figure 3

Temperature dependence of the modes frequency, ${\omega }_0,$ damping, $\gamma$, and the initial relaxation time, $\tau$, from the DCE fit for the $x=0.2$ sample. Arrows indicate cooling/warming cycles. The blue symbols correspond to the warming scan in the H phase after application of an above-threshold optical switching pulse at $T=30$ K.

Figure 4

The switching-pulse fluence dependence of the low-fluence transient reflectivity FTA spectra at 30 K for $x=0.2$ sample.

Figure 5

Reflectivity transients FTA spectra in the C and H phase at $T=30$ K for different Se doping levels, $x$.

Figure 6

(a) The H-phase $x-T$ stability diagram obtained from transient reflectivity data. The parentheses indicate warming. Open symbols are data from Refs. [10, 11]. $T_{\mathrm{H}}$ is the temperature up to which the H phase is stable on the timescale of the pump-probe scan $(\sim 30$ min). The boundary between the T and NC phases on warming is tentative for $x>0$ since it is not resolved in the transient reflectivity. (b) The H-phase threshold fluence as a function of $x.$

Figure 7

(a) Temperature dependence of the 2.40-THz mode frequency on warming after switching into the H phase at 30K. The empty symbols refer to the frequency in the C phase. (b) Se doping dependence of the 2.4(5)-THz mode damping and (c) the frequency at $T=30$ K in the C and H phases. The dotted lines are guide lines. The error bars correspond to the errors obtained from the DCE fit.