Nonthermal Melting of a Charge Density Wave in ${\mathrm{TiSe}}_2$

We use time-resolved optical reflectivity and x-ray diffraction with femtosecond resolution to study the dynamics of the structural order parameter of the charge density wave phase in ${\mathrm{TiSe}}_2$. We find that the energy density required to melt the charge density wave nonthermally is substantially lower than that required for thermal suppression and is comparable to the charge density wave condensation energy. This observation, together with the fact that the structural dynamics take place on an extremely fast time scale, supports the exciton condensation mechanism for the charge density wave in ${\mathrm{TiSe}}_2$.

Figure 1

(a) Temperature dependence of the transient reflectivity signal of ${\mathrm{TiSe}}_2$ measured at a laser fluence of $3\mu \mathrm{J}/{\mathrm{cm}}^2$. The inset shows the power spectrum of the oscillatory component of the data and the peak positions as extracted from a fit to a damped oscillator model. (b) ${\mathrm{TiSe}}_2$ unit cell at $T<T_c$. Arrows indicate the atomic displacements associated with the $A{1}_g$ amplitude mode of the CDW phase [22]. (c) Power spectrum of the oscillatory part of the reflectivity measured at $T=80\mathrm{K}$ for different excitation fluences. Open circles indicate peak positions of the $A_{1g}$ mode of the CDW.

Figure 2

Time evolution of the $(\frac{1}{2}\frac{1}{2}\frac{1}{2})$ diffraction peak after laser excitation at 90 and 140 K. The inset shows laser excitation energy per u.c. in the sample vs distance from the sample surface. Regions $\mathbf{I}$ and $\mathbf{I}\mathbf{I}$ correspond to melted and perturbed CDW phases, respectively.

Figure 3

Normalized intensity of the $(\frac{1}{2}\frac{1}{2}\frac{1}{2})$ SL peak at 100 ps after excitation vs incident laser fluence at 140 (●) and 90 K (▴). The threshold fluences ($I_{\mathrm{th}}$) are determined from the extrema of the numerical derivatives of the curves. Gray lines correspond to the calculated maximum thermal contribution to the drop in the diffraction intensity.