We study the non-equilibrium structural dynamics of the incommensurate and nearly commensurate charge-density wave (CDW) phases in 1T-${\text{TaS}}_2$. Employing ultrafast low-energy electron diffraction with 1 ps temporal resolution, we investigate the ultrafast quench and recovery of the CDW-coupled periodic lattice distortion (PLD). Sequential structural relaxation processes are observed by tracking the intensities of main lattice as well as satellite diffraction peaks and the diffuse scattering background. Comparing distinct groups of diffraction peaks, we disentangle the ultrafast quench of the PLD amplitude from phonon-related reductions of the diffraction intensity. Fluence-dependent relaxation cycles reveal a long-lived partial suppression of the order parameter for up to 60 ps, far outlasting the initial amplitude recovery and electron-phonon scattering times. This delayed return to a quasi-thermal level is controlled by lattice thermalization and coincides with the population of zone-center acoustic modes, as evidenced by a structured diffuse background. The long-lived non-equilibrium order parameter suppression suggests hot populations of CDW-coupled lattice modes. Finally, a broadening of the superlattice peaks is observed at high fluences, pointing to a non-linear generation of phase fluctuations.

中文翻译：

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超快速低能电子衍射跟踪的不相称电荷密度波的结构动力学。

我们研究了1 T-中不相称和近相称的电荷密度波（CDW）相的非平衡结构动力学-${\text{钽}}_2$。利用具有1 ps时间分辨率的超快速低能量电子衍射，我们研究了CDW耦合的周期性晶格畸变（PLD）的超快速猝灭和恢复。通过跟踪主晶格的强度以及卫星衍射峰和扩散散射背景，观察到顺序的结构弛豫过程。比较不同的衍射峰组，我们可以将PLD振幅的超快猝灭与声子相关的衍射强度降低区分开来。取决于注量的弛豫循环揭示了对阶数参数的长期局部抑制，最高可达60 ps，远超过了初始振幅恢复和电子-声子散射时间。这种延迟返回准热水平是由晶格热化控制的，并且与区域中心声模的总数一致，这由结构化的扩散背景所证明。长期的非平衡阶参数抑制表明CDW耦合晶格模式的热种群。最后，在高通量下观察到超晶格峰变宽，表明相位波动是非线性产生的。