Coherent order parameter dynamics in SmTe3

We present ultrafast optical pump-probe and ultrafast x-ray diffraction measurements of the charge density wave dynamics in SmTe3 at 300 K. We performed ultrafast x-ray diffraction measurements at the Linac Coherent Light Source to directly probe the dynamics of the finite-wave-vector order parameter. The dynamics reveal coherent oscillations at similar to 1.6 THz that become overdamped with increasing fluence. We identify this oscillation with the lattice component of the amplitude mode. Furthermore, our data allow for a clear identification of the amplitude mode frequency in the optical pump-probe data. In both measurements, the system reaches the symmetric phase at high fluence, where the order parameter vanishes and the response (reflectivity and x-ray intensity) is quadratic in the order parameter. This is observed in the x-ray diffraction as a small overdamped modulation near zero intensity. Similar overdamped features are observed in the optical reflectivity at high fluence. A time-dependent Ginzburg-Landau model captures qualitatively the essential features of the experimental observations.