Ultrafast formation of topological defects and transient charge density waves in rare earth tri-tellurides

Ultrafast excitations in solids can lead to novel dynamics and to the emergence of material phases that are out of reach in thermal equilibrium. We use ultrafast x-ray diffraction to study the dynamics of charge density waves (CDW) in LaTe₃, where a “competing” CDW has been observed following laser excitation. LaTe₃ is a layered compound exhibiting a CDW along its ‘c’ axis. Upon laser excitation we measure suppression of the super-lattice Bragg peak and the formation of domain walls. These walls break down, forming 1D topological defects (vortex lines), the signatures of which we resolve using the extremely high temporal and momentum resolution provided by the free electron laser. The suppression of the stable CDW induces a transient enhancement of the intensity at a perpendicular wave-vector (‘a’ axis) with a timescale of a few picoseconds; However, our results strongly suggest that this modulation never develops long range order. By studying the dynamics of the ‘a’ and ‘c’ modulations we find indications that the vortex lines of the ‘c’ order prolong the lifetime of the ‘a’ axis fluctuations. Our results provide an important insight into the delicate interplay between competing phases and suggest routes to control emergent phases through topological defects.