Ultrafast dynamics of vortex strings in a charge density wave*

Excitation of materials with ultrafast light pulses can induce novel states of matter not accessible in thermodynamic equilibrium. Visualizing the pathways of transformation into those transient states can inspire rational ways to control materials’ functionality. Fast non-adiabatic transients between two structures typically leads to proliferation of topological defects, which govern the return to equilibrium as well as the metastability of competing orders in systems with intertwined orders. Yet, the dynamics of these defects in ultrafast phase transitions have been elusive.

We use ultrafast x-ray diffraction at an x-ray free electron laser (XFEL) to capture the dynamics of the charge density waves (CDW) of SmTe₃ and LaTe₃ after strong photoexcitation above the CDW gap. Exploiting the unique momentum- and time-resolution of the XFEL we obtain direct experimental evidence of the creation and evolution of topological defects of this incommensurate CDW. The scaling properties of the structure factor S(k, t) show unequivocally that the scattering originates from vortex strings characteristic of the topological defects of a complex order parameter in three dimensions, which correspond to dislocations of the CDW. While most features of the scattering can be explained by a simple Ginzburg-Landau model with a complex order parameter, certain scaling properties suggest additional conserved quantities. These observations and analysis show the unique capabilities of ultrafast x-ray scattering as an incisive tool to characterize nonequilibrium states.