Topological defects play a key role in nonequilibrium phase transitions, ranging from birth of the early universe to quantum critical behavior of ultracold atoms. In solids, transient defects are known to generate a variety of hidden orders not accessible in equilibrium, but how defects are formed at the nanometer lengthscale and femtosecond timescale remains unknown. In this work, we discovered the sub-picosecond formation of 1D topological defects in a two-dimensional charge density wave using ultrafast electron diffraction. We discovered a dual-stage growth of 1D domain walls which takes place within 1 ps which is mediated by nonthermal lattice vibrations. This work constitutes the first visualization of topological defect formation process in the femtosecond timescale. Our work provides a framework for ultrafast engineering of topological defects based on selective excitation of collective modes, opening new avenues for dynamical control of nonequilibrium phases in correlated materials.

This work was done in collaboration with researchers from Shanghai Jiao Tong University, Brookhaven National Laboratory, ShanghaiTech University, University of Amsterdam and UCLA.

Pre-print available here:
https://arxiv.org/abs/2211.05748