Ultrafast formation of topological defects in a two-dimensional charge density wave

Topological defects are crucial in understanding the behavior of dynamic systems experiencing non-adiabatic transitions, particularly in solid materials. The formation of these defects through femtosecond laser excitation has become an increasingly fascinating area of study, offering insights into phase transitions and the emergence of hidden orders not present in thermal equilibrium. Despite their prevalence in non-equilibrium states, the speed at which these defects can appear in solids and the mechanisms behind their creation at such rapid timescales have remained elusive. In a groundbreaking study, researchers utilized ultrafast electron diffraction to investigate the reciprocal-space manifestations of transient defects in a two-dimensional charge density wave. By simultaneously tracking the dynamics of both defects and phonons, the team gained a microscopic perspective on defect formation in the femtosecond timeframe. Remarkably, they observed the generation of one-dimensional domain walls just nanoseconds after photoexcitation, with the growth of defects driven not by the order parameter's magnitude, but by a non-thermal population of longitudinal optical phonons. This research establishes a framework for the precision engineering of topological defects linked to specific collective modes, offering a pathway for dynamically influencing non-equilibrium phases in correlated materials. By shedding light on the ultrafast creation of defects and their interaction with underlying phonons, this study opens up new possibilities for manipulating material properties and exploring novel phases of matter. For more information, the full article can be found at https://www.nature.com/articles/s41567-023-02279-x.