Graphic: Artist’s impression of the birth of a defect. How fast can a defect form in solids? Using ultrafast electron diffraction, scientists recorded a video of the birth of one-dimensional domain walls in a charge density wave, which is illustrated by the wavy structure. After photoexcitation, these topological defects are found to form well within a picosecond (1 picosecond = 10–12 second) — the intrinsic time scale of atomic vibration. Specifically, it is found that defect formation is tied to a specific type of atomic movement, as illustrated by the vibrating spheres in the foreground.
Quest for understanding topological defects
Topological defects play an important role in phase transitions. Taking the theory of the early universe as an example, after the Big Bang, the universe rapidly cooled, triggering a series of symmetry-breaking phase transitions. Theoretical physicists like Tom Kibble proposed that topological defects, known as cosmic strings, would accompany these quench-induced phase transitions. Since direct observation of the formation process of cosmic strings in the universe is challenging because one has to create a condition similar to the one immediately after the Big Bang, researchers had to resort to other systems to study the dynamics of topological defects. In this regard, solid crystals provide an ideal platform for investigating the formation process of topological defects at the microscopic level. In crystals, topological defects not only occur during a thermal quench but can also be generated by femtosecond light pulses, which can induce exotic material properties or phase transitions that do not exist in equilibrium. Similar to the problem of cosmic strings, the dynamic formation process of optically induced topological defects has lacked a direct observation at the microscopic lengthscale and ultrafast timescale, and there is no consensus on the exact time required for the formation of topological defects.
To study the formation process of these defects on a nanoscale spatial scale and a femtosecond time scale, an international collaboration between researchers from the Zuerch group at UC Berkeley, Shanghai Jiao Tong University, Brookhaven National Laboratory, UC Los Angeles, University of Amsterdam, and ShanghaiTech University utilized state-of-the-art megaelectron-volt electron diffraction beamline in Shanghai and observed in real-time the dynamic formation process of topological defects in a charge-density wave (CDW) material 1T-TiSe2 under optical excitation. Their work titled "Ultrafast formation of topological defects in a two-dimensional charge density wave" was recently published in Nature Physics and was featured in journal’s News & Views.