Topological defects play a crucial role in the phase transition process. Taking the theory of early universe formation as an example, after the Big Bang, the universe cooled rapidly, which triggered a series of spontaneous phase transitions. theoretical physicists, such as Tom Kibble, have proposed that topological defects would be generated along with these phase transitions at plummeting temperatures, and these defects are known as cosmic strings. Since it is still difficult to directly observe the formation process of cosmic strings by current experimental means, people are also exploring the use of other systems to study topological defects, and quantum materials provide an ideal platform for studying the formation process of topological defects at the microscopic level. In the study of quantum materials, topological defects are not only generated in temperature dips, but also transient topological defects can be generated by femtosecond beam excitation, and these defects often induce properties or phase transitions that do not exist in equilibrium, such as light-induced insulator-metal phase transitions and superconducting-like behavior. Similar to the problem of cosmic strings, the dynamical formation of photo-induced topological defects has lacked experimental observations at microscopic scales and ultrashort time scales, and there is a lack of consensus on the exact time required for topological defect formation.
Image Figure 1: Femtosecond laser-induced generation of topological defects
In order to be able to study the formation process of these defects at both the spatial scale of nanometers and the time scale of femtoseconds, a group led by Prof. Wizard of the School of Physics and Astronomy/Zhangjiang Institute of Advanced Studies and Academician Jie Zhang of the School of Physics and Astronomy/Li Zhendao Research Institute at Shanghai Jiao Tong University recently collaborated with researchers from the Shanghai University of Science and Technology (SUSTech), the Universities of California, Berkeley, and Los Angeles, Brookhaven National Laboratory (BNL), and the University of Amsterdam (UA). In collaboration with researchers at the University of California, Berkeley, and Los Angeles, Brookhaven National Laboratory, and the University of Amsterdam, the group has utilized a mega-volt ultrafast electron diffraction system developed independently with the support of the National Research Instrumentation Program of the China Foundation for Science and Technology (CNRI), and has observed, in real time and at the atomic scale, the dynamics of topological defects formation in the charge-density-wave material 1T-TiSe2 under optical excitation (Fig. 1). The work was recently published in Nature Physics under the title "Ultrafast formation of topological defects in a 2D charge density wave".
Unlike the direct imaging of defects in real space, this experiment utilizes diffraction to obtain structural information of defects, as different defect structures form different diffraction fingerprints in inverse space (Fig. 2). After analyzing and simulating the diffraction peaks as well as the diffuse scattering signals, the research team successfully decoded the dynamic process of material structure and topological defects after light excitation.
Image 2: Schematic representation of diffraction spot information corresponding to different structure distributions
The experiments were conducted on the 2D quantum material 1T-TiSe2, which undergoes a charge density wave (CDW) phase transition near 200 K. The team found in past experiments that the structure of the CDW in some of the layers can be controlled in an orderly manner by a weak femtosecond laser to cause an inversion of the whole layer when the temperature is below 200 K. As a result, a domain with a 2D electronic state can be formed at the interface of the original CDW and the CDW of the inverted layer. domain wall with two-dimensional electronic states [Nature 595,239(2021)]. As the energy density of the pump laser continues to increase, the number of CDW layers undergoing structural inversion behavior gradually increases, and the three-dimensional CDW is completely transformed into a two-dimensional CDW with no correlation between the layers [Nature Communications 13, 963 (2022)].
In this study, the team chose the measurement temperature above 200 K, which is the state in which only in-plane 2D CDW exists in 1T-TiSe2. By analyzing the diffuse scattering signal in the diffraction spot, which is about 1000 times weaker than the conventional Bragg peak signal (Fig. 3), the team found that the two-dimensional CDW within the facet also undergoes a similar inversion process as the three-dimensional CDW, i.e., there exists a single-stranded CDW inversion within the facet, and that this inversion process induces the formation of a one-dimensional domain wall, i.e., one-dimensional topological defects, within the layer (see schematic diagram in the lower left corner of Fig. 2).
Thanks to the ultra-high temporal resolution and signal-to-noise ratio of the system, while measuring the 1D domain walls, the group also found that the same time scale of the defect formation is accompanied by some diffuse scattering signals with special distributions in the inverse space. Combined with theoretical simulations of the diffuse scattering signals and related dynamics, the team found that these signals come from longitudinal acoustic phonons generated by optical excitation, and that these longitudinal acoustic phonons are the triggering factor for the formation of the chain domain wall defects mentioned above.
This work shows for the first time the defect formation process in sub-picosecond time scale and the key role of lattice vibration in the process, which will provide important information for the future understanding of the nature of non-equilibrium matter and the role of topological defects, and the phonon dynamics analysis method can also help to further understand the energy conversion mechanism in quantum materials, thermoelectric materials, and other new energy materials.
This work was supported by the National Key Research and Development Program of China (No. 2021YFA1400202), the National Natural Science Foundation of China (NNSFC) (No. 11925505, 12005132, 11504232 and 11721091), the Major Program of Shanghai Municipal Commission of Science and Technology (No. 16DZ2260200), the U.S. Department of Energy (DOE) and the U.S. National Science Foundation (NSF). Foundation (NSF). Dr. Yun Cheng (graduated) from Shanghai Jiaotong University and Dr. Alfred Zong, Miller Fellow at the University of California, Berkeley (soon to be Assistant Professor at Stanford University) are the co-first authors of the article.
