Materials Science Research Lecture

Abstract:

Topological structures in ferroic materials can emerge as particle-like objects such as skyrmions and merons, with real-space swirling arrangements of the order parameter that not only have mathematical beauty but hold promise for potential applications in next generation nanodevices. As those ferroic textures are intrinsically nm-scale and dynamic, developing methods for visualizing and characterizing their detailed 3D structure is a critical step in understanding their properties and exploring possible phase transitions. In this talk, I will show how the measurement of structural information such as polarization, strain, chirality, electric or magnetic fields was made possible by new imaging methods, i.e., four-dimensional scanning transmission electron microscopy (4D-STEM) diffraction imaging.

I will demonstrate how new science is enabled by 4D-STEM-based approach, which was otherwise not possible, by presenting examples for both polar and magnetic textures. We report the observation of room temperature Néel-type skyrmion in a van der Waals ferromagnet accompanied by a change in crystallographic symmetry and chemical order. Second, I report the emergence of achiral polar meron lattice (topological charge of +1/2) from disordered but chiral skyrmion (topological charge of +1) phase transition driven by elastic boundary conditions. Further, using multislice electron ptychography, the 3D structural distortions of unknown polar textures in complex oxide heterostructures can be resolved at unprecedented resolution and precision.

More about the Speaker:

Yu-Tsun Shao's primary research focuses on understanding the interplay between spin, lattice, polarization, and charge in quantum materials by developing and employing novel electron microscopy techniques, specifically 4D-STEM. He applies 4D-STEM to study (multi-)ferroic crystals with the aim to elucidate the microscopic origin of interactions among local polar/magnetic order, strain, and chiralities during topological phase transitions. Before joining USC, Yu-Tsun did postdoctoral work in Professor David Muller's group at Cornell University and received his Ph.D. in Materials Science and Engineering at the University of Illinois at Urbana-Champaign in 2018, under the mentorship of Professor Jian-Min Zuo. He is a recipient of the Robert P. Apkarian Postdoctoral Scholar Award (2021) and Presidential Student Award (2016) of the Microscopy Society of America, and the Ludo Frevel Crystallography Scholarship Award of the International Centre for Diffraction Data (2016).