MSE 298 Seminar: Imaging of Real-Space Topological Textures and Their Order Parameters

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 and electric or magnetic fields was made possible by new imaging methods, i.e., four-dimensional scanning transmission electronmicroscopy (4D-STEM) diffraction imaging. I will demonstrate how new science is enabled by 4D-STEM, which was otherwise not possible, by presenting examples for both polar and magnetic textures. First, 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. Second, I 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. 

Bio: 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 and strain and chiralities during topological phase transitions. Yu-Tsun received his doctorate in materials science and engineering at the University of Illinois at Urbana-Champaign in 2018 and was a postdoctoral researcher in David Muller’s group at Cornell University. He is a recipient of the Robert P. Apkarian Postdoctoral Scholar Award (2021), 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).