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. 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 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.
Speaker Bio
Yu-Tsun Shao studies quantum materials by novel electron microscopy techniques, specifically
4D-STEM. He studies the (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.