Professor Shuolong Yang
University of Chicago
Layer-by-layer distribution of electronic wavefunctions in topological materials
Location: 1080 Physics Research Building
Faculty Host: Chris Hill
Abstract: Layer-by-layer engineering of ultrathin topological materials has given rise to a plethora of exciting quantum phenomena, such as the Quantum Spin Hall Effect and Quantum Anomalous Hall Effect. In such endeavors, understanding and controlling the electronic wavefunction across different layers is key to engineering the band topology. In this talk, I will show how we employ a “seeing-by-listening” scheme in time-resolved ARPES to achieve this goal. In a superlattice structure of magnetic topological insulator, MnBi4Te7, we launch intralayer vibrations in the MnBi2Te4 and Bi2Te3 constituent layers. Transforming the time-dependent band energy oscillations into the frequency domain allows us to link different bands to different frequencies, and ultimately to different layers. This study reveals a surprising wavefunction relocation of the topological surface state from the top magnetic layer to the second non-magnetic layer, which reconciles the missing broken-symmetry gap in this material. For the second part, I will discuss how to engineer the electronic structures using millimeter-scale carpets of two-dimensional topological insulators: two-quintuple-layer Bi2Te3 and MnBi2Te4/Bi2Te3 bilayers. These carpet-like materials extend coherently over millimeters with minimal disruptions from the substrate step edges, and exhibit inverted bandgaps over 100 meV. The unusual uniformity and continuity lead to a layer-by-layer correspondence between the ARPES spectra and first-principles calculations. Utilizing time-resolved photoemission, we modulate the interlayer distance in two-quintuple-layer Bi2Te3 and observe coherent band energy oscillations. The band-specific and momentum-dependent oscillations can only be understood in the inverted bandgap scheme, which is a dynamical signature of the 2D topological insulator phase.
Bio: Shuolong Yang is an expert in photoemission spectroscopy and thin film synthesis on quantum materials. Being a pioneer in combining femtosecond temporal control and atomic-layer engineering, Yang significantly advanced the physics understanding of unconventional superconductors and topological materials. Yang obtained his B.S. in Physics at Stanford University in 2010. He continued to finish his Ph.D. in Applied Physics at Stanford in 2016. Before starting at the University of Chicago, he was a postdoctoral fellow at Cornell University, working at the intersection between condensed matter physics and materials science. Yang has been the recipient of several prestigious scholarships and fellowships, including the Larry Yung Scholarship, the J. E. Wallace Sterling Award for Scholastic Achievement at Stanford, and the Stanford Graduate Fellowship. He was awarded the Kavli postdoctoral fellowship at Cornell. He was a recipient of the NSF CAREER Award and the DOE Early Career Award.
