The advent of intrinsic magnetic topological insulators enables us to envisage various low-dimensional topological orders, such as the quantum anomalous Hall insulators and the axion insulators, at realistic cryogenic temperatures. These materials are represented by MnBi 2 Te 4 and its derived superlattices MnBi 2n Te 3n+1 . However, it has been controversial whether these materials exhibit the key ingredient for magnetic topological phases: an energy gap due to the time-reversal symmetry breaking. Moreover, the construction of high-quality magnetic topological insulators at the ultrathin limit has met significant challenges. In this talk, I will present a new technique, layer-encoded frequency-domain photoemission spectroscopy, which allows us to decipher the layer origins of various electronic states. By encoding layer indices with intralayer phonon frequencies, we measure the strengths of coupling with layer-specific phonons. This experiment reveals that the topological surface states on antiferromagnetic MnBi 4 Te 7 are partially relocated to the nonmagnetic layers, reconciling the mystery of vanishing broken-symmetry gaps. Moreover, I will present our recent progress on the “carpet-growth” of Bi 2 Te 3 ultrathin films and MnBi 2 Te 4 /Bi 2 Te 3 heterostructures using molecular beam epitaxy. These thin films extend coherently across a millimeter spatial scale without disruptions by substrate step edges. Angle-resolved photoemission spectroscopy studies yield unprecedentedly sharp electronic structures in agreement with first-principles calculations layer-by-layer, and suggest opportunities to realize the quantum spin Hall effect at near-ambient temperatures.

Bio: Shuolong Yang pioneered in combining femtosecond temporal control and atomic-layer engineering of quantum 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 is an NSF CAREER awardee, a DOE Early Career Awardee, and Moore foundation investigator.