Quantum Matter Seminar- Shuolong Yang (University of Chicago)- Layer-by-layer distribution of electronic wavefunctions in topological materials

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.