Physics Colloquium: “Designing and probing exciton quantum phase transitions in 2D semiconductors” by Yusong Bai - Brown University

Designing and harnessing quantum phases of matter is a central goal in modern physical sciences and information technologies. Quantum phases of matter are predominantly characterized as interacting many-body electronic systems. They often feature intricate interplay between strong correlation, magnetism, and topology, which are underpinned by charge, spin, and orbital interactions. In this talk, I will present examples from our recent research efforts in realizing exciton (H-atom-like electron-hole pair) quantum phase transitions. Particularly, the crystallization of dipolar and quadrupolar excitons in 2D van der Waals heterostructures of transition metal dichalcogenide will be discussed. The goal is to understand how the fundamental factors, such as Coulomb interactions, kinetic energies, Fermionic exchange interactions, and quantum fluctuations, drive the formation of ordered ground states. Along this line, we further aim to develop “quantum knobs” that tune charge, spin, and orbital degrees of freedom by controlling interfacial interactions amongst molecular-to-nanoscale material entities and map the resultant emergent quantum effects using spectro-microscopies under cryogenic and magnetic conditions. I will survey a number of recently observed exotic quantum phases of matter based on 2D materials and hint the future opportunities in this area of research. Professor Yusong Bai is currently a faculty member in the Chemistry Department at Brown University. He received his PhD degree in 2017 from Duke University, where his research focused on the design, synthesis, and spectroscopy of porphyrin-based supermolecules, as well as spectroscopic studies of the many-body photophysics in carbon nanotubes. Following his doctoral studies, he moved to Columbia University, where he began investigating 2D materials and conducted research on exciton phase transitions in such systems. In 2022, Dr. Bai joined the Chemistry Department at Brown University. His current research integrates expertise gained from his PhD and postdoctoral work, with a focus on designing new quantum materials based on mixed-dimensional material systems. He is particularly interested in studying the exciton quantum phase transitions and quantum optical properties in these new material systems.