Seminar: Excitons in 2D materials: Time-dependent phenomena, nonlinear optical responses, and excitonic insulators

Abstract:

Two-dimensional van der Waals materials exhibit novel photoexcited states, intriguing field-driven and time-dependent phenomena, and exotic ground states. Here, we present some recent theoretical studies on some remarkable exciton phenomena in these systems and their moiré structures. Our ab initio studies reveal a self-driven exciton-Floquet effect in the time-resolved, angle-resolved photoemission spectroscopy of 2D materials, where exciton-induced satellite features and band renormalization emerge, analogous to the optical Floquet effect driven by photons. We show that strong excitonic physics in 2D semiconductors can greatly enhance their nonlinear optical responses, such as shift currents and higher-order harmonic generation. This led to the discovery of the formation of light-induced shift current vortex crystals in transition metal dichalcogenide (TMD) moiré systems. Moreover, with an ab initio GW-BSE-BCS formalism, we predict that, at low temperature, spontaneously formed excitons in 1T′ monolayer MoS₂ condense into an unconventional p-wave excitonic insulator phase, characterized by emergent k-space spin textures and other telltale spectroscopic signatures.

Biography:

Steven G. Louie is a Distinguished Professor of the Graduate School at the University of California, Berkeley (UC Berkeley), and a Senior Faculty Scientist at the Lawrence Berkeley National Laboratory. He received his PhD in Physics from UC Berkeley in 1976. Before joining the UC Berkeley Physics faculty in 1980, he worked at the IBM Watson Research Center, Bell Labs, and the University of Pennsylvania.

Professor Louie is an elected member of the National Academy of Sciences, the American Academy of Arts & Sciences, Academia Sinica (Taiwan), the European Academy of Sciences, and a foreign member of the Chinese Academy of Sciences. He is also a fellow of the American Physical Society (APS), the American Association for the Advancement of Science, and the Materials Research Society (MRS). His numerous further honors include the APS Rahman Prize, APS Davisson-Germer Prize, APS Dresselhaus Prize, MRS Materials Theory Award, and the Foresight Institute Richard Feynman Prize.

Professor Louie’s research focuses on condensed matter physics and nanoscience. With over 700 publications, he is widely recognized for his pioneering development of the ab initio GW method and for his groundbreaking studies of novel bulk and reduced-dimensional systems.