Optical Probes of Correlated Electrons, Spins, and Dipoles
Correlation plays a central role in emergent phenomena. Examples include, among others, quantum ground states and collective excitations. Here, I will discuss what we can learn from time-domain views of correlation in two dimensional (2D) van der Waals (vdW) materials. In the 2D vdW magnetic semiconductor, CrSBr, excitonic transition is found to strongly couple to magnetic order [1] and this allows the easy detection of low energy (GHz-THz) magnons by visible-NIR light [2]. In twisted bilayer MoTe2, we apply pump-probe spectroscopy as hitherto the most sensitive probe of electron correlation in moiré quantum matter [3] to discover a zoo of quantum phases at fractional fillings in moiré super-lattices [4]. In the 2D vdW magnetic semiconductor, NbOI2 [5], we report the experimental discovery of a quasi-particle, the ferron, which may form the basis for new modes of information processing and control [6].
[1] Nature Mater. 2021, 20, 1657-1663.[2] Nature 2022, 609, 282-286.[3] Phys. Rev. Lett. 2024, 132, 126501.[4] Nature 2025, 641, 1149-1155.[5] Nature Mat. 2025, 24, 1203-1208.[6] arXiv preprint 2025, https://arxiv.org/abs/2505.22559.
Bio: Xiaoyang Zhu is the Howard Family Professor of Nanoscience and a professor of chemistry at Columbia University. He received a B.S. degree from Fudan University in 1984 and a Ph.D. from the University of Texas at Austin in 1989. After doing postdoctoral research with Gerhard Ertl at the Fritz-Haber-Institute, he joined the faculty at Southern Illinois University as an assistant professor in 1993. In 1997, he moved to the University of Minnesota as a tenured associate professor, later a full professor, and a Merck endowed professor. In 2009, he returned to the University of Texas at Austin as the Vauquelin Regents Professor and served as director of the DOE Energy Frontier Research Center and the Center for Materials Chemistry. In 2013, he moved to Columbia University. His honors include a Dreyfus New Faculty Award, a Cottrell Scholar Award, a Friedrich Wilhelm Bessel Award, APS Fellow, a Vannevar Bush Faculty Fellow Award from DOD, an Ahmed Zewail Award from the American Chemical Society, and an APS Earle K. Plyler Prize. Among his professional activities, he serves on the editorial/advisory boards of Science Advances, Chemical Physics, and Progress in Surface Science, and as a scientific advisor to the Fritz-Haber-Institute of the Max-Planck Society.