Inelastic Light Scattering from Disordered Crystals

Abstract
My group investigated the structural dynamics of semiconducting, ion-conducting, and insulating crystals over the past several years. We studied many effects, such as soft modes, order-disorder phase transitions, and local fluctuations. All of these are related to the interaction between vibrational normal modes of the crystals (i.e., anharmonic effects). Anharmonic effects have significant implications on the functional properties of the crystals at finite temperatures (e.g. dielectric response, charge carrier lifetimes, and ion mobilities). These effects also relax the selection rules for Raman scattering derived from the symmetry of the crystal’s unit cell. Therefore, the resulting Raman spectra are difficult to interpret, on the one hand, but are rich in information regarding the crystal properties, on the other hand. In this talk, I will review our journey to unlock the mysteries of atomic motion in crystals at finite temperatures using Raman spectra. First, I will demonstrate and discuss a common discrepancy between x-ray diffraction data showing perfect single crystals and Raman data showing disorder. Next, I will explain how we use generalized scattering models to settle this discrepancy. Finally, I will show that the intensity of the scattered light, an underused experimental observable, can be utilized to learn much about crystal properties.

Biography
Dr. Yaffe is currently a senior scientist (Equiv. Assistant Professor) in the Chemical and Biological Physics Department at the Weizmann Institute of Science, Israel. He obtained a dual B.Sc. in chemistry and chemical engineering at Ben Gurion University in 2005. He then completed his Ph.D. in the Department of Materials and Interfaces at the Weizmann Institute of Science in 2012, working with Prof. David Cahen. From 2013 to 2016, he worked as a postdoctoral fellow with Prof. Louis Brus and Prof. Tony Heinz at Columbia University, Energy Frontier Research Center. His group includes physicists, engineers, and chemists that use optical spectroscopy to investigate the structure-function relationship in functional materials such as semiconductors, ionic-conductors, and ferroelectrics. Specifically, they are interested in phenomena that stem from strongly anharmonic thermal motion.