Yimei Zhu
Brookhaven National Laboratory, USA
Title: Disentangle phonon modes using ultrafast electron diffraction and timely-resolved electron crystallography
Biography
Biography: Yimei Zhu
Abstract
Polaron transport, in which electron motion is strongly coupled to the underlying atomic lattice, is crucial to understanding the electrical conductivity in many solids. The accompanying atomic displacements are themselves coupled through phonons, but the specific phonon modes responsible for the dynamics of polaron motion have rarely been identified. In this presentation, I will first give an overview on the 2.8 MeV ultrafast electron diffraction instrument and the time resolved electron crystallography method we developed at BNL, then focus on its application to understand charge, orbital and lattice coupling and interaction in strongly correlated electron systems. A detailed example will be given on quantifying the dynamics of both electronic and atomic motion in the LaSr2Mn2O7 manganite. Using photoexcition to set the electronic system in motion, we find that Jahn-Teller-like O, La/Sr, and Mn4+ displacements dominate the lattice response and exhibit a dichotomy in behavior overshoot-and-recovery for one sub-lattice versus normal behavior for the other. This dichotomy, attributed to slow electronic relaxation, proves that polaron transport is a key process in doped manganites. Our technique with the access to high-order reflections and being sensitive to phonons promises to be applicable for specifying the nature of electron-phonon coupling in many complex materials.
