Speaker: Professor Yimei Zhu (Department of Condensed Matter Physics，Brookhaven National Laboratory)

Time: October 09, 2017 10:30

Place: 理科楼C302

Abstract: MeV-ultrafast electron diffraction (MeV-UED), taking advantage of its high-sensitivity to phonons and the large interaction cross-sections of electrons with matter, has been identified as one of the frontiers and future directions of modern electron microscopy.  It enables us to reveal competing orders of electrons and phonons as well as hidden states far-from equilibrium to address some key issues in condensed matter physics. In this presentation, I will give two examples of our recent studies. The first is on observations of strong electron-lattice interaction and how to disentangle various phonon modes in charge/orbital ordered LaSr₂Mn₂O₇ manganite using time-resolved electron crystallography method we developed.  The work provides direct evidence of polaron formation and transport as the key mechanism for the colossal magnetoresistance in the system. The second example is the study of electron and lattice dynamics of optimally doped Bi₂Sr₂CaCu₂O_8+ō. By quantitatively measuring the diffuse scattering after photoexcitation our work illustrates the preferential coupling of electrons to the in-plane Cu-O bond-stretching phonon. The observed unique electron-phonon interactions beyond the N-temperature model provide insight in resolving longstanding controversies surrounding equilibrium interactions in the high-T_c superconductor.

Bio: Prof. Yimei Zhu is Senior Physicist at Brookhaven National Laboratory (BNL) and Adjunct Professor at Columbia University and Stony Brook University. He received his BS from Shanghai Jiaotong University in 1982, MS and PhD from Nagoya University in 1987. He joined BNL as Assistant Scientist in 1988, rising through the rank to become Tenured Senior Physicist in 2002. He is the founding director of the Institute for Advanced Electron Microscopy at BNL, and currently holds the largest DOE core-research program on electron scattering and microscopy in the United States. His research interests include condensed matter physics of strongly correlated electron systems and advanced electron microscopy including ultrafast microscopy instrumentation. Through the years, Prof. Zhu has received many honors and awards and served on many government committees and adversary boards. He is Inaugural Fellow of Microscopy Society of America, and Elected Fellow of APS and AAAS.  He has published more than 500 peer-reviewed journal articles and delivered more than 300 invited talks at international conferences.