9:30 AM - *QN04.09.04
Ultrafast X-Ray Scattering Measurements of Electron-Phonon and Phonon-Phonon Coupling
The ultrashort pulse duration and high brightness of x-ray free-electron lasers (XFEL) enables high-resolution measurments of lattice dynamics in the time-domain. Here I will present recent femtosecond optical pump and hard x-ray probe XFEL-based experiments and describe how we can obtain information about phonon dynamics spanning the entire Brillouin zone without the use of high-resolution monochromators or crystal analyzers . Such time domain experiments are begining to yield important information on the excited-state phonon dispersion, as well as electron-phonon and phonon-phonon coupling. In particular, I will focus on two recent experiments. In the first experiment , we measure phonon dynamics along the Γ-X direction in photoexcited PbTe and demonstrate that the ferroelectric instability is driven by electron-phonon interactions. In the second experiment  we measure mode-dependent anharmonic decay channels of the coherent A1g phonon in photo-excited bisumth to pairs of high-wavevector acoustic modes. We extract coupling constants that are within an order of magnitude of first-principles-based calculation.
This work was primarily supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences through the Division of Materials Sciences and Engineering under Contract No. DE-AC02-76SF00515. Measurements were carried out at the Linac Coherent Light Source, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences.
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