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Lattice Dynamics with a big “D”: a case for ultrafast x-ray scattering

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Date and time Feb 05, 2015
from 04:10 PM to 05:00 PM
Location McLennan Physics (MP) 102
Host Arun Paramekanti

David Reis

Stanford University

ReisThe x-ray free electron laser is shaping up to be a revolutionary new tool for advancing our understanding of the dynamics of matter at the level of electrons and atoms.  Their utility stems from extremely bright x-ray beams comprising a large number of photons concentrated in femtosecond pulses with a high-degree transverse coherence and relatively low repetition-rate.    A challenge for scientists is to devise experiments that make effective use of these unique properties, in an environment characterized by large shot-to-shot fluctuations and potential for x-ray induced damage.  This may seem like precisely the wrong source for high-resolution spectroscopy of elementary excitations in solids.  In this colloquium, I will make a case for femtosecond x-ray scattering as a powerful new tool for studying lattice dynamics, particularly out of equilibrium.  I will draw on two recent examples from my group where we perform high-resolution inelastic x-ray scattering in the time-domain using a femtosecond optical laser to prepare broadband two-phonon coherences in the sample. In the first experiment, we extract the transverse acoustic phonon dispersion in the prototypical semiconductor germanium with sub-meV resolution--- without the use of an analyzer. In the second experiment, we investigate the strongly anharmonic lattice dynamics in PbTe and present evidence for strong photo-induced mode-coupling spanning the Brillouin zone.

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