The advent of XFEL sources in the hard x-ray range has opened up many new applications of time-resolved x-ray diffraction with femtosecond time resolution. The specificity and directness of x-ray diffraction methods to quantify structural dynamics on these time scales provides a uniquely efficient view on the role of coherent lattice excitations in a variety of materials.
In this seminar I will describe two recent examples of this. The first is a study of the wide-gap ferroelectric Sn2P2S6, where we use strong-field THz pulses to drive coherent phonons that modulate the ferroelectric polarization as we tune the temperature across the ferroelectric-paraelectric phase transition. The resulting data show strong evidence of an order-disorder character to the transition. I also describe some related preliminary work on the binary ferroelectric GeTe where time-resolved SHG and THz spectroscopy show evidence of nonlinear dynamics of the polar mode linked to the phase transition.
The second example is a recent experiment at the SwissFEL free-electron laser where we study coherent phonon dynamics in WTe2, a candidate type-II Weyl semimetal. Building on previous work using relativistic electron diffraction at SLAC , we measure the properties of the quasi-equilibrium structure induced by strong electronic excitation. The resulting data suggest an unusually strong coherent modulation of the transport properties of this material from the structural dynamics.
 Sie et al. Nature 656, 61 (2019)
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