European XFEL Science Seminar

Ultrafast modulation of order with coherent phonons: novel pathways to control of solids

by Prof. Steven Johnson (Paul Scherrer Institute)

Zoom meeting

Zoom meeting


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 [1], 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.

[1] Sie et al. Nature 656, 61 (2019)


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Meeting ID: 951 6094 5350

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