Ultrafast x-ray diffraction of a ferroelectric soft mode driven by   broadband terahertz pulses
  Intense, few-cycle pulses in the terahertz frequency range have strong potential for schemes of control over vibrational modes in solid-state materials in the electronic ground-state. Here we report an experiment using single cycle terahertz pulses to directly excite lattice vibrations in the ferroelectric material $\mathrm{Sn_2P_2S_6}$ and ultrafast x-ray diffraction to quantify the resulting structural dynamics. A model of a damped harmonic oscillator driven by the transient electric field of the terahertz pulses describes well the movement of the Sn$^{2+}$ ion along the ferroelectric soft mode. Finally, we describe an anharmonic extension of this model which predicts coherent switching of domains at peak THz-frequency fields of 790 kV/cm. 
S. Gr\"ubel, J. A. Johnson, P. Beaud, C. Dornes, A. Ferrer, V.   Haborets, L. Huber, T. Huber, A. Kohutych, T. Kubacka, M. K...
cond-mat.mtrl-sci