Bereiche | Tage | Auswahl | Suche | Downloads | Hilfe

DF: Fachverband Dielektrische Festkörper

DF 8: Dielectric and Ferroelectric Thin Films and Nanostructures II

DF 8.5: Vortrag

Mittwoch, 28. März 2007, 16:10–16:30, H11

Ultrafast Structure and Polarization Dynamics in Nanolayered Perovskites Studied by Femtosecond X-Ray Diffraction — •Clemens von Korff Schmising¹, Matias Bargheer¹, Mareike Kiel¹, Nicolai Zhavoronkov¹, Michael Woerner¹, Thomas Elsaesser¹, Ionela Vrejoiu², Dietrich Hesse², and Marin Alexe² — ¹Max Born Institut, Berlin — ²Max Planck Institut, Halle

We apply ultrafast x-ray diffraction with 100 femtosecond temporal and 100 femtometer spatial resolution to study optically induced polarization dynamics in a nanolayered PbZr_0.2Ti_0.8O₃/SrTiO₃ (PZT/SRO) superlattice. Displacive ferroelectricity of crystals with a perovskite structure is essentially determined by two lattice coordinates, the tetragonal distortion η (i.e. the ratio of the out- and in-of-plane lattice constant) and the ion displacement ξ within the unit cell, which directly causes the macroscopic polarization P. Optical excitation of the metallic SRO layers generates ultrafast giant stress at the 1 GPa level, which compresses the ferroelectric PZT layers by up to 2%, thus directly modulating the tetragonal distortion η. The measured time-dependent x-ray intensity changes of two Bragg reflections provide complementary information on the coupled dynamics of the tetragonal distortion η and the ion displacement ξ. The evaluation of these transient Bragg reflections yield a maximal change of η after 1.5 ps. As a result, the ferroelectric polarization P is reduced by up to 100 percent with a slight delay that is due to the pronounced anharmonic coupling of the two modes. The resulting change of ξ reaches a maximum after 2 ps.