1. Condensed-Matter Physics & Materials Science Seminar

    "Nanosecond dynamics of polarization domains and lattice structure in novel ferroelectric"

    Presented by Pice Chen, University of Wisconsin

    Tuesday, June 4, 2013, 4 pm
    Bldg. 480 conference room

    Hosted by: Yimei Zhu

    Novel ferroelectric materials including ferroelectric/dielectric superlattices provide additional means to modify the electronic and structural properties. These functional properties are closely linked to the response of the spontaneous polarization to external stimuli. Insight into the coupling of the polarization to other degrees of freedom can be inferred from the dynamics of ferroelectrics under electric field and optical excitations. We have studied the switching mechanism of polarization domains at a nanosecond timescale in ferroelectric/dielectric PbTiO3/SrTiO3 superlattices using time-resolved xray microdiffraction. In a superlattice with weakly-coupled component layers, the competition between the energy associated with the depolarization field and the energy of domain walls leads to the formation of striped domains. The dielectric layers are polarized with a weaker polarization than the ferroelectric layers. The striped domains and the non-equal distribution of the polarization have important consequences in the response of the superlattice to applied electric fields. We found that the switching of the striped domains occurs heterogeneously over the areas under applied electric fields, with a nanosecond timescale. Each component layer, however, responses different to applied electric fields. The dielectric SrTiO3 layers are less stable and show larger distortion of domains than the ferroelectric PbTiO3 layers at the early-time regime of switching. A larger piezoelectric expansion in the SrTiO3 layers is found at the late-time regime, commensurate with polarization change due to the elimination of striped domains. We have also studied the dynamics of structural modification in multiferroic BiFeO3 thin films under above-bandgap femtosecond optical excitation. Understanding the