Switching kinetics of the ferroelectric transition in K2SeO4 studied by stroboscopic γ-ray diffraction.

The kinetics of the ferroelectric lock-in transition in potassium selenate (K2SeO4) was studied on a millisecond timescale using high-resolution γ-ray diffraction. A large change of the line width and wavevector of the first order satellite is observed during the switching process. This is attributed to a loss of long-range order under the influence of the electric field. In addition, the incommensurate phase is stabilized by the pulsed field and the transition to the pure commensurate phase is shifted to lower temperatures. Strains that may build up during the rapid switching process are supposed to be the reason for this behaviour.

Low frequency excitations in multiferroic MnWO4.

Dynamic anomalies have been found in the magnetically ordered phases of multiferroic MnWO(4) using polarized Raman scattering. Strong phonon damping is observed for several B(g) modes within the ferroelectric phase and has been attributed to spin-phonon interactions. Moreover, a new low frequency excitation was detected near 33 cm(-1) that grows in intensity on cooling into the antiferromagnetic phases. It is argued that this signal is most probably due to a two-magnon process.

Phonon-lifetimes in demixing systems.

The dynamics of silver-alkali halide mixed single crystals (Ag(x)Na(1-x)Br, x = 0.23, 0.35, 0.40 and 0.70) were studied by inelastic neutron scattering during the process of spinodal decomposition. Using the thermal three-axes spectrometer PUMA as well as the time-of-flight spectrometer IN5, the time evolution of phonons was observed in time-resolved, stroboscopic measurements. Complementary to the study of long wavelength acoustic phonons, as studied previously, we extended these investigations to Brillouin-zone boundary modes that are particularly sensitive to variations of the local structure. Starting from the homogeneous mixed phase the behaviour of these modes during demixing is observed in real-time. A simple dynamical model based on local structure variants helps to interpret the results. It is shown that the phonon lifetimes vary strongly during the phase separation and increase drastically during the coarsening process. Up to a critical size of precipitates of about 10 nm, zone-boundary modes are found to be strongly damped, while beyond the line widths are reduced to the experimental resolution. This finding leads to the conclusion that the typical mean free path of these modes is of the order of 10 nm, which corresponds to 20 unit cells.

Switching behaviour of modulated ferroelectrics: the kinetics of the field induced lock-in transition in K2SeO4.

The field induced switching process across the ferroelectric lock-in transition in K(2)SeO(4) has been studied on a millisecond timescale using stroboscopic neutron diffraction. The time evolution of both the first and the third order satellites was examined. The time constants are found to vary with temperature between 0.2 and 1.2 ms. This is an order of magnitude faster than in the isostructural Rb(2)ZnCl(4). Moreover, the time dependence of the satellite's linewidth provides information about the evolution of the coherence of the modulated structure.

Quantitative determination of domain distribution in SrTiO3-competing effects of applied electric field and mechanical stress.

Below its ordering temperature at 105 K, perovskite-type SrTiO(3) exhibits a tetragonal phase with three different structural domains that are strongly influenced by the application of uniaxial mechanical stresses and electric fields. A careful neutron diffraction study of superlattice reflections provides full quantitative information about the varying domain distributions under external loads as a function of temperature. It is shown that electric field and uniaxial stress exhibit competitive effects and the simultaneous application leads to a complex redistribution behaviour of the tetragonal domains. The results are discussed in the context of the formation of a field induced ferroelectric phase at low temperatures. The experimental findings demonstrate that its polarization is always perpendicular to the tetragonal axis and the polar phase has orthorhombic symmetry.

Switching behaviour of modulated ferroelectrics I: kinetics of the field-induced lock-in transition of Rb(2)ZnCl(4).

The structural changes of ferroelectric Rb(2)ZnCl(4) have been studied in real time under the influence of pulsed electric fields by high-resolution gamma-ray diffraction close to the lock-in transition at 198.3 K. A well-defined field-induced switching between the incommensurate and the commensurate phase is observed only in virgin crystals on a time-scale of milliseconds. Crystals aged under high electric fields, however, exhibit an intermediate diffuse phase that is attributed to pinning effects of discommensurations, i.e. the walls of nano-domains in the incommensurate phase. Surprisingly, the incommensurate phase itself is stabilized and the transition into the ferroelectric phase is shifted to lower temperatures as soon as the frequency of the cycled electric field is increased. This unusual finding is discussed in terms of stress fields associated with the motion of discommensurations.

Pressure induced phase transitions of TTAB-micellar solutions studied by SANS and Raman spectroscopy.

Using small angle neutron scattering in combination with optical Raman spectroscopy, phase transitions in the micellar system tetradecyl-trimethylammonium bromide (TTAB) have been investigated in a wide range of temperatures, pressures and surfactant concentrations. Allowing for the sluggish kinetics of crystallisation, the phase diagram in the p-T-c space could be determined that is dominated by a pronounced two-phase region of coexisting liquid and solid phases at elevated pressures.