Bruno Berge

Nonlinear optical investigation of the bulk ferroelectric polarization in a vinylidene fluoride/trifluoroethylene copolymer

The ferroelectric polarization of vinylidene fluoride‐trifluoroethylene copolymers (70/30 molu2009%) is investigated using complementary measurements of the surface charge and the second harmonic intensity (at 530 nm) generated in the polymer from a Nd:YAG laser beam (at 1060 nm). Due to the nonzero electrical conductivity of the polymer (especially above room temperature), the nonlineear optical technique provides better measurement of the bulk polarization, its changes with time, the applied electric field, and temperature. First, we present optical results obtained on polarized films after removal of the electrodes: they confirm the proportionality between the second harmonic intensity and the square of the ferroelectric polarization, and the centrosymmetric character of the paraelectric phase; they also show the temperature dependence of the remanent polarization in the crystal phase. Second, we present simultaneous measurements of the surface charge and of the second harmonic intensity under very low‐frequency applied voltage through transparent electrodes. The field dependence of the measured polarization is discussed in terms of a microstructural analysis of the dielectric properties in the semicrystalline material. It is also shown that unipolar voltages applied during long periods of time are able to produce charge injection and space charge in the vicinity of the electrodes which can result in screening of the applied electric field.The ferroelectric polarization of vinylidene fluoride‐trifluoroethylene copolymers (70/30 molu2009%) is investigated using complementary measurements of the surface charge and the second harmonic intensity (at 530 nm) generated in the polymer from a Nd:YAG laser beam (at 1060 nm). Due to the nonzero electrical conductivity of the polymer (especially above room temperature), the nonlineear optical technique provides better measurement of the bulk polarization, its changes with time, the applied electric field, and temperature. First, we present optical results obtained on polarized films after removal of the electrodes: they confirm the proportionality between the second harmonic intensity and the square of the ferroelectric polarization, and the centrosymmetric character of the paraelectric phase; they also show the temperature dependence of the remanent polarization in the crystal phase. Second, we present simultaneous measurements of the surface charge and of the second harmonic intensity under very low‐fre...

Stress-Induced “3-q”-“1-q” Incommensurate Phase Transition in Quartz

The influence of a symmetry breaking field on the triangular 3-q incommensurate phase of quartz is discussed. The appearance of a 1-q stripe phase between the β phase and the 3-q phase is expected when a uniaxial stress σ is applied in the (001) plane. In the (σ, T)-plane the 1-q phase is bound by two transition lines which meet at Ti. Elastic-neutron-scattering experiments demonstrate that such a stress-induced 1-q phase is actually observed in quartz and the experimental phase diagram is in qualitative agreement with theory.

Synchrotron Radiation Diffraction from Two-Dimensional Protein Crystals at the Air/Water Interface

Protein structure determination by classical x-ray crystallography requires three-dimensional crystals that are difficult to obtain for most proteins and especially for membrane proteins. An alternative is to grow two-dimensional (2D) crystals by adsorbing proteins to ligand-lipid monolayers at the surface of water. This confined geometry requires only small amounts of material and offers numerous advantages: self-assembly and ordering over micrometer scales is easier to obtain in two dimensions; although fully hydrated, the crystals are sufficiently rigid to be investigated by various techniques, such as electron crystallography or micromechanical measurements. Here we report structural studies, using grazing incidence synchrotron x-ray diffraction, of three different 2D protein crystals at the air-water interface, namely streptavidine, annexin V, and the transcription factor HupR. Using a set-up of high angular resolution, we observe narrow Bragg reflections showing long-range crystalline order in two dimensions. In the case of streptavidin the angular range of the observed diffraction corresponds to a resolution of 10 A in plane and 14 A normal to the plane. We show that this approach is complementary to electron crystallography but without the need for transfer of the monolayer onto a grid. Moreover, as the 2D crystals are accessible from the buffer solution, the formation and structure of protein complexes can be investigated in situ.

Phonon Dispersion and Mode Coupling in High-Quartz Near the Incommensurate Phase Transition

In order to test the possible dynamical origin of the incommensurate phase of quartz, the dispersion of low-energy phonon branches in high quartz has been determined by inelastic neutron scattering. The optical sof tmode associated with the α-β transition and the transverse acoustic mode corresponding to uxy strains exhibit a large anisotropy within the hexagonal plane. The relations of these results to the phenomenological gradient coupling model and to microscopic lattice dynamical models are discussed.

Inelastic neutron scattering studies of the origin of the incommensurate phase of quartz

Abstract The origin of several incommensurate phases has been attributed to a gradient coupling between the order parameter and strain. High resolution inelastic neutron scattering measurements performed in the high temperature phase of quartz show that the origin of the incommensurate phase of quartz can be explained by this gradient coupling.

Study of the incommensurate phases of quartz by means of a coupled γ-ray double crystal neutron diffraction technique

Abstract The coupled γ-ray double crystal neutron diffraction technique is shown to be a very powerful method for high resolution studies of incommensurate phase. For quartz, it allows a test of theoretical prediction and makes possible a precise determination of parameters involved in the Landau expansion.

Raman scattering investigation of the α−β transition and of the incommensurate phase in quartz

Abstract The temperature dependence of the low frequency light scattering spectrum of quartz has been studied in the vicinity of the α−β transition and in the incommensurate phase New excitations have been observed in the range 0 – 20 cm−1.