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Colloidal particles trapped at the interface between two immiscible fluids usually display various bidimensional phases depending on the competition between direct and capillary interactions [1]. On the other hand, the physics of colloids confined in a nematic cell has been thoroughly investigated in the recent years. The structures formed by embedded solid spheres strongly depend on the nematic elasticity, the nematic topological defects and the anchoring at interfaces. Only few studies are devoted to colloids at nematic interfaces [2-5] where these interactions combine. Here, we examine the behavior of spherical solid particles at nematic liquid crystal (NLC) interfaces with different geometries. First, we will detail the phase behavior of beads with a strong homeotropic anchoring at an air-NLC interface [6]. Depending on their area density, the nematic thickness and the anchoring, colloids spontanously form various structures (Fig.1). Using optical tweezers, we determine the pair potential and we probe the different roles of capillary and elastic forces. Subsequently, we report the behavior of silica beads at more complex NLC interfaces. By controlling the geometry of the interfaces and the colloids density with microfluidic techniques, several structures have been observed. The competition between anchoring conditions and liquid crystal elasticity, but also the topology of the curved surface is responsible of long-range interactions and the formation of new ordered structures. In particular we examine how the behavior of colloids trapped at the interface of a curved nematic interface modifies the overall director field (Fig.2).

PZT is the most commonly used piezoelectric material. In spite of intensive experimental and theoretical studies, no model at the atomic scale has been proposed to explain the large piezoelectric effect in PZT, (d33= 520 pC/N), compared to the low piezoelectric response in the simple end-member lead titanate PbTiO3 with no-Zr (d33= 35 pC/N). X-ray absorption spectroscopy (XAS) appears as the technique of choice not only to clarify the role of Zr, but also to quantify the Zr displacement through the Ferroelectric-Paraelectric (F-P) transition. We clearly show evidence of the polar character of the Zr-atoms in PZT with a Zr-shift which will produce a small polarization. Such an atomic configuration for one type of atoms leads to relatively easy switching, i.e. relatively low electric field to align the Zr-polar atoms, which will create a favorable energetic situation for the cooperative switching of the strongly polar Ti-O dipole and would therefore account for the large piezoelectric effect in PZT1. In mixed B-cation systems of the perovskite ABO3 systems, such local configurations for one type of B cations leads to relatively easy switching giving rise to exceptional ferro/piezoelectric properties. Our recent “in-situ” XAS measurements of the giant piezoelectric effect in PbZn1/3Nb2/3O3 single crystal as a function of electric field confirms that large Zn atoms (small polarization) are easily poled and cooperatively align the strong Nb-O dipoles. 1 PRB 81, 174110 (2010)

Hyper-Raman scattering is a non-linear-optic spectroscopy where two incident photons scatter one photon after interaction with an excitation in the media [1]. One major interest of this technique is its high sensitivity to all polar vibrations, including soft polar modes [2], and to excitations that are inactive in both infrared absorption and Raman scattering. These specificities provide hyper-Raman with a very powerful tool for the investigation of local and average structure of ferroelectric-type materials such as relaxors or multiferroic systems. We will focus on hyper-Raman results obtained on three single crystals of the relaxor PbMg1/3Nb2/3O3 (PMN)[3,4]. The relative scattering intensities of the band near 250 cm−1 in various polarization geometries are fully compatible with the hyper-Raman tensor of the F2u ‘silent' mode of the parent Oh cubic structure. The temperature dependence of the three F1usymmetry polar modes was investigated between 20 K and 800 K. Some of the transverse (TO) and longitudinal (LO) components are splitted up to the highest temperatures, confirming the existence of local lattice distortions from the cubic symmetry well above the Burns temperature Td ≈ 620 K. The splitting of the LO2-mode strongly increases below Td, a behaviour which likely relates to the growth of the nano-domains. The soft TO-mode is also clearly observed, with a frequency decreasing to zero near Td. Same behaviours have been observed in another relaxor-type compound, PbMg1/3Ta2/3O3. Finally, a weak but clear transition-like anomaly in the temperature dependence of the lowest frequency LO-mode (LO1) is observed near the Curie temperature Tc = 210 K. These experimental observations will be compared to Raman and neutron scattering literature data, and confronted to the proposed scenarios for the evolution of the local structure of PMN with temperature.

In the past few years, we have tried to employ the Hyper'Raman Scattering technique in the investigations of the prototype relaxor crystal ' lead magnoniobate, PMN. This nonlinear technique is usually applicable only in the paralecric phase of ferroelectric substances, but is is often quite conveninent as the Hyper'Raman selection rules allways allow scattering by polar modes. In case of realoxors, investigations can be pursued well below the Burns temperature. Recently, the technique has been sucessfully applied to the investigation of the soft polar mode. We shall demonstrate that the lowest'frequency polar mode observed is the \"primary\" polar soft mode of PMN, responsible for the Curie'Weiss behavior of its dielectric permittivity above the Burns temperature.

Lattice vibrations of PbMg1/3Nb2/3O3 relaxor and re- lated materials have been already subject of numerous theoretical and experimental studies, but despite of these efforts, so far none of the traditional techniques was capa- ble to disclose a comprehensive and broadly acceptable picture of the dynamical processes associated with the order parameter dynamics in such systems. For com- putational methods starting from first-principles, this is a difficult task since the characteristic time and length- scales of the structural fluctuations, which are believed to be responsible for the relaxor behavior, are beyond the capabilities of the standard brut-force methods. Perhaps the most definite observation is the very existence of a soft dynamics in the system. The "royal" experi- mental lattice dynamics technique, the inelastic neutron scattering, has brought the first demonstration that the A-site cation vibration modes are showing at least partially the classical softening and hardening behavior at high and low temperatures, and these observations were later fully supported also by the "most proper" technique for studies of ferroelectric soft modes, the far-infrared spectroscopy. Nevertheless, there is a more than a hundred K broad range of the intermediate temperatures, where the situation is far from being clear. Among others, it was obscured by the continuing discussions about the so called waterfall effect and various spurious phenomena. But the analysis is mainly difficult because the soft mode response cannot be well modeled by a single damped harmonic oscillator response: the mode seems to be split and/or coupled to other excitations of vibrational and/or relaxational nature (central peaks). Moreover, for both far-infrared and inelastic neutron scattering techniques it is technically very di±culties to disclose neatly the be- havior at very low frequencies in these materials. At the same time, several rather fine spectral features could be identified in Brillouin and Raman spectra, and very detailed temperature-dependent experiments could reveal anomalies linked to the characteristic "critical" temperatures established independently from other techniques. Unfortunately, there is no consensus about the assignment of these features and precise meaning of these temperatures. In fact, even the mechanism responsible for activation of the Raman scattering is a matter of controversy. In order to broaden the perspective for the considerations about this subject, we shall offer an overview of our recent studies of PMN lattice dynamics by the hyper-Raman scattering technique, which, according to our opinion, provides rather interesting new insights in the relaxor soft mode problem. Hyper-Raman scattering (HRS) spectroscopy is based on a non-linear optical effect where two incident pho- tons produce one scattered photon after interaction with a phonon or other excitation. One of the interest of this technique is that its selection rules are different from RS and IR ones. For example, in the Pm3m simple cubic perovskite, the polar F1u modes are active both in IR and HRS, while the "silent" F2u mode is active only in HRS. Here we mostly exploit the fact that hyper-Raman technique combines advantage of infrared spectroscopy to probe the polar modes irrespectively on the structural °uctuation in the structure (in contrast with the the Raman spectroscopy), that the same modes can be inves- tigated in several nonequivalent geometries (similarly to neutron scattering), and, at the same time, this technique allows to probe the spectral profiles with a sufficient de- tail down to frequencies of a few cm-1. The key results to be discussed are the following: 1. there is a remarkable correspondence between the LO band intensities (!) in the HRS and Im[1/epsilon] spectra 2. experiments reveal enhancement of the intensity of the LO modes in the forbidden backscattering geometry, 3. the pronounced mode observed in cross-polarized spectra near 250 cm-1 is assigned as F2u mode, 4. soft mode is splitting is clearly observed and the soft mode spectra could be followed well below the Burns temperature, 5. temperature dependence of soft mode intensity re- veals freezing of the relaxor order parameter in a very similar way as the refractive index data does it in the original works of Burns and coworkers. Although all the observations are not fully understood, we are convinced that the HRS scattering has a potential to become extremely useful tool for studying polar modes in lead-based relaxor materials.

Due to their exceptional ferroelectric properties, lead-based perovskite materials (PbB'B''O3) have been the subject of numerous studies. The different solid solutions exhibit their highest electromechanical responses for compositions lying at the so-called morphotropic phase boundary (MPB) between the rhombohedral (R3m) and tetragonal (P4mm) phases. Ferroelectric properties are dependent on 3 variables: applied electric field, temperature and mechanical stress σij. The effect of hydrostatic pressure on the physical properties of ferroelectrics is actually under intense investigations. In technological applications, large stress fields can be observed in miniaturized components or in ferroelectric dots. Additionally in the literature, experiments and theoretical calculations have indicated the existence of an intrinsic short range dynamical disorder over nearly the entire lead-based perovskite materials systems. Therefore the structural and dynamical behavior of lead-based perovskite materials as a function of P,T,E has to be entirely considered in the fundamental understanding of the ferroelectric-to-ferroelectric and ferroelectric-to-paraelectric phase transitions of such important technological materials. Based on neutron diffraction, X-ray diffraction, dielectric measurements, resonance Raman spectroscopy and X-Ray absorption Spectroscopy, we will present an overview of our new results.

Similarly to preformed polymeric phases such as 1D-Pmnn [O] or 2D-Immm [T], and in contrast to monomeric FCC C60, dimeric C60 [D or R'] is expected to yield new polymerized phases of higher dimensions when subjected to high pressure. We have carried out an in-situ high-pressure powder diffraction experiment in a diamond anvil cell using MoKagr laboratory X-rays, on a pure dimeric sample previously carefully characterized at ambient pressure using synchrotron X-ray HR XRPD and Raman spectroscopy. A transition is indeed observed from 1.2 to 8.6 GPa. A robust analysis shows that the volume per C60 unit decreases from 689 Aring3 (0 GPa) to 550 Aring3 (8.6 GPa), resulting in a 20% effect. The new phase can no longer be described by a cubic (FCC) unit cell, whereas it can now be indexed and LeBail-refined using a trigonal/rhombohedral unit cell predicted by Dzyabchenko et al. in 1999.

Aqueous dispersions of exfoliated, bile-salt stabilized single-wall carbon nanotubes exhibit a first order transition to a nematic liquid-crystalline phase. The nematic phase presents itself in the form of micron-sized nematic droplets also known as tactoids, freely floating in the isotropic host dispersion. The nematic droplets are spindle shaped and have an aspect ratio of about four, irrespective of their size. We attribute this to a director field that is uniform rather than bipolar, which is confirmed by polarization microscopy. It follows that the ratio of the anchoring strength and the surface tension must be about four, which is quite larger than predicted theoretically but in line with earlier observations of bipolar tactoids. From the scatter in the data we deduce that the surface tension of the coexisting isotropic and nematic phases must be extremely low, that is, of the order of nN/m., Comment: 4 pages, 5 figures

The boson peak (BP) of silica at several densities and of lithium borate glasses has been measured using hyper-Raman scattering (HRS). In the former case, the HRS spectral responses greatly differ from Raman ones but match very well the neutron-scattering measurements. In borate glasses, the Raman and hyper-Raman BPs are rather similar. These results are consistent with the picture that librational motions of rigid structural units (i.e. tetrahedra in silica and triangles and rings in borates) contribute to the excess of low frequency modes. In disordered networks, rigid librations are expected to couple with rigid translations (the second family of “external” modes), leading to extended hybrid excitations. The temperature dependence of the BP of vitreous silica was also measured from 20 to 800 K. It can be scaled with the very high frequency limit of the sound velocity. This indicates that this limit velocity provides a measure of temperature-dependent structural changes in silica.

International audience; Lyotropic nematic aqueous suspensions of single-wall carbon nanotubes can be uniformly aligned in thin cells by shearing. Homogeneous anisotropic thin films of nanotubes can be prepared by drying the nematic. Optical transmission between parallel or crossed polarizers is measured and described in order to estimate the dichroic ratio. The order parameter is measured using polarized Raman spectroscopy and found to be quite weak due to entanglement of the nanotubes and/or to an intrinsic viscoelastic behavior of the nanotube suspensions.

International audience; In this paper, the important role of neutron spectroscopy for the study of the dynamics of C-60 molecules hosted inside single-walled carbon nanotubes (SWNT) is described. We find that the intramolecular vibrations of the fullerene are very sensitive to temperature and that an excess of low frequency modes is observed in the density of states below T=50K. These are strong indications of some coupling between vibrational modes and whole molecule mobility of the confined fullerene. However, no libration modes of the C-60 inside the nanotubes are observed, down to a temperature of 2K, indicating that the order-disorder transition, if it exists, does not show up in the same way that it does for cubic C-60. The lack of such ordering of the C-60 chains inside the nanotubes could be explained by the low dimensionality of this system added to a smooth nanotubes field potential.

International audience; There exists a special interest in investigating acoustical and optical vibrations of glasses in the one THz frequency region. The main experimental tools that are used to this effect are briefly discussed. The current understanding of the boson-peak vibrations and of their resonance with acoustic branches are explained. Specific new results on the origin of the boson peak and on the end of acoustic branches are presented for borates.

0031-9090; International audience; The current understanding of sound properties in glasses is reviewed and compared to that in crystals. Sound damping in oxide glasses is controlled mainly by three processes which in increasing order of the frequency at which they dominate are: the thermal relaxation of defects, relaxation via anharmonic interactions with the thermal bath, and hybridization with optic-like vibrations. The former two also affect distinctly the velocity of sound. The latter one relates to the boson peak and the low temperature plateau generally observed in the thermal conductivity. In addition, in silica and other tetrahedrally coordinated glasses, the sound velocity tends to increase with temperature, presumably owing to a progressive structural change.

The short- and long-range structure of a series of single and mixed aluminophosphate glasses with the general composition [xNa(2)O (46-x)Li2O], [y,Al2O3 (54 -y)P2O5] is analyzed using P-31 and Al-21 magic-angle spinning (MAS) NMR as well as small-angle X-ray scattering. These series of glasses allow analyzing both the effect of alumina incorporation in these glasses, for small alumina content (y = 0, 4, 8). and the structural changes associated with the so-called mixed alkali effect (,v = 0, 11.5, 23, 34.5, 46). Our results indicate that aluminum is mainly octalledrally coordinated in these glasses and that there is most likely some segregation of the Al(OP)(6) species. In the pure phosphate glasses, we observe a "classical" continuous variation of the structural properties with the relative alkali content, but in the aluminophosphate, both local and long-range structural results reveal for the first time some nonlinear change as a function of the relative alkali content.