In situ measurements of density fluctuations and compressibility in silica glass as a function of temperature and thermal history

In this paper, small-angle X-ray scattering measurements are used to determine the different compressibility contributions, as well as the isothermal compressibility, in thermal equilibrium in silica glasses having different thermal histories. Using two different methods of analysis, in the supercooled liquid and in the glassy state, we obtain respectively the temperature and fictive temperature dependences of the isotheraml compressibility. The values obtained in the glass and supercooled liquid states are very close to each other. They agree with previous determinations of the literature. The compressibility in the glass state slightly decreases with increasing fictive temperature. The relaxational part of the compressibility is also calculated and compared to previous determinations. We discussed the small differences between the different determinations

Characterization of the glass transition in vitreous silica by temperature scanning small-angle X-ray scattering

The temperature dependence of the x-ray scattering in the region below the first sharp diffraction peak was measured for silica glasses with low and high OH content (GE-124 and Corning 7980). Data were obtained upon scanning the temperature at 10, 40 and 80 K/min between 400 K and 1820 K. The measurements resolve, for the first time, the hysteresis between heating and cooling through the glass transition for silica glass, and the data have a better signal to noise ratio than previous light scattering and differential thermal analysis data. For the glass with the higher hydroxyl concentration the glass transition is broader and at a lower temperature. Fits of the data to the Adam-Gibbs-Fulcher equation provide updated kinetic parameters for this very strong glass. The temperature derivative of the observed X-ray scattering matches that of light scattering to within 14%.Comment: EurophysicsLetters, in pres

Minimal Surfaces, Screw Dislocations and Twist Grain Boundaries

Large twist-angle grain boundaries in layered structures are often described by Scherk's first surface whereas small twist-angle grain boundaries are usually described in terms of an array of screw dislocations. We show that there is no essential distinction between these two descriptions and that, in particular, their comparative energetics depends crucially on the core structure of their screw-dislocation topological defects.Comment: 10 pages, harvmac, 1 included postscript figure, final versio

The Cone Phase of Liquid Crystals: Triangular Lattice of Double-Tilt Cylinders

We predict the existence of a new defect-lattice phase near the nematic - smectic-C (NC) transition. This tilt- analogue of the blue phase is a lattice of double-tilt cylinders. We discuss the structure and stability of the cone phase. We suggest that many `nematics' exhibiting short range layering and tilt order may in fact be in the molten cone phase, which is a line liquid.Comment: 4 Pages, 3 Figure

Universal behavior of internal friction in glasses below T : anharmonicity vs relaxation

Comparison of the internal friction at hypersonic frequencies between a few K and the glass transition temperature Tg for various glasses brings out general features. At low temperature, internal friction is only weakly dependent on the material. At high temperature but still below Tg the internal friction for strong glasses shows a T-independent plateau in a very wide domain of temperature; in contrast, for fragile glass, a nearly linear variation of internal friction with T is observed. Anharmonicity appears dominant over thermally activated relaxational processes at high temperature.Comment: accepted in Physical Review

Chiral and herringbone symmetry breaking in water-surface monolayers

We report the observation from monolayers of eicosanoic acid in the L′2 phase of three distinct out-of-plane first-order diffraction peaks, indicating molecular tilt in a nonsymmetry direction and hence the absence of mirror symmetry. At lower pressures the molecules tilt in the direction of their nearest neighbors. In this region we find a structural transition, which we tentatively identify as the rotator-herringbone transition L2d−L2h

Harmonic Vibrational Excitations in Disordered Solids and the "Boson Peak"

We consider a system of coupled classical harmonic oscillators with spatially fluctuating nearest-neighbor force constants on a simple cubic lattice. The model is solved both by numerically diagonalizing the Hamiltonian and by applying the single-bond coherent potential approximation. The results for the density of states $g(\omega)$ are in excellent agreement with each other. As the degree of disorder is increased the system becomes unstable due to the presence of negative force constants. If the system is near the borderline of stability a low-frequency peak appears in the reduced density of states $g(\omega)/\omega^2$ as a precursor of the instability. We argue that this peak is the analogon of the "boson peak", observed in structural glasses. By means of the level distance statistics we show that the peak is not associated with localized states

ÉTUDE DE L'ORDRE LOCAL LIÉ A LA ROTATION DES MOLÉCULES DANS LA PHASE SMECTIQUE B

Il a déjà été prouvé en particulier par des mesures de diffusion inélastique de neutrons que, dans la phase smectique B, les molécules tournent autour de leur grand axe. La répartition de l'intensité des rayons X diffusés dans le plan réciproque équatorial (perpendiculaire aux molécules) montre que cette rotation se fait par sauts orientationnels de π/3, les rotations de molécules voisines étant corrélées. On a ainsi des domaines de fluctuations dont la taille croît lorsque la température s'abaisse ; mais, si dans la phase smectique BA toutes les positions occupées sont équivalentes, au contraire, dans la phase smectique BC du TBBA, lorsqu'on abaisse la température, les molécules occupent de plus en plus souvent les positions qui sont celles de la phase smectique EC de basse température.From inelastic neutron scattering experiments, it has been proved that there is a rotational motion of molecules about their long axis in the smectic B phase. From the repartition of the X ray diffuse scattering intensity in the equatorial reciprocal plane (perpendicular to the molecules), it is shown that in fact orientational jumps of π/3 take place with correlations between neighbouring molecules. So there are fluctuations domains the size of which is increasing with decreasing temperature. In the smectic BA phase all occupied positions are equivalent ; on the contrary in the smectic BC phase of TBBA when the temperature is decreasing, the molecules occupied more often the positions corresponding to the smectic EC lower temperature phase

Structure of a disk-like mesophase

We have obtained X-ray diffraction patterns from the mesomorphic phase of two derivatives of terphenylene. The observations agree with the structure already proposed for the mesomorphic phase of a similar molecule : the molecules are stacked in columns and the columns form a regular hexagonal array. In addition it is possible to give a more accurate description of the positions of the terphenyl groups in a column.Nous avons effectué des diagrammes de diffraction des rayons X de la phase mésomorphe des deux dérivés hexasubstitués du terphénylène. Nous confirmons la structure déjà proposée pour la phase mésomorphe d'une molécule analogue : les molécules forment des colonnes régulièrement disposées aux nœuds d'un réseau hexagonal. En outre, il est possible de décrire plus précisément l'arrangement des groupes terphényl à l'intérieur d'une colonne