A DSC study of precipitation hardening in a WE43 Mg alloy

The phase transformations leading to hardening a Mg-Y-Nd-Zr (WE43) alloy submitted to thermal treatments are followed by calorimetric and microhardness measurements. A double-stage thermal treatment is adopted, the first at 210°C for 8 h and the second at 150°C, on samples quenched from a temperature higher than the conventional one. A secondary precipitation of the metastable phase β″ in the second stage makes the hardness increase with respect to the primary precipitation.     

A FTIR study on low hydration saccharide amorphous matrices: Thermal behaviour of the Water Association Band

We report a study on the thermal behaviour of the infrared Water Association Band (WAB) in dry binary saccharide-water systems (containing trehalose, sucrose, maltose, and raffinose). This is a follow-up of preceding studies on analogous carboxymyoglobin-saccharide-water ternary systems, which pointed out a mutual protein-matrix influence (coupling). A comparison between binary and ternary systems, for all the saccharides studied, evidences a reduction in the residual water content in the latter and, except for trehalose, a sizable modification in the thermal behaviour, which is discussed in terms of structure and hydrogen bonding properties of the sugars. The study allowed us also to improve the definition of Spectra Distance, which is useful to roughly evaluate the system thermal evolution.     

The structure of liquid lead: EXAFS and MD studies

Accurate L-edge EXAFS (Extended X-ray Absorption Fine Structure) measurements of liquid Pb have been collected and analyzed using an advanced data-analysis technique (GNXAS) based on exact spherical wave multiple-scattering calculation of the absorption cross-section. Structural models obtained using molecular-dynamics (MD) simulations with an empirical and an embedded-atom potential, as well as structural data on liquid lead obtained from neutron and X-ray diffraction by other authors, have been put to a test using our new experimental results. The empirical two body potential, originally designed for the liquid state, reproduces quite accurately our experimental results. At the same time, the embedded atom potential, originally designed for the solid state, has turned out to inappropriately reproduce the first-neighbor g(r) peak. A comparison with previous XD and ND studies has shown that EXAFS analysis can improve our knowledge of the first-neighbor distribution. Previous observations of an apparent reduction in the coordination number in liquid Pb detected by EXAFS are briefly discussed.     

Search for nematic biaxiality in bent-core mesogens: an X-ray diffraction perspective

ABSTRACT

Since its theoretical prediction in 1970, the search for the biaxial nematic phase in thermotropic systems has challenged generations of liquid crystal scientists. Over the last 10 years, bent-core mesogens have drawn much interest as promising candidates for nematic biaxiality. However, despite a number of disputed claims, conclusive evidence of proper (spontaneous and macroscopic) biaxial order in these materials is still missing. By contrast, it is now widely recognised that biaxiality exists on a local scale, in the form of nano-sized clusters of molecules (cybotactic groups) possessing smectic-like positional order and biaxial orientational order. This article provides a review of X-ray diffraction studies on biaxiality and cybotaxis in bent-core nematics, discussing the most relevant issues related to this research field.     

Large-scale effects on meso-scale modeling for scalar transport

The transport of scalar quantities passively advected by velocity fields with a component at small scale ℓ can be modeled at scales larger than ℓ by means of an effective drift and an effective diffusivity, which can be determined by means of multiple-scale techniques. We show that the presence of a weak flow at large scales Lℓ induces interesting effects on the scalar transport at the meso-scales (i.e. at scales intermediate between ℓ and L). In particular, it gives rise to non-isotropic and non-homogeneous corrections to the meso-scale drift and diffusivity. We discuss an approximation that allows us to retain the second-order effects caused by the large-scale flow. This provides a rather accurate meso-scale modeling for both asymptotic and pre-asymptotic scalar transport properties. Numerical simulations in model flows are used to illustrate the importance of such large-scale effects.     

Absorbance spectra of polycrystalline samples and twinned crystals of oligothiophenes

We propose optical absorption technique at oblique incidence as one of the spectroscopic tools that allow experimentally recognizing the macroscopic order and structural features of molecular solids of conjugated molecules, from single crystals to polycrystalline or twinned samples. We apply this spectroscopy to quaterthiophene as representative of a wide class of materials that usually possess optical transitions of Frenkel exciton origin with strong directional dispersion. The comparison between experimental and simulated data gives evidence of the high sensitivity of this technique for determining quantitatively the polycrystallinity of the measured samples, whose domains may show mirror-like orientation of the unit cell with respect to one of its faces.Frenkel exciton; Oligothiophene, Optical properties     

The limits of the rotating wave approximation in electromagnetic field propagation in a cavity

We consider three two-level atoms inside a one-dimensional cavity, interacting with the electromagnetic field in the rotating wave approximation (RWA). One of the three atoms is initially excited, and the other two are in their ground state. We numerically calculate the propagation of the field emitted by the excited atom and scattered by the second atom, as well as the excitation probability of the second and third atom. Our numerical results clearly show the limits of the RWA in subtle problems such as relativistic causality in the atom–field interaction.     

The Asymptotic Behaviour of the Fourier Transforms of Orthogonal Polynomials I: Mellin Transform Techniques

The Fourier transform of orthogonal polynomials with respect to their own orthogonality measure defines the family of Fourier–Bessel functions. We study the asymptotic behaviour of these functions and of their products, for large real values of the argument. By employing a Mellin analysis we construct a general framework to exhibit the relation of the asymptotic decay laws to certain dimensions of the orthogonality measure, that are defined via the divergence abscissas of suitable integrals. The unifying r?le of Mellin transform techniques in deriving classical and new results is underlined. Submitted: November 5, 2004. Accepted: January 6, 2006.     

Long-living BLOCH oscillations of matter waves in periodic potentials

The dynamics of matter waves in linear and nonlinear optical lattices subject to a spatially uniform linear force is studied both analytically and numerically. It is shown that by properly designing the spatial dependence of the scattering length it is possible to induce long-living Bloch oscillations of gap-soliton matter waves in optical lattices. This occurs when the effective nonlinearity and the effective mass of the soliton have opposite signs for all values of the crystal momentum in the Brillouin zone. The results apply to all systems modeled by the periodic nonlinear Schr?dinger equation, including propagation of light in photonic and photorefractive crystals with tilted band structures.