Confinement-induced orientational order in a ferroelectric liquid crystal containing dispersed aerosils

The study of the smectic-A to chiral smectic-C(*) phase transition of the liquid crystal S-(+)-[4-(2(')-methyl butyl) phenyl 4(')-n-octylbiphenyl-4-carboxylate] (CE8) containing dispersed hydrophilic aerosils reveals novel properties, important to understanding quenched disorder and confinement in ferroelectric liquid crystals. Smectic layer compression leads to a distribution of transition temperatures inducing smearing of the macroscopic data across the transition. A pronounced confinement-induced pretransitional tilted order is observed.     

Precession and motional slowing of spin evolution in a high mobility two-dimensional electron gas

Optical spin-dynamic measurements in a high-mobility n-doped GaAs/AlGaAs quantum well show oscillatory evolution at 1.8 K consistent with a quasi-collision-free D'yakonov-Perel'-Kachorovskii regime. Above 5 K evolution becomes exponential as expected for collision-dominated spin dynamics. Momentum scattering times extracted from Hall mobility and Monte Carlo simulation of spin polarization agree at 1.8 K but diverge at higher temperatures, indicating the importance of electron-electron scattering and an intrinsic upper limit for the spin-relaxation rate.     

Crossover from glassy to inhomogeneous-ferroelectric nonlinear dielectric response in relaxor ferroelectrics

The temperature dependence of the dielectric nonlinearities in a PMN single crystal and in 9/65/35 PLZT ceramics has been determined by measuring the first and third harmonic response as well as the dielectric behavior as a function of the dc electric field. In zero field a paraelectric-to-glass, and, in a high enough dc field, a glass-to-ferroelectriclike crossover in the temperature dependence of the nonlinear response have been observed. Both crossovers agree with the predictions of the spherical random-bond-random-field model. Relaxors thus undergo in zero field a transition to a spherical glass, while above the critical field a transition into a ferroelectric state occurs.     

In situ monitoring of hydrogen loss during pyrolysis of wood by neutron imaging

Hydrogen is an element of fundamental importance for energy but hard to quantify in bulk materials. Neutron radiography was used to map in situ loss of elemental hydrogen from beech tree wood samples during pyrolysis. The samples consisted of three wood cylinders (finished dowel or cut branch) of approximately 1 cm in length. The samples were pyrolyzed under vacuum in a furnace vessel that was placed inside a cold neutron imaging beamline using a temperature ramp of 5 °C/min from ambient up to 400 °C. Neutron radiographs with exposures of 30 s were sequentially recorded with a charge-coupled device over the course of the experiment. Relative absorbance/scattering of the neutron beam by each sample was based on intensity (or brightness) values as a function of pixel position. The much larger neutron cross section for hydrogen compared to carbon and oxygen enables almost direct conversion of neutron attenuation into sample hydrogen content for each time step during the pyrolysis experiment. Target and vessel temperatures were recorded concurrently with collection of the radiographs so that changes could be directly correlated to different states of pyrolysis. The most visible change appeared at the initial phase of the 400 °C plateau as evidenced by strong hydrogen loss and primarily diametric shrinking of the samples. The loss of elemental hydrogen between initial and final states of pyrolysis was estimated to be about 70%.     

Type-II Clathrate Na24-δGe136 from a Redox-Preparation Route

The metastable type-II clathrate Na_24-δGe₁₃₆ was obtained from Na₁₂Ge₁₇ by applying a two-step procedure. At first, Na₁₂Ge₁₇ was reacted at 70 °C with a solution of benzophenone in the ionic liquid (IL) 1,3-dibutyl-2-methylimidazolium-bis(trifluoromethylsulfonyl) azanide. The IL was inert towards Na₁₂Ge₁₇, but capable of dissolving the sodium salts formed in the redox reaction. By annealing at 340 °C under an argon atmosphere, the X-ray amorphous intermediate product was transformed to crystalline Na_24-δGe₁₃₆ (δ≈2) and α-Ge in an about 1 : 1 mass ratio. The product was characterized by X-ray powder diffraction, chemical analysis, and ²³Na solid-state NMR spectroscopy. Metallic properties of Na_24-δGe₁₃₆ were revealed by a significant Knight shift of the ²³Na NMR signals and by a Pauli-paramagnetic contribution to the magnetic susceptibility. At room temperature, Na_24-δGe₁₃₆ slowly ages, with a tendency to volume decrease and sodium loss.     

Structural investigation of ternary La/alkaline earth phosphate (La(1−x)MxP3Oy) (M = Ba,Ca, Sr) glasses

The structure of La-based metaphosphate glasses modified with aliovalent alkaline earth elements (M = Ba, Ca, Sr), with P:(M + La) ratio kept approximately at 3, were analyzed using Raman and IR spectroscopy. Increasing the concentration of the alkaline earths monotonically decreased the glass density and glass transition temperature, and shifted some vibrational modes by as much as 40 wavenumbers. The Raman frequencies of the symmetric POP and the PO vibrations depended on the M/La ratio, but not on the type of alkaline earth addition; however, the type of alkaline earth cation significantly changed the symmetric PO₂ vibration. The average cation-oxygen coordination number remained near ～7 except for a slight decrease for high concentrations of Ca and Sr substitutions. The POP/PO₂ intensity ratio increased with increasing alkaline earth content. The replacement of divalent elements for trivalent La in a metaphosphate glass structure was partially accommodated by incorporation of protons as charge compensating defects. The protons were found to be in close proximity to the divalent cations.     

A direct nonlinear theory of elastic membranes

Summary The authors develop a nonlinear theory of elastic membranes which accounts for thickness effects. By assuming the existence of a strain energy function and postulating a principle of virtual work which governs the mechanical behavior of an elastic directed surface, the field equations and the general constitutive relations are obtained for both the compressible and incompressible case. These equations are identical with those arrived at by a suitable limiting process applied to the three dimensional equations of nonlinear elasticity.

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Sommario Gli autori sviluppano una teoria non lineare delle membrane elastiche che tiene conto degli effetti dello spessore. Assumendo l'esistenza di una funzione energia di deformazione e postulando un principio di lavoro virtuale che governa il comportamento meccanico di una superficie elastica orientata si ottengono le equazioni di campo e le relazioni costitutive generali sia per il caso compressibile che per l'incompressibile. Queste equazioni sono identiche a quelle a cui si arriva con un opportuno processo di limite applicato alle equazioni tridimensionali della elasticità non lineare.