Experimental adiabatic vortex ratchet effect in Nb films with asymmetric pinning trap

Nb films grown on top of an array of asymmetric pinning centers show a vortex ratchet effect. A net flow of vortices is induced when the vortex lattice is driven by fluctuating forces on an array of pinning centers without reflection symmetry. This effect occurs in the adiabatic regime and it could be mimiced only by reversible DC driven forces.     

Comments on the Paper astro-ph/0103335 by C. Rubano and P. Scudellaro

We briefly comment on a paper by Rubano and Scudellaro [Gen. Rel. Grav. 34 (2002) 307, astro-ph/0103335] where they found general exact solutions for two classes of exponential potentials in a scalar field model for quintessence. In that paper the authors were led to some interesting conclusions after a proper choice of the integration constants. By using dimensionless variables we show that the integration constants can be found explicitly without additional assumptions. In consequence we revise some results and conclusions in that paper. We also reproduce observations for Type-Ia supernovae with good accuracy.     

Switch in regioselectivity of epoxide ring-opening by changing the organometallic reagent

The regio- and stereoselective ring-opening of a 2-(2'-oxiranyl)-1,2,3,6-tetrahydropyridine using organometallic reagents is reported. The choice of the organometallic reagent determines the formation of either 2-[(R)-1-hydroxyalkyl]- or 2-[(S)-2-hydroxy-1-alkyl]-1,2,3,6-tetrahydropyridines. The formation of 2-[(S)-2-hydroxy-1-alkyl]-1,2,3,6-tetrahydropyridines is a rare example of epoxide ring-opening with retention of configuration. The process has been applied to the asymmetric synthesis of β-(+)-conhydrine and to the formal synthesis of (2S,2'R)-erythro-methylphenidate from a common precursor. Extension of the structural diversity of the process has allowed the synthesis of several β-(+)-conhydrine analogs.     

Study of Polymer-Solvent Interactions of Complex Polysiloxanes Using Dissipative Particle Dynamics

The mesoscopic modeling of three polysiloxanes in solution is reported in this work, with the purpose of predicting their physicochemical properties as functions of the quality of the solvent, so that a judicious choice of polymer/solvent can be made for various applications. The polymers studied were the following polysiloxanes: polydimethylsiloxane (PDMS), polysiloxane with a bulky alkyl side group (PMHS) and a siloxane copolymer with a hydrophilic polar side group (P2DMPAS). The model used and solved through numerical simulations is the one known as dissipative particle dynamics. Density profiles and radial distribution functions were calculated for each system. We analyzed how the polymers behave in the presence of solvents of varying quality and compared their behavior with experimental data. We observed that we could replicate the behavior in good solvents for PDMS and PMHS. We also observed in the simulation box the formation of pseudo-micelles for P2DMPAS.     

Optimización de forma de modelos paramétricos bidimensionales basada en ponderación geométrica y reglas evolutivas

This paper presents a method for shape optimization of two-dimensional models subjected to simple or multiple load cases. The optimization is performed iteratively using evolutionary rules, based on the stress level. These rules determine regions on the boundary where the material is underused or overused; the objective is that the model evolves to a minimum weight model with a high and homogeneous stress level. The evolution is performed by modifying the boundary of the model slowly. Since the boundary is defined by parametric B-spline curves, generated changes result in a smooth boundary. The main proposal of the method is that boundary modifications are given by a set of displacements that, its magnitude and direction, take into account the geometrical information of the neighborhood, unlike other methods that simply generate displacement perpendicular to the boundary. Finally, the paper presents the design of a frame bike in order to show the good performance of the method.     

Low energy shifted photoluminescence of Er3+ incorporated in amorphous hydrogenated silicon–germanium alloys

Low energy shifted photoluminescence from isolated erbium ions incorporated into a-SiGe:H thin films is reported. The Er³⁺ are thermally diffused from an a-SiGe:H:Er layer to a-SiGe:H subsequently grown, both by magnetron sputtering. The photoluminescence observed is associated with transitions produced by erbium emission centers activated by the oxidation in a 1 h annealing process in air at 250 °C. The resultant Er³⁺ concentration observed from the a-SiGe:H is affected by the hydrogen concentration already present in the layer. It is observed that at higher hydrogen concentrations in a-SiGe:H the resultant amount of diffused Er³⁺ decreases. As a consequence of the resultant smaller density of erbium ions, the probability of having isolated Er³⁺ ions increases. In this last regime, a correlation with stronger photoluminescence is observed.     

Preparation and characterisation of calcein-doped thin coatings

Silica sol-gel coatings doped with calcein (CA) were prepared and optically charecterised by means of UV-VIS spectroscopies (optical absorption and luminescence). In addition, the stability of samples against thermal treatment, UV exposure and alkali attack (chemical durability) were studied. Microstructure was verified using transmission electron microscopy and specific surface area was measured using the BET method. Results highlighted the ability of sol-gel coatings to react reversibly to pH changes when immersed in liquid media at different pH values. The limits for maximum optical absorption sensitivity were determined to be 5<pH<9, while the 8.5<pH<14.0 and 0<pH<2ranges could be detected using photoluminescence spectroscopy. Stability studies showed good behaviour against UV (maximum loss of 50% in peak intensity after 30 h of continuous exposure) and good chemical durability under alkaline conditions (80 h attack time was necessary to reduce the percentage absorption to 50%). Thermal stability of doped films was good up to 130 °C (in this range a reduction of about 25% in the intensity of the initial peak absorption is lost). Received: 26 September 2000 / Accepted: 29 January 2001 / Published online: 30 August 2001     

An Approach to the Use of Glycol Alkoxysilane–Polysaccharide Hybrids in the Conservation of Historical Building Stones

Stone consolidants have been widely used to protect historical monuments. Consolidants and hydrophobic formulations based on the use of tetraethoxysilane (TEOS) and alkylalkoxysilanes as precursors have been widely applied, despite their lack of solubility in water and requirement to be applied in organic media. In the search for a “greener” alternative based on silicon that has potential use in this field, the use of tetrakis(2-hydroxyethyl)silane (THEOS) and tris(2-hydroxyethyl)methyl silane (MeTHEOS) as precursors, due their high water solubility and stability, is proposed in this paper. It is already known that THEOS and MeTHEOS possess remarkable compatibility with different natural polysaccharides. The investigated approach uses the water-soluble silanes THEOS–chitosan and MeTHEOS–chitosan as a basis for obtaining hybrid consolidants and hydrophobic formulations for the conservation of siliceous and calcareous stones. In the case of calcareous systems, their incompatibility with alkoxysilanes is known and is expected to be solved by the developed hybrid consolidant. Their application in the conservation of building stones from historical and archeological sites from Guanajuato, México was studied. The evaluation of the consolidant and hydrophobic formulation treatment was mainly conducted by determining the mechanical properties and contact angle measurements with satisfactory results in terms of the performance and compatibility with the studied stones.