Magnetic field induced orbital polarization in cubic YbInNi4: determining the quartet ground state using x-ray linear dichroism

We have been able to induce a linear dichroic signal in the Yb M(5) x-ray absorption white line of cubic YbInNi(4) by the application of a magnetic field. The nonzero integrated intensity of the magnetic field induced dichroic spectrum indicates a net noncubic 4f orbital polarization. A quantitative analysis of the temperature and field strength dependence establishes that the crystal-field ground state is a Γ(8) quartet. The results demonstrate the potential of magnetic field induced linear dichroism as a new powerful approach for the investigation of the degeneracy and orbital degrees of freedom of cubic heavy-fermion and Kondo systems.     

Spin structure of surface-supported single-molecule magnets from isomorphous replacement and X-ray magnetic circular dichroism

Surface-supported arrays of Fe(4)-type Single-Molecule Magnets retain a memory effect and are of current interest in the frame of molecule-based information storage and spintronics. To reveal the spin structure of [Fe(4)(L)(2)(dpm)(6)] (1) on Au, an isomorphous compound [Fe(3)Cr(L)(2)(dpm)(6)] was synthesized and structurally and magnetically characterized (H(3)L is tripodal ligand 11-(acetylthio)-2,2-bis(hydroxymethyl)undecan-1-ol and Hdpm is dipivaloylmethane). The new complex contains a central Cr(3+) ion and has a S = 6 ground state as opposed to S = 5 in 1. Low-temperature X-ray Magnetic Circular Dichroism studies at Fe- and Cr-L(2,3) edges revealed that the antiparallel alignment between Fe and Cr spins is preserved on surfaces. Moreover, the different Fe-L(2,3) spectral features found in the homo- and heterometallic species disclose the opposing contribution of the central Fe(3+) ion in the former compound, proving that its ferrimagnetic spin structure is retained on surfaces.     

Théorèmes de point fixe pour les applications contractantes et anticontractantes

Sans résumé     

A conserved karyotype of Sternopygus macrurus (Sternopygidae, Gymnotiformes) in the Amazon region: differences from other hydrographic basins suggest cryptic speciation

We studied the karyotypes of 35 Sternopygus macrurus fishes of four localities from rivers of the Eastern Amazon basin. In these four places the karyotypes have 2n = 46 chromosomes, NF = 92, where 30 are metacentric (M) and 16 submetacentric (SM). The constitutive heterochromatin (CH) is found in the centromeric region of most chromosomes and in the pericentromeric region of pairs 5, 17 and 19. Pair 1 has a large and not common heterochromatic block in the short arm, useful as a marker for this species if not found in other Sternopygus taxa. The NOR is located in the distal region of the short arm of pair 1, showing a size heteromorphism in some specimens. The CMA₃ and DAPI fluorochrome bandings and the fluorescent in situ hybridization (FISH), using pantelomeric human probes techniques are described for the first time for this species. DAPI has banding coincident with the C-banded regions, which suggests that the CH is AT base-pair-rich. CMA₃ banding is coincident with the NOR, meaning that this region is GC base-pair-rich. The FISH showed that the probes hybridized only with the telomeric regions, without any sign of interstitial telomeric regions. The karyotype of the samples from different places in the Amazon basin is quite conserved, probably because of the gene flow among the populations. The karyotype differences among the Sternopygus macrurus from the Amazon basin and the São Francisco and Paraná rivers suggest that these taxa may be different species.     

Influence of pre-shearing on rheometric measurements of an oil-based drilling fluid

Drilling fluids are suspensions of solid particles and present thixotropic and elastoviscoplastic behaviors simultaneously, which turn their rheological characterization into a challenging task. Rotational rheometers are widely employed to determine the properties of these fluids, and one major challenge in rheometric tests carried out with thixotropic fluids is to obtain repeatable results. Submitting the fluid specimen to a known shear history by performing a pre-shear procedure is commonly used to achieve repeatable results with thixotropic fluids. In this paper, the effect of different pre-shearing conditions on the rheological measurements of an oil-based drilling fluid was investigated using cross-hatched parallel plates. Firstly, an adequate aging time to ensure complete fluid restructuring was determined by monitoring the storage modulus, G′, over an oscillatory time sweep experiment. Shear-rate controlled start-up tests were then conducted to evaluate the gel strength and the steady-state shear stress. Nine different pre-shearing conditions were evaluated, and each experiment was performed three times. Finally, the results were analyzed by using two statistical tools: a two-way analysis of variance (ANOVA) and the Tukey’s range test. The analysis shows that despite the good repeatability achieved when the same pre-shear condition is applied, the pre-shearing affects not only the gel strength but also the steady-state shear stress. The ANOVA also revealed that the effect of the pre-shearing shear rate on the fluid rheological behavior is statistically more significant than the effect of the pre-shearing time. The same analysis was performed after 10 s and 10 min of aging, and similar results were obtained.     

Microfabricated porous glass channels for electrokinetic separation devices

Electrically insulated porous SiO2 channels for electrokinetic separation devices were fabricated based on a mask-less etching process for creation of high aspect ratio needles in silicon. The silicon needles are converted to SiO2 by oxidation and integrated within the interior of a fluidic channel network. The channels are about 5 microm high with a pore size of 0.5+/-0.2 microm. An electrophoretic separation of a mixture of fluorescein and 5-carboxyfluorescein using epi-fluorescence detection was performed to verify proper electrokinetic transport in the porous channels. The plate height was about 170,000 m-1 for a field strength of 170 V cm-1. In the near future, it is intended to extend the fabrication scheme to include an array of porous pillars for capillary electrochromatography experiments.     

Determination of cadmium,copper and lead in alumina based catalysts by direct solid sampling graphite furnace atomic absorption spectrometry

Trace impurities of Cd, Cu and Pb were determined in alumina based catalysts using direct solid sampling graphite furnace atomic absorption spectrometry (DSS-GF AAS). The analyzed catalysts are widely used in petrochemical processes. The following analytical parameters were evaluated: pyrolysis and atomization temperatures, feasibility of calibration with aqueous solutions, the necessity for palladium as chemical modifier and the sample mass introduced into the atomizer. Test samples between 0.05 and 8.5 mg were used. Palladium was investigated as chemical modifier but no improvement in analytical performance was obtained and its use was considered unnecessary for all elements. The results obtained by DSS-GF AAS were compared with those of inductively coupled plasma optical emission spectrometry (ICP OES) and also with conventional solution analysis by GF AAS (Sol-GF AAS). Characteristic masses were 1.4, 9 and 20 pg, for Cd, Cu and Pb, respectively. Using DSS-GF AAS the relative standard deviation was always less than 10% and the results agreed with those obtained by Sol-GF AAS and ICP OES. Calibration using aqueous solutions showed good linearity within the working range (R² better than 0.99). Limits of detection (3σ, n = 10) for Cd, Cu and Pb using the proposed procedure were 0.2, 22, and 1.2 ng g^− 1, respectively.     

Spin state transition in LaCoO3 studied using soft x-ray absorption spectroscopy and magnetic circular dichroism

Using soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co-L(2,3) edge, we reveal that the spin state transition in LaCoO3 can be well described by a low-spin ground state and a triply degenerate high-spin first excited state. From the temperature dependence of the spectral line shapes, we find that LaCoO3 at finite temperatures is an inhomogeneous mixed-spin state system. It is crucial that the magnetic circular dichroism signal in the paramagnetic state carries a large orbital momentum. This directly shows that the currently accepted low- or intermediate-spin picture is at variance. Parameters derived from these spectroscopies fully explain existing magnetic susceptibility, electron spin resonance, and inelastic neutron data.