Magnetic resonance imaging of rigid polymers at elevated temperatures with SPRITE

Magnetic resonance imaging has rarely been applied to rigid polymeric materials, due primarily to the strong dipolar coupling and short signal lifetimes inherent in these materials. SPRITE (single point ramped imaging withT ₁ enhancement) (B. J. Balcom, R. P. MacGregor, S. D. Beyea, D. P. Green, R. L. Armstrong, T. W. Bremner: J. Magn. Reson. A123, 131–134, 1996) is particularly well suited to imaging solid materials. With SPRITE, the only requirement is thatT ₂^* be long enough so that the signal can be phase-encoded. The minimum phase encoding time is limited by the maximum gradient strength available and by the instrument deadtime. At present this is usually tens of microseconds and will only improve with refinements in technology. We have used the SPRITE sequence in conjunction with raising the sample temperature to obtain images of rigid polymers that have largely frustrated conventional imaging methods. This approach provides a straightforward and reliable method for imaging a class of samples that, up until now, have been very difficult to image.     

MO analysis of macroscopic supercurrent flow in MgB 2

We present results from an extended magneto-optical (MO) analysis of two samples cut from high-density pellets of MgB₂. The first sample was studied in order to show that no matter how large the sample is and despite the bulk granularity, the material enters into a critical state in a crystal-like fashion. The second sample was chosen for the quantitative analysis. A numerical approach based on an inverted 2D Biot-Savart model was used to calculate the current paths across the homogeneous polycrystalline bulk, as well as in the vicinity and across some morphological defects. Local current densities in the homogeneous part were estimated as a function of the applied magnetic field at different temperatures, in three regimes: below full penetration, at full penetration and above full penetration, respectively. A hypothesis of interpretation of the apparent absence of magnetic granularity inside the polycrystalline microstructure is presented. It is related to a critical state likely reached by a network of strongly coupled Josephson junctions. Received 31 May 2001 and Received in final form 5 December 2001     

Diffractive heavy vector meson production from the BFKL equation

Diffractive heavy vector meson photoproduction accompanied by proton dissociation is studied for arbitrary momentum transfer. The process is described by the non-forward BFKL equation, for which a complete analytical solution is found, giving the scattering amplitude. The impact of non-leading corrections to the BFKL equation is also analysed. Results are compared to the HERA data on production. Received: 2 July 2002 / Revised version: 29 August 2002 / Published online: 18 October 2002     

Study of the Surface Processes in Vapor-Phase Epitaxial GaAs: Asymmetric Trapping of Atoms at the Step

Experimental proofs of asymmetric trapping of atoms at the growth step in vapor-phase epitaxy of gallium arsenide in the GaAs–AsCl₃–H₂ system are given. The data obtained confirm the important role of the surface diffusion mass transfer in the growth of epitaxial GaAs layers on vicinals in the neighborhood of (111)A. The effective diffusion length is estimated.     

A method of computing the gradient and the Hessian of the quality criterion in parametric optimization of continuous-discrete stochastic systems

We propose an effective method of numerical determination of the gradient vector and the Hessian matrix of a quadratic quality criterion in parametric optimization of continuous-discrete stochastic dynamic control systems based on the application of adjoint equations. Translated fromDinamicheskie Sistemy, No. 13, 1994, pp. 20–25.     

Exciton states in In x Ga1−x As/GaAs double quantum wells: Normalized reflection spectra

Il Nuovo Cimento D - Normalized reflection spectra in GaInAs/GaAs quantum wells are shown for two sets of samples with different alloy concentration (x=9% and 18.5%) and well thickness ranging from...     

Preparation and Eh--pH diagrams of Fe(II)--Fe(III) green rust compounds; hyperfine interaction characteristics and stoichiometry of hydroxy-chloride, -sulphate and -carbonate

Fe(II)--Fe(III) hydroxy-chloride, -sulphate and -carbonate were prepared by oxidation of a ferrous hydroxide precipitate in anion-containing aqueous solutions. The compounds are characterized by monitoring the redox potential E_h and the pH of stochiometric suspension vs time with the appropriate concentration ratios. X-ray diffraction allows us to characterize the crystal structure by distinguishing “green rust one” (GR1) from “green rust two” (GR2). Since green rusts (GRs) are of a pyroaurite-sjögrenite-like structure, i.e., consisting of intercalated foreign anions and water molecules in the interlayers between the brucite-like layers of Fe(OH)₂, their chemical formulae can be determined from the Mössbauer spectra. Three quadrupole doublets are observed: D₁ and D₂ correspond to a ferrous state with isomershift IS of about 1.27 mm s^-1 and quadrupole splittings QS of about 2.85 and 2.60 mm s^-1, respectively, whereas D₃ corresponds to a ferric state with IS and QS of about 0.4 mm s^-1. The hyperfine parameters of these doublets are similar from one green rust to another but their intensity ratios vary considerably. Finally, E_h and pH equilibrium diagrams of the Fe species in the presence of chloride, sulphate and carbonate anions contained within the water solution are drawn and the thermodynamic conditions of existence and degrees of oxidation of green rusts are discussed.     

Projective-type axioms for the hyperbolic plane

It is shown that the Laws of Pappus and Desargues may replace the Axiom of Projectivities in Menger's development of hyperbolic geometry from axioms of alignment.     

Magnetic resonance imaging: a new window into industrial processing.

Although the most important use of nuclear magnetic resonance imaging (MRI) continues to be for diagnostic medicine, recognition is being gained for many nonmedical applications. Examples include the following areas: petrogeology, food, agriculture, polymers and polymer-composites, and pharmaceuticals. These areas all involve studies of species that have short spin-spin relaxation times, and consequently need far fast gradient switching. These technical details are discussed and typical applications given.