Influence of disorder on superconductivity in non-magnetic rare-earth nickel borocarbides

The effect of substitutional disorder on the superconducting properties of YNi₂B₂C was studied by partially replacing yttrium and nickel by Lu and Pt, respectively. For the two series of (Y, Lu)Ni₂B₂C and Y(Ni, Pt)₂B₂C compounds, the upper critical field H _c2(T) and the specific heat c _p(T, H) in the superconducting mixed state have been investigated. Disorder is found to reduce several relevant quantities such as T _c, the upper critical field H _c2(0) at T=0 and a characteristic positive curvature of H _c2(T) observed for these compounds near T _c. The H _c2(T) data point to the clean limit for (Y, Lu) substitutions and to a transition to the quasi-dirty limit for (Ni, Pt) substitutions. The electronic specific heat contribution γ(H) exhibits significant deviations from the usual linear γ(H) law. These deviations reduce with growing substitutional disorder but remain even in the quasidirty limit which is reached in the Y(Ni_1−x , Pt _x )₂B₂C samples for x=0.1.     

Wavelength stabilization and spectrum narrowing of high-power multimode laser diodes and arrays by use of volume Bragg gratings

Spectral line narrowing (by a factor of 8) and stabilization of the emission wavelength (by a factor of 30) of multimode high-power laser diodes and arrays is demonstrated by use of volume Bragg gratings fabricated in high-stability inorganic photorefractive glasses. Applications include stabilization of pump laser diodes and arrays for solid-state lasers and metal-vapor lasers, spin hyperpolarization of noble gases used in medical imaging, and others.     

Non-resonant microwave absorption studies of superconducting MgB2 and MgB2 + MgO

Non-resonant microwave absorption (NRMA) studies of superconducting MgB₂ and a sample containing ∼10% by weight of MgO in MgB₂ are reported. The NRMA results indicate near absence of intergranular weak links in the pure MgB₂ sample. A linear temperature dependence of the lower critical field H _c1 is observed indicating a non-s wave superconductivity. However, the phase reversal of the NRMA signal which could suggest d wave symmetry is also not observed. In the MgB₂ + MgO sample, much larger low field dependent absorption is observed indicating the presence of intergranular weak links. The hysteretic behavior of NRMA is compared and contrasted in the two samples. In the pure MgB₂ sample, a large hysteresis is observed between the forward and the reverse scans of the magnetic field indicating strong pinning of flux lines. This hysteresis saturates a few degrees below T _c while in the MgB₂ + MgO sample, a much slower increase of hysteresis with decreasing temperature is observed, a signature of weaker pinning.     

Electron impact induced rearrangement processes for some 1-trimethylsilyl-2-methoxycarbonyl-3-alkyl-Δ1–3-cyclopropenes

The electron impact induced decomposition of five of the title compounds with methyl, ethyl, n-propyl, n-butyl or n-pentyl groups, has been investigated in detail. The observed fragmentation can be characterized by two outstanding features: (a) interactions of the silicon centre with the ethereal oxygen atom of the methoxy carbonyl group, (b) strong directing effects of the cyclopropene ring. As a consequence of (a), the loss of a (CH₃)₃SiOCH₃ molecule represents the main decomposition pathway of the molecular ion. It was shown to be a low energy, single dissociation step reaction, not depending on the radical site or charge localization. In spite of the rigid skeleton, ring opening does not take place during this process. As to (b), beside the β-fission, highly selective and intense δ- and ?-cleavages of the alkyl chain have been observed following the (CH₃)₃SiOCH₃ elimination. On the basis of energetic and kinetic considerations, these interesting reactions can be rationalized in terms of through-space interactions and intramolecular ortho substitution (cyclization) processes.     

Inverse stereochemical effects in the mass spectra of carboalkoxycyclopropylsilanes

The electron impact induced decomposition of the cis and trans isomeric pairs of five 1-trialkylsilyl-2-alkoxy-carbonylcyclopropanes were investigated. Compared with carbon analogues, the silicon atom inhibited ring opening and dramatically altered the fragmentation. The main primary decomposition routes were alkyl loss from silicon, and trialkylsilyl ion formation as well as trialkylalkoxysilane elimination. An unexpected inverse stereochemical effect was found for this latter reaction which was far less significant for the cis isomers, where the interacting SiQ₃ and OR groups are close to each other, than for the trans isomers. This is explained by assuming a hidden silicon-oxygen bond formation, which is possible only in the cis isomers for geometric reasons. This interaction favours alkyl elimination over other fragmentations, and has a marked effect on the further decomposition of the [M—alkyl]⁺ ions, where observed differences in abundance as great as three orders of magnitude are accounted for.