Exiguolide, a new macrolide from the marine sponge Geodia exigua

A new 20-membered macrolide designated exiguolide has been isolated from the marine sponge Geodia exigua, and its structure determined by interpretation of spectroscopic data. Exiguolide specifically inhibited fertilization of sea urchin (Hemicentrotus pulcherrimus) gametes but not embryogenesis of the fertilized egg.     

The new hydrides CeNiGeH1.6 and CeCuGeH1.0 crystallizing in the derivative hexagonal ZrBeSi-type structure

The ternary germanides CeNiGe and CeCuGe absorb hydrogen in the temperature range 393-473 K. X-ray powder diffraction and transmission electron microscopy show that the hydride CeNiGeH_1.6(1) adopts the hexagonal ZrBeSi-type whereas CeCuGeH_1.0(1) crystallizes in a superstructure of this type. Magnetization, electrical resistivity and thermopower measurements reveal that CeNiGeH_1.6(1) is an intermediate valence compound having a Kondo temperature ≅220(10) K smaller than that observed ≅600(20) K for CeNiGe. On the contrary, a transition from ferromagnetism to non-magnetic ordering above 1.8 K is evidenced during the hydrogenation of CeCuGe.     

Reinvestigation and Structural Approach of the Ba–Pt–O System for <Y=Ba/Pt<

A reinvestigation of the Ba-rich region of the Ba–Pt–O system has been undertaken. The existence of a solid solution, already mentioned, has been confirmed, and its true formulation has been determined: Ba₁₂[Ba_xPt_3−x]Pt₆O₂₇with 0≤x≤3. The structure of these phases can be schematically considered as being built up from the periodic stacking along thecdirection of one close-packing [Ba₃O₉] layer, similar to those of the hexagonal perovskite, and three [Ba₃A′O₆] layers, withA′=Ba, Pt. The stacking of these mixed layers is of hexagonal type, and then columns parallel tocare formed. They consist of octahedra and trigonal prisms, sharing faces, which are occupied byA′ cations. For 0≤x≤1, the structure appears to be incommensurate and should be described as a composite crystal based on two subsystems: [Ba] and [(Ba, Pt)O₃]. Forx>1, the X-ray diffraction pattern can be explained using a three-dimensional cell, but the structure refinement attempts as well as the electron diffraction patterns seem to indicate these phases are commensurate modulated structures.     

A tilted rotatory frame method for the measurement of nuclear spin diffusion coefficients in solids doped with paramagnetic centers: Mn-Doped CaF<Subscript>2</Subscript>

Nuclear magnetic resonance (NMR) experiments recording the recovery of the magnetization of the nuclei in one phase, following the excitation of the nuclei in the other phase, is a classical way of studying blends inhomogeneous at the nanometer scale. Interpretation of the time recovery in terms of the spatial dimension requires knowledge of the two-phase spin diffusion coefficientsD ₀. A new method of measurement of_{D 0} is proposed on the basis of variable angle-tilted rotatory frame relaxation in homogeneous samples doped with paramagnetic centers. The choice of the tilt angle allows one to finely balance the direct relaxation by the paramagnetic center and the spin diffusion. The shape of the relaxation is analyzed with the solution for the diffusion-limited regimeM(t)/M(0)=exp[?(r ₂ t)^1/2?r ₁ t] andD ₀ then calculated fromr ₁,r ₂ and the concentration of paramagnetic centers. Conditions where reliable results can be obtained both theoretically and numerically are explored. The method has been implemented and applied to polycrystalline Mn-doped CaF₂ leading toD ₀=540±60 nm²/s, in agreement with existing values on this model compound.     

Determination of the α parameter for the decay Ξ− → Λπ−

The α parameter for Ξ^? → Λπ^? decay has been determined to be α_Ξ^?=?0.462 ± 0.015, from a sample of 1.5 × 10⁵ events recorded in the CERN SPS hyperon beam. This value is 2.5 standard deviations higher than the current world average.     

Critical behavior of light in mode-locked lasers

Light is shown to exhibit critical and tricritical behavior in passively mode-locked lasers with externally injected pulses. It is a first and unique example of critical phenomena in a one-dimensional many-body light-mode system. The phase diagrams consist of regimes with continuous wave, driven parapulses, spontaneous pulses via mode condensation, and heterogeneous pulses, separated by phase transition lines that terminate with critical or tricritical points. Enhanced non-Gaussian fluctuations and collective dynamics are present at the critical and tricritical points, showing a mode system analog of the critical opalescence phenomenon. The critical exponents are calculated and shown to comply with the mean field theory, which is rigorous in the light system.     

First observation ofΩ * resonances

In a experiment at the CERN SPS charged hyperon beam using incident Ξ^?, we have obtained evidence for the production of two Ω^* resonances decaying into Ξ^?π⁺ K ^?, with the following parameters:M ₁=2251±12,Γ ₁=48±20 MeV/c², 78 ±23 events, andM ₂=2284±12,Γ ₂=26±23 MeV/c² 45±10 events. The first state is also observed as a 4.2σ effect in a subsample which contains an additionalK ⁺ orK ⁰ in the final state. Production cross sections and branching ratios to Ξ^* K ^? and Ξ^? K ^* are presented.     

Influence of hydrostatic pressure on the diffusion of hydrogen in n-GaAs : Si

Hydrogen diffusion experiments have been performed in buried silicon doped GaAs epilayers under hydrostatic pressure. The deuterium diffusion profile in n-GaAs : Si depends on the hydrostatic pressure: a plateau followed by a steep decrease progressively appears as the pressure is increased. This has been interpreted as being due to the increasing importance of the trapping-detrapping process of H⁻ on Si⁺ donors during the hydrogen diffusion. This increase has been attributed to a deepening of the hydrogen acceptor level with respect to the bottom of the Γ conduction band of GaAs as the hydrostatic pressure increases.     

A new upper limit for the branching ratio (Ξ− → nπ−)(Ξ− → Λπ−)

From a sample of 735 000 Ξ^? decays, we have obtained a new upper limit for the branching ratio $\text{(Ξ}{}^{\mathrm{?}}\text{→}\text{n}\text{π}{}^{\mathrm{?}}\text{)}\text{(Ξ}{}^{\mathrm{?}}\text{→ Λπ}{}^{\mathrm{?}}\text{)}$. The result is 1.9 × 10^?5 at the 90% confidence level.