Coordination et selectivite dans les reductions des amino-2 cyclopentanones

Contrary to reduction of amino-2 cyclohexanones, those for amino-2 cyclopentanones proceed via preferential trans attack of the hydrogen atom. A cation effect is also observed, but this effect is opposite for the two sets of compounds, and depends on the reduction stereochemistry. An interpretation is suggested with two competitive paths, implying either a cyclic intermediate, or a dipolar transition state.     

Temperature dependence of the penetration depth in Sr2RuO4: evidence for nodes in the gap function

We report measurements of the magnetic penetration depth in single crystals of Sr2RuO4 down to 0.04 K using a tunnel-diode based, self-inductive technique. We observe a power law temperature dependence below 0.8 K, with no sign of a second phase transition nor of a crossover predicted for a multiband superconductor. A power law dependence suggests that the gap function has nodes, inconsistent with candidate p-wave states. We argue that nonlocal effects, rather than impurity scattering, can explain the observed T2 dependence instead of the T-linear behavior expected for line nodes.     

Derivations, isomorphisms, and second cohomology of generalized Witt algebras

Generalized Witt algebras, over a field of characteristic , were defined by Kawamoto about 12 years ago. Using different notations from Kawamoto's, we give an essentially equivalent definition of generalized Witt algebras over , where the ingredients are an abelian group , a vector space over , and a map which is linear in the first variable and additive in the second one. In this paper, the derivations of any generalized Witt algebra
, with the right kernel of being , are explicitly described; the isomorphisms between any two simple generalized Witt algebras are completely determined; and the second cohomology group for any simple generalized Witt algebra is computed. The derivations, the automorphisms and the second cohomology groups of some special generalized Witt algebras have been studied by several other authors as indicated in the references.

     

Observation of superfluorescent emission of the B-X system in I2

Transient superfluorescent emission has been observed on the I₂ (B-X) system when I₂ was pumped by a broadband pulsed dye laser. Emission was observed from 1.1–1.3 μm for pump wavelengths in the range 510–600 nm.     

Control of density fluctuations and electron transport in the reversed-field pinch

Contrary to what has been observed thus far collision-induced light scattering (CILS) can be completely polarized. This exceptional behavior characterizes the very far wing of the binary CILS spectrum by gaseous helium. This conclusion is drawn from an experimental study of the depolarization ratio of He (2) in a much extended, previously unexplored, spectral domain. Our analysis shows that this property, unique thus far, is mainly due to an almost perfect cancellation between polarization and exchange pair polarizability contributions to the depolarized spectrum, taking place at internuclear distances shorter than the atomic diameter.     

New high-energy luminescence bands from Co2+ in ZnSe

Three sharp absorption features in the energy range 2.36–2.55 eV have been detected in the transmission spectrum of Co-diffused ZnSe, and a number of luminescence transitions originating from the lowest of these states at 2.361 eV have been observed. Photoluminescence excitation spectra prove that these are high energy excited states of the Co²⁺_Zn impurity, a conclusion confirmed by comparison of measured and predicted luminescence energies. This represents the first identification of luminescence branching from a higher excited state of a transition metal ion in any semiconductor. The sharp, weakly phonon-coupled transitions involve either intra-impurity excitation or transitions from the impurity to localised states split off from a minimum in the conduction band. The implications of these observations for the mechanism of host-impurity energy transfer and for the nature of the excited state wavefunctions are discussed.     

Geometrical optimization of a surface DBD powered by a nanosecond pulsed high voltage

In this study, surface Dielectric Barrier Discharge (DBD) actuators powered by nanosecond pulsed high voltage are investigated. The goal is to experimentally characterize the surface DBD actuators in terms of electrical and geometrical parameters.The actuators are made of two conducting electrodes separated by a thin dielectric (Kapton films) and arranged asymmetrically. The active electrode is connected to a pulsed high voltage power supply (voltage up to ±10 kV, rise and fall times of 50 ns and pulse width of 250 ns) and the second electrode is grounded.The experimental results show that the energy per pulse (normalized by the length of the active electrode) is smaller when one increases the inter-electrode spacing between 1 and 3 mm, the thickness of the dielectric barrier between 120 and 360 μm or the length of the electrodes between 10 and 50 cm, for both applied voltage polarities.Optical characterization of the plasma layer for different electrode gaps has been investigated by using an ICCD camera. Results indicate that the plasma produced by positive and negative rising voltage propagates in a streamer-like regime with numerous and well-distributed channels, for any electrode gap distance. However, the positive and negative falling voltage produces similar discharges only for large electrode gaps. In this case, the plasma layer starts from a corona spot in contact with the active electrode and expands in the direction of the grounded electrode in a plume shape.     

Qualitative properties of modified equations

Suppose that a consistent one-step numerical method of orderr is applied to a smooth system of ordinary differential equations.Given any integer m 1, the method may be shown to be of orderr + m as an approximation to a certain modified equation. Ifthe method and the system have a particular qualitative propertythen it is important to determine whether the modified equationsinherit this property. In this article, a technique is introducedfor proving that the modified equations inherit qualitativeproperties from the method and the underlying system. The techniqueuses a straightforward contradiction argument applicable toarbitrary one-step methods and does not rely on the detailedstructure of associated power series expansions. Hence the conclusionsapply, but are not restricted, to the case of Runge-Kutte methods.The new approach unifies and extends results of this type thathave been derived by other means: results are presented forintegral preservation, reversibility, inheritance of fixed points.Hamiltonian problems and volume preservation. The techniquealso applies when the system has an integral that the methodpreserves not exactly, but to order greater than r. Finally,a negative result is obtained by considering a gradient systemand gradient numerical method possessing a global property thatis not shared by the associated modified equations.