Random cluster models for icosahedral phase alloys

We present results on computer generated random cluster models for icosahedral phase alloys. By the application of physically motivated constraints on the local atomic cluster configurations, the model achieves long range translational order comparable to, or greater than, that found in simple icosahedral alloys such asi-AlMnSi andi-AlLiCu. The parallel and perpendicular space structures are explored in some detail including a comparison with experimental powder diffraction patterns and an examination of the phason fluctuations. The latter are shown to decrease markedly with increased constraint on the local environment, but the remaining phason strain would seem finally not to vanish with increasing model size. Our model is compared with other cluster models for quasicrystalline materials and is shown to possess a density and connectivity very close to those of Elser's (best) model and those predicted by Henley for a canonical tiling. The relation of this model to recently discovered icosahedral phase alloys with resolution limited diffraction peak widths, which are essentially free of phason strain, is also discussed.     

Critical point wetting in NbH0.31

The local α-α' phase distribution in a thin Nb single crystal loaded with the critical H-concentration was investigated with X-rays. The sample used in the experiment showed a macroscopic hydrogen density mode, leading to a curvature of the whole crystal plate with the α-phase on the concave side and the α'-phase on the convex side. In addition an α-phase wetting layer was found on top of the α'-phase separated from the bulk α'-phase by a mixed α-α' phase region. In a simple model their thickness were determined to about $\text{\$}\text{?}$1.0 and 7?0μm, resp. This thin wetting layer leads to a new interpretation of the phase transition of H and Nb, involving the familiar long-range elastic interaction as well as a short-range surface free energy contribution.     

Non-dynamical stochastic resonance: Theory and experiments with white and various coloured noises

Summary We describe the simplest system which shows stochastic resonance. A linear(ized) theory for white and (almost) arbitrarily coloured noise is presented. The presented new system has new, unique properties which originate from itsnon-dynamical character; for example, the strength and phase shift of periodic response of the system is independent of the frequency. Experiments have been carried out with the following noise processes: (physical) white noise, (physical) Lorentzian noise and (physical) 1/f noise. With a small extension of the system, its linear-response regime can be significantly increased. As the system is similar to some simple models of neurones, the new results might have not only physical but also biological importance. Paper presented at the International Workshop “Fluctuations in Physics and Biology: Stochastic Resonance, Signal Processing and Related Phenomena”, Elba, 5–10 June 1994.     

Spin flip in inelastic scattering: A possible explanation of the observed backward-angle peak

Utilizing an analogy between the spin-orbit field and a magnetic field, it is shown that a plane-wave Born-approximation calculation in two dimensions gives a good qualitative account of the observed features of the probability of spin flip in inelastic scattering. Comparisons of the two-dimensional model and the collective model DWBA are given for the reaction ⁵⁸Ni(p, p′)⁵⁸Ni(2⁺) at 20 MeV and 100 MeV.     

Differentiation of electrophilic and ambiphilic carbenes

Comparative selectivities toward alkenes of CH₃CCl, CCl₂, and CH₃OCCl establish the former two carbenes as electrophiles; the latter is an ambiphile.     

Thermal rearrangement of divinylcyclopropane systems. A new formal total synthesis of (±)-quadrone

The key step of a new approach to the total synthesis of the structurally and physiologically interesting sesquiterpenoid (±)-quadrone (1) involves thermal (Cope) rearrangement of the highly functionalized divinylcyclopropane derivative 8. The product 9 of this process is converted into 19, thus completing a formal total synthesis of (±)-1.