Electron-dislocation scattering and negative deviations from matthiessen's rule

It is shown that the negative deviations from Matthiessen's rule observed recently for the electrical resistivity of Al and of Ag can be explained quantitatively by including the effect of anisotropic electron-dislocation scattering.     

Backscattered indicatrix from surfaces with random impedance

Intensity enhancement in the retroreflection direction is studied for waves scattered by a random interface which has a bound state (surface wave). We have found that an enhanced backscattered peak appears as the result of competition of two mechanisms: multiple scattering of a surface wave along the interface, and its leakage to upper half-space. The last one is the important distinction of surface scattering, which shifts the diffuse pole and eliminates in a natural way the divergence of second moments, which is the crucial difficulty in existing theories. New results, valid when there is no real absorption in the medium, are obtained for the diffuse scattered intensity as a function of the angles near the antispecular direction.     

General model of the transverse dielectric constant of GaAs-AlAs superlattices

A general model of the transverse dielectric constant of GaAs-AlAs superlattices is presented. The model is based on treating separately the individual contributions from the Λ, X, and L valleys, enabling us to understand better the general trends of the overall dielectric constant. An accurate · band calculation is used to determine the bulk band structure around the three symmetry points and a realistic Kronig-Penney model is used to calculate the influence of the superlattice periodicity on the bulk band structure. The resulting dielectric constant shows a small polarization effect due to the anisotropy of the superstructure and a large amount of fine structure corresponding to different transitions between the quantized levels. The influence of the barrier and well thicknesses, L_B and L_Z, respectively, is also important and it is shown that for a constant value of =L_B/(L_B+L_Z), the real part of the zero frequency dielectric constant increases as a function of L_B. Finally, the real parts of the dielectric constants of a superlattice and the corresponding AlGaAs alloy, characterized by the same value of , are compared and it is found that at zero frequency the superlattice dielectric constant is slightly larger with the difference increasing for higher frequencies.     

Rhodium-catalysed tetradehydro-Diels–Alder reactions of enediynes via a rhodium-stabilized cyclic allene

Efficient methods for the synthesis of fused-aromatic rings is a critical endeavour in the creation of new pharmaceuticals and materials. A direct method for preparing these systems is the tetradehydro-Diels–Alder reaction, however this is limited by the need for harsh reaction conditions. A potential, but underdeveloped, route to these systems is via transition metal-catalysed cycloaromatisation of ene-diynes. Herein, tethered unconjugated enediynes have been shown to undergo a facile room-temperature Rh^I-catalysed intramolecular tetradehydro-Diels–Alder reaction to produce highly substituted isobenzofurans, isoindolines and an indane. Furthermore, experimental and computational studies suggest a novel mechanism involving an unprecedented and complex Rh^I/Rh^III/Rh^I/Rh^III redox cycle involving the formation of an unusual strained 7-membered rhodacyclic allene intermediate and a Rh^III-stabilized 6-membered ring allene complex.

Room temperature Rh-catalysed tetradehydro-Diels–Alder reaction via an unusual Rh-stabilised allene.     

Complete intersections in spherical varieties

Let G be a complex reductive algebraic group. We study complete intersections in a spherical homogeneous space G / H defined by a generic collection of sections from G-invariant linear systems. Whenever nonempty, all such complete intersections are smooth varieties. We compute their arithmetic genus as well as some of their \(h^{p,0}\) numbers. The answers are given in terms of the moment polytopes and Newton–Okounkov polytopes associated to G-invariant linear systems. We also give a necessary and sufficient condition on a collection of linear systems so that the corresponding generic complete intersection is nonempty. This criterion applies to arbitrary quasi-projective varieties (i.e., not necessarily spherical homogeneous spaces). When the spherical homogeneous space under consideration is a complex torus \((\mathbb {C}^*)^n\), our results specialize to well-known results from the Newton polyhedra theory and toric varieties.