Recent Theoretical Developments on the Formation of Liesegang Patterns

When an electrolyte A diffuses into a gel containing another electrolyte B, the eventual formation of a rhythmic pattern of precipitate by the moving chemical reaction front is known as the Liesegang phenomenon. Although the Liesegang phenomenon has been studied for a century, the mechanisms responsible for these structures are still under discussion. However, recently, important theoretical progresses have been made towards a theoretical understanding of this phenomena. A critical analysis of the present state of the art as well as a discussion of some open problems is presented.     

有机化合物的化学发光

在过去十多年里,有关有机化合物化学发光的研究增长很快,在三个方面取得了新进展:(1)1,2-二氧杂丁烷体系化学发光的发现和研究,有助于人们了解单分子化学激发作用;(2)开展了电子转移化学发光的研究,用电生化学发光技术,研究电子转移的化学激发过程;(3)确认了化学引发电子变换发光(CIEEL)为一般化学发光机理。 本文将介绍化学发光有机反应的分类,说明化学发光的一般要求,叙述三个化学发光机理以及列出一些重要的化学发光体系。 在过氧化草酸芳酯体系中,我们提出了一种合成草酸芳酯的新方法,其反应如下: 2ArOH+(COOH)2+POCl3→(COOAr)2+HPO3+3HCl     

A note on the intensities of N.M.R. absorption lines

A proof is given of the proposition that the intensity of the N.M.R. absorption line from a group of n equivalent protons is proportional to n. To do this, it has been necessary to satisfy the Pauli exclusion principle by classifying states according to the irreducible representations of the symmetric group. As far as spin is concerned, this classification is equivalent to one according to the value of I, the total spin. Degeneracy with respect to I, which would otherwise occur, is prohibited by the exclusion principle, and a correlation (not one-to-one) is established between I and the symmetries of the rotational states.     

Helium and point defect accumulation: (ii) kinetic modelling

The main outstanding issues regarding modeling He diffusion and defect accumulation in α-iron are reviewed. During recent years, first principles calculations have provided a better understanding of defect stability and migration properties in pure α-iron, and accurate values of energetics of He migration and He-vacancy interactions. Such information has been used by several authors to study damage evolution under different irradiation conditions using both kinetic Monte Carlo and rate theory models. In this article a review of the main results is provided, in particular for He desorption. The influence of impurities such as carbon is discussed as well as the main challenges ahead for modeling. To cite this article: M.J. Caturla et al., C. R. Physique 9 (2008).     

Synchrotron X‐ray CT characterization of titanium parts fabricated by additive manufacturing. Part II. Defects

Synchrotron X‐ray tomography (SXRT) has been applied to the study of defects within three‐dimensional printed titanium parts. These parts were made using the Arcam EBM^® (electron beam melting) process which uses powdered titanium alloy, Ti64 (Ti alloy with approximately 6%Al and 4%V) as the feed and an electron beam for the sintering/welding. The experiment was conducted on the Imaging and Medical Beamline of the Australian Synchrotron. The samples represent a selection of complex shapes with a variety of internal morphologies. Inspection via SXRT has revealed a number of defects which may not otherwise have been seen. The location and nature of such defects combined with detailed knowledge of the process conditions can contribute to understanding the interplay between design and manufacturing strategy. This fundamental understanding may subsequently be incorporated into process modelling, prediction of properties and the development of robust methodologies for the production of defect‐free parts.     

14N pulsed nuclear quadrupole resonance. 3. Effect of a pulse train. Optimal conditions for data averaging

The calculations developed in this paper aim at determining the optimal conditions of a NQR experiment when a transition is monitored by means of a pulse train with pulses of identical duration and signal acquisition after each pulse; coherences are assumed to vanish by effective transverse relaxation prior to every new pulse. These calculations demonstrate that, as in NMR, a steady state is effectively reached for any value of the recycle time. However, by contrast with NMR, it is shown that, for optimal data averaging under steady state conditions, the recycle time T can be kept as low as possible (the only limitation is the acquisition time). Nutation curves (signal amplitude versus pulse length) calculated in the steady state case are shown to depend strongly on the ratio T/T ₁ (T ₁: longitudinal relaxation time). The signal growth as a function of T/T ₁under averaging of the first transients has been evaluated as well as the number of pulses necessary for reaching a steady state.     

Contribution of the Malpighian tubules for the maintenance of symbiotic microorganisms in cephalotes ants

Given the physiological importance of the Malpighian tubules to homeostasis in ants, this study aimed to characterize the enzymology, histology, histochemistry, and ultramorphology of the Malpighian tubules of Cephalotes atratus, C. clypeatus, and C. pusillus, as a contribution for the understanding of this organ, as well as to examine its role in the maintenance of symbiontic microorganisms in the ileum of these ants.     

A new slant on hadron structure

Rather than regarding the restriction of current lattice QCD simulations to quark masses that are 5–10 times larger than those observed as a problem, we note that this presents a wonderful opportunity to deepen our understanding of QCD. Just as it has been possible to learn a great deal about QCD by treating N _c as a variable, so the study of hadron properties as a function of quark mass is leading us to a deeper appreciation of hadron structure. As examples we cite progress in using the chiral properties of QCD to connect hadron masses, magnetic moments, charge radii and structure functions calculated at large quark masses within lattice QCD with the values observed physically.     

A reexamination of theI.S. results for the cluster compound Au55(PPh3)12Cl6

The observed lowI.S. values [1] for the four distinct gold sites in the cluster compound Au₅₅(PPh₃)₁₂Cl₆ have been explained in terms of a decrease of the 6s electron density at the nucleus, due either to a lattice expansion or to a delocalization of 6s electrons over the ligand shell surrounding the gold core. Recent EXAFS measurements [2] indicate a single average distance between the gold atoms, about 3.5% less than for bulk gold. This not only excludes the lattice expansion hypothesis, it effectively increases the density of 6s electrons, making an explanation for the observedI.S. even more difficult. For an understanding of theI.S. values it is necessary to reconsider the probable occupation of the 5d orbitals within the framework provided by the XPS surface atom core level shifts developed by Citrin and Wertheim [3]. Partial confirmation has been found in preliminary XPS results [5]. The consequences for thef-factor and specific heat results [1] will also be examined.     

Ion-acoustic solitary waves in e-p-i plasmas with (r,q)-distributed electrons and kappa-distributed positrons

In this paper, we have studied the propagation of non-linear ion-acoustic waves in a plasma comprising of (r, q) -distributed electrons and kappa-distributed positrons. We have investigated the effect of complete electron distribution profile on the propagation of small, as well as arbitrary, amplitude solitons (via pseudopotential technique) by using generalized (r, q) distribution, which exhibits a spiky and flat top nature at low energies and a super-thermal tail at high energies. Interestingly, for negative values of r , solitons are formed with both polarities, positive (compressive) and negative (rarefactive), separately within a small amplitude limit and exist simultaneously in an arbitrary amplitude limit. We also found that the propagation of solitons has been affected by the change in parameters r , q , positron concentration, and electron to positron temperature ratio. The results presented in this study add to the fundamental understanding of the complete profile of the electron distribution function, high- and low-energy parts, and in the formation of compressive and rarefactive small and finite amplitude solitons in both space and astrophysical plasmas.     

Onset of the nonlinear dielectric response of glasses in the two-level system model

We have calculated the real part of the nonlinear dielectric susceptibility of amorphous insulators in the kHz range, by using the two-level system model and a nonperturbative numerical quantum approach. At low temperature T, it is first shown that the standard two-level model should lead to a decrease of when the measuring field E is raised, since raising E increases the population of the upper level and induces Rabi oscillations cancelling the ones induced from the ground level. This predicted E-induced decrease of is at odds with experiments. However, a better, though still not perfect, agreement with low-frequency experimental nonlinear data is recovered if, in our fully quantum simulations, interactions between defects are taken into account by a new relaxation rate whose efficiency increases as , as was proposed recently by Burin et al. [Phys. Rev. Lett. 86, 5616 (2001)]. In this approach, the behavior of at low T is mainly explained by the efficiency of this new relaxation channel. Since a quantitative understanding of glasses is still missing, we finally discuss experiments whose results should yield a refined understanding of this new relaxation mechanism: i) a completely new nonlinear behavior should be found for samples whose thickness is ≃ 10 nm; ii) a decrease of nonequilibrium effects should be found when E is increased. Received 19 September 2002 / Received in final form 4 December 2002 Published online 14 March 2003     

A.C. loss and related effects in type II superconductors

This review gathers together information important to the understanding of superconducting materials under a.c. conditions, and points out some features which are at present not too well understood. The idea is to try to build a consistent picture of the performance of superconductors rather than to present a historical account of the subject.

The basic behaviour of fluxons in the bulk of type II superconductors can be explained by the critical state model, but some of the details of flux pinning and the method of flux motion are in doubt. Flux instabilities under d.c. conditions have received a lot of attention and so have the methods of stabilization, but instabilities under a.c. conditions have not been studied in so much detail, and the problem of stabilization is more difficult than for f.c.

Generally surface effects are less well understood than bulk effects, partly because there are a number of possible phenomena to contend with. A.C. loss in the Meissner state can be explained by field enhancement and flux penetration at peaks in the surface. For fields between H _c3 and H _c2 present solutions of the Ginzburg-Landau equations for the currents in the surface sheath do not give sufficient agreement with experiment and some alternative explanation, such as flux pinning, may be necessary. There are a number of effects between H _c2 and H _c1, but the most important are Meissner currents and flux pinning.     

Study of exchange and dipolar interaction in MCuX 3 systems by E.P.R.

Compounds of the ABX ₃ type (where A = K, NH₄, Cs, Rb, B = Cu, Ni, Cr, X = Cl, Br, F, I) make an interesting study by the presence of BX ₆, B ₂ X ₄, B ₃ X ₉, linear chains which also show interesting magnetic properties and crystal structures. Single crystals of KCuCl₃, NH₄CuCl₃, RbCuCl₃ and CsCuCl₃, in the polycrystalline state have been studied by E.P.R. measurements. These compounds show simple E.P.R. spectra but have interesting dependences of line widths on temperature and g values. The line width dependence has been shown to arise from unresolved hyperfine and dipolar interactions, confirmed by second moment calculations. The quasi axial nature of the g value has been explained as being due to antiferro-distortive ordering of the Cu₂ X 6/2- octahedra. The ammonium ion acts as a better exchange pathway than potassium in the two respective crystals. The E.P.R. intensities have a Curie type dependence and has no contribution from the dimeric coppers.     

Holstein-Primakoff/Bogoliubov Transformations and the Multiboson System

As an aid to understanding the displacement operator definition of squeezed states for arbitrary systems, we investigate the properties of systems where there is a Holstein-Primakoff or Bogoliubov transformation. In these cases the ladder-operator or minimum-uncertainty definitions of squeezed states are equivalent to an extent displacement-operator definition. We exemplify this in a setting where there are operators satisfying [A, A^å] = 1, but the A's are not necessarily the Fock space a's; the multiboson system. It has been previously observed that the ground state of a system often can be shown to to be a coherent state. We demonstrate why this must be so. We close with a discussion of an alternative, effective definition of displacement-operator squeezed states.     

N.M.R. spectra of oriented molecules

By analysis of the high resolution N.M.R. spectrum of 1,2,2,3-tetrachloro-propane partially oriented in a liquid-crystal solvent, it has been shown that this molecule takes up predominantly the G′G′ type of conformation.     

MAP kinase cascades regulating axon regeneration in C. elegans

Mitogen-activated protein kinase (MAPK) signaling cascades are activated by diverse stimuli such as growth factors, cytokines, neurotransmitters and various types of cellular stress. Our evolving understanding of these signal cascades has been facilitated by genetic analyses and physiological characterization in model organisms such as the nematode Caenorhabditis elegans. Genetic and biochemical studies in C. elegans have shed light on the physiological roles of MAPK cascades in the control of cell fate decision, neuronal function and immunity. Recently it was demonstrated that MAPK signaling is also important for axon regeneration in C. elegans, and the use of C. elegans as a model system has significantly advanced our understanding of the largely conserved molecular mechanisms underlying axon regeneration. This review summarizes our current understanding of the role and regulation of MAPK signaling in C. elegans axon regeneration.     

On Decomposing N=2 Line Bundles as Tensor Products of N=1 Line Bundles

We obtain the existence of a cohomological obstruction to expressing N=2 line bundles as tensor products of N=1 bundles. The motivation behind this paper is an attempt at understanding the N=2 super KP equation via Baker functions, which are special sections of line bundles on supercurves.     

The Stochastic Traveling Salesman Problem: Finite Size Scaling and the Cavity Prediction

We study the random link traveling salesman problem, where lengths l _ij between city i and city j are taken to be independent, identically distributed random variables. We discuss a theoretical approach, the cavity method, that has been proposed for finding the optimum tour length over this random ensemble, given the assumption of replica symmetry. Using finite size scaling and a renormalized model, we test the cavity predictions against the results of simulations, and find excellent agreement over a range of distributions. We thus provide numerical evidence that the replica symmetric solution to this problem is the correct one. Finally, we note a surprising result concerning the distribution of k th-nearest neighbor links in optimal tours, and invite a theoretical understanding of this phenomenon.     

Vibrational relaxation of KrF* and XeCl* by rare gases

A steady-state, chemiluminescence technique has been used to measure effective rate constants for vibrational relaxation as a function of vibrational level for KrF^* in collisions with He, Ne, and Ar and XeCl^* with Ar. The effective rate constants reported include contributions to relaxation due to intersystem crossing between theB andC states, in addition to direct relaxation within theB state. The relevance of these results to the understanding of previous measurements in KrF and XeCl lasers is discussed.     

The effects of macroscopic inhomogeneities on the magnetotransport properties of the electron gas in two dimensions

In experiments on electron transport, the macroscopic inhomogeneities in the sample play a fundamental role. In this paper and a subsequent one, we introduce and develop a general formalism that captures the principal features of sample inhomogeneities (density gradients, contact misalignments) in the magnetoresistance data taken from low-mobility heterostructures. We present detailed assessments and experimental investigations of the different regimes of physical interest, notably the regime of semiclassical transport at weak magnetic fields, the plateau–plateau transitions as well as the plateau–insulator transition that generally occurs at much stronger values of the external field only.It is shown that the semiclassical regime at weak fields plays an integral role in the general understanding of the experiments on the quantum Hall regime. The results of this paper clearly indicate that the plateau–plateau transitions, unlike the plateau–insulator transition, are fundamentally affected by the presence of sample inhomogeneities. We propose a universal scaling result for the magnetoresistance parameters. This result facilitates, amongst many other things, a detailed understanding of the difficulties associated with the experimental methodology of H.P. Wei et al. in extracting the quantum critical behavior of the electron gas from the transport measurements conducted on the plateau–plateau transitions.