Growth and characteristics of ZnO nano-aggregates electrodeposited onto p-Si(1 1 1)

In this paper we study nanocrystalline zinc oxide thin films produced by oxidation of electrodeposited zinc nanolayers on a monocrystalline p-Si(1 1 1) substrate.The electrolyte used is ZnCl₂, an aqueous solution of 4 × 10⁻² mol/l concentration. Several deposits were made for various current densities, ranging from 13 mA/cm² to 44 mA/cm², flowing through the solution at room temperature. A parametric study enabled us to assess the effect of the current density on nucleation potential and time as well as zinc films structure. The grazing incidence X-ray diffraction (GIXD) revealed that both Zn and ZnO films are polycrystalline and nanometric. After 1-h oxidation of zinc films at 450 °C in the open air, the structural analyses showed that the obtained ZnO films remained polycrystalline with an average crystal size of about 47 nm and with (1 0 0), (0 0 2) and (1 0 1) as preferential crystallographic orientations.     

A DNR perspective on mathematics curriculum and instruction. Part II: with reference to teacher’s knowledge base

Two questions are on the mind of many mathematics educators; namely: What is the mathematics that we should teach in school? and how should we teach it? This is the second in a series of two papers addressing these fundamental questions. The first paper (Harel, 2008a) focuses on the first question and this paper on the second. Collectively, the two papers articulate a pedagogical stance oriented within a theoretical framework called DNR-based instruction in mathematics. The relation of this paper to the topic of this Special Issue is that it defines the concept of teacher’s knowledge base and illustrates with authentic teaching episodes an approach to its development with mathematics teachers. This approach is entailed from DNR’s premises, concepts, and instructional principles, which are also discussed in this paper.     

Two-dimensional spectroscopy can distinguish between decoherence and dephasing of zero-quantum coherences

Recent experiments on a variety of photosynthetic antenna systems have revealed that coherences among electronic states persist longer than previously anticipated. In an ensemble measurement, the observed dephasing of a coherent state can occur because of either disorder across the ensemble or decoherence from interactions with the bath. Distinguishing how much such disorder affects the experimentally observed dephasing rate is paramount for understanding the role that quantum coherence may play in energy transfer through these complexes. Here, we show that two-dimensional electronic spectra can distinguish between the limiting cases of homogeneous dephasing (decoherence) and inhomogeneous dephasing by examining how the quantum beat frequency changes within a cross peak. For the antenna complex LH2 isolated from Rhodobacter sphaeroides , we find that dephasing of the coherence between the B850 and B800 rings arises predominantly from inhomogeneity. In contrast, within the Fenna-Matthews-Olson (FMO) complex from Chlorobium tepidum , dephasing of the coherence between the first two excitons appears quite homogeneous. Thus, the observed dephasing rate sets an upper bound on decoherence for the LH2 complex while establishing both an upper and lower bound for the FMO complex.     

The influence of intense control laser pulses on homodyne-detected rotational wave packet dynamics in O2 by degenerate four-wave mixing

We illustrate how the preparation and probing of rotational Raman wave packets in O(2) detected by time-dependent degenerate four-wave mixing (TD-DFWM) can be manipulated by an additional time-delayed control pulse. By controlling the time delay of this field, we are able to induce varying amounts of additional Rabi cycling among multiple rotational states within the system. The additional Rabi cycling is manifested as a change in the signal detection from homodyne detected to heterodyne detected, depending on the degree of rotational alignment induced. At the highest laser intensities, Rabi cycling among multiple rotational states cannot account for the almost complete transformation to a heterodyne-detected signal, suggesting a second mechanism involving ionization. The analysis we present for these effects, involving the formation of static alignment by Rabi cycling at moderate laser intensities and possibly ion gratings at the highest intensities, appears to be consistent with the experimental findings and may offer viable explanations for the switching from homodyne to heterodyne detection observed in similar DFWM experiments at high laser field intensities (>10(13) W/cm(2)).     

Sharp and simple bounds for the Erlang delay and loss formulae

We prove some simple and sharp lower and upper bounds for the Erlang delay and loss formulae and for the number of servers that invert the Erlang delay and loss formulae. We also suggest simple and sharp approximations for the number of servers that invert the Erlang delay and loss formulae. We illustrate the importance of these bounds by using them to establish convexity proofs. We show that the probability that the M/M/s queue is empty is a decreasing and convex function of the traffic intensity. We also give a very short proof to show that the Erlang delay formula is convex in the traffic intensity when the number of servers is held constant. The complete proof of this classical result has never been published. We also give a very short proof to show that the Erlang delay formula is a convex function of the (positive integer) number of servers. One of our results is then used to get a sharp bound to the Flow Assignment Problem.     

Parametric excitation of traveling waves in a circular non-dispersive medium

This paper discusses a special type of propagating waves created by parametric excitation in a circular taught string. The string, being a non-dispersive medium propagates deformations in a similar manner to electromagnetic waves in vacuum, both have simple wavelength–frequency relationship that play an important role here. Nonlinear equations are derived under the assumption of finite deformations, whose solution produces a square-wave like, limited-amplitude, traveling wave. Closed-form expressions are obtained for the parametric excitation characteristics of the nonlinear system and the steady-state traveling waves are described by a generalized eigenvalue problem. The latter relates the nonlinear elongation of the neutral axis to the participating wavelengths forming the propagating wave. Detailed numerical simulations are provided to validate the solution and to illustrate graphically the waveforms. It is shown that propagating sinusoidal parametric excitation gives rise to various square-wave like deformation shapes which a unique phenomenon is arising in non-dispersive media.     

Comportement asymptotique des fonctions de répartition perturbées pour des processus non stationnaires et absolument réguliers

The object is to study the asymptotic normality of the statistics associated to the perturbed empirical distribution function via the slow convergence of multivariate U-statistic. We extend the results of Sun (1993) from the case of identically distributed absolutely regular random variables to the case of nonstationary absolutely regular random vectors. To cite this article: M. Harel, E. Elharfaoui, C. R. Acad. Sci. Paris, Ser. I 340 (2005).