A spin transition molecular material with a wide bistability domain

[Fe(hyptrz)3](4-chloro-3-nitrophenylsulfonate)22 H2O (1; hyptrz=4-(3-hydroxypropyl)-1,2,4-triazole) has been synthesized and its physical properties have been investigated by several physical techniques including magnetic susceptibility measurements, calorimetry, and M?ssbauer, optical, and EXAFS spectroscopy. Compound 1 exhibits a spin transition below room temperature, together with a very wide thermal hysteresis of about 50 K. This represents the widest hysteresis loop ever observed for an FeII-1,2,4-triazole spin transition material. The cooperativity is discussed on the basis of temperature-dependent EXAFS studies and of the structural features of a CuII analogue. The EXAFS structural model of (1) in both spin states is compared to that obtained for a related material whose spin transition occurs above room temperature. EXAFS spectroscopy suggests that 1,2,4-triazole chain compounds retain a linear character whatever the spin state of the iron(II).     

Solid-state synthesis of boron subnitride, B6N: myth or reality?

Solid-state synthesis of boron subnitride, B₆N, as a result of chemical interaction between boron and boron nitride at 7.5 GPa and 1700 °C has been previously reported by Hubert et al. However, a critical analysis of the results has shown that the evidence for the formation of boron subnitride with B₆O-like structure is inconclusive. We have studied in situ the interaction between boron and BN at the same p–T conditions using X-ray diffraction with synchrotron radiation. At 7.4 GPa and 1700 °C the formation of a new phase has not been observed. At the same time, HP–HT treatment has resulted in strong and unpredictable preferred orientation of boron crystallites. This leads to the rise of some weak boron reflections that might be erroneously attributed to the appearance of a new phase. To cite this article: V.L. Solozhenko et al., C. R. Chimie 9 (2006).     

Computational methods for discrete hidden semi-Markov chains

We propose a computational approach for implementing discrete hidden semi-Markov chains. A discrete hidden semi-Markov chain is composed of a non-observable or hidden process which is a finite semi-Markov chain and a discrete observable process. Hidden semi-Markov chains possess both the flexibility of hidden Markov chains for approximating complex probability distributions and the flexibility of semi-Markov chains for representing temporal structures. Efficient algorithms for computing characteristic distributions organized according to the intensity, interval and counting points of view are described. The proposed computational approach in conjunction with statistical inference algorithms previously proposed makes discrete hidden semi-Markov chains a powerful model for the analysis of samples of non-stationary discrete sequences. Copyright © 1999 John Wiley & Sons, Ltd.     

Influence of the shallow impurity environment on the luminescence of single-quantum dots

The evolution of low-temperature photoluminescence (PL) spectra of single GaAs/AlGaAs quantum dots (QD) is studied as a function of laser excitation power. At very low powers, where multi-exciton occupation of the QD can be excluded, an unexpected and pronounced spectral evolution is observed. In this weak excitation regime, a significant difference in the fine structure of single-QD spectra is observed not only among different, structurally identical QDs of the same sample, but also among spectra taken from the same single QD excited above and below the AlGaAs barrier. A time-resolved, two-color pump and probe PL experiment on a single QD indicates relaxation times between the different spectral shapes in the ms-range. A model, taking into account the influence of the shallow impurities in the environment of each QD, explains the experimental results.     

Sur la complexité des nombres algébriques

Let b?2 be an integer. We prove that real numbers whose b-ary expansion satisfies some given, simple, combinatorial condition are transcendental. This implies that the b-ary expansion of any algebraic irrational number cannot be generated by a finite automaton. To cite this article: B. Adamczewski et al., C. R. Acad. Sci. Paris, Ser. I 339 (2004).     

On a relaxation approximation of the incompressible Navier-Stokes equations

We consider a hyperbolic singular perturbation of the incompressible Navier Stokes equations in two space dimensions. The approximating system under consideration arises as a diffusive rescaled version of a standard relaxation approximation for the incompressible Euler equations. The aim of this work is to give a rigorous justification of its asymptotic limit toward the Navier Stokes equations using the modulated energy method.

     

Dynamical Janssen effect on granular packing with moving walls

Apparent mass (M(app)) measurements at the bottom of granular packings inside a vertical tube in relative motion are reported. They demonstrate that Janssen's model is valid over a broad range of velocities v. The variability of the measurements is lower than for static packings and the theoretical exponential increase of M(app) with the height of the packing is precisely followed (the corresponding characteristic screening length is of the order of the tube diameter). The limiting apparent mass at large heights is independent of v and significantly lower than the static value.