Structural characterisation of GaAlN/GaN HEMT heterostructures

(GaN/GaAlN/GaN)//Al₂O₃(00.1) HEMT heterostructures have been studied by X-ray scattering techniques, transmission electron microscopy and atomic force microscopy. X-ray reflectometry has been used to determine with a high accuracy both the individual layer thicknesses and the interfacial roughness, in spite of the weak electronic density contrast between layers. From the Fourier inversion method and using a simulation software, the roughness of the interface corresponding to the two-dimensional electron gas location has been determined equal to 0.5 nm. Both high resolution X-ray diffraction and transmission electron microscopy experiments have shown the excellent crystallinity of the heterostructures. Finally, the surface morphology has been inferred using atomic force microscopy experiments.     

Inverse Problems for Time-Dependent Singular Heat Conductivities: Multi-Dimensional Case

We consider an inverse boundary value problem for the heat equation ? _t u = div (γ? _x u) in (0, T) × Ω, u = f on (0, T) × ?Ω, u| _t=0 = u ₀, in a bounded domain Ω ? ? ⁿ , n ≥ 2, where the heat conductivity γ(t, x) is piecewise constant and the surface of discontinuity depends on time: γ(t, x) = k ² (x ∈ D(t)), γ(t, x) = 1 (x ∈ Ω?D(t)). Fix a direction e* ∈ 𝕊 ^n?1 arbitrarily. Assuming that ?D(t) is strictly convex for 0 ≤ t ≤ T, we show that k and sup {e*·x; x ∈ D(t)} (0 ≤ t ≤ T), in particular D(t) itself, are determined from the Dirichlet-to-Neumann map : f → ?_ν u(t, x)|_{(0, T)×?Ω}. The knowledge of the initial data u ₀ is not used in the proof. If we know min_0≤t≤T (sup _x∈D(t) x·e*), we have the same conclusion from the local Dirichlet-to-Neumann map. Numerical examples of stationary and moving circles inside the unit disk are shown. The results have applications to nondestructive testing. Consider a physical body consisting of homogeneous material with constant heat conductivity except for a moving inclusion with different conductivity. Then the location and shape of the inclusion can be monitored from temperature and heat flux measurements performed at the boundary of the body. Such a situation appears for example in blast furnaces used in ironmaking.     

Resonances for multistratified acoustic waveguides

The equation of acoustic oscillations in multistratified waveguides is considered. It is assumed that the properties of the medium do not depend on the longitudinal coordinate in a neighbourhood of infinity and may be different at different ends of the waveguides. It is proved that the truncated resolvent of the corresponding operator admits an analytical continuation through the continuous spectrum. The singularities (poles, branching points) of the truncated resolvent on the continuous spectrum are investigated. The large time asymptotic behavior of the compulsory oscillations due to periodic forces is obtained.     

Photocatalyzed E→Z Contra-thermodynamic Isomerization of Vinyl Boronates with Binaphthol

The photocatalytic contra-thermodynamic E→Z isomerization of vinyl boronates by using a binaphthol catalyst is disclosed. The reaction, based on the transient formation of a suitable chromophore with a BINOL derivative as the catalyst, allowed geometrical isomerization in good-to-excellent Z/E ratio and excellent-to-quantitative yields. The mechanism of this E→Z contra-thermodynamic isomerization was studied, and the formation of a transient chromophore species is suggested.     

Slow epimerization of stereochemically rigid diastereomers of the equatorially substituted cluster [Os3H2(mu 3-S)(CO)8((S)-PhCHMeNH2)

Turnstile rotation is suppressed in the equatorially substituted cluster [Os3(mu-H)2(mu 3-S)(CO)8((S)-PhCHMeNH2)] which was separated by HPLC into two diastereomers which do not interconvert at room temperature and epimerize only slowly at 90 degrees C.     

Structural characterization of glassy phases in the system Na2O-Ga2O3-P2O5 by MAS-NMR, EXAFS and vibrational spectroscopy. I. Cations coordination

The cationic coordinations of phosphate based gallium sodium glasses in the system Na2O-Ga2O3-P2O5 have been studied by several techniques (71Ga and 23Na MAS-NMR, EXAFS and vibrational spectroscopies) in order to study the relationship between the structure and the chemical composition. We found that three different environments are available for the gallium ions while it is very difficult to get accurate information on the sodium coordinations. Our data show that in orthophosphate glasses, gallium is mainly tetrahedral but when the mean phosphate chain length increases, its coordination becomes more and more octahedral. In these glassy structure, it becomes then possible to dissolve large amounts of typically octahedral cations like Fe3+ or Cr3+.     

Gas-phase dynamics of spiropyran and spirooxazine molecules

The gas-phase dynamics of two classes of photochromic molecules, three spiropyrans and one spirooxazine, have been investigated here using both time-resolved mass spectrometry and photoelectron spectroscopy approaches. It is, to our knowledge, the first gas-phase experiment done of these kinds of molecules. The molecules are excited at 266 nm and probed at 800 nm. The comparison of the dynamics of these four molecules has been used to propose a sequential photoisomerization mechanism involving four steps occurring in the first 100 ps. Each of these steps is discussed and related to the observed condensed-phase dynamics and to theoretical calculations.     

Dichloroenol Ethers X-ray Analysis in the Mechanistic Elucidation of Ynol Ethers Formation

Abstract  

X-ray crystallographic data are provided for dichloroenol ethers (S,E)-2-(1-(1,2-dichlorovinyloxy)ethyl)-1,3,5-triisopropylbenzene (2) and (S,E)-2-(1-(1,2-dichloroprop-1-enyloxy)ethyl)-1,3,5-triisopropylbenzene (3). The former forms colorless crystals (orthorhombic, P2₁2₁2₁ space group) and exhibits the following cell parameters: a = 10.212(5) ?; b = 10.359(8) ?; c = 18.217(6) ?. The latter also affords colorless crystals (monoclinic, P2₁ space group) with a = 13.558(2) ?; b = 10.891(1) ?; c = 15.260(2) ?; β = 115.65(1)°. The data complement those recently reported for two other dichloroenol ethers, ((1R,2S)-2-((E)-1,2-dichlorovinyloxy)cyclohexyl)benzene (5) and ((1R,2S)-2-((E)-1,2-dichloroprop-1-enyloxy)cyclohexyl)benzene (6). The X-ray analyses of these dichloroenol ethers, the only reported to date, establish unambiguously the trans stereochemistry of the chlorides in these and, by extension, similarly prepared enol ethers. This information was required for the complete mechanistic understanding of ynol ethers formation from dichloroenol ethers. Structural comparison of these dichloroenol ethers with some carbon (dichloroalkene) and nitrogen (dichloroenamine) analogues is also presented.     

The effect of attractions on the local structure of liquids and colloidal fluids

We revisit the role of attractions in liquids and apply these concepts to colloidal suspensions. Two means are used to investigate the structure; the pair correlation function and a recently developed topological method. The latter identifies structures topologically equivalent to ground state clusters formed by isolated groups of 5 ≤ m ≤ 13 particles, which are specific to the system under consideration. Our topological methodology shows that, in the case of Lennard-Jones, the addition of attractions increases the system's ability to form larger (m ≥ 8) clusters, although pair-correlation functions are almost identical. Conversely, in the case of short-ranged attractions, pair correlation functions show a significant response to adding attraction, while the liquid structure exhibits a strong decrease in clustering upon adding attractions. Finally, a compressed, weakly interacting system shows a similar pair structure and topology.