Tail Behavior of a Threshold Autoregressive Stochastic Volatility Model

We consider a threshold autoregressive stochastic volatility model where the driving noises are sequences of iid regularly random variables. We prove that both the right and the left tails of the marginal distribution of the log-volatility process (α_t)_t are regularly varying with tail exponent −α with α > 0. We also determine the exact values of the coefficients in the tail behaviour of the process (α_t)_t. AMS 2000 Subject Classification. Primary—62G32, 62PO5     

Effect of boundary layer losses on 2D detonation cellular structures

We evaluate the effect of boundary layer losses on two-dimensional H2/O2/Ar cellular detonations obtained in narrow channels. The experiments provide the details of the cellular structure and the detonation speed deficits from the ideal CJ speed. We model the effect of the boundary layer losses by incorporating the flow divergence in the third dimension due to the negative boundary layer displacement thickness, modeled using Mirels’ theory. The cellular structures obtained numerically with the resulting quasi-2D formulation of the reactive Euler equations with two-step chain-branching chemistry are found in excellent agreement with experiment, both in terms of cell dynamics and velocity deficits, provided the boundary layer constant of Mirels is modified by a factor of 2. A significant increase in the cell size is found with increasing velocity deficit. This is found to be very well captured by the induction zone increase in slower detonations due to the lower temperatures in the induction zone.     

Na7Y2(P2O7)2 (P3O10) – A New Layer and Tunnel Structure

A new layered phosphate, Na₇Y₂(P₂O₇) ₂(P₃O₁₀), containing both [P₂O₇]^4? and [P₃O₁₀]^5? groups, has been prepared. It crystallizes in the monoclinic space group P2/c with a = 16.205(4), b = 5.3746(9), c = 12.309(4) Å, β = 97.96(2)°, V = 1061.7(5) ų and Z = 2. The structure was solved and refined to R₁ = 0.0298 and wR₂ = 0.0698 for 1844 independent reflections with I > 2σ(I). It consists of layers of corner and edge-sharing YO₇ polyhedra, P₂O₇ and P₃O₁₀ groups. Each layer is built up from two parallel [YP₂O₇] slabs, held together by the P₃O₁₀ groups. This arrangement gives rise to intersecting tunnels within the layer. The Na⁺ cations are located in the tunnels and between the layers. Both P₂O₇ and P₃O₁₀ groups contain unshared oxygen atoms directed toward the interlayer space and toward the tunnels. The P₂O₇ groups show a staggered configuration. In addition to this original layered framework, the title compound provides the third example of a compound containing a mixed anion of [P₂O₇]^4? and [P₃O₁₀]^5?. The structure was compared with the two previously reported ones, containing such a mixed anion: NH₄Cd₆(P₂O₇) ₂(P₃O₁₀) [6] and Cs₂Mo₅O₂(P₂O₇)₃ · (P₃O₁₀) [7].     

Monitoring deprotonation of gallic acid by Raman spectroscopy

Gallic acid (GA) is involved in many biochemical processes and reactions and is of great importance in environment thanks to its antioxidant and pro‐oxidant properties. We present in this paper a comprehensive study of GA deprotonation by means of Raman spectroscopy. In the Raman spectra, we identified the characteristic bands that were sensitive to the five pH‐dependent GA forms. From these results, we extracted the GA pKa values that are consistent with the reference values. These results permit the monitoring of GA forms for applications. Copyright © 2015 John Wiley & Sons, Ltd.     

Low temperature homoepitaxy of GaN by LP-MOVPE using Dimethylhydrazine and nitrogen

Thin films of GaN with the V/III≈10 ratio were grown by low-pressure metal organic vapour phase epitaxy (LP-MOVPE) using N₂ and Dimethylhydrazine (DMHy) as a carrier gas and nitrogen precursor, respectively. For the growth temperatures in the range from 550 to 690 C the GaN layers exhibited good surface morphology. In the temperature range from approximately 550 to 610 C, the growth rate increases with increasing temperature, characteristic of the process limited by surface kinetics with the activation energy of approximately 36 kcal/mol. For the temperatures between 620 and 690 C, the growth rate was nearly independent of temperature, which is indicative of a mass transport limited growth. The activation energy was about 4.6 kcal/mol. Micro Raman spectroscopy revealed a significant relaxation of the selection rules for the scattering by the optical phonons in the films grown at lower temperatures. Variation of the intensity ratio for and E₁ phonon modes has been attributed to the changes in the structural quality of the films grown at different temperatures.     

MOVPE growth study of BxGa(1−x)N on GaN template substrate

BGaN materials with good structural quality and surface morphology have been successfully grown on GaN template substrates by low pressure metal organic vapour phase epitaxy. TEB and NH3 were used as precursors of boron and nitrogen respectively. All the growths were performed under 100% N₂ process gas. Boron concentration was estimated by HRXD measurements combined with SIMS analysis. Single-crystal layers of BGaN with B content as high as 3.6% have been obtained.     

Solid state and solution study of some phosphoramidate derivatives containing the P(O)NHC(O) bifunctional group: Crystal structures of CCl2HC(O)NHP(O)(NCH3(CH2C6H5))2, p-ClC6H4C(O)NHP(O)(NCH3(CH2C6H5))2, CCl2HC(O)NHP(O)(N(CH2C6H5)2)2 and p-BrC6H4C(O)NHP(O)(N(CH2C6H5)2)2

Synthetic methods for several novel phosphoramidate compounds containing the P(O)NHC(O) bifunctional group were developed. These compounds with the general formula R₁C(O)NHP(O)(N(R₂)(CH₂C₆H₅))₂, where R₁ = CCl₂H, p-ClC₆H₄, p-BrC₆H₄, o-FC₆H₄ and R₂ = hydrogen, methyl, benzyl, were characterized by several spectroscopic methods and analytical techniques. The effects of phosphorus substituents on the rotation rate around the P–N_amine bond were also investigated. ¹H NMR study of the synthesized compounds demonstrated that the presence of bulky groups attached to the phosphorus center and electron withdrawing groups in the amide moiety lead to large chemical-shift non-equivalence (Δδ_H) of diastereotopic methylene protons. The crystal structures of CCl₂HC(O)NHP(O)(NCH₃(CH₂C₆H₅))₂, p-ClC₆H₄C(O)NHP(O)(NCH₃(CH₂C₆H₅))₂, CCl₂HC(O)NHP(O)(N(CH₂C₆H₅)₂)₂ and p-BrC₆H₄C(O)NHP(O)(N(CH₂C₆H₅)₂)₂ were determined by X-ray crystallography using single crystals. The coordination around the phosphorus center in these compounds is best described as distorted tetrahedral and the P(O) and C(O) groups are anti with respect to each other. In the compound Br-C₆H₄C(O)NHP(O)(N(CH₂C₆H₅)₂)₂ (with two independent molecules in the unit cell), two conformers are connected to each other via two different N–H?O hydrogen bonds forming a non-centrosymmetric dimer. In the crystalline lattice of other compounds, the molecules form centrosymmetric dimers via pairs of same N–H?O hydrogen bonds. The structure of CCl₂HC(O)NHP(O)(N(CH₂C₆H₅)₂)₂ reveals an unusual intramolecular interaction between the oxygen of CO group and amine nitrogen.