Controlling the thermal stability of thin films by interfacial engineering

The quantized electronic structure in Pb films on Si(111) varies substantially as the film thickness increases. The changes in electronic energy cause the thermal stability of the films to oscillate with an approximate bilayer period. The phase of the oscillations can be controlled by interfacial engineering. Comparison of Pb films prepared on Si(111) terminated by In, Au, and Pb as interfactants reveals a phase reversal. For , films made of odd numbers of atomic layers (5, 7, and 9) are more stable than the even ones. This trend is reversed for the other two cases.     

Nonlinear evolution of the lower-hybrid drift instability in a current sheet

The lower-hybrid drift instability is simulated in an ion-scale current sheet using a fully kinetic approach with values of the ion to electron mass ratio up to m(i)/m(e)=1836. Although the instability is localized on the edge of the layer, the nonlinear development increases the electron flow velocity in the central region resulting in a strong bifurcation of the current density and significant anisotropic heating of the electrons. This dramatically enhances the collisionless tearing mode and may lead to the rapid onset of magnetic reconnection for current sheets near the critical scale.     

Synthesis of alpha-hydrazino ketones via regio- and stereoselective electrophilic amination of manganese enolates and enamines

A straightforward procedure for the regio- and stereoselective synthesis of alpha-hydrazino ketones is described. Manganese enolates and manganese enamines derived from ketones and from the corresponding N-sulfinylimines react with azodicarboxylate esters (DTBAD and DEAD) in a regioselective fashion to afford in good to excellent yields the kinetic alpha-hydrazino ketones as sole or highly prevalent products. When enantiopure N-sulfinyl manganese enamines were used the stereoselectivity of these reactions ranged from 40% to 68% ee.     

Ions in water: the microscopic structure of a concentrated HCl solution

A neutron diffraction experiment with isotopic H/D substitution on a concentrated HCl/H2O solution is presented. The full set of partial structure factors is extracted, by combining the diffraction data with a Monte Carlo simulation. This allows us to investigate both the changes of the water structure in the presence of ions and their solvation shell, overcoming the limitations of standard diffraction experiments. It is found that the interaction with the solutes affects the tetrahedral network of hydrogen bonded water molecules, in a manner similar to the application of an external pressure to pure water, although HCl seems less effective than other solutes, such as NaOH, at the same concentration. Consistent with experimental and theoretical data, the number of water molecules in the solution is not sufficient to completely dissociate the acid molecule. As a consequence, both dissociated H+ and Cl- ions and undissociated HCl molecules coexist in the sample, and this mixture is correctly reproduced in the simulation box. In particular, the hydrated H+ ions, forming a H3O+ complex, participate in three strong and short hydrogen bonds, while a well-defined hydration shell is found around the chlorine ion. These results are not consistent with the findings of early diffraction experiments on the same system and could only be obtained by combining high quality experimental data with a proper computer simulation.     

Paramagnetic defect centers at the MgO surface. An alternative model to oxygen vacancies

On the basis of embedded cluster calculations, we propose a new model for the structure of paramagnetic color centers at the MgO surface usually denoted as F(S)(H)(+) (an electron trapped near an adsorbed proton). These centers are produced by exposing the surface of polycrystalline MgO to H(2) followed by UV irradiation. We demonstrate that properties of H atom absorbed at surface sites such as step edges (MgO(step)) and reverse corner sites (MgO(RC)), formed at the intersection of two step edges, are compatible with a number of features observed for F(S)(H)(+). Our calculations suggest that (i) H(2) dissociates at the reverse corner site heterolytically and that there is no barrier for this exothermic reaction; (ii) the calculated vibrations of the resulting MgO(RC)(H(+))(H(-)) complex are fully consistent with the measured ones; (iii) desorption of a neutral H atom from the diamagnetic precursor requires UV light and leads to the formation of stable neutral paramagnetic centers at the surface, MgO(step)(H(+))(e(-))(trapped) and MgO(RC)(H(+))(e(-))(trapped). The computed isotropic hyperfine coupling constants and optical transitions of these centers are in broad agreement with the existing experimental data. We argue that these centers, which do not belong to the class of "oxygen vacancies", are two of the many possible forms of the F(S)(H)(+) defect center.     

Cholinesterase sensors based on screen-printed electrodes for detection of organophosphorus and carbamic pesticides

Cholinesterase sensors based on screen-printed electrodes modified with polyaniline, 7,7,8,8-tetracyanoquinodimethane (TCNQ), and Prussian blue have been developed and tested for detection of anticholinesterase pesticides in aqueous solution and in spiked grape juice. The influence of enzyme source and detection mode on biosensor performance was explored. It was shown that modification of the electrodes results in significant improvement of their analytical characteristics for pesticide determination. Thus, the slopes of the calibration curves obtained with modified electrodes were increased twofold and the detection limits of the pesticides were reduced by factors of 1.6 to 1.8 in comparison with the use of unmodified transducers. The biosensors developed make it possible to detect down to 2×10^–8 mol L^–1 chloropyrifos-methyl, 5×10^–8 mol L^–1 coumaphos, and 8×10^–9 mol L^–1 carbofuran in aqueous solution and grape juice. The optimal conditions for grape juice pretreatment were determined to diminish interference from the sample matrix.Abbreviations ChE Cholinesterase - TCNQ 7,7,8,8-Tetracyanoquinodimethane - ChO Choline oxidase - AChE Acetylcholinesterase - BChE Butyrylcholinesterase - BSA Bovine serum albumin - 2-PAM 2-Pyridine aldoxime methiodide     

Solvability of second-order left-invariant differential operators on the Heisenberg group satisfying a cone condition

We discuss local solvability of operators of the form where theV _j are left-invariant vector fields on the Heisenberg group such that [V _j ,V _j+n ]=U for 1≤j≤n andA=(a _jk )=A ₁+iA ₂ is a complex symmetric matrix satisfying the “cone condition” |A ₂|≤CA ₁. The authors acknowledge the support for this work by the European Commission through the European HCM-program “Fourier Analysis” and the TMR network “Harmonic Analysis”.     

Maximal functions with polynomial densities in lacunary directions

Given a real polynomial in one variable such that , we consider the maximal operator in ,

0\,,\,i,j\in \mathbb{Z}}\frac{1... ...t f\big (x_{1}-2^{i}p(t),x_{2}-2^{j}p(t)\big )\big \vert\,dt . \end{displaymath}">

We prove that is bounded on for 1$"> with bounds that only depend on the degree of .

     

A Nonlinear Problem Associated to the Motion of a Membrane

A nonlinear model associated to the motion of a membrane is considered as limit of a sequence of approximate models, for which a global existence and uniqueness theorem can be proved. The paper investigates the relationship between the solutions of the real and approximate models.     

Symbolic Computation of Newton Sum Rules for the Zeros of Polynomial Eigenfunctions of Linear Differential Operators

A symbolic algorithm based on the generalized Lucas polynomials of first kind is used in order to compute the Newton sum rules for the zeros of polynomial eigenfunctions of linear differential operators with polynomial coefficients.