Electron surface layer at the interface of a plasma and a dielectric wall

We study the potential and the charge distribution across the interface of a plasma and a dielectric wall. For this purpose, the charge bound to the wall is modelled as a quasi-stationary electron surface layer which satisfies Poisson's equation and minimizes the grand canonical potential of the wall-thermalized excess electrons constituting the wall charge. Based on an effective model for a graded interface taking into account the image potential and the offset of the conduction band to the potential just outside the dielectric, we specifically calculate the potential and the electron distribution for magnesium oxide, silicon dioxide and sapphire surfaces in contact with a helium discharge. Depending on the electron affinity of the surface, we find two vastly different behaviors. For negative electron affinity, electrons do not penetrate into the wall and an external surface charge is formed in the image potential, while for positive electron affinity, electrons penetrate into the wall and a space charge layer develops in the interior of the dielectric. We also investigate how the electron surface layer merges with the bulk of the dielectric.Comment: 15 pages, 9 figures, accepted versio

Influence of the First Preparation Steps on the Properties of GaN Layers Grown on 6H-SIC by Mbe

AbstractThe Gan heteroepitaxy on 6H-SiC is affected by the bad morphology of the substrate surface. We performed a hydrogen etching at 1550°C on the 6H-SiC(0001) substrates to obtain atomically flat terraces. An improvement of the structural properties of GaN grown by MBE on such substrates after deposition of a LT-AIN buffer layer is observed. A value of less than 220 arcsec of the FWHM of the XRD rocking curve, showing a reduced screw dislocations density, is comparable with the best results reported until now for thick GaN samples. Photoluminescence showed a structured near band edge emission spectrum with evidence of the A, B and C free exciton recombinations

Extraction of |V ub | with Reduced Dependence on Shape Functions

Using BABAR measurements of the inclusive electron spectrum in B → Xueν decays and the inclusive photon spectrum in B → Xsγ decays, we extract the magnitude of the CKM matrix element V ub . The extraction is based on theoretical calculations designed to reduce the theoretical uncertainties by exploiting the assumption that the leading shape functions are the same for all b → q transitions (q is a light quark). The results agree well with the previous analysis, have indeed smaller theoretical errors, but are presently limited by the knowledge of the photon spectrum and the experimental errors on the lepton spectrum

Electronic structure of Pd multi-layers on Re(0001): the role of charge transfer

Understanding the origin of the properties of metal-supported metal thin films is important for the rational design of bimetallic catalysts and other applications, but it is generally difficult to separate effects related to strain from those arising from interface interactions. Here we use density functional (DFT) theory to examine the structure and electronic behavior of few-layer palladium films on the rhenium (0001) surface, where there is negligible interfacial strain and therefore other effects can be isolated. Our DFT calculations predict stacking sequences and interlayer separations in excellent agreement with quantitative low-energy electron diffraction experiments. By theoretically simulating the Pd core-level X-ray photoemission spectra (XPS) of the films, we are able to interpret and assign the basic features of both low-resolution and high-resolution XPS measurements. The core levels at the interface shift to more negative energies, rigidly following the shifts in the same direction of the valence d-band center. We demonstrate that the valence band shift at the interface is caused by charge transfer from Re to Pd, which occurs mainly to valence states of hybridized s-p character rather than to the Pd d-band. Since the d-band filling is roughly constant, there is a correlation between the d-band center shift and its bandwidth. The resulting effect of this charge transfer on the valence d-band is thus analogous to the application of a lateral compressive strain on the adlayers. Our analysis suggests that charge transfer should be considered when describing the origin of core and valence band shifts in other metal / metal adlayer systems

Net Charge on a Noble Gas Atom Adsorbed on a Metallic Surface

Adsorbed noble gas atoms donate (on the average) a fraction of an electronic charge to the substrate metal. The effect has been experimentally observed as an adsorptive change in the electronic work function. The connection between the effective net atomic charge and the binding energy of the atom to the metal is theoretically explored.Comment: ReVvTeX 3.1 format, Two Figures, Three Table

Automated Generation of User Guidance by Combining Computation and Deduction

Herewith, a fairly old concept is published for the first time and named "Lucas Interpretation". This has been implemented in a prototype, which has been proved useful in educational practice and has gained academic relevance with an emerging generation of educational mathematics assistants (EMA) based on Computer Theorem Proving (CTP). Automated Theorem Proving (ATP), i.e. deduction, is the most reliable technology used to check user input. However ATP is inherently weak in automatically generating solutions for arbitrary problems in applied mathematics. This weakness is crucial for EMAs: when ATP checks user input as incorrect and the learner gets stuck then the system should be able to suggest possible next steps. The key idea of Lucas Interpretation is to compute the steps of a calculation following a program written in a novel CTP-based programming language, i.e. computation provides the next steps. User guidance is generated by combining deduction and computation: the latter is performed by a specific language interpreter, which works like a debugger and hands over control to the learner at breakpoints, i.e. tactics generating the steps of calculation. The interpreter also builds up logical contexts providing ATP with the data required for checking user input, thus combining computation and deduction. The paper describes the concepts underlying Lucas Interpretation so that open questions can adequately be addressed, and prerequisites for further work are provided.Comment: In Proceedings THedu'11, arXiv:1202.453

Resonant electron transfer between quantum dots

An interaction of electromagnetic field with a nanostructure composed of two quantum dots is studied theoretically. An effect of a resonant electron transfer between the localized low-lying states of quantum dots is predicted. A necessary condition for such an effect is the existence of an excited bound state whose energy lies close to the top of the barrier separating the quantum dots. This effect may be used to realize the reversible quantum logic gate NOT if the superposition of electron states in different quantum dots is viewed as the superposition of bits 0 and 1.Comment: 8 pages, 1 EPS-figure, submitted to Phys. Rev.

Consumers' Willingness to Pay for Quality and Safety in Clams

The aim of the research is to estimate the potential demand for certified clams in Italy and to investigate the determinants of maximum amount that respondents are willing to pay for this product. Quantitative analysis was used based on 1,067 face-to-face interviews collected in 3 Italian regions in the north bordering the Adriatic Sea (Friuli-Venezia Giulia, Veneto, and Emilia-Romagna) carried out during 2008. The consumers' willingness to pay (WTP) is measured using a contingent valuation method. In order to estimate separately the determinants of the probability that respondents are willing to pay and the maximum that they are willing to pay, a generalization of Tobit model was adopted. The results indicate that consumers are willing to pay a premium price mainly to purchase better quality products. The research provides some initial insight into consumers' WTP that can be useful for certified fish farming. © 2014 Taylor & Francis Group, LLC