A first-principles DFT+GW study of spin-filter and spin-gapless semiconducting Heusler compounds

Among Heusler compounds, the ones being magnetic semiconductors (also known as spin-filter materials) are widely studied as they offer novel functionalities in spintronic/magnetoelectronic devices. The spin-gapless semiconductors are a special case. They possess a zero or almost-zero energy gap in one of the two spin channels. We employ the $GW$ approximation, which allows an elaborate treatment of the electronic correlations, to simulate the electronic band structure of these materials. Our results suggest that in most cases the use of $GW$ self energy instead of the usual density functionals is important to accurately determine the electronic properties of magnetic semiconductors.Comment: Final version as publishe

The surface adhesion parameter: a measure for wafer bondability

A theory is presented which describes the initial direct wafer bonding process. The effect of surface microroughness on the bondability is studied on the basis of the theory of contact and adhesion of elastic solids. An effective bonding energy, the maximum of which is the specific surface energy of adhesion, is proposed to describe the real binding energy of the bonding interface including the influence of the wafer surface microroughness. Both the effective bonding energy and the real area of contact between rough surfaces depend on a dimensionless surface adhesion parameter, &thetas;. Using the adhesion parameter as a measure, three kinds of wafer contact interfaces can be identified with respect to their bondability; viz. the non-bonding regime (&thetas;>&ap;12), the bonding regime (&thetas;<&ap;1), and the adherence regime (1<&thetas;<12). Experimental data are in agreement with this theor

Quasiparticle band structure of the almost-gapless transition-metal-based Heusler semiconductors

Transition-metal-based Heusler semiconductors are promising materials for a variety of applications ranging from spintronics to thermoelectricity. Employing the $GW$ approximation within the framework of the FLAPW method, we study the quasi-particle band structure of a number of such compounds being almost gapless semiconductors. We find that in contrast to the \textit{sp}-electron based semiconductors such as Si and GaAs, in these systems the many-body corrections have a minimal effect on the electronic band structure and the energy band gap increases by less than 0.2~eV, which makes the starting point density functional theory (DFT) a good approximation for the description of electronic and optical properties of these materials. Furthermore, the band gap can be tuned either by the variation of the lattice parameter or by the substitution of the \emph{sp}-chemical element

Nabla discrete fractional Grüss type inequality

Properties of the discrete fractional calculus in the sense of a backward difference are introduced and developed. Here, we prove a more general version of the Grüss type inequality for the nabla fractional case. An example of our main result is given

Multiple detection using the eigenvalues of the spectral matrix

In this study we treat the problem of detecting from multidimensional data, the number of uncorrelated signais in passive array treatment as it is the case in underwater acoustics, array processing and seismology . We use four detection criteria. Some of them are known, like AIC and MDL criteria where direct Kullback's divergence is the information measure; we prolong them using the inverse Kullback's divergence. We also adapt a new criterion using the logarithm of the likelihood ratio that has a chi square distribution and we suggest a simplified threshold criterion that uses the eigenvalues of the spectral matrix of the data . We study and compare the performances of these criteria in realistic simulations . The first one is inspired by the problems of array processing and the second one by seismic problems. Finally we study the robustness of these criteria when the classical hypothesis of uncorrelated noises having equal variances is not fulfilled . Thus we outline some application limits of these criteria .Critères de détection, résultats sur des simulation

Improvement of passive array treatment by estimation of the spectral matrix of noises

Array processing aims to characterize impinging sources front recorded data ; a model of the noise spectral matrix is necessary for the treatment . One usually suppose either that this matrix is known or that the noises are uncorrelated and have equal variances on each sensor. We present here an algorithm to estimate the noise spectral matrix when the noises are uncorrelated and have différent variances on each sensor. It needs technics of the principal components analysis; thus it uses the eigensystem of the spectral matrix of the received signais (the number of impinging signais is assumed known) . We show on simulations that, if the spectral matrix of the noises is estimated with this algorithm, the following array processing treatments give improved results .Présentation d'une méthode pour estimer la matrice spectrale des bruits lorsqu'ils sont non corrélés et ont des puissances différentes sur les capteurs. Utilisation des techniques d'analyse en composantes principales et donc des éléments propres de la matrice spectrale des signaux reçus. Simulations justifiant l'emploi de cet algorithm

Numerical ordinality in a wild nectarivore

This work was supported by the Association for the Study of Animal Behaviour (S.D.H.), the University of Lethbridge, and the Natural Sciences and Engineering Research Council of Canada (RGPIN 121496-2003; T.A.H.)Ordinality is a numerical property that nectarivores may use to remember the specific order in which to visit a sequence of flowers, a foraging strategy also known as traplining. In this experiment, we tested whether wild, free-living rufous hummingbirds (Selasphorus rufus) could use ordinality to visit a rewarded flower. Birds were presented with a series of linear arrays of 10 artificial flowers; only one flower in each array was rewarded with sucrose solution. During training, birds learned to locate the correct flower independent of absolute spatial location. The birds' accuracy was independent of the rewarded ordinal position (1st, 2nd, 3rd or 4th), which suggests that they used an object-indexing mechanism of numerical processing, rather than a magnitude-based system. When distance cues between flowers were made irrelevant during test trials, birds could still locate the correct flower. The distribution of errors during both training and testing indicates that the birds may have used a so-called working up strategy to locate the correct ordinal position. These results provide the first demonstration of numerical ordinal abilities in a wild vertebrate and suggest that such abilities could be used during foraging in the wild.PostprintPeer reviewe

Nonequilibrium Electron Interactions in Metal Films

Ultrafast relaxation dynamics of an athermal electron distribution is investigated in silver films using a femtosecond pump-probe technique with 18 fs pulses in off-resonant conditions. The results yield evidence for an increase with time of the electron-gas energy loss rate to the lattice and of the free electron damping during the early stages of the electron-gas thermalization. These effects are attributed to transient alterations of the electron average scattering processes due to the athermal nature of the electron gas, in agreement with numerical simulations

Stiction, Adhesion Energy and the Casimir Effect in Micromechanical Systems

We measure the adhesion energy of gold using a micromachined doubly-clamped beam. The stress and stiffness of the beam are characterized by measuring the spectrum of mechanical vibrations and the deflection due to an external force. To determine the adhesion energy we induce stiction between the beam and a nearby surface by capillary forces. Subsequent analysis yields a value $\gamma =0.06$ J/m$^{2}$ that is a factor of approximately six smaller than predicted by idealized theory. This discrepancy may be resolved with revised models that include surface roughness and the effect of adsorbed monolayers intervening between the contacting surfaces in these mesoscopic structures.Comment: RevTex, 4 pages, 4 eps figure