Surface electrons at plasma walls

In this chapter we introduce a microscopic modelling of the surplus electrons on the plasma wall which complements the classical description of the plasma sheath. First we introduce a model for the electron surface layer to study the quasistationary electron distribution and the potential at an unbiased plasma wall. Then we calculate sticking coefficients and desorption times for electron trapping in the image states. Finally we study how surplus electrons affect light scattering and how charge signatures offer the possibility of a novel charge measurement for dust grains.Comment: To appear in Complex Plasmas: Scientific Challenges and Technological Opportunities, Editors: M. Bonitz, K. Becker, J. Lopez and H. Thomse

Anomalous resistivity and the origin of a heavy mass in the two-band Hubbard model with one narrow band

We search for marginal Fermi-liquid behavior in the two-band Hubbard model with one narrow band. We consider the limit of low electron densities in the bands and strong intraband and interband Hubbard interactions. We analyze the influence of electron polaron effect and other mechanisms of mass-enhancement (related to momentum dependence of the self-energies) on effective mass and scattering times of light and heavy components in the clean case (electron - electron scattering and no impurities). We find the tendency towards phase-separation (towards negative partial compressibility of heavy particles) in a 3D case for large mismatch between the densities of heavy and light bands in a strong coupling limit. We also observe that for low temperatures and equal densities the resistivity in a homogeneous state R(T) ~ T^2 - behaves in a Fermi-liquid fashion both in 3D and 2D cases. For temperatures higher then effective bandwidth for heavy electrons T > Wh* the coherent behavior of heavy component is totally destroyed. The heavy particles move diffusively in the surrounding of light particles. At the same time the light particles scatter on the heavy ones as if on immobile (static) impurities. In this regime the heavy component is marginal, while the light one is not. The resistivity goes on saturation for T > Wh* in the 3D case. In 2D the resistivity has a maximum and localization tail due to weak - localization corrections of Altshuler - Aronov type. Such behavior of resistivity in 3D could be relevant for some uranium-based heavy-fermion compounds like UNi2Al3 and in 2D for some other mixed-valence compounds possibly including the layered manganites. We also consider briefly the superconductive (SC) instability in the model. The leading instability is towards p-wave pairing and is governed by enhanced Kohn - Luttinger mechanism of SC at low electron density.Comment: 37 pages, 9 figures, accepted for JET

Brownian motion in a periodic potential under an applied bias : the transition from hopping to free conduction

We consider particles in an arbitrary large bias, subject to a periodic potential ; the friction is such that the oscillation in the potential wells is underdamped. We develop a stochastic formulation based on the assumption that the friction is Markovian from one well to the next. The relevant parameter is the mean energy δ lost in crossing one well, compared to either T or to the drop in bias energy y. We show that a rapid change of regime occurs at y = δ. When δ >> y, T, we recover the old results of Frenkel. When δ > y, T, nous retrouvons les résultats classiques de Frenkel. Dans la limite opposée δ << T, la mobilité augmente; nous donnons dans ce cas une solution détaillée pour y quelconque. Nous généralisons ainsi les résultats obtenus par Ambegaokar et Halperin [2] dans le cas suramorti

A simple stochastic description of desorption rates

The kinetic theory of desorption rates for an adatom on a solid surface is reconsidered from a stochastic point of view. The results of Kramers, obtained within the framework of a Fokker-Planck equation, are thereby generalized. For a deep potential well, the desorption rate is related to the average sticking probability of an incoming thermal particle. The latter in turn depends only on the average energy loss δ of the particle to the substrate during a single oscillation. It is shown that in most cases the loss to lattice vibrations is so strong as to justify a posteriori the absolute rate theory of Eyring. The theory is extended to more complicated situations (adparticle with internal degrees of freedom, activated barrier, ...). Finally, a fully time-dependent formulation, valid for shallow wells, is sketched; it is shown how, for deep wells, it reduces to the above simple formulation, based on detailed balance only.La théorie cinétique des vitesses de désorption d'un adatome en surface est reprise sous un angle stochastique, permettant de généraliser les résultats de Kramers (fondés sur une équation de Fokker Planck). Pour un puits de potentiel profond, 1a vitesse de désorption est reliée à la probability de piégeage moyenne d'un atome thermique incident. Celle-ci dépend à son tour de l'énergie moyenne cédée au substrat par la particule pendant une oscillation. En général, le transfert d'énergie aux phonons est si efficace qu'il justifie a posteriori le modèle absolu de Eyring. On peut généraliser le modèle à des situations plus compliquées (adparticule avec degré de liberté interne, barrières activées, etc...). Enfin, un formalisme dépendant du temps, valable pour des puits peu profonds, est esquissé — pour des puits profonds, ce formalisme général se réduit au modèle simple du depart

Behavioral consequences of dopamine deficiency in the Drosophila central nervous system

The neuromodulatory function of dopamine (DA) is an inherent feature of nervous systems of all animals. To learn more about the function of neural DA in Drosophila, we generated mutant flies that lack tyrosine hydroxylase, and thus DA biosynthesis, selectively in the nervous system. We found that DA is absent or below detection limits in the adult brain of these flies. Despite this, they have a lifespan similar to WT flies. These mutants show reduced activity, extended sleep time, locomotor deficits that increase with age, and they are hypophagic. Whereas odor and electrical shock avoidance are not affected, aversive olfactory learning is abolished. Instead, DA-deficient flies have an apparently “masochistic” tendency to prefer the shock-associated odor 2 h after conditioning. Similarly, sugar preference is absent, whereas sugar stimulation of foreleg taste neurons induces normal proboscis extension. Feeding the DA precursor l-DOPA to adults substantially rescues the learning deficit as well as other impaired behaviors that were tested. DA-deficient flies are also defective in positive phototaxis, without alteration in visual perception and optomotor response. Surprisingly, visual tracking is largely maintained, and these mutants still possess an efficient spatial orientation memory. Our findings show that flies can perform complex brain functions in the absence of neural DA, whereas specific behaviors involving, in particular, arousal and choice require normal levels of this neuromodulator