Conduction electrons localized by charged magneto-acceptors A2−^{2-} in GaAs/GaAlAs quantum wells

A variational theory is presented of A$^{1-}$ and A$^{2-}$ centers, i.e. of a negative acceptor ion localizing one and two conduction electrons, respectively, in a GaAs/GaAlAs quantum well in the presence of a magnetic field parallel to the growth direction. A combined effect of the well and magnetic field confines conduction electrons to the proximity of the ion, resulting in discrete repulsive energies above the corresponding Landau levels. The theory is motivated by our experimental magneto-transport results which indicate that, in a heterostructure doped in the GaAs well with Be acceptors, one observes a boil-off effect in which the conduction electrons in the crossed-field configuration are pushed by the Hall electric field from the delocalized Landau states to the localized acceptor states and cease to conduct. A detailed analysis of the transport data shows that, at high magnetic fields, there are almost no conducting electrons left in the sample. It is concluded that one negative acceptor ion localizes up to four conduction electrons.Comment: 8 pages, 5 figure

Nanofibers as a Vehicle for the Synthetic Attactant TRIMEDLURE to be Used for Ceratitis capitata Wied: (Diptera, Tethritidae) Capture

 The Mediterranean fruit fly (medfly), Ceratitis capitata (Wied.) is considered a serious pest of citrus fruits in the southeast of Brazil because of the direct economic impact they have on fruit production and quarantine restrictions for fruit exports. Monitoring and detection of the medfly, using food bait and synthetic attractants, is a key step towards signalling the need for management and control. The aim of this study has been to verify the attractiveness of innovative nanofiber formulations with Trimedlure (TML) for the male of C. capitata in laboratory and field cage tests.Material and Methods: The nanofibers were produced by solution or emulsion electrospinning, containing TML and polymers, such as polycaprolactone, PEG-polycaprolactone, ethyl cellulose and polyvinyl acetate-PVP.Results: At the laboratory the electrophysiological responses were accessed by the eletroanntenogram technique and in the field by the cage test. The bioactivity of C. capitata antennae was highest when stimulated with all TML nanofiber treatments rather than their controls. There were no differences among the TML nanofiber treatments. In the field cages the same number of medflies were found on the adhesive traps baited with one of each of the TML nanofibers

Integration of P2Y receptor-activated signal transduction pathways in G protein-dependent signalling networks

The role of nucleotides in intracellular energy provision and nucleic acid synthesis has been known for a long time. In the past decade, evidence has been presented that, in addition to these functions, nucleotides are also autocrine and paracrine messenger molecules that initiate and regulate a large number of biological processes. The actions of extracellular nucleotides are mediated by ionotropic P2X and metabotropic P2Y receptors, while hydrolysis by ecto-enzymes modulates the initial signal. An increasing number of studies have been performed to obtain information on the signal transduction pathways activated by nucleotide receptors. The development of specific and stable purinergic receptor agonists and antagonists with therapeutical potential largely contributed to the identification of receptors responsible for nucleotide-activated pathways. This article reviews the signal transduction pathways activated by P2Y receptors, the involved second messenger systems, GTPases and protein kinases, as well as recent findings concerning P2Y receptor signalling in C6 glioma cells. Besides vertical signal transduction, lateral cross-talks with pathways activated by other G protein-coupled receptors and growth factor receptors are discussed

Two-electron states localized by charged acceptors in GaAs/GaAlAs quantum wells in ultra-quantum regime of magnetic fields

International audienceWe demonstrate theoretically that bound acceptors states of a two dimensional electron gas in quantizing magnetic field can localize two electrons on a same site. We calculate the energies and wave functions of the second quantum state, and show that they are close to calculations of the first quantum state found previously

New edge magnetoplasmon interference like photovoltage oscillations and their amplitude enhancement in the presence of an antidot lattice

We present new photovoltage oscillation in a pure two dimensional electron gas (2DEG) and in the presence of circular or semicircular antidot lattices. Results were interpreted as EMPs-like photovoltage oscillations. We observed and explained the photovoltage oscillation amplitude enhancement in the presence of an antidot lattice with regard to the pure 2DEG. The microwave frequency excitation range is 139 – 350 GHz. The cyclotron and magnetoplasmon resonances take place in the magnetic field range 0.4 – 0.8 T. This original experimental condition allows edge magnetoplasmons EMPs interference like observation at low magnetic field, typically B < Bc where Bc is the magnetic field at which the cyclotron resonance takes place. The different oscillation periods observed and their microwave frequency dependence were discussed. For 139 and 158 GHz microwave excitation frequencies, a unique EMPs-like interference period was found in the presence of antidots whereas two periods were extracted for 295 or 350 GHz. An explanation of this effect is given taking account of strong electron interaction with antidot at low magnetic field. Indeed, electrons involved in EMPs like phenomenon interact strongly with antidots when electron cyclotron orbits are larger than or comparable to the antidot diameter

Magnetic thaw down and boil-off of electrons in the quantum Hall effect regime due to magnetoacceptors in GaAs/GaAlAs heterostructures

International audienceThe quantum Hall effect (QHE) and the Shubnikov-de Haas effect in the QHE regime are investigated experimentally using modulation doped n-type GaAs/GaAlAs quantum wells additionally doped in the well with beryllium. It is known that acceptor states introduced by Be atoms have a localized character in the conduction band due to a combined effect of the well and a magnetic field parallel to the growth direction and that they possess discrete energies above the corresponding conduction Landau levels. It is presently shown that the localized magnetoacceptor (MA) states lead to two observable effects in magnetotransport in the ultraquantum limit: magnetic thaw down and magnetic boil-off of two-dimensional (2D) electrons. Both effects are related to the fact that electrons occupying localized MA states cannot conduct. Thus in the thaw down effect the electrons fall down from the MA states to the free Landau states, which leads to a shift of the Hall plateau toward higher magnetic fields as a consequence of an increase of the 2D electron density Ns. In the boil-off effect the electrons are pushed from the free Landau states to the MA states which leads to a dramatic increase of resistance, as a consequence of the decrease of Ns. Differences between the above effects and those induced by magnetodonors in 2D systems are emphasized. We analyze the magnetic boil-off effect theoretically assuming that it is caused by the quantum Hall electric field present in our experiments. It is demonstrated that a sufficiently strong electric field in the crossed-field configuration can indeed populate localized MA states above the Landau levels

Effects of repulsive and attractive ionized impurities on the resistivity of semiconductor heterostructures in the quantum Hall regime

International audienceWe have investigated experimentally and theoretically the effect of repulsive and attractive ionized impurities on the resistivity components (ρxx and ρxy) in the quantum Hall effect regime. GaAs/GaAlAs asymmetric modulation-doped quantum wells with additional delta doping (by Si donor atoms or Be acceptor atoms) in the GaAs channel or at the AlGaAs/GaAs interface has been grown using molecular beam epitaxy technique. Magnetotransport experiments, performed on samples doped with Si-attractive atoms, showed a plateau width increasing toward lower magnetic field at even filling factor. However, when samples were delta doped with Be repulsive atoms, the increase was observed in the opposite side. Part of the results was explained using a model based on the fifth Klauder's approximations where we demonstrate that the asymmetrical increase of the Hall plateaus with even filling factor (Landau gaps) is related to the asymmetry induced in the density of states by the additional impurities: the resulting disorder short range potential broadens and shifts the Landau levels but also creates impurity bands on the lower energy side of the Landau levels in the case of donors and on the upper energy side of the Landau levels in the case of acceptors. We notice that this asymmetrical behavior was not experimentally observed for odd filling factor plateaus (exchange gaps). We have also experimentally underscored the screening effect by free two-dimensional electrons of this disorder short range potential. Moreover, for delta-doped Be samples, the whole ν=1 Hall plateau was shifted toward higher magnetic field with respect to the classical Hall effect. This shift, observed for all samples, cannot be explained by the asymmetry of the density of states but rather by a magnetic delocalization of electrons from the upper energy impurity band associated with the last Landau level (n=0) into the free n=0 Landau states when this impurity band overtakes the Fermi level at the end of the ν=2 plateau. This magnetic delocalization effect is the opposite effect of the magnetic freeze out. © 2009 The American Physical Societ

Chemically assisted vapour transport for bulk ZnO crystal growth

International audienceA chemically assisted vapour phase transport (CVT) method is proposed for the growth of bulk ZnO crystals. Thermodynamic computations have confirmed the possibility of using CO as a sublimation activator for enhancing the sublimation rate of the feed material in a large range of pressures (10(-3) to 1 atm) and temperatures (800-1200 degrees C). Growth runs in a specific and patented design yielded single ZnO crystals up to 46 mm in diameter and 8 mm in thickness, with growth rates up to 400 mu m/h. These values are compatible with an industrial production rate. N type ZnO crystals (mu = 182 cm(2)/(V s) and n=7 10(15) cm(-3)) obtained by this CVT method (Chemical Vapour Transport) present a high level of purity (10-30 times better than hydrothermal ZnO crystals), which may be an advantage for obtaining p-type doped layers ([Li] and [Al] < 10(+15) cm(-3)). Structural (HR-XRD), defect density (EPD), electrical (Hall measurements) and optical (photoluminescence) properties are presented