Anharmonicity and asymmetry of Landau levels for a two-dimensional electron gas

We calculate the density of states of a two dimensional electron gas located at the interface of a GaAlAs/GaAs heterojunction. The disorder potential which is generally created by a single doping layer behind a spacer, is here enhanced by the presence of a second delta doped layer of scatterers which can be repulsive or attractive impurities. We have calculated the density of states by means of the Klauder's approximation, in the presence of a magnetic field of arbitrary strength. At low field either band tails or impurity bands are observed for attractive potentials, depending on the impurity concentration. At higher field, impurity bands are observed for both repulsive and attractive potentials. We discuss the effect of such an asymmetrical density of states on the transport properties in the quantum Hall effect regime.Comment: 22 pages, 12 figures. submitted to Phys. Rev.

The influence of ant-attendance on aphid behaviour investigated with the electrical penetration graph technique

For themutualistic interaction between the aphidMetopeurum fuscoviride Stroyan (Homoptera:Aphididae) and the ant Lasius niger L. (Hymenoptera: Formicidae) it has been shown that ant-tended aphids develop faster, reproduce at a higher rate, and live longer than aphids not tended by ants. We used electrical penetration graphs (EPG) to investigate if behavioural patterns differ between ant-tended and untended M. fuscoviride during 8 h experiments. Measurements were made on adult aphids from four different ant-tended colonies that continued to be tended by L. niger during the experiments, and from four different colonies where ant workers were excluded several days before the start of the experiment and that were also not tended by ants during the experiments. Ants readily tended wired aphids and ant tending did not interfere with the EPG measurements. There were no significant differences in the duration of sieve element penetration or in any other analysed feeding-related EPG parameters between anttended and untended individuals. However, the quality of the EPG recordings did not allow the distinction between the EPG-waveform E1 (salivation only) and E2 (salivation and ingestion). These results suggest that the changes in life-history traits of ant-tended aphids do not result from changes in time of sieve element penetration waveforms. Alternative mechanisms may involve an increase in the rate of sap uptake or a higher effectiveness in nutrient uptake in the presence of ants. Our study demonstrates that the EPG technique is a useful tool to investigate the feeding behaviour of aphids during interactions with ants

Low Frequency Admittance Measurements in the Quantum Hall Regime

International audienceIn this paper we present an ac-magneto-transport study of a two-dimensional electron gas (2DEG) in the quantum Hall effect (QHE) regime, for frequencies in the range [100Hz, 1MHz]. We present an approach to understand admittance measurements based in the Landauer-Buttiker formalism for QHE edge channels and taking into account the capacitance and the topology of the cables connected to the contacts used in the measurements. Our model predicts an universal behavior with the a-dimensional parameter R H Cω where R H is the 2 wires resistance of the 2DEG, C the capacitance cables and the angular frequency, in agreement with experiments. For a specific configuration, we measure the electrochemical capacitance of the quantum Hall edge channels as predicted by Christen and Büttiker

Effets tunnels dans des nano capteurs de Hall

Dans le cadre de cette thèse, nous avons étudié une famille de capteurs à effet Hall, composé de croix de Hall submicroniques et de microgradiomètres réalisée à partir d'hétérostructures à puits quantiques optimisées. De par leurs performances, ces capteurs pourraient permettre d'aborder de façon plus efficace, l'étude à l'échelle nanométrique, des propriétés magnétiques de la matière micro- et nanostructurée, ainsi que celles des nanocomposants. La première partie de cette thèse est consacrée à la fabrication et à la caractérisation des capteurs . On étudiera tout d'abords les résistances de contacts et les largeurs de déplétion limitant le fonctionnement des capteurs puis on s'intéressera à la métrologie des composants. Dans la seconde partie, on étudie le passage par effet tunnel des électrons entre les bords de l'échantillon. En régime d'effet Hall Quantique, il apparaît des fluctuations aux extrémités des plateaux à la fois sur les résistances transverses et longitudinales. Les pics sur le côté fort champ de la transition sont différent de ceux situé sur le côté faible champ de la transition. Il diffère aussi par leur comportement en température. Du côté des fort facteurs de remplissage, l'évolution en température des pics permet de les relier à un effet tunnel résonnant à travers un état créé par un des antidots se formant lors du remplissage des niveaux de Landau. Du côté des faibles facteurs de remplissage, la dépendance en température est différente et est reliable à un processus de "Variable Range Hopping". Ces processus peuvent être reliés à l'assymétrie de la densité d'étatsAs part of this thesis, we studied a family of Hall effect sensors , composed of submicrometers Hall cross and microgradiometers made from optimized quantum wells. The first one left this thesis is dedicated to manufacture and to characterization of sensors. They will study everything of manners the resistance of contacts and the depletion lenght restricting the functioning of sensors In second part, we study the tunnel effect of electrons between the edges of the sample. In the presence of a quantizing magnetic field, at the transition between two quantized Hall plateaus, a succession of sharp peaks is detected in the Hall signal RH and in the longitudinal resistance RL. The peaks appearing on the high-í side of the RL transition appear to be different from the peaks appearing on the low-í side. They mainly differ by their temperature dependence. On the high-í side of the RL transition, the temperature evolution of the peaks is typical for resonant tunneling through a single state in one of the antidots that are progressively formed when the initially occupied LL is emptied. On the low í of the transition, by contrast, the temperature dependence is different. This may be related to the asymmetry of the density of statesMONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Vers un interféromètre électronique de type Fabry-Pérot

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Encyclop’Aphid: a website on aphids and their natural enemies

International audienceBecause aphids represent major crop pests worldwide and are intensively studied for their agricultural impacts and unusual biological features, there is a constant need to disseminate the knowledge accumulating on these insects to a large audience. Encyclop'Aphid, an online encyclopedia, in both English and French, devoted to aphids and their natural enemies, is intended to meet this need. The website includes five main sections describing 1) aphid traits (taxonomy, morphology, biology), 2) their interactions with other organisms (antagonists, mutualists), 3) their economical impact, 4) some current research themes and 5) a species-by-species presentation of common aphid species and their natural enemies, i.e., pathogenic fungi, insect predators, parasitoids and hyperparasitoids. Encyclop'Aphid also contains several taxonomic identification keys for aphids that attack cultivated plants. It offers freely accessible iconographies and a video library. The website is intended to evolve and to be constantly enriched by new contributions from different international specialists

Landau levels analysis by using symmetry properties of mesoscopic Hall bars

We use the resistance fluctuations (RFs) appearing in the integer quantum Hall regime to scan the density of states of a very thin Hall bar. By applying a dc voltage on a top gate, we analyze the correlation properties of the various resistances as a function of the magnetic field and the carrier density. In the gate voltage-magnetic field plane, these RFs follow lines with slopes quantized in unit of filling factor and the slope of these RFs depends on their correlation properties

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

Characterization of dextran particle size: How frit-inlet asymmetrical flow field-flow fractionation (FI-AF4) coupled online with dynamic light scattering (DLS) leads to enhanced size distribution

International audienc