A quantum Hall Mach-Zehnder interferometer far beyond the equilibrium

We experimentally realize quantum Hall Mach-Zehnder interferometer which operates far beyond the equilibrium. The operation of the interferometer is based on allowed intra-edge elastic transitions within the same Landau sublevel in the regime of high imbalances between the co-propagating edge states. Since the every edge state is definitely connected with the certain Landau sublevel, the formation of the interference loop can be understood as a splitting and a further reconnection of a single edge state. We observe an Aharonov-Bohm type interference pattern even for low-size interferometers. This novel interference scheme demonstrates high visibility even at millivolt imbalances and survives in a wide temperature range.Comment: As accepted by PR

Präparation von nanostrukturierten zweidimensionalen Elektronengasen und die Charakterisierung ihrer linearen und nichtlinearen Transporteigenschaften

In dieser Arbeit wird der nichtlineare Transport in zweidimensionalen Elektronengasen anhand von drei Experimenten untersucht, wobei das nichtlineare Verhalten der untersuchten Bauteile auf einer besonderen Geometrie beruht. Im ersten Experiment wird die Gleichrichtung an einem Symmetrie-gebrochenen zweidimensionalen Elektronengas analysiert, wobei der Symmetriebruch durch eine Dichtemodulation im Elektronengas entlang der Stromrichtung herbeigeführt wird. Mit einem Thermodiffusions-Modell wird gezeigt, dass die Gleichrichtung durch diffusive Thermoelektrik bewirkt wird. Zudem wird eine neue Methode eingeführt, mit welcher die inelastische Streuzeit des zweidimensionalen Elektronengases bestimmt werden kann. Mit der inelastischen Streuzeit werden die Elektronentemperaturen in dem dichtemodulierten System abgeschätzt, welche für die Berechnung der Thermospannung benötigt werden. Für die Realisierung eines ballistischen Transistors wird im zweiten Experiment das lineare und nichtlineare Transportverhalten der magnetischen Elektronen-Fokussierung untersucht. Um das nichtlineare Verhalten für hohe Injektionsenergien zu erklären, wird ein Einzelelektronen-Billard-Modell der Elektronen-Fokussierung präsentiert. Mit dem Modell kann gezeigt werden, dass die Elektron-Elektron-Streuung, welche stark von der Injektionsenergie abhängig ist, für das nichtlineare Verhalten verantwortlich ist. Auf den Ergebnissen dieser Arbeit basierend wird das Konzept eines ballistischen Transistors vorgeschlagen. Im letzten Experiment wird eine steuerbare Selbst-Schaltende-Diode und ihre Wirkungsweise vorgestellt. Die Funktionsweise dieser Diode beruht auf einem Rückkopplungs-Effekt, wobei die Steuerung mit Seiten-Gates erfolgt. Im selbst-sperrenden Regime funktioniert das Bauteil wie ein perfekter Halbwellen-Gleichrichter. Obwohl alle drei Bauteile aus dem gleichen Material bestehen, verursachen in allen Experimenten unterschiedliche (und teilweise ungewöhnliche) Effekte das beobachtete nichtlineare Transportverhalten. Zudem wird ständig nach neuen, fortgeschrittenen Strukturierungsmethoden gesucht, z.B. um Formgebungs-Effekte in neuen Materialsystemen untersuchen zu können. Solch eine neue Methode zur Strukturierung von GaAs und Graphen auf der Nanometerskala wird mit dem Gas-unterstützten Elektronenstrahl-induzierten lokalen Ätzen vorgestellt. Bei dieser Methode handelt es sich um eine präzise Methode mit einer sehr hohen Auflösung, welche zudem sehr schonend für das zu strukturierende Material ist. Die neue Strukturierungsmethode ist insbesondere für das neue Materialsystem Graphen interessant, um ballistischen Transport bei Raumtemperatur zu realisieren.This thesis investigates the non-linear transport in two-dimensional electron gases on the basis of three experiments. In all three cases the non-linear behavior is caused by a special device geometry. In the first experiment the rectification in a symmetry-broken two-dimensional electron gas is analyzed. The broken symmetry in the device is induced by a density modulation of the electron channel along the current direction. A diffusion thermopower model shows that the observed rectification is caused by a thermoelectric effect. In addition, a new method is introduced, which allows the determination of the inelastic scattering time in a two-dimensional electron gas. With the inelastic scattering time it is possible to estimate the electron temperatures in the density modulated system, which are needed for the calculation of the thermovoltage. As a preliminary work for a ballistic transistor the second experiment investigates the linear and non-linear transport behavior of the ballistic electron focusing. In order to explain the non-linear behavior of the electron focusing devices for high injection energies, a single electron billiard model of the electron focusing is developed. The model shows that electron-electron interaction, which is highly dependent on the injection energy, is causing the observed non-linear behavior. A concept of a ballistic transistor, based on the results presented in this thesis, is suggested. The last experiment deals with a voltage-tunable self-switching in-plane diode and its working principle. An applied voltage along the channel modulates the effective width of the conducting channel, resulting in a diode-like IV-characteristic. In the normally-off state the device works as an ideal half-wave rectifier. Although all three devices consist of the same material, the non-linear transport behavior in all three experiments is caused by entirely different effects. In addition to that, the search for new, more advanced patterning methods is going on, for example to allow the investigation of finite size effects in new material systems. Such a new method to pattern GaAs and graphene at the nanometer scale is introduced with the gas-assisted electron beam induced local etching. This method is non-destructive for the patterned material and has a very high resolution. This patterning method is particularly interesting for the material system graphene to realize ballistic transport at room temperature

Transverse rectification in density-modulated two-dimensional electron gases

We demonstrate tunable transverse rectification in a density-modulated two-dimensional electron gas (2DEG). The density modulation is induced by two surface gates, running in parallel along a narrow stripe of 2DEG. A transverse voltage in the direction of the density modulation is observed, i.e. perpendicular to the applied source-drain voltage. The polarity of the transverse voltage is independent of the polarity of the source-drain voltage, demonstrating rectification in the device. We find that the transverse voltage $U_{y}$ depends quadratically on the applied source-drain voltage and non-monotonically on the density modulation. The experimental results are discussed in the framework of a diffusion thermopower model.Comment: 8 pages, 9 figures, published in PR

A comparison between bright field and phase-contrast image analysis techniques in activated sludge morphological characterization

Different approaches using microscopy image analysis procedures were employed for characterization of activated sludge systems. The approaches varied mainly on the type of visualization and acquisition method used for collection of data. In this context, this study focused on the comparison of the two most common acquisition methods: bright field and phase-contrast microscopy. Images were acquired from seven different wastewater treatment plants for a combined period of two years. Advantages and disadvantages of each acquisition technique and the results are discussed. Bright field microscopy proved to be more simple and inexpensive and provided the best overall results.Fundação para a Ciência e a Tecnologia (FCT) - SFRH/BD/32329/2006, POCI/AMB/57069/200

Monitoring of activated sludge settling ability through image analysis : validation on full-scale wastewater treatment plants

In recent years, a great deal of attention has been focused on the research of activated sludge processes, where the solid–liquid separation phase is frequently considered of critical importance, due to the different problems that severely affect the compaction and the settling of the sludge. Bearing that in mind, in this work, image analysis routines were developed in Matlab environment, allowing the identification and characterization of microbial aggregates and protruding filaments in eight different wastewater treatment plants, for a combined period of 2 years. The monitoring of the activated sludge contents allowed for the detection of bulking events proving that the developed image analysis methodology is adequate for a continuous examination of the morphological changes in microbial aggregates and subsequent estimation of the sludge volume index. In fact, the obtained results proved that the developed image analysis methodology is a feasible method for the continuous monitoring of activated sludge systems and identification of disturbances.Empresa de Águas, Efluentes e Resíduos de Braga, Portugal - EM (AGERE)Fundação para a Ciência e a Tecnologia (FCT

Enhancing the removal of hazardous pollutants from coke making wastewater by dosing activated carbon to a pilot‐scale activated sludge process

Powdered activated carbon (PAC) was investigated for its ability to remove 6 polycyclic aromatic hydrocarbons (Σ6PAHs) (fluoranthene, benzo[b + J]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, indeno[1,2,3-cd]pyrene, benzo[g,h,i]perylene), trace metals and colour from coke making wastewater when dosed to a pilot-plant activated sludge process (ASP). The ASP had a volume of 0.68 m3 and was operated to simulate the full-scale ASP treating coke wastewater from a steel works. Operational conditions included a flow rate of 0.78 m3/day, a hydraulic retention time of 21 hours, a sludge retention time of 38 days and a temperature of 27°C. The ASP was operated for a control period before PAC was dosed directly into the aeration cell at a dose of 400 mg/L. Powdered activated carbon addition resulted in a 20% increase in removal efficiency of the Σ6PAHs. Removal efficiency of trace metals was variable, but increased for nickel, chromium and cadmium by 22.6%, 20.5% and 12.4%, respectively. Improvement in colour removal efficiency was marginal at 5%. PAC addition allowed the improvement of treatment efficiencies in the ASP process at relatively low capital and operational costs, which may assist in reaching tighter effluent emission limits set for the industry

Options for Producing a Warm-Water Fish in the UK: limits to "Green-Growth"?

This paper explores the development of a sustainable production system for tilapia and the research implications involved with ensuring commercial viability of such a system for UK farmers. The tilapia is a warm water fish with firm texture, white flesh and mild taste quite similar to a cod or haddock. Whilst tropical in origin it is thought to be highly suitable for low cost aquaculture in temperate zones with the potential to be a more sustainable source of food with fewer environmental impacts than other substitutes. Drawing on a literature review and findings from technical trials the paper will review and compare two production systems - novel Activated Suspension Technology (AST) and conventional Recirculating Aquaculture Systems (RAS) - considering their feasibility in terms of potential and financial viability for scaling up to commercial production of tilapia and their environmental and sustainability benefits. The review concludes that AST based only on microbial floc is currently uncompetitive with RAS in a UK context although the approach has benefits that might be incorporated in a new generation of mixed systems. Refinement of such systems needs to occur with potential adopters and could be part of diversification of mixed farms. Such development might further enhance the ethical values of fish produced in small-scale, modular RAS

Quantitative image analysis for the characterization of microbial aggregates in biological wastewater treatment : a review

Quantitative image analysis techniques have gained an undeniable role in several fields of research during the last decade. In the field of biological wastewater treatment (WWT) processes, several computer applications have been developed for monitoring microbial entities, either as individual cells or in different types of aggregates. New descriptors have been defined that are more reliable, objective, and useful than the subjective and time-consuming parameters classically used to monitor biological WWT processes. Examples of this application include the objective prediction of filamentous bulking, known to be one of the most problematic phenomena occurring in activated sludge technology. It also demonstrated its usefulness in classifying protozoa and metazoa populations. In high-rate anaerobic processes, based on granular sludge, aggregation times and fragmentation phenomena could be detected during critical events, e.g., toxic and organic overloads. Currently, the major efforts and needs are in the development of quantitative image analysis techniques focusing on its application coupled with stained samples, either by classical or fluorescent-based techniques. The use of quantitative morphological parameters in process control and online applications is also being investigated. This work reviews the major advances of quantitative image analysis applied to biological WWT processes.The authors acknowledge the financial support to the project PTDC/EBB-EBI/103147/2008 and the grant SFRH/BPD/48962/2008 provided by Fundacao para a Ciencia e Tecnologia (Portugal)

Stanovení teplotních mezí výbušnosti

Import 20/04/2006Prezenční výpůjčkaVŠB - Technická univerzita Ostrava. Fakulta bezpečnostního inženýrství. Katedra (030) požární ochrany a ochrany obyvatelstv