Gel'fand-Levitan equation for deletion of states from Coulomb spectra

The Gel'fand-Levitan formalism is used to study how a selected set of bound states can be eliminated from the spectrum of the Coulomb potential and also how to construct a bound state in the Coulomb continuum. It is demonstrated that a sizeable quantum well can be produced by deleting a large number of levels from the s spectral series and the edge of the Coulomb potential alone can support the von Neumann-Wigner states in the continuum. © 1998 Elsevier Science B.V.

Poços quânticos e transferência de elétrons

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Investigação óptica de pontos quânticos de InAs confinados em poços quânticos de In0,15Ga0,85As

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Estados de impurezas em semicondutores e nanoesetruturas sob campos externos

Pós-graduação em Ciência e Tecnologia de Materiais - FC

Equações de taxa aplicadas na modelagem de dispositivos fotodetectores baseados em poços quânticos

In this work, it is presented a novel method for calculating some intrinsic parameters such as capture mean time, thermic emission mean time, capture probability and noise gain on quantum well infrared photodetectors. Such devices are built by depositing layers of semiconductors of different energy gap, forming quantum wells. The present method uses rate equations to describe the occupation of discrete states of the quantum wells, that together with noise gain equations given in literature, are solved self consistently. The input data of the method is experimental measurement of dark current (current measured with no relevant incident photon) versus applied bias. In order to validate this approach, the values obtained were compared with results from literature

Estudo de flutuações de potenciais em poços quânticos de InGaAsN

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Estudo de flutuações de potenciais em poços quânticos de InGaAsN

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Nonlocal Transport Near Charge Neutrality Point in a Two-Dimensional Electron-Hole System

Nonlocal resistance is studied in a two-dimensional system with a simultaneous presence of electrons and holes in a 20 nm HgTe quantum well. A large nonlocal electric response is found near the charge neutrality point in the presence of a perpendicular magnetic field. We attribute the observed nonlocality to the edge state transport via counterpropagating chiral modes similar to the quantum spin Hall effect at a zero magnetic field and graphene near a Landau filling factor nu = 0.

Supramolecular order and dynamics of discotic materials studied by solid-state NMR recoupling methods

The topic of this thesis is the investigation of structure,order and dynamics in discotic mesogens by advancedsolid-state NMR spectroscopy. Most of the discotic mesogensunder investigation are hexa-peri-hexabenzocoronene (HBC)derivatives which are of particular interest for potentialdevice applications due to their high one-dimensional chargecarrier mobilities. The supramolecular stacking arrangement of the discoticcores was investigated by 2D 1H-1H double-quantum (DQ)methods, which were modified by incorporating the WATERGATEsuppression technique into the experiments in order toovercome severe phase problems arising from the strongsignal of the long alkyl sidechains. Molecular dynamics and sample orientation was probed throughthe generation of sideband patterns by reconversion rotorencoding in 2D recoupling experiments. These experimentswere extended by new recoupling schemes to enable thedistinction of motion and orientation effects. The solid-state NMR studies presented in this work aim tothe understanding of structure-property relationships in theinvestigated discotic materials, while the experimentsapplied to these materials include new recoupling schemeswhich make the desired information on molecular orientationand dynamics accessible without isotope labelling.

Characterisation of supramolecular structures by novel recoupling methods in solid-state NMR

In this work, solid-state NMR methods suitable for the investigation of supramolecular systems were developed and improved. In this context, special interest was focussed on non-covalent interactions responsible for the formation of supramolecular structures, such as pi-pi interacions and hydrogen-bonds. In the first part of this work, solid-state NMR methods were presented that provide information on molecular structure and motion via the investigation of anisotropic interactions, namely quadrupole and dipole-dipole couplings, under magic-angle spinning conditions. A two-dimensional 2H double quantum experiment was developed, which is performed under off magic-angle conditions and correlates 2H isotropic chemical shifts with quasistatic DQ-filtered line shapes. From the latter, the quadrupole coupling parameters of samples deuterated at multiple sites can be extracted in a site-selective fashion. Furthermore, 7Li quadrupole parameters of lithium intercalated into TiO2 were determined by NMR experiments performed under static and MAS conditions, and could provide information on the crystal geometry. For the determination of 7Li-7Li dipole-dipole couplings, multiple-quantum NMR experiments were performed. The 1H-13C REREDOR experiment was found to be capable of determining strong proton-carbon dipole-dipole couplings with an accuracy of 500~Hz, corresponding to a determination of proton-carbon chemical-bond lengths with picometer accuracy In the second part of this work, solid-state NMR experiments were combined with quantum-chemical calculations in order to aid and optimise the interpretation of experimental results. The investigations on Calix[4]hydroquinone nanotubes have shown that this combined approach can provide information on the presence of disordered and/or mobile species in supramolecular structures. As a second example, C3-symmetric discs arranging in helical columnar stacks were investigated. In these systems, 1H chemical shifts experience large pi-shifts due to packing effects, which were found to be long-ranged. Moreover, quantum-chemical calculations revealed that helicity in these systems is induced by the propeller-like conformation of the core of the molecules.

NMR spectroscopy and imaging of hyperpolarized gases

Since the discovery of the nuclear magnetic resonance (NMR) phenomenon, countless NMR techniques have been developed that are today indispensable tools in physics, chemistry, biology, and medicine. As one of the main obstacles in NMR is its notorious lack of sensitivity, different hyperpolarization (HP) methods have been established to increase signals up to several orders of magnitude. In this work, different aspects of magnetic resonance, using HP noble gases, are studied, hereby combining different disciplines of research. The first part examines new fundamental effects in NMR of HP gases, in theory and experiment. The spin echo phenomenon, which provides the basis of numerous modern experiments, is studied in detail in the gas phase. The changes of the echo signal in terms of amplitude, shape, and position, due to the fast translational motion, are described by an extension of the existing theory and computer simulations. With this knowledge as a prerequisite, the detection of intermolecular double-quantum coherences was accomplished for the first time in the gas phase. The second part of this thesis focuses on the development of a practical method to enhance the dissolution process of HP 129Xe, without loss of polarization or shortening of T1. Two different setups for application in NMR spectroscopy and magnetic resonance imaging (MRI) are presented. The continuous operation allows biological and multidimensional spectroscopy in solutions. Also, first in vitro MRI images with dissolved HP 129Xe as contrast agent were obtained at a clinical scanner.

Thermoelektrische Eigenschaften dünner Schichten und Übergitterstrukturen von IV-VI- und V-VI-Verbundhalbleitern

Als ein vielversprechendes Konzept zur Erhöhung der thermoelektrischen Effizienz wird seit Anfang der 90er Jahre die Nutzung niederdimensionaler Systeme angesehen. Aus theoretischen Arbeiten von Hicks und Dresselhaus folgt, dass in ein- und zweidimensionalen Systemen eine Erhöhung der thermoelektrischen Effizienz möglich ist, die einen Durchbruch für die Anwendung thermoelektrischer Wandler zur Folge haben könnte. Die Realisierung solcher niederdimensionaler Systeme ist in geeigneten Mehrlagenstrukturen und durch Verwendung von Halbleiterverbindungen mit unterschiedlicher Energiebandlücke möglich. Ziel des Verbundprojektes Nitherma war es Mehrfachschichtsysteme mit 2-dimensionalem Transportverhalten aus thermoelektrischen Materialien (Pb1-xSrxTe, Bi2(SexTe1-x)3) herzustellen und auf die erwartete hohe thermoelektrische Effizienz zu untersuchen. Diese wurde messtechnischrndurch die Bestimmung der elektrischen Leitfähigkeit, des Seebeck-Koeffizienten und der Wärmeleitfähigkeit parallel zu den Schichtebenen (in-plane-Transporteigenschaft) ermittelt. Ziel dieser Arbeit war einerseits die Verbesserung der Präparations- und Messtechnik bei der Untersuchung der Wärmeleitfähigkeit von Schichten und Schichtsystemen sowie die Demonstration der Reproduzierbarkeit, andererseits die Interpretation der an niederdimensionalen Strukturen ermittelten Transportmessungen. Um den Einfluß der Niederdimensionalität auf die Wärmeleitfähigkeit zu ermitteln, wurden umfangreiche Messungen an unterschiedlich dimensionierten Übergitter- und Multi-Quantum-Well-Strukturen (MQW-Strukturen) durchgeführt. Die Verifizierung der von den Projektpartnern durchgeführten Transportmessungen wurde durch die Messung des Seebeck-Koeffizienten unterstützt.Neben der Charakterisierung durch Transportmessungen erfolgte die Bestimmung der thermoelektrischen Effizienz.

Unveiling electrotransformation of Moraxella catarrhalis as a process of natural transformation

The human respiratory tract pathogen Moraxella catarrhalis is a naturally competent microorganism. However, electrotransformation has long been used to introduce foreign DNA into this organism. This study demonstrated that electrotransformants obtained with linear or circular nonreplicating plasmid DNA originated exclusively from natural transformation processes taking place during the recovery phase after the application of current. Only replicating plasmid DNA could be introduced into M. catarrhalis by electrotransformation, in a type IV pilus-independent manner. Electrotransformation with homologous genomic DNA indicated that restriction of double-stranded DNA was independent of type III restriction-methylation systems. Nontransformability of M. catarrhalis by electrotransformation was observed using double- as well as single-stranded DNA. In addition, the study showed that natural competence is a very constant feature of M. catarrhalis.

Realistic performance prediction in nanostructured solar cells as a function of nanostructure dimensionality and density

The behavior of quantum dot, quantum wire, and quantum well InAs/GaAs solar cells is studied with a very simplified model based on experimental results in order to assess their performance as a function of the low bandgap material volume fraction fLOW. The efficiency of structured devices is found to exceed the efficiency of a non-structured GaAs cell, in particular under concentration, when fLOW is high; this condition is easier to achieve with quantum wells. If three different quasi Fermi levels appear with quantum dots the efficiency can be much higher.

Application of photoreflectance to advanced multilayer structures for photovoltaics

Photoreflectance (PR) is a convenient characterization tool able to reveal optoelectronic properties of semiconductor materials and structures. It is a simple non-destructive and contactless technique which can be used in air at room temperature. We will present experimental results of the characterization carried out by means of PR on different types of advanced photovoltaic (PV) structures, including quantum-dot-based prototypes of intermediate band solar cells, quantum-well structures, highly mismatched alloys, and III?V-based multi-junction devices, thereby demonstrating the suitability of PR as a powerful diagnostic tool. Examples will be given to illustrate the value of this spectroscopic technique for PV including (i) the analysis of the PR spectra in search of critical points associated to absorption onsets; (ii) distinguishing signatures related to quantum confinement from those originating from delocalized band states; (iii) determining the intensity of the electric field related to built-in potentials at interfaces according to the Franz?Keldysh (FK) theory; and (v) determining the nature of different oscillatory PR signals among those ascribed to FK-oscillations, interferometric and photorefractive effects. The aim is to attract the interest of researchers in the field of PV to modulation spectroscopies, as they can be helpful in the analysis of their devices.