Vertically coupled double quantum rings at zero magnetic field

Within local-spin-density functional theory, we have investigated the `dissociation' of few-electron circular vertical semiconductor double quantum ring artificial molecules at zero magnetic field as a function of inter-ring distance. In a first step, the molecules are constituted by two identical quantum rings. When the rings are quantum mechanically strongly coupled, the electronic states are substantially delocalized, and the addition energy spectra of the artificial molecule resemble those of a single quantum ring in the few-electron limit. When the rings are quantum mechanically weakly coupled, the electronic states in the molecule are substantially localized in one ring or the other, although the rings can be electrostatically coupled. The effect of a slight mismatch introduced in the molecules from nominally identical quantum wells, or from changes in the inner radius of the constituent rings, induces localization by offsetting the energy levels in the quantum rings. This plays a crucial role in the appearance of the addition spectra as a function of coupling strength particularly in the weak coupling limit.Comment: 18 pages, 8 figures, submitted to Physical Review

A study of the inclusion of prelayers in InGaN/GaN single- and multiple-quantum-well structures

We report on the effects on the optical properties of blue-light emitting InGaN/GaN single- and multiple-quantum-well structures including a variety of prelayers. For each single-quantum-well structure containing a Si-doped prelayer, we measured a large blue shift of the photoluminescence peak energy and a significant increase in radiative recombination rate at 10 K. Calculations of the conduction and valence band energies show a strong reduction in the built-in electric field across the quantum well (QW) occurs when including Si-doped prelayers, due to enhancement of the surface polarization field which opposes the built-in field. The reduction in built-in field across the QW results in an increase in the electron–hole wavefunction overlap, increasing the radiative recombination rate, and a reduction in the strength of the quantum confined Stark effect, leading to the observed blue shift of the emission peak. The largest reduction of the built-in field occurred for an InGaN:Si prelayer, in which the additional InGaN/GaN interface of the prelayer, in close proximity to the QW, was shown to further reduce the built-in field. Study of multiple QW structures with and without an InGaN:Si prelayer showed the same mechanisms identified in the equivalent single-quantum-well structure.This work was carried out with the financial support of the United Kingdom Engineering and Physical Sciences Research Council under Grant Numbers EP/I012591/1 and EP/H011676/1.This is the accepted manuscript. The final version's available from Wiley at http://dx.doi.org/10.1002/pssb.20145153

Excitons and charged excitons in semiconductor quantum wells

A variational calculation of the ground-state energy of neutral excitons and of positively and negatively charged excitons (trions) confined in a single-quantum well is presented. We study the dependence of the correlation energy and of the binding energy on the well width and on the hole mass. The conditional probability distribution for positively and negatively charged excitons is obtained, providing information on the correlation and the charge distribution in the system. A comparison is made with available experimental data on trion binding energies in GaAs-, ZnSe-, and CdTe-based quantum well structures, which indicates that trions become localized with decreasing quantum well width.Comment: 9 pages, 11 figure

Renormalization approach for quantum-dot structures under strong alternating fields

We develop a renormalization method for calculating the electronic structure of single and double quantum dots under intense ac fields. The nanostructures are emulated by lattice models with a clear continuum limit of the effective-mass and single-particle approximations. The coupling to the ac field is treated non-perturbatively by means of the Floquet Hamiltonian. The renormalization approach allows the study of dressed states of the nanoscopic system with realistic geometries as well arbitrary strong ac fields. We give examples of a single quantum dot, emphasizing the analysis of the effective-mass limit for lattice models, and double-dot structures, where we discuss the limit of the well used two-level approximation.Comment: 6 pages, 7 figure

Quantum phases in artificial molecules

The many-body state of carriers confined in a quantum dot is controlled by the balance between their kinetic energy and their Coulomb correlation. In coupled quantum dots, both can be tuned by varying the inter-dot tunneling and interactions. Using a theoretical approach based on the diagonalization of the exact Hamiltonian, we show that transitions between different quantum phases can be induced through inter-dot coupling both for a system of few electrons (or holes) and for aggregates of electrons and holes. We discuss their manifestations in addition energy spectra (accessible through capacitance or transport experiments) and optical spectra.Comment: 29 pages, 8 figures (ps and eps), LaTeX 2e, ELSART package. To appear in Solid State Communications - Special Issue on Spin Effects in Mesoscopic System

Dissociation of vertical semiconductor diatomic artificial molecules

We investigate the dissociation of few-electron circular vertical semiconductor double quantum dot artificial molecules at 0 T as a function of interdot distance. Slight mismatch introduced in the fabrication of the artificial molecules from nominally identical constituent quantum wells induces localization by offsetting the energy levels in the quantum dots by up to 2 meV, and this plays a crucial role in the appearance of the addition energy spectra as a function of coupling strength particularly in the weak coupling limit.Comment: Accepted for publication in Phys. Rev. Let

A vertical diatomic artificial molecule in the intermediate coupling regime in a parallel and perpendicular magnetic field

We present experimental results for the ground state electrochemical potentials of a few electron semiconductor artificial molecule made by vertically coupling two quantum dots, in the intermediate coupling regime, in perpendicular and parallel magnetic fields up to 5 T. We perform a quantitative analysis based on local-spin density functional theory. The agreement between theoretical and experimental results is good, and the phase transitions are well reproduced.Comment: Typeset using Revtex, 13 pages and 8 Postscript figure

Organic matter cycling along geochemical, geomorphic and disturbance gradients in forests and cropland of the African Tropics – Project TropSOC Database Version 1.0

The African Tropics are hotspots of modern-day land-use change and are, at the same time, of great relevance for the cycling of carbon (C) and nutrients between plants, soils and the atmosphere. However, the consequences of land conversion on biogeochemical cycles are still largely unknown as they are not studied in a landscape context that defines the geomorphic, geochemically and pedological framework in which biological processes take place. Thus, the response of tropical soils to disturbance by erosion and land conversion is one of the great uncertainties in assessing the carrying capacity of tropical landscapes to grow food for future generations and in predicting greenhouse gas fluxes (GHG) from soils to the atmosphere and, hence, future earth system dynamics. Here, we describe version 1.0 of an open access database created as part of the project &ldquo;Tropical soil organic carbon dynamics along erosional disturbance gradients in relation to variability in soil geochemistry and land use&rdquo; (TropSOC). TropSOC v1.0 contains spatial and temporal explicit data on soil, vegetation, environmental properties and land management collected from 136 pristine tropical forest and cropland plots between 2017 and 2020 as part of several monitoring and sampling campaigns in the Eastern Congo Basin and the East African Rift Valley System. The results of several laboratory experiments focusing on soil microbial activity, C cycling and C stabilization in soils complement the dataset to deliver one of the first landscape scale datasets to study the linkages and feedbacks between geology, geomorphology and pedogenesis as controls on biogeochemical cycles in a variety of natural and managed systems in the African Tropics. The hierarchical and interdisciplinary structure of the TropSOC database allows for linking a wide range of parameters and observations on soil and vegetation dynamics along with other supporting information that may also be measured at one or more levels of the hierarchy. TropSOC&rsquo;s data marks a significant contribution to improve our understanding of the fate of biogeochemical cycles in dynamic and diverse tropical African (agro-)ecosystems. TropSOC v1.0 can be accessed through the supplementary material provided as part of this manuscript or as a separate download via the websites of the Congo Biogeochemistry observatory and the GFZ data repository where version updates to the database will be provided as the project develops.</p

Localization, disorder, and polarization fields in wide-gap semiconductor quantum wells

In der vorliegenden Arbeit werden verschiedene Aspekte des Einflusses von Lokalisation, Unordnung und Polarisationsfeldern auf Elektron-Loch Zustände in Quantengräben (QWs von engl. quantum wells) aus Halbleitern mit großer Bandlücke theoretisch untersucht. Unter Verwendung eines Schwerpunktseparationsansatzes wird das Verhalten von QW Exzitonen und Biexzitonen im Grenzfall schwacher Lokalisation beschrieben. Es zeigt sich, daß die Lokalisationsenergie des Biexzitons mehr als doppelt so groß ist wie die des Exzitons. Dies wird verursacht durch ein universelles Gesetz der Lokalisation in schwachen zwei-dimensionalen Potentialen, welches lediglich durch das "Potentialvolumen" und die Masse des lokalisierten Teilchens bestimmt wird. Ein einfaches Modell des QW Biexzitons wird entwickelt, dessen Ergebnisse gut mit jenen übereinstimmen, die man mit Hilfe eines aufwendigeren numerischen Modells erhält. Der Grenzfall starker Lokalisation von QW Exzitonen und höheren Exzitonenkomplexen wird mittels einer Dichtefunktionalrechnung untersucht. Es wird gezeigt, daß Zustände bis mindestens zum X4 in den nm-großen Potentialminima lokalisieren können, die durch Phasenseparation in (In,Ga)N/GaN QWs enstehen. Es wird das Übergangsspektrum des sukzessiven Zerfalls eines lokalisierten X4 berechnet. Auf Grundlage der selbstkonsistenten Lösung von Poisson- und Schrödinger-Gleichung wird der Einfluß des Probendesigns von (In,Ga)N/GaN QW-Strukturen auf den makroskopischen Verlauf des Polarisationsfeldes in Wachstumsrichtung und somit auf optische Übergangsenergie und Oszillatorstärke systematisch untersucht. Besondere Bedeutung kommt dabei der Abschirmung der Felder durch Raumladungszonen zu. Es wird gezeigt, daß die Position des QW bezüglich einer ausgedehnten Oberflächen-Verarmungszone - die in n-dotierten, Ga-polarisierten Proben auftritt - erheblichen Einfluß auf Übergangsenergie und Oszillatorstärke hat. Durch die räumliche Variation der Polarisationsfeldstärke in dieser Verarmungszone kann das optische Übergangsspektrum eines Mehrfach-QW Schultern oder mehrere Maxima aufweisen. Indium Oberflächen-Segregation ruft eine Blauverschiebung der Übergangsenergie hervor, die bis zu einem Drittel der vom Polarisationsfeld verursachten quantum confined Stark-Verschiebung kompensiert. Diese Blauverschiebung wird von einer Verringerung des Elektron-Loch Überlapps begleitet. Die Polarisationsfelder in (In,Ga)N/GaN Mehrfach-QWs verschmieren das stufenförmige Einteilchen-Absorptionsspektrum. Durch die Aufhebung der näherungsweisen Diagonalität von Inter-Subband Übergängen und durch die Miniband-Dispersion in höheren, gekoppelten Zuständen haben diese Felder, neben dem Beitrag von Potentialfluktuationen, einen entscheidenden Einfluß auf die Form des Absorptionsspektrums. Ein in der Literatur diskutierter Mechanismus, der allein durch Polarisationsfelder eine Verbreiterung optischer Spektren hervorruft, kann nicht bestätigt werden. Unter Annahme einer unkorrelierten Zusammensetzung von (In,Ga)N und einer lateral korrelierten Grenzflächenrauhigkeit von einer Monolage in jeder Grenzfläche zeigt die spektrale Breite des Exzitonen-Schwerpunktpotentials eine Verschmälerung mit zunehmendem Feld. Diese wird verursacht durch das Eindringen der Teilchen in die binären Barrieren und durch ein vergrößertes Exzitonenvolumen. Im Fall einer langreichweitigen Grenzfächenrauhigkeit findet man eine Aufspaltung des Spektrums in einzelne Linien.In this thesis, various aspects of the influence of localization, disorder, and polarization fields on electron-hole states in wide-gap semiconductor quantum wells (QWs) are investigated theoretically. A theoretical treatment of quantum well exciton and biexciton states in the limit of weak localization is presented, using a center-of-mass separation ansatz. It shows that the localization energy of the biexciton is more than twice as large as that of the exciton due to the universal behaviour of localization in weak two-dimensional potentials which is ruled only by the potential "volume" and the mass of the localized particle. A useful simple model of the QW biexciton wavefunction is developed which provides good agreement with the results obtained with an extensive numerical solution. The limit of strong localization of QW excitons and higher exciton complexes is investigated with a density functional calculation. It is demonstrated that states at least up to X4 may localize in nm-scale potential boxes caused by indium phase separation in (In,Ga)N/GaN QWs. The transition spectrum of the successive recombination of a localized X4 is calculated. A systematic investigation of the influence of the sample design of (In,Ga)N/GaN QW structures on optical transition energy and oscillator strength reveals the importance of space charge layers with regard to screening of polarization fields along the QW-axis. Based on a self-consistent solution of the Schrödinger-Poisson equations, the overall situation of the macroscopic spontaneous and piezoelectric polarization fields is discussed in dependence on various substantial sample parameters. It is found that the position of a QW in the sample with respect to an extended surface depletion layer - which is shown to exist in n-type Ga-face grown material - severely affects transition energy and electron-hole overlap. Due to the spatial variation of the field strength in this surface depletion layer, the optical transition spectrum of a Ga-face grown multiple-QW can display shoulders or even a multiple-peak structure. Indium surface segregation results in a blueshift of the transition energy compensating up to one third of the quantum confined Stark shift produced by the polarization field. This blueshift is accompanied by a decrease of the electron-hole overlap. Polarization fields in (In,Ga)N/GaN multiple-QWs result in a smoothing of the step-like single-particle absorption spectrum. Apart from the contribution of compositional fluctuations, the fields have significant influence on the shape of the spectrum via the abrogation of the nearly diagonality of inter-subband transitions and via the mini-band dispersion of higher coupled states in case of a periodic structure. A line broadening-mechanism due to polarization fields in (In,Ga)N/GaN QWs, as sometimes discussed in literature, could not be confirmed. Assuming uncorrelated (In,Ga)N alloy and in-plane-correlated interface roughness of one monolayer in each interface, the calculation of the spectral width of the QW exciton center-of-mass potential yields a narrowing with increasing average field. This is a result of the penetration of the carriers into the barriers and of an increasing exciton volume. In case of a long-range interface roughness, a splitting of the spectrum into individual lines can be predicted

OFDM für die Datenübertragung über Glasfasern bei hoher spektraler Effizienz

Data traffic within networks worldwide increases exponentially – different authors give different numbers; some mention growth rates up to 50 percent per year. This increasing traffic load gives rise for various kinds of challenges for the network operators. Especially, they have to offer higher bitrates in the core networks, which work on the basis of optical data transmission over silica fibers. There have been many major steps in technical progress which allowed for increasing the datarates per wavelength to 10 and 40 GBit/s within the last decade. Currently, transponders offering 100 GBit/s become available. The new equipment generation is facilitated by coherent receivers, which enable linear downconversion of the optical receive signal to the electrical baseband. Signal distortion can be compensated for with the help of digital signal processing. Furthermore, two data streams can be transmitted on orthogonally polarized optical carriers of the same frequency (polarization-division multiplexing, PDM). In the future, a further raise of bitrates is enabled by increasing the spectral efficiency. In this context, orthogonal frequency-division multiplexing (OFDM) is a promising technique as it offers efficient compensation of linear signal distortion caused by dispersive channels. This work investigates the usage of OFDM for high bitrate data transmission over silica fibers at high spectral efficiency. For this purpose, at first the relevant sources of signal distortion in the optical waveguide are discussed. Linear dispersive effects determine how to choose the OFDM system parameters. Polarization dependent loss causes a difference between the signal-to-noise power ratios of both PDM channels. By means of polarization-time coding the influence on the bit error ratio can be delimited. When implementing OFDM special attention has to be payed to non-linear effects. Most dominantly the fiber channel is affected by the Kerr effect causing non-linear signal distortion along with crosstalk between channels at different wavelengths. In this work the transmission system shall be modeled as a weakly non-linear system implying that non-linear signal distortion is considered as a noise-like additive distortion. This approach allows for an estimation of the channel capacity – and thereby approximating the maximum achievable spectral efficiency. Moreover, in this system model sources of distortion can be included, which are caused by the non-ideal implementation of transmitters and receivers. Laser phase noise, clipping and quantization turn out to be crucial effects. In the weakly non-linear system model their contributions are regarded as further uncorrelated sources of noise, and thus, the estimated noise variances are increased by respective summands. Doing so, the estimation of the channel capacity can be repeated and it turns out that the mentioned impairments delimit the achievable spectral efficiency especially for short transmission distances. The power of the Kerr effect induced distortion can be separated into two contributions: The first one is only considering non-linear signal distortion of the useful signal. The second part corresponds to non-linear crosstalk of neighboring channels. System parameters are identified for which the first contribution dominates. For that scenario methods are discussed which help to compensate non-linear signal distortion leading to improved transmission performance.Der Datenverkehr in den weltweiten Netzen wächst exponentiell – verschiedene Quellen nennen unterschiedliche Zahlen, wobei Zuwächse von bis zu 50% pro Jahr ermittelt werden. Aus diesem zunehmenden Datenaufkommen ergeben sich viele Herausforderungen für die Netzbetreiber. Insbesondere müssen ständig höhere Datenraten in den Kernnetzen, welche mit optischer Übertragung über Glasfasern arbeiten, zur Verfügung gestellt werden. Es gab viele technologische Entwicklungen, die im letzten Jahrzehnt eine Steigerung der Datenrate pro Wellenlänge auf 10 bzw. 40 GBit/s erlaubten. Aktuell werden die ersten Geräte mit 100 GBit/s verfügbar. Die letzte Erhöhung wird vor allem durch den Einsatz von kohärenten Empfängern ermöglicht. Dabei wird das optische Empfangssignal linear umgesetzt in ein elektrisches Basisbandsignal. Signalverzerrungen werden mittels digitaler Signalverarbeitung kompensiert. Ferner können zwei Datensignale auf orthogonal polarisierten optischen Trägern gleicher Frequenz transportiert werden (Polarisationsmultiplex, PolMux ). In der Zukunft wird eine weitere Erhöhung der Datenraten möglich, indem die spektrale Effizienz gesteigert wird. In diesem Kontext ist “orthogonal frequency-division multiplexing” (OFDM) eine interessante Option, da diese Technik eine effiziente Kompensation von Signalverzerrungen, die durch dispersive Kanäle hervorgerufen werden, erlaubt. Die vorliegende Arbeit untersucht den Einsatz von OFDM zur hochbitratigen Datenübertragung über Glasfasern bei hoher spektraler Effizienz. Dazu werden zunächst die relevanten Fasereffekte diskutiert. Die linearen dispersiven Effekte bestimmen die Systemparameterwahl. Es zeigt sich, dass polarisationsabhängige Dämpfung eine Verschiebung der Störabstände der beiden PolMux-Teilkanäle verursacht. Mittels “polarization-time coding” kann die Auswirkung auf die Fehlerrate eingegrenzt werden. Besondere Beachtung beim Einsatz von OFDM muss nichtlinearen Effekten gelten. Für den optischen Faserkanal ist insbesondere der Kerr-Effekt zu nennen, der zu nichtlinearen Signalverzerrung und Übersprechen zwischen Nachbarkanälen auf Trägern unterschiedlicher Wellenlänge führt. In dieser Arbeit wird das OFDM-Übertragungssystem als schwach nichtlineares System modelliert und nichtlineare Signalverzerrungen als additive Störung charakterisiert. Dieser Ansatz erlaubt eine Abschätzung der Kanalkapazität und damit der maximal erreichbaren spektralen Effizienz. Auch gestattet das Systemmodell die Hinzunahme von Störungen, die bedingt sind durch nichtideale Implementierung von Sender und Empfänger. Als kritisch stellen sich Laserphasenrauschen, Amplitudenlimitierung und Quantisierung heraus. Die verursachten Signalstörungen werden in der Systemmodellierung als weitere Quelle für Rauschen betrachtet und die geschätzten Störleistungen erhalten entsprechend weitere Beiträge. Damit kann die Abschätzung der Kanalkapazität wiederholt werden und es zeigt sich, dass die genannten Effekte die erreichbare spektrale Effizienz insbesondere auf kurzen Übertragungsstrecken limitieren. Die Leistung des vom Kerr-Effekt hervorgerufenen Störsignals lässt sich separieren in einen Anteil, der lediglich nichtlineare Verzerrungen des eigentlichen Nutzsignals betrifft und einen weiteren Beitrag, der durch nichtlineares Übersprechen hervorgerufen wird. Es werden Systemparameter identifiziert, bei denen der erstgenannte Effekt überwiegt. Für diesen Fall werden Methoden zur Kompensation der nichtlinearen Störung diskutiert, durch welche die Leistungsfähigkeit des Systems verbessert werden kann