Dissertations / Theses on the topic 'Quantum nanostructures'
To see the other types of publications on this topic, follow the link: Quantum nanostructures.
Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles
Consult the top 50 dissertations / theses for your research on the topic 'Quantum nanostructures.'
Next to every source in the list of references, there is an 'Add to bibliography' button. Press on it, and we will generate automatically the bibliographic reference to the chosen work in the citation style you need: APA, MLA, Harvard, Chicago, Vancouver, etc.
You can also download the full text of the academic publication as pdf and read online its abstract whenever available in the metadata.
Browse dissertations / theses on a wide variety of disciplines and organise your bibliography correctly.
1
Todorov, Tchavdar N. "Quantum transport in nanostructures." Thesis, University of Oxford, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.334909.
Full text
2
Leadbeater, Mark. "Quantum dynamics of superconducting nanostructures." Thesis, Lancaster University, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.337369.
Full text
3
Barbosa, Jose Camilo. "Quantum transport in semiconductor nanostructures." Thesis, University of Warwick, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.263288.
Full text
4
Reina, Estupin̄án John-Henry. "Quantum information processing in nanostructures." Thesis, University of Oxford, 2002. http://ora.ox.ac.uk/objects/uuid:6375c7c4-ecf6-4e88-a0f5-ff7493393d37.
Full textAbstract:
Since information has been regarded as a physical entity, the field of quantum information theory has blossomed. This brings novel applications, such as quantum computation. This field has attracted the attention of numerous researchers with backgrounds ranging from computer science, mathematics and engineering, to the physical sciences. Thus, we now have an interdisciplinary field where great efforts are being made in order to build devices that should allow for the processing of information at a quantum level, and also in the understanding of the complex structure of some physical processes at a more basic level. This thesis is devoted to the theoretical study of structures at the nanometer-scale, "nanostructures," through physical processes that mainly involve the solid-state and quantum optics, in order to propose reliable schemes for the processing of quantum information. Initially, the main results of quantum information theory and quantum computation are briefly reviewed. Next, the state-of-the-art of quantum dots technology is described. In so doing, the theoretical background and the practicalities required for this thesis are introduced. A discussion of the current quantum hardware used for quantum information processing is given. In particular, the solid-state proposals to date are emphasised. A detailed prescription is given, using an optically-driven coupled quantum dot system, to reliably prepare and manipulate exciton maximally entangled Bell and Greenberger-Horne-Zeilinger (GHZ) states. Manipulation of the strength and duration of selective light-pulses needed for producing these highly entangled states provides us with crucial elements for the processing of solid-state based quantum information. The all-optical generation of states of the so-called Bell basis for a system of two quantum dots (QDs) is exploited for performing the quantum teleportation of the excitonic state of a dot in an array of three coupled QDs. Theoretical predictions suggest that several hundred single quantum bit rotations and controlled-NOT gates could be performed before decoherence of the excitonic states takes place. In addition, the exciton coherent dynamics of a coupled QD system confined within a semiconductor single mode microcavity is reported. It is shown that this system enables the control of exciton entanglement by varying the coupling strength between the optically-driven dot system and the microcavity. The exciton entanglement shows collapses and revivals for suitable amplitudes of the incident radiation field and dot-cavity coupling strengths. The results given here could offer a new approach for the control of decoherence mechanisms arising from entangled "artificial molecules." In addition to these ultrafast coherent optical control proposals, an approach for reliable implementation of quantum logic gates and long decoherence times in a QD system based on nuclear magnetic resonance (NMR) is given, where the nuclear resonance is controlled by the ground state "magic number" transitions of few-electron QDs in an external magnetic field. The dynamical evolution of quantum registers of arbitrary length in the presence of environmentally-induced decoherence effects is studied in detail. The cases of quantum bits (qubits) coupling individually to different environments ("independent decoherence"), and qubits interacting collectively with the same reservoir ("collective decoherence") are analysed in order to find explicit decoherence functions for any number of qubits. The decay of the coherences of the register is shown to strongly depend on the input states: this sensitivity is a characteristic of both types of coupling (collective and independent) and not only of the collective coupling, as has been reported previously. A non-trivial behaviour - "recoherence" - is found in the decay of the off-diagonal elements of the reduced density matrix in the specific situation of independent decoherence. The results lead to the identification of decoherence-free states in the collective decoherence limit. These states belong to subspaces of the system's Hilbert space that do not become entangled with the environment, making them ideal elements for the engineering of "noiseless" quantum codes. The relations between decoherence of the quantum register and computational complexity based on the new dynamical results obtained for the register density matrix are also discussed. This thesis concludes by summarising and pointing out future directions, and in particular, by discussing some biological resonant energy transfer processes that may be useful for the processing of information at a quantum level.
5
Shortell, Matthew P. "Zinc oxide quantum dot nanostructures." Thesis, Queensland University of Technology, 2014. https://eprints.qut.edu.au/76335/4/Matthew_Shortell_Thesis.pdf.
Full textAbstract:
Zinc oxide (ZnO) is one of the most intensely studied wide band gap semiconductors due to its many desirable properties. This project established new techniques for investigating the hydrodynamic properties of ZnO nanoparticles, their assembly into useful photonic structures, and their multiphoton absorption coefficients for excitation with visible or infrared light rather than ultraviolet light. The methods developed are also applicable to a wide range of nanoparticle samples.
6
Nemec, Norbert. "Quantum transport in carbon-based nanostructures." [S.l.] : [s.n.], 2007. http://deposit.ddb.de/cgi-bin/dokserv?idn=985358963.
Full text
7
Smeeton, Timothy Michael. "The nanostructures of InGaN quantum wells." Thesis, University of Cambridge, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.614901.
Full text
8
Boese, Daniel. "Quantum transport through nanostructures : quantum dots, molecules, and quantum wires = Quantentransport durch Nanostrukturen /." Aachen : Shaker, 2002. http://swbplus.bsz-bw.de/bsz096321318abs.htm.
Full text
9
Wesslén, Carl-Johan. "Many-Body effects in Semiconductor Nanostructures." Licentiate thesis, Stockholms universitet, Fysikum, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-102344.
Full textAbstract:
Low dimensional semiconductor structures are modeled using techniques from the field of many-body atomic physics. B-splines are used to create a one-particle basis, used to solve the more complex many-body problems. Details on methods such as the Configuration Interaction (CI), Many-Body Perturbation Theory (MBPT) and Coupled Cluster (CC) are discussed. Results from the CC singles and doubles method are compared to other high-precision methods for the circular harmonic oscillator quantum dot. The results show a good agreement for the energy of many-body states of up to 12 electrons. Properties of elliptical quantum dots, circular quantum dots, quantum rings and concentric quantum rings are all reviewed. The effects of tilted external magnetic fields applied to the elliptical dot are discussed, and the energy splitting between the lowest singlet and triplet states is explored for varying geometrical properties. Results are compared to experimental energy splittings for the same system containing 2 electrons.
10
Pang, Hongliang, and 庞鸿亮. "Quantum control of spins in semiconductor nanostructures." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2014. http://hdl.handle.net/10722/208042.
Full textAbstract:
Spins localized in semiconductor nanostructures have been intensively investigated for quantum spintronics. These include the spin of single electron localized by quantum dots or impurities, and spins of the lattice nuclei. These localized spins can be exploited as carriers of quantum information, while in some circumstances they also play the role of deleterious noise sources for other quantum objects through their couplings. Quantum control of the spins in semiconductor nanostructures is therefore of central interest for quantum applications. In this thesis, we address several problems related to the quantum control of electron or hole spin and nuclear spins in semiconductor quantum dots and impurity centers. The first problem studied is the control of nuclear spin bath for a hole spin qubit in III-V semiconductor quantum dot. In quantum dots formed on III-V compounds, the direct band gap of the host material allows ultrafast optical addressability of a single electron or hole spin qubit. However, nonzero nuclear spins of group III and group V elements result in a large statistical fluctuation in the Zeeman splitting of the spin qubit which then dephases in nanosecond time scale. We present a novel feedback scheme to suppress the statistical fluctuation of the nuclear spin field for enhancing the coherence time of the hole spin qubit. We also find positive feedback control which can amplify the magnitude of the nuclear field, so that a bimodal distribution can develop, realizing a quantum environment that can not be described by a single temperature. The second problem addressed here is the control of donor spin qubits in silicon architecture which have ultra-long quantum coherence time. We developed the quantum control scheme to realize the quantum metrology of magnetic field gradient, based on the celebrated Kane’s architecture for quantum computation. The scheme can also be generalized to calibrate the locations of the donors. In the third part of the thesis, we investigate a novel type of quantum dot formed in a new class of two-dimensional semiconductors, monolayer transition metal dichalcogenides (TMDs), which exhibit interesting spin and pseudospin physics. This novel quantum dot system may offer new opportunity for quantum spintronics in the ultimate 2D limit, and we investigate here the valley pseudospin as a possible quantum bit carrier. A main finding is that, contrary to the intuition, the lateral confinement by the quantum dot potential does not lead to noticeable valley hybridization, and therefore the valley pseudospin in monolayer TMDs QD can well inherit the valley physics such as the valley optical selection rules from the 2D bulk which implies a variety of quantum control possibilities.published_or_final_version
Physics
Doctoral
Doctor of Philosophy
11
Dellow, Mark Winston. "Quantum and classical transport in semiconductor nanostructures." Thesis, University of Nottingham, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.334765.
Full text
12
Lee, Young-Su Ph D. Massachusetts Institute of Technology. "Electronic structure and quantum conductance of nanostructures." Thesis, Massachusetts Institute of Technology, 2006. http://hdl.handle.net/1721.1/37371.
Full textAbstract:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006.Includes bibliographical references (p. 145-158).
This thesis is dedicated to development and application of a novel large-scale first-principles approach to study the electronic structure and quantum conductance of realistic nanoscale materials. Electron transport at the nanometer scale involves phenomena which are beyond the realm of classical transport theory: the wave character of the electrons becomes central, and the Schrddinger equation needs to be solved explicitly. First-principles calculations can nowadays deal with systems containing hundreds of electrons, but simulations for nanostructures that contain thousands of atoms or more need to rely on parametrized Hamiltonians. The core of our approach lies in the derivation of exact and chemically-specific Hamiltonians from first-principles calculations, in a basis of maximally-localized Wannier functions, that become explicit tight-binding orbitals. Once this optimal basis is determined, the Hamiltonian matrix becomes short-ranged, diagonally-dominant, and transferable - i.e. a large nanostructure can be constructed by assembling together the Hamiltonians of its constitutive building block. This approach is first demonstrated for pristine semiconducting and metallic nanotubes, demonstrating perfect agreement with full first-principles calculations in a complete planewave basis.
(cont.) Then, it is applied to study the electronic structure and quantum conductance of functionalized carbon nanotubes. The first class of functionalizing addends, represented by single-bond covalent ligands (e.g. hydrogens or aryls), turns out to affect very strongly the back-scattering and the conductance, since sp3 rehybridization at the sidewall carbon where a group is attached dramatically perturbs the conjugated [pi]-bonding network. Inspection on the shape and the on-site energy of MLWFs before and after functionalizations leads to the conclusion that the effect of sp3 rehybridization is essentially identical to removing a "half-filled" p-orbital from the [pi]-manifold. In this perspective, the chemical difference between functional groups (e.g. different electronegativity of the residues) is relatively minor, even if, of course, will lead to different doping of the tube. We also find that these single-bond ligands tend to cluster, and are more stable when two groups are located nearby (incidentally, the degree of perturbation at the Fermi level becomes weaker when such paired configuration is assumed). The second class of functionalizing addends, represented by cycloaddition functionalizations (e.g. carbenes and nitrenes), demonstrates a radically different behavior.
(cont.) These addends are bonded to two neighboring sidewall carbon atoms, creating a three-membered ring structure. On narrow-diameter tubes, cleaving of the sidewall bond takes place to release the high strain energy of a three-membered ring. In the process, the two sidewall carbons recover their original sp2 hybridization. This step is crucial, since the quantum conductance of a metallic nanotube then recovers almost perfectly the ideal limit of a pristine tube: the bond cleavage restores a transparent conduction manifold. Bond cleavage is controlled by the chemistry of the functional groups and the curvature of the nanotubes. High-curvature favors bond opening, whereas in graphene the bond is always closed; in between the two limits, chemistry determines the critical curvature at which the open-to-closed transition takes place. The preference for bond opening or closing has been screened extensively for different classes of functional groups, using initially some molecular homologues of the nanotubes. It is found that a subclass of addends, exemplified by dicyanocarbene, can assume both the open and closed form in the same tube around a narrow range of diameters.
(cont.) While these two forms are very similar in energy, and separated by a small barrier (hence they can be considered "fluxional" tautomers), the quantum conductance in the closed case is found to be significantly lower than that in the open case. Interconversion between the two minima could then be directed by optical or electrochemical means, in turn controlling the conductance of the functionalized tubes. We envision thus that this novel class of functionalization will offer a practical way toward non-destructive chemistry that can either preserve the metallic conductance of the tubes, or modulate it in real-time, with foreseeable applications in memories, sensors, imaging, and optoelectronic devices.
by Young-Su Lee.
Ph.D.
13
Toft, Ian. "Fibre optic micro-photoluminescence of quantum nanostructures." Thesis, University of Cambridge, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.614103.
Full text
14
Schönherr, Piet. "Growth and characterisation of quantum materials nanostructures." Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:7dca792e-4236-4d19-aa59-7c9c3cb5d0e4.
Full textAbstract:
The three key areas of this thesis are crystal synthesis strategies, growth mechanisms, and new types of quantum materials nanowires. The highlights are introduction of a new catalyst (TiO2) for nanowire growth and application to Bi2Se3, Bi2Te3, SnO2, and Ge nanowires; demonstration of step-flow growth, a new growth mechanism, for Bi2Te3 sub-micron belts; and the characterisation of the first quasi-one dimensional topological insulator (orthorhombic Sb-doped Bi2Se3) and topological Dirac semimetal nanowires (Cd3As2). Research into new materials has been one of the driving forces that have contributed to the progress of civilisation from the Bronze Age four thousand years ago to the age of the semiconductor in the 20th century. At the turn to the 21st century novel materials, so-called quantum materials, started to emerge. The fundamental theories for the description of their properties were established at the beginning of the 20th century but expanded significantly during the last three decades based, for example, on a new interpretation of electronic states by topological invariants. Hence, topological insulator (TI) materials such as mercury-telluride are one manifestation of a quantum material. In theory, TIs are characterised by an insulating interior and a surface with spin-momentum locked conduction. In real crystals, however, the bulk can be conducting due to crystal imperfections. Nanowires suppress this bulk contribution inherently by their high surface-to-volume ratio. Additionally, trace impurity elements can be inserted into the crystal to decrease the conductance further. These optimised TI nanowires could provide building blocks for future electronic nanodevices such as transistors and sensors. Initial synthesis efforts using vapour transport techniques and electronic transport studies showed that TI nanowires hold the promise of reduced bulk contribution. This thesis expands the current knowledge on synthesis strategies, crystal growth mechanisms, and new types of quantum materials nanowires. Traditionally, gold catalyst nanoparticles were used to grow TI nanowires. We demonstrate that they are suitable to produce large amounts of nanowires but have undesired side-effects. If a metaloxide catalyst nanoparticle is used instead, quality and even quantity are significantly improved. This synthesis strategy was used to produce a new TI which is built from chains of atoms and not from atomic layers as in case of previously known TIs. The growth of large nanowires with a layered crystal structure leads to step-flowgrowth, an intriguing phenomenon in the growth mechanism: New layers grow on top of previous layers with a single growth frontmoving fromthe root to the tip. These wires are ideal for further electronic characterisation that requires large samples. The nanowire growth of tin-oxide will also be discussed, a side project that arose from my growth studies, which is useful for sensor applications. Under certain conditions it forms tree-like structures in a single synthesis step. All of the aforementioned growth studies are carried out at atmospheric pressure. A separate growth study is carried out in ultra-high vacuum to assess the transferability of the growth process towards the cleanliness requirements of the semiconductor industry. If two quantum materials are joined together, exotic physics may emerge at the interface. One of the goals of TI research is the experimental observation of Majorana fermions, exotic particles which are their ownantiparticles with potential applications in quantum computing that may appear in superconductor/TI hybrid structures. We have synthesised such structures and initial characterisation suggests that the resistivity increases when they are cooled below the critical temperature of the superconductor. Beyond TIs, a new type of quantum material, called a topological Dirac semimetal, opens new realms of exotic physics to be discovered. Nanowires are grownfroma material which has recently been discovered to be a topological Dirac semimetal. Their growth mechanism is characterised and an extremely high electron mobility at room temperature is measured. The contribution of this thesis to the field is summarised in Fig. 1. Its core is the study of the growth mechanism of quantum materials which will be vital for future development of applications and fundamental research.
15
Vezvaee, Arian. "Quantum spins in semiconductor nanostructures: Hyperfine interactions and optical control." Diss., Virginia Tech, 2021. http://hdl.handle.net/10919/104870.
Full textAbstract:
Quantum information technologies offer significantly more computational power for certain tasks and secure communication lines compared to the available classical machines. In recent years there have been numerous proposals for the implementation of quantum computers in several different systems that each come with their own advantages and challenges. This dissertation primarily focuses on challenges, specifically interactions with the environment, and applications of two of such systems: Semiconductor quantum dots and topological insulators. The first part of the dissertation is devoted to the study of semiconductor quantum dots as candidates for quantum information storage and sources of single-photon emission. The spin of the electron trapped in a self-assembled quantum dot can be used as a quantum bit of information for quantum technology applications. This system possesses desirable photon emission properties, including efficiency and tunability, which make it one of the most advanced single-photon emitters. This interface is also actively explored for the generation of complex entangled photonic states with applications in quantum computing, networks, and sensing. First, an overview of the relevant developments in the field will be discussed and our recent contributions, including protocols for the control of the spin and a scheme for the generation of entangled photon states from coupled quantum dots, will be presented. We then look at the interaction between the electron and the surrounding nuclear spins and describe how its interplay with optical driving can lead to dynamic nuclear polarization. The second part of the dissertation follows a similar study in topological insulators: The role of time-reversal breaking magnetic impurities in topological materials and how spinful impurities enable backscattering mechanisms by lifting the topological protection of edge modes. I will present a model that allows for an analytical study of the effects of magnetic impurities within an experimental framework. It will be discussed how the same platform also enables a novel approach for applications of spintronics and quantum information, such as studying the entanglement entropy between the impurities and chiral modes of the system.Doctor of Philosophy
Quantum information science has received special attention in recent years due to its promising advantages compared to classical machines. Building a functional quantum processor is an ongoing effort that has enjoyed enormous advancements over the past few years. Several different condensed matter platforms have been considered as potential candidates for this purpose. This dissertation addresses some of the major challenges in two of the candidate platforms: Quantum dots and topological insulators. We look at methods for achieving high-performance optical control of quantum dots. We further utilize quantum dots special ability to emit photons for specific quantum technology applications. We also address the nuclear spin problem in these systems which is the main source of destruction of quantum information and one of the main obstacles in building a quantum computer. This is followed by the study of a similar problem in topological insulators: Addressing the interaction with magnetic impurities of topological insulators. Included with each of these topics is a description of relevant experimental setups. As such, the studies presented in this dissertation pave the way for a better understanding of the two major obstacles of hyperfine interactions and the optical controllability of these platforms.
16
Wen, Xiaoming, and n/a. "Ultrafast spectroscopy of semiconductor nanostructures." Swinburne University of Technology, 2007. http://adt.lib.swin.edu.au./public/adt-VSWT20070426.110438.
Full textAbstract:
Semiconductor nanostructures exhibit many remarkable electronic and optical properties.
The key to designing and utilising semiconductor quantum structures is a physical understanding
of the detailed excitation, transport and energy relaxation processes. Thus the nonequilibrium
dynamics of semiconductor quantum structures have attracted extensive attention in recent years.
Ultrafast spectroscopy has proven to be a versatile and powerful tool for investigating transient
phenomena related to the relaxation and transport dynamics in semiconductors.
In this thesis, we report investigations into the electronic and optical properties of various
semiconductor quantum systems using a variety of ultrafast techniques, including up-conversion
photoluminescence, pump-probe, photon echoes and four-wave mixing. The semiconductor
quantum systems studied include ZnO/ZnMgO multiple quantum wells with oxygen ion
implantation, InGaAs/GaAs self-assembled quantum dots with different doping, InGaAs/InP
quantum wells with proton implantation, and silicon quantum dots. The spectra of these
semiconductor nanostructures range from the ultraviolet region, through the visible, to the
infrared. In the UV region we investigate excitons, biexcitons and oxygen implantation effects in
ZnO/ZnMgO multi-quantum wells using four-wave mixing, pump-probe and photoluminescence
techniques. Using time-resolved up-conversion photoluminescence, we investigate the relaxation
dynamics and state filling effect in InGaAs self-assembled quantum dots with different doping,
and the implantation effect in InGaAs/InP quantum wells. Finally, we study the optical properties
of silicon quantum dots using time-resolved photoluminescence and photon echo spectroscopy on
various time scales, ranging from microseconds to femtoseconds.
17
Smeu, Manuel. "Quantum transport modeling of atomic nanostructures on silicon." Thesis, McGill University, 2012. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=107818.
Full textAbstract:
Surface effects can adversely influence the performance of a nanoelectronic device,but may also lead to new functionality. The focus of this thesis is to theoreticallystudy the role of surfaces in nanoelectronics. Our theoretical analysis is from atomicfirst principles achieved by combining density functional theory with the Keldyshnonequilibrium Green's function approach. This technique allows for all atoms in asystem to be treated on an equal footing without any phenomenological parameters.The first part of the thesis considers conduction through a molecule with no substrateto illustrate the sort of system typically modeled in transport calculations. Two Auelectrodes are bridged by a substituted benzenediamine molecule (R = CH3, NH2,OH) where an H atom is removed to form a radical that may behave as a spin filter,depending on the R group. Next is considered a π–stacked line of ethylbenzenemolecules on the Si(100) surface, where the Si atoms are explicitly included in thecalculation. Although the molecules conduct electrons at certain energies, a channeloccurs through the substrate, which can dominate the conductance. The use ofsubstituent groups to modulate the electron transport properties of such wires is alsoinvestigated, showing that the conductance of the molecular wire could be tuned todominate over the substrate. Finally, the conductance of the Si(111)–7 × 7 metallicsurface is studied. Inspired by experiments suggesting that atomic steps reduce thesurface conductance, the atomic structure and transport properties of such steps areexamined, revealing that dimer atom buckling along the step edges is the primaryculprit since it leads to an opening of a local band gap at the step.Les effets de surface peuvent affecter la performance d'un dispositif nanoélectronique, mais peuvent aussi conduire à de nouvelles fonctionnalités. L'objectif de cette thèse est d'effectuer une étude théorique sur le rôle des surfaces en nanoélectronique. Notre analyse, de type premiers principes atomiques, est effectuée en combinant la théorie de la fonctionnelle de la densité avec les fonctions de Green hors-équilibre. Cette technique permet de traiter tous les atomes de manière égale sans utiliser de paramètres phénoménologiques. La première partie de cette thèse considère la conduction à travers une molécule sans substrat, afin d'illustrer le genre de systèmes typiquement modélisés dans les calculs de transport. Deux électrodes en Au sont mises en contactavec une molécule benzènediamine substituée (R = CH3, NH2, OH), où un atome H est retiré pour former un radical qui peut se comporter comme un filtre de spin, dépendant du groupe R. Ensuite, nous nous concentrons sur une ligne formée d'éthylbenzènes empilées–π sur la surface de Si(100), où les atomes de silicium sont explicitement inclus dans le calcul. Quoique les molécules permettent le transport d'électronsà certaines énergies, un canal se forme à travers le substrat qui peut dominer la conductance. Nous étudions aussi comment certains substituants peuvent moduler les propriétés de transport électronique de ces fils moléculaires. Nous trouvons que la conductance du fil moléculaire peut être modifiée pour dominer l'effet du substrat.Enfin, la conductance de la surface métallique Si(111)–7 × 7 est analysée. Dans lebut d'expliquer théoriquement les expériences suggérant que les marches atomiques réduisent la conductance de la surface, la structure atomique et les propriétés de transport de ces marches ont été examinées. Les résultats révèlent que c'est la déformation atomique des dimères le long des marches qui cause ce phénomène, en raison de la formation d'une bande interdite localisée proche de la marche.
18
Petej, Ivan. "Coulomb blockade and quantum conductance in ferromagnetic nanostructures." Thesis, University of Oxford, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.270647.
Full text
19
Gillies, Patrick R. "Path integral quantum Monte Carlo for semiconductor nanostructures." Thesis, Heriot-Watt University, 2007. http://hdl.handle.net/10399/2033.
Full textAbstract:
Path integral quantum Monte Carlo (PI-QMC) is a powerful technique, which can be used to model the properties of multiple interacting particles at finite temperatures. In this work path integral quantum Monte Carlo has been applied to the problem of few particle interactions in quantum dots and other semiconductor nanostructures. Quantum dots are currently the subject of much research and in order to further understand their properties it is necessary to perform theoretical modelling. In this work, the method by which the problem of the attractive Coulomb potential was overcome is detailed. Following that, comparisons are made between . experimental data and PI-QMC results for excitonic complexes in 111-V dots. Both the energies and voltage extents were found to show good agreement between experiment and theory. Comparisons are also between theory and experiment of II-VI, with experimental data using a harmonic potential to model the dot. Again, good agreement is seen. Finally, as an example of the power of PI-QMC, the behaviour of electrons and holes is modelled for alternative nanostructures, such as coupled quantum dots, quantum rings and core-shell structures. With some simple modifications, the same PI-QMC method could be used.
20
Gauger, E. M. "Applications of quantum coherence in condensed matter nanostructures." Thesis, University of Oxford, 2010. http://ora.ox.ac.uk/objects/uuid:fb792980-bfc4-4771-b5d5-b9ecc7d40cd8.
Full textAbstract:
This thesis is concerned with studying the fascinating quantum properties of real-world nanostructures embedded in a noisy condensed matter environment. The interaction with light is used for controlling and manipulating the quantum state of the systems considered here. In some instances, laser pulses also provide a way of actively probing and controlling environmental interactions. The first two research chapters assess two different ways of performing all-optical spin qubit gates in self-assembled quantum dots. The principal conclusion is that an `adiabatic' control technique holds the promise of achieving a high fidelity when all primary sources of decoherence are taken into account. In the next chapter, it is shown that an optically driven quantum dot exciton interacting with the phonons of the surrounding lattice acts as a heat pump. Further, a model is developed which predicts the temperature-dependent damping of Rabi oscillations caused by bulk phonons, finding an excellent agreement with experimental data. A different system is studied in the following chapter: two electron spin qubits with no direct interaction, yet both exchange-coupled to an optically active mediator spin. The results of this study show that these general assumptions are sufficient for generating controlled electron spin entanglement over a wide range of parameters, even in the presence of noise. Finally, the Radical Pair model of the avian compass is investigated in the light of recent experimental results, leading to the surprising prediction that the electron spin coherence time in this molecular system seems to approach the millisecond timescale.
21
Schaffry, Marcus C. "Creation and manipulation of quantum states in nanostructures." Thesis, University of Oxford, 2011. http://ora.ox.ac.uk/objects/uuid:3d38fd34-041a-45be-aee0-2038d94b31ed.
Full textAbstract:
Nanostructures are promising building blocks for quantum technologies due to their reproducible nature and ability to self-assemble into complex structures. However, the need to control these nanostructures represents a key challenge. Hence, this thesis investigates the manipulation and creation of quantum states in certain nanostructures. The results of this thesis can be applied to quantum information processing and to extremely sensitive magnetic-field measurements. In the first research chapter, we propose and examine methods for entangling two (remote) nuclear spins through their mutual coupling to a transient optically excited electron spin. From our calculations we identify the specific molecular properties that permit high entangling power gates for different protocols. In the next research chapter, we investigate another method to create entanglement; this time between two remote electronic spins. This method uses a very sensitive magnetic-field sensor based on a crystal defect that allows the detection of single magnetic moments. The act of sensing the local field constitutes a two-qubit projective measurement. This entangling operation is remarkably robust to imperfections occurring in an experiment. The third research chapter presents an augmented sensor consisting of a nitrogen-vacancy centre for readout and an `amplifier' spin system that directly senses tiny local magnetic fields. Our calculations show that this hybrid structure has the potential to detect magnetic moments with a sensitivity and spatial resolution far beyond that of a sensor based on only a nitrogen-vacancy centre, and indeed this may be the physical limit for sensors of this class. Finally, the last research chapter investigates measurements of magnetic-field strength using an ensemble of spin-active molecules. Here, we describe a quantum strategy that can beat the common standard strategy. We identify the conditions for which this is possible and find that this crucially depends on the decoherence present in the system.
22
Xu, Fuming, and 许富明. "Quantum transport study of mesoscopic systems and nanostructures." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2011. http://hub.hku.hk/bib/B4691772X.
Full text
23
Hu, Hsiu-Lien. "Quantum transport of energetic electrons in ballistic nanostructures." Virtual Press, 2000. http://liblink.bsu.edu/uhtbin/catkey/1178341.
Full textAbstract:
The various electronic phenomena of electrons in the quasi-one-dimensional semiconducotor heterostructures have been largely investigated in the past research, due to its importance both on the theoretical understanding and the design of nanodevices. In particular, most research is currently based on the GaAs-AIGaAs material system with a 2-DEG interface. From the study of Hua Wu, following the Bohm's interpretation of quantum mechanics, energetic electrons approximately approach the classical behavior. The goal of this theoretical study is to investigate how the flow of energetic electrons may be controlled by the use of a tunable reflector. When encountering hard potential walls, energetic electrons in the nanostructure nearly follow the law of reflection. In addition, if the hard potential walls function as a reflector, the bouncing ball trajectory is also predicted. In this project, the fact that energetic electrons demonstrate semi-classical periodical flow motion is conceptually verified.The quantum wire (QW) with a tab and a notch nanostructure is selected as the practical model to achieve the project's goal. The resonant properties of the QW with a tab and the QW with a notch are individually investigated. The tight-binding recursive Green's function method is the theory underlying the numerical computation of the conductance in a nanodevice.Department of Physics and Astronomy
24
Kumar, A. S. (Arvind S. ). "Single electron charging effects in quantum dot nanostructures." Thesis, Massachusetts Institute of Technology, 1994. http://hdl.handle.net/1721.1/41328.
Full text
25
Veyrat, Louis. "Quantum Transport Study in 3D Topological Insulators Nanostructures." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-210217.
Full textAbstract:
In this thesis, we investigate the quantum transport properties of disordered three dimensional topological insulator (3DTI) nanostructures of BiSe and BiTe in detail. Despite their intrinsic bulk conductivity, we show the possibility to study the specific transport properties of the topological surface states (TSS), either with or without quantum confinement. Importantly, we demonstrate that unusual transport properties not only come from the Dirac nature of the quasi-particles, but also from their spin texture.
Without quantum confinement (wide ribbons), the transport properties of diffusive 2D spin-helical Dirac fermions are investigated. Using high magnetic fields allows us to measure and separate all contributions to charge transport. Band bending is investigated in BiSe nanostructures, revealing an inversion from upward to downward bending when decreasing the bulk doping. This result points out the need to control simultaneously both the bulk and surface residual doping in order to produce bulk-depleted nanostructures and to study TSS only. Moreover, Shubnikov-de-Haas oscillations and transconductance measurements are used to measure the ratio of the transport length to the electronic mean free path ltr/le. This ratio is measured to be close to one for bulk states, whereas it is close to 8 for TSS, which is a hallmark of the anisotropic scattering of spin-helical Dirac fermions.
With transverse quantum confinement (narrow wires or ribbons), the ballistic transport of quasi-1D surface modes is evidenced by mesoscopic transport measurements, and specific properties due to their topological nature are revealed at very low temperatures. The metallic surface states are directly evidenced by the measure of periodic Aharonov-Bohm oscillations (ABO) in 3DTI nanowires. Their exponential temperature dependence gives an unusual power-law temperature dependence of the phase coherence length, which is interpreted in terms of quasi-ballistic transport and decoherence in the weak-coupling regime. This remarkable finding is a consequence of the enhanced transport length, which is comparable to the perimeter. Besides, the ballistic transport of quasi-1D surface modes is further evidenced by the observation of non-universal conductance fluctuations in a BiSe nanowire, despite the long-length limit (L > ltr) and a high metallicity (many modes). We show that such an unusual property for a mesoscopic conductor is related to the limited mixing of the transverse modes by disorder, as confirmed by numerical calculations. Importantly, a model based on the modes' transmissions allows us to describe our experimental results, including the full temperature dependence of the ABO amplitude.
26
Pegolotti, Giulia. "Quantum engineering of collective states in semiconductor nanostructures." Paris 7, 2014. http://www.theses.fr/2014PA077136.
Full textAbstract:
Ce travail de thèse est centré sur l'étude théorique de la réponse optique de gaz d'électrons denses confinés dans des puits quantiques semiconducteurs. Dans ces systèmes, les spectres d'absorption présentent des résonances optiques à des énergies complètement différentes par rapport aux transitions électroniques du puits quantique. Ces résonances correspondent à des excitations collectives, rénormalisées par l'interaction de Coulomb. La partie principale de ce travail concerne le développement d'un modèle de la réponse optique qui décrit les effets collectifs dans des systèmes des puits quantiques couplés par effet tunnel. L'interaction lumière-matière est calculée en deux étapes. Nous commençons par considérer les polarisations microscopiques associées avec les transitions électroniques entre niveaux confinés dans les puits quantiques. Le couplage dipôle-dipôle entre polarisations électroniques donne lia. à des états collectifs, dont nous calculons successivement l'interaction avec le champ électromagnétique. Le spectre d'absorption est donc exprimé au travers de courants microscopiques, qui décrivent les oscillations collectives de charge. Le modèle théorique est appliqué à des systèmes pertinents et ses prédictions sont comparées aux résultats expérimentaux. Comme les états collectifs sont issus de la superposition cohérente de plusieurs excitations électroniques, ils ont les propriétés d'états superradiants. Ils représentent ainsi un système prometteur pour la réalisation de sources lumineuses efficaces dans les régions spectrales du moyen et lointain infrarougeThe main focus of this PhD work is the theoretical study of the optical response of dense electron gases confined in semiconductor quantum wells. In such systems, the absorption spectra present optical resonances at completely different energies with respect to the quantum well electronic transitions. These resonances are associated with the excitation of collective states, renormalized by Coulomb interaction. Most of this work is devoted to the development of a model of the optical response accounting for collective effects in systems of tunnel-coupled quantum wells. The light-matter interaction is calculated in two steps. We start from the microscopic polarizations associated with the electronic transitions between confined levels of the wells. Dipole-dipole coupling between electronic polarizations causes the appearance of collective states, whose interaction with the electromagnetic field is then considered. As a result, the absorption spectrum is expressed in terms of microscopic currents, describing the collective charge oscillations. The theoretical model is applied to a series of relevant systems, and its outcomes are compared with experimental results. As the collective states are issued from the coherent superposition of several electronic excitations, they have the properties of superradiant states. They are thus a promising entity for the realization of efficient light emitters in the mid- and far - infrared frequency range
27
Yoshie, Tomoyuki Scherer Axel. "Planar photonic crystal nanocavities with active quantum nanostructures /." Diss., Pasadena, Calif. : California Institute of Technology, 2004. http://resolver.caltech.edu/CaltechETD:etd-05272004-095431.
Full text
28
Cothrel, Helen M. "Photolithography for the Investigation of Nanostructures." Ohio University Honors Tutorial College / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=ouhonors1429719171.
Full text
29
Lok, Shu Kin. "MBE grown Fe-based nanostructures /." View abstract or full-text, 2010. http://library.ust.hk/cgi/db/thesis.pl?PHYS%202010%20LOK.
Full text
30
Wen, Xiaoming. "Ultrafast spectroscopy of semiconductor nanostructures." Australasian Digital Thesis Program, 2007. http://adt.lib.swin.edu.au/public/adt-VSWT20070426.110438/index.html.
Full textAbstract:
Thesis (PhD) - Swinburne University of Technology, Centre for Atom Optics and Ultrafast Spectroscopy, 2007.Thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy, Centre for Atom Optics and Ultrafast Spectroscopy, Swinburne University of Technology, 2007. Typescript. Bibliography: p. 122-144.
31
Racec, Paul Nicolae. "Transport phenomena and capacitance of open quantum semiconductor nanostructures." [S.l.] : [s.n.], 2002. http://deposit.ddb.de/cgi-bin/dokserv?idn=965463613.
Full text
32
Wickles, Christian [Verfasser]. "Quantum Transport in Non-Collinear Magnetic Nanostructures / Christian Wickles." Konstanz : Bibliothek der Universität Konstanz, 2011. http://d-nb.info/102321041X/34.
Full text
33
Nazir, Ahsan. "Optical schemes for quantum information processing in semiconductor nanostructures." Thesis, University of Oxford, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.409819.
Full text
34
Sanvito, Stefano. "Giant magnetoresistance and quantum transport in magnetic hybrid nanostructures." Thesis, Lancaster University, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.484207.
Full text
35
Iamraksa, Phansak. "Near-infrared photodetectors based on Si/SiGe quantum nanostructures." Thesis, University of Southampton, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.438676.
Full text
36
Baek, Jinyoung. "Microchemical systems for the synthesis of nanostructures : quantum dots." Thesis, Massachusetts Institute of Technology, 2012. http://hdl.handle.net/1721.1/76476.
Full textAbstract:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2012.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 153-152).
We have developed a continuous multi-stage high-temperature and high-pressure microfluidic system. High-pressure conditions enabled the use low molecular weight solvents that have previously not been available for quantum dot (QD) synthesis such as hexane or octane. The use of supercritical phase provided excellent mixing, which was critical in producing high quality QDs. In addition, the microfluidic system allowed precise control of synthetic conditions for the fast screening of reaction parameters. The continuous multi-stage microfluidic system enabled separating of reaction conditions such as mixing and aging steps, which was not possible in batch synthesis, as a result it was possible to conduct systematic investigation of the synthesis of indium phosphide (InP) QDs. We investigated synthesis of InP QDs with a continuous 3-stage high-temperature and high-pressure microreactor system without incorporating any batch manipulations between the synthesis steps. By separating the mixing process from the following aging process, we found that InP QD synthesis were mainly dominated by coalescence processes. Indium to fatty acid ratio showed the largest effect on particle size due to enhanced inter-particle processes. Concentrations or mixing temperatures changes, which are important reaction parameters of cadmium selenide (CdSe) QD growth, had no significant impact. We also synthesized larger (>3.2 nm) InP QDs with a sequential injection microreactor consisting of 6 sequential alternative monomer injections similar to the successive ion layers adsorption and reaction (SILAR) method. We obtained InP QDs with size distributions as narrow or narrower than the InP QDs synthesized via the ripening process. Indium phosphide / zinc sulfide (InP / ZnS) core-shell QDs were obtained with a 5 or 6 -stage microreactor system consisting of additional shell growth reactors, in addition to the three-step InP growth system. We were able to obtain narrow emissions with high quantum yield. This system was also used for the synthesis of indium phosphide / cadmium sulfide (InP / CdS), indium arsenide / indium phosphide (InAs / InP), and indium arsenide / cadmium sulfide (InAs / CdS) core-shell QDs. We also investigated the growth of InAs QDs using the same system for InP QD synthesis. We found that the InAs growth from indium myristate (In(MA) 3) and tristrimethylsilyl arsine ((TMS) 3As) precursors showed similar behavior as InP growth. However, different from the growth of InP nanocrystals, the amount of excess fatty acid did not affect on the growth of InAs nanocrystals. Indium phosphide arsenide (InPxAs1 -) alloy nanocrystals were also synthesized by precise control of phosphorus (P) and arsenic (As) precursor amounts. Mixing two anionic and cationic precursors at an elevated temperature followed by fast heating up to the reaction zone is very important for InPxAsl1x alloy nanocrystal synthesis. A multistage microfluidic system with a mixing reactor with gradient temperature was a useful tool for this synthesis. InPxAs - alloy nanocrystals were characterized with optical measurements and wide angle X-ray diffraction scattering. We investigated growth of InAs nanocrystals from a less reactive arsenic precursor, tris(trimethygermyl) arsine (TMG3As). We obtained InAs nanocrystals with better size distribution than those synthesized from TMS3As. We also compared the growth behavior of InAs nanocrystals synthesized from those two different arsenic precursors. With TMG3As, we observed a growth behavior potentially following a similar nucleation and growth model to that of growth of II-VI QDs.
by Jinyoung Baek.
Ph.D.
37
Foxman, Ethan Bradley 1966. "Single electron charging and quantum effects in semiconductor nanostructures." Thesis, Massachusetts Institute of Technology, 1993. http://hdl.handle.net/1721.1/72770.
Full text
38
Fruchtman, Amir. "Theory and modelling of energy transport in quantum nanostructures." Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:9c00d93c-c839-4342-9dc1-c2917c71a670.
Full textAbstract:
This thesis is concerned with quantum properties of excitonic energy transport in nanostructures that are embedded in a noisy environment. Of principal interests are ways to exploit this environment to facilitate the transport of energetic excitations. The first research chapter deals with an extension to the 'standard' open quantum system picture, where the Hilbert space is split into three: system, environment, and a wider universe. This division is natural for many biological and artificial nanostructures. A new analytical method, based on a phase space representation of the density matrix, is developed for studying such division. The effects of the wider universe are shown to be captured by a simple correction of the environmental response function. The second research chapter addresses the question: when do second-order perturbative approaches to open quantum systems, which are intuitive and simple to compute, provide adequate accuracy? A simple analytical criterion is developed, and its validity is verified for the case of the much-studied FMO dynamics as well as the canonical spin-boson model. In the third research chapter, an intuitive model of a photocell is studied. The model comprises two light-absorbing molecules coupled to an idealised reaction centre, showing asymmetric dimers are capable of providing a significant enhancement of light-to-current conversion under ambient conditions. This is done by 'parking' the energy of an absorbed photon in a dark state which neither absorbs nor emits light. In the final research chapter, a basic model for what can be thought as a "quantum brachistochrone" problem is investigated. Exotic energy configurations are found to yield considerable enhancement to the exciton's transfer probability, due to similar mechanisms studied in the previous chapter.
39
Al-Galiby, Qusiy. "Quantum theory of sensing and thermoelectricity in molecular nanostructures." Thesis, Lancaster University, 2016. http://eprints.lancs.ac.uk/80279/.
Full textAbstract:
This thesis presents a series of studies into the electronic and thermoelectric properties of molecular junction single organic molecules: They include perylene Bisimide (PBIs), naphthalenediimide (NDI), metallo-porphryins and a large set of symmetric and asymmetric molecules. Two main techniques will be included in the theoretical approach, which are Density Functional Theory, which is implemented in the SIESTA code [1], and the Green’s function formalism of elctron transport (Chapter 2), which is implemented in the GOLLUM code [2], it is a next-generation code, born out of the non-equilibrium transport code SMEAGOL code [3]. Both techniques are used to extensively to study a family of perylene bisimide molecules (PBIs) (Chapter 3) to understand the potential of these molecules for label-free sensing of organic molecules by investigating a change in the electronic properties of PBI derivatives. Also, these techniques are used to simulate electrochemical gating of a single molecule naphthalenediimide (NDI) junction (Chapter 4) using a strategy to control the number of electrons on the molecule by modelling different forms of charge double layers comprising positive and negative ions. Chapter 5 will deal with the thermoelectric properties of the single organic molecule. I will demonstrate that varying the transition metal-centre of a porphyrin molecule over the family of metallic atoms allows the molecular energy levels to be tuned relative to the Fermi energy of the electrodes and that leads to the ability to tune the thermoelectric properties of metallo-porphryins. Chapter 6 will present our new approach to materials discovery for electronic and thermoelectric properties of single-molecule junctions. I will deal with a large set of symmetric and asymmetric molecules to demonstrate a general rule for molecular-scale quantum transport, which provides a new route to materials design and discovery. The rule of this approach that “the conductance of an asymmetric molecule is the geometric mean of the conductance of the two symmetric molecules derived from it and the thermopower of the asymmetric molecule is the algebraic mean of their thermopowers”.
40
Beyer, Jan. "Spin Properties in InAs/GaAs Quantum Dot based Nanostructures." Doctoral thesis, Linköpings universitet, Funktionella elektroniska material, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-75097.
Full textAbstract:
Semiconductor quantum dots (QDs) are a promising building block of future spin-functional devices for applications in spintronics and quantum information processing. Essential to the realization of such devices is our ability to create a desired spin orientation of charge carriers (electrons and holes), typically via injection of spin polarized carriers from other parts of the QD structures. In this thesis, the optical orientation technique has been used to characterize spin generation, relaxation and detection in self-assembled single and multi-QD structures in the InAs/GaAs system prepared by modern molecular beam epitaxy technique. Optical generation of spin-oriented carriers in the wetting layer (WL) and GaAs barrier was carried out via circularly polarized excitation of uncorrelated electron-hole pairs from band-to-band transitions or via resonant excitation of correlated electron-hole pairs, i.e. excitons. It was shown that the generation and injection of uncorrelated electron-hole pairs is advantageous for spin-preserving injection into the QDs. The lower spin injection efficiency of excitons was attributed to an enhanced spin relaxation caused by the mutual electron-hole Coulomb exchange interaction. This correlation affects the spin injection efficiency up to elevated temperatures of around 150 K. Optical orientation at the energy of the WL light-hole (lh) exciton (XL) is accompanied by simultaneous excitation from the heavy-hole (hh) valence band at high ~k-vectors. Quantum interference of the two excitation pathways in the spectral vicinity of the XL energy resulted in occurrence of an asymmetric absorption peak, a Fano resonance. Complete quenching of spin generation efficiency at the resonance was observed and attributed to enhanced spin scattering between the hh and lh valence bands in conjunction with the Coulomb exchange interaction in the XL. This mechanism remains effective up to temperatures exceeding 100 K. In longitudinal magnetic fields up to 2 T, the spin detection efficiency in the QD ensemble was observed to increase by a factor of up to 2.5 in the investigated structures. This is due to the suppression of two spin depolarization mechanisms of the QD electron: the hyperfine interaction with the randomly oriented nuclear spins and the anisotropic exchange interaction with the hole. At higher magnetic fields, when these spin depolarization processes are quenched, only anisotropic QD structures (such as double QDs, aligned along a specific crystallographic axis) still exhibit a rather strong field dependence of the QD electron spin polarization under non-resonant excitation. Here, an increased spin relaxation in the spin injector, i.e. the WL or GaAs barrier, is suggested to lead to more efficient thermalization of the spins to the lower Zeeman-split spin state before capture to the QD. Finally, the influence of elevated temperatures on the spin properties of the QD structures was studied. The temperature dependence of dynamic nuclear polarization (DNP) of the host lattice atoms in the QDs and its effect on the QD electron spin relaxation and dephasing were investigated for temperatures up to 85 K. An increase in DNP efficiency with temperature was found, accompanied by a decrease in the extent of spin dephasing. Both effects are attributed to an accelerating electron spin relaxation, suggested to be due to phonon-assisted electronnuclear spin flip-flops driven by the hyperfine interaction. At even higher temperatures, reaching up to room temperature, a surprising, sharp rise in the QD polarization degree has been found. Experiments in a transverse magnetic field showed a rather constant QD spin lifetime, which could be governed by the spin dephasing time T*2. The observed rising in QD spin polarization degree could be likely attributed to a combined effect of shortening of trion lifetime and increasing spin injection efficiency from the WL. The latter may be caused by thermal activation of non-radiative carrier relaxation channels.
41
Sabathil, Matthias. "Opto-electronic and quantum transport properties of semiconductor nanostructures /." Garching : Verein zur Förderung des Walter Schottky Instituts der Technischen Universität München, 2005. http://www.loc.gov/catdir/toc/fy1002/2008380872.html.
Full text
42
Moaied, Modjtaba. "A Generalized Non-local Quantum Theory for Plasmonic Nanostructures." Thesis, The University of Sydney, 2017. http://hdl.handle.net/2123/18433.
Full textAbstract:
The propagation of light and its interaction with metallic nanostructures at the scale that is smaller than the wavelength of light (subwavelength) is an interesting phenomenon. In recent years, confining light at a subwavelength scale by very small metallic nanoparticles, such that quantum effects cannot be neglected, has generated interest in the scientific community. The light-matter interactions at this level require a careful quantum mechanical treatment to be correctly characterized, hence evolving in a new field named Quantum Plasmonics. In metallic nanostructures with sizes below 10 nm, the collective and coherent oscillations of electrons (plasmon resonance), cannot be described by classical models since the quantum-mechanical effects start dominating and become relevant with changing the plasmons oscillation frequency. Such changes have so far been poorly understood and the experimental measurements that have been carried out, have struggled to be correctly interpreted. Therefore, a quantum model of metal permittivity is required to understand the size-dependent optical properties of very small nanostructures. Here we present the nonlocal quantum model, obtained by applying the Wigner equation with the collision term in the kinetic theory of metals. Our results suggest that the probability of finding electrons at higher energy levels increases in the excitation of quantum plasmons, since their wave functions overlap, and therefore, the quantum tunnelling effect increases. The dispersion relation, damping rate, and decay length of surface and bulk plasmon resonances are investigated in thin metal film slabs and small silver nanoparticles with the various diameter down to atomic size and plasmon wave functions are shown for solutions of infinite quantum well at various quantum levels.
43
Hellström, Staffan. "Exciton-plasmon interactions in metal-semiconductor nanostructures." Doctoral thesis, KTH, Teoretisk kemi och biologi, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-93306.
Full textAbstract:
Semiconductor quantum dots and metal nanoparticles feature very strong light-matter interactions, which has led to their use in many photonic applications such as photodetectors, biosensors, components for telecommunications etc.Under illumination both structures exhibit collective electron-photon resonances, described in the frameworks of quasiparticles as exciton-polaritons for semiconductors and surface plasmon-polaritons for metals.To date these two approaches to controlling light interactions have usually been treated separately, with just a few simple attempts to consider exciton-plasmon interactions in a system consisting of both semiconductor and metal nanostructures.In this work, the exciton-polaritons and surface \\plasmon-polaritons are first considered separately, and then combined using the Finite Difference Time Domain numerical method coupled with a master equation for the exciton-polariton population dynamics.To better understand the properties of excitons and plasmons, each quasiparticle is used to investigate two open questions - the source of the Stokes shift between the absorption and luminescence peaks in quantum dots, and the source of the photocurrent increase in quantum dot infrared photodetectors coated by a thin metal film with holes. The combined numerical method is then used to study a system consisting of multiple metal nanoparticles close to a quantum dot, a system which has been predicted to exhibit quantum dot-induced transparency, but is demonstrated to just have a weak dip in the absorption.QC 20120417
44
Righetto, Marcello. "Optical Nanostructures for Excitonic Devices." Doctoral thesis, Università degli studi di Padova, 2018. http://hdl.handle.net/11577/3425292.
Full textAbstract:
Unrelenting advances in the field of nanoscience are fostering the progress in diverse research fields, ranging from light-emitting to medicine and diagnostics, from energy conversion to communication technologies. Besides representing the most paradigmatic example of nanoscience, semiconductor quantum dots (QDs) avowedly brought revolutions in many of the research fields mentioned above. Nowadays, some QDs-based devices and applications reported efficiencies almost as good as current state-of-the-art technologies. The founding concept of QDs is the application of quantum confinement effects on excitons, i.e., the main players of optical properties in bulk semiconductors. Among the wealth of ensuing properties, the size- and shape- tunability of the electronic excitations and increased coupling with light field aroused much interest. Also, the colloidal approach endows QDs with high processability and low cost, thereby encouraging their implementation in existing technologies and extending their impact to other fields. Howbeit, despite three decades of investigations, the bottom line has not been reached yet, and researchers are still delving deeper into the photophysics of these nanosystems. Though many of the low hanging fruit of QDs have been harvested, higher-lying ones seem to be even more succulent.
This thesis deals with the quest for highly performing nanostructures, as a prerequisite for some high impact optoelectronic applications, e.g., QD-Lasers and QD-Solar Cells. Within this framework, the struggle against fast Auger recombinations and trapping of either hot carriers or cold excitons was addressed mainly by sophisticated core/shell technologies. Thus, the first part of the thesis reports how tuning different shell parameters (e.g., the smoothness of the interface potential, the height of the confining potential, and the interfacial strain) it is possible to exert control on these detrimental recombination processes. Though often disregarded, even the role of organic capping ligand is reconsidered in promoting the outcoupling of QDs excited states and addressing their interaction. Besides the useful and technologically relevant advice gathered within these studies, the primary inheritance of the first part is the comprehensive photophysical scenario, portrayed by a phenomenological model that successfully describes many aspects of the exciton dynamics in QDs. This amount of knowledge was capitalized in the second part of this thesis, dealing with the quest for novel materials, potentially outpacing conventional CdSe-based QDs. Perovskite-based QDs reported promising results, whereas some pitfall in the conventional characterization of carbon-based QDs were discovered. The rationalization of both nature and dynamics of this materials is expected to expedite their development as alternative (and potentially superior) technologies concerning those studied in the first part.---
45
Wang, Cheng. "Modulation dynamics of InP-based quantum dot lasers and quantum cascade lasers." Thesis, Rennes, INSA, 2015. http://www.theses.fr/2015ISAR0009/document.
Full textAbstract:
Le besoin incessant de débits toujours plus élevés dans les systèmes de télécommunications a un impact sur tous les éléments composant la chaine de transmission. Ainsi, pour faire face à l’augmentation croissante du volume de données échangées à travers le monde, le développement de nouvelles sources optiques semi-conductrices est absolument nécessaire. La modulation directe de lasers nanostructurés constitue une alternative bas coût et à faible consommation énergétique qui permettra de remplacer progressivement les diodes lasers à puits quantiques actuelles. De nombreux efforts en recherche ont été consacrés au cours des dernières années en vue d’améliorer les performances dynamiques des lasers nanostructurés notamment en terme de bande passante, de facteur de couplage phase-amplitude (facteur α) et de dérive de fréquence (chirp). Pour les applications aux très grands réseaux et systèmes de communication, la croissance d’îlots ou de fils quantiques déposés sur substrat InP permet de réaliser des dispositifs nanostructurés émettant dans le proche infra-rouge autours de 1550 nm. Dans ce mémoire, la dynamique de modulation des lasers nanostructuré est étudiée en régime de modulation directe. Les caractéristiques analysées comprennent: la modulation en amplitude (AM) et en fréquence (FM), le chirp, et les réponses en régime grandsignal. Grâce à une approche semi-analytique, il est démontré que la bande passante et l’amortissement sont fortement limités par les phénomènes de capture et de relaxation des porteurs de charge dans les nanostructures. Afin d’étudier les propriétés du facteur α et du chirp, un nouveau modèle dynamique a été proposé, prenant en compte la contribution à l’indice optique des porteurs de charge dans des états hors résonance. Il est ainsi montré que, contrairement au cas des lasers à puits quantiques, le facteur α dépend fortement du courant de pompe et de la fréquence de modulation. Le facteur α reste constant à basses fréquences (<0,1 GHz) et supérieur aux valeurs obtenues à hautes fréquences (au-delà de quelques GHz) à partir de la technique FM/AM. Ces caractéristiques sont essentiellement attribuées aux contributions des porteurs dans les états hors résonance. Les simulations montrent que le facteur α peut être réduit en augmentant la séparation énergétique entre l’état fondamental résonant (GS) et les états hors résonance. En particulier, un effet laser sur 1’état excité des nanostructures (ES) constitue une solution prometteuse pour améliorer les performances dynamiques, en accroissant notamment la bande passante de modulation et en réduisant le facteur α d’environ 40%. Les techniques d’injection optique sont également intéressantes pour régénérer les performances dynamiques des lasers. Le couplage phase-amplitude et le gain optique y sont substantiellement modifiés via le contrôle de l’amplitude et du désaccord en fréquence du faisceau injecté. Dans ce cadre, ce travail propose une nouvelle technique dérivée de la méthode Hakki-Paoli, permettant de mesurer, sous injection optique, le facteur α à la fois en dessous et au-dessus du seuil. Les lasers à cascade quantique (QCL) sont basés sur des transitions électroniques inter-sous-bandes dans des hétérostructures à puits quantiques. Ces lasers présentent une bande passante (AM) relativement de quelques dizaines de GHz et sans résonance ce qui est prometteur pour les transmissions en espace libre. De manière surprenante, les calculs montrent que les QCL présentent une largeur de bande FM extrêmement large de l’ordre quelques dizaines de THz, environ trois ordres de grandeur supérieure à la largeur de bande AM. L’injection optique dans ces lasers présente les mêmes avantages que ceux procurés dans les lasers à transitions interbandes. Des désaccords positifs ou négatifs en fréquence augmentent notamment la largeur de la bande passanteHigh performance semiconductor lasers are strongly demanded in the rapidly increasing optical communication networks. Low dimensional nanostructure lasers are expected to be substitutes of their quantum well (Qwell) counterparts in the next-generation of energy-saving and high-bandwidth telecommunication optical links. Many efforts have been devoted during the past years to achieve nanostructure lasers with broad modulation bandwidth, low frequency chirp, and reduced linewidth enhancement factor. Particularly, 1.55-μm InP-based quantum dash (Qdash)/dot (Qdot) lasers are preferable for long-haul transmissions in contrast to the 1.3-μm laser sources. In this dissertation, we investigate the dynamic characteristics of InPbased nanostructure semiconductor lasers operating under direct current modulation, including the amplitude (AM) and frequency (FM) modulation responses, the linewidth enhancement factor (also known as α-factor), as well as large-signal modulation responses. Using a semi-analytical analysis of the rate equation model, it is found that the modulation bandwidth of the quantum dot laser is strongly limited by the finite carrier capture and relaxation rates. In order to study the α- factor and chirp properties of the quantum dot laser, we develop an improved rate equation model, which takes into account the contribution of carrier populations in off-resonant states to the refractive index change. It is demonstrated that the α-factor of quantum dot lasers is strongly dependent on the pump current as well as the modulation frequency, in comparison to the case of Qwell lasers. The α-factor remains constant at low modulation frequencies (<0.1 GHz) and higher than the value derived at high modulation frequencies (beyond several GHz) from the FM/AM technique. These unique features are mostly attributed to the carrier populations in off-resonant states. Further simulations show that the α-factor can be reduced by enlarging the energy separation between the resonant ground state (GS) and off-resonant states. Lasing from the excited state (ES) can be a promising alternative to enhance the laser’s dynamic performance. The laser exhibits a broader modulation response and the α-factor can be reduced by as much as 40%. The optical injection technique is attractive to improve the laser’s dynamical performance, including bandwidth enhancement and chirp reduction. These are demonstrated both theoretically and experimentally. The phase-amplitude coupling property is altered as well in comparison with the free-running laser and the optical gain depends on the injection strength and the frequency detuning. This work proposes a new method derived from the Hakki-Paoli method, enabling to measure the α-factor of semiconductor lasers under optical injection both below and above threshold. In addition, it is demonstrated theoretically that the α-factor in nanostructure lasers exhibits a threshold discontinuity, which is mainly attributed to the unclamped carrier populations in the off-resonant states. It is a fundamental limitation, preventing the reduction of the α-factor towards zero. Quantum cascade (QC) lasers rely on intersubband electronic transitions in multi-quantum well heterostructures. QC lasers show flat broadband AM response (tens of GHz) without resonance, which constitutes promising features for free-space communications. Surprisingly, calculations show that the QC laser exhibits an ultrabroad FM bandwidth on the order of tens of THz, about three orders of magnitude larger than the AM bandwidth. Optically injection-locked QC lasers also exhibit specific characteristics by comparison to interband semiconductor lasers. Both positive and negative frequency detunings enhance the modulation bandwidth
46
Tian, Heng, and 田恒. "Application of hierarchical equations of motion to time dependent quantum transport." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2012. http://hub.hku.hk/bib/B47869446.
Full textAbstract:
Within the exact framework established recently, which is a successful marriage
between the time dependent density functional theory for open electronic system
and quantum dissipation theory formulated in the hierarchical equations of motion,
an entirely new scheme is proposed in this thesis to simulate the time-dependent
quantum transport in nano-devices at both zero and finite temperature equally
without relying on the pole structure of the Fermi distribution function. Neither
does it depend on any non-unique parametrization of the line-width matrix, hence,
this new practical approach can be integrated with the first principles simulations
seamlessly.
Beyond the exact framework, a reliable method which works under the Wide-
Band-Limit approximation at zero temperature is also developed. At the price of
loss of some non-Markovian memory effects on the dynamics, a set of equations
of motion which terminates at the first tier instead of the second tier is obtained.
Benefiting from the latest advancement of numerical analysis, a hybrid fourth-order
Runge-Kutta algorithm is proposed to solve this set of equations of motion which
comprises stiff ones. Based on this result, an alternative scheme is considered to
deal with the same approximation at finite temperature.
As an illustration of these new approaches, the transient current of the one
dimensional tight-binding periodical chain with and without a single impurity, driven
by some time alternating and/or static bias voltages, are investigated. The influence
of temperature and switch-on rate of bias voltage is exemplified. Particularly, in the
one dimensional tight-binding chain with a single impurity which breaks its perfect
periodicity, an asymmetry between the left and right transient current is found.
Comparison between the results under the Wide-Band-Limit approximation and
those with the exact description is carried out.published_or_final_version
Chemistry
Doctoral
Doctor of Philosophy
47
Rho, Young Gyu. "Quantum-Confined CdS Nanoparticles on DNA Templates." Thesis, University of North Texas, 1998. https://digital.library.unt.edu/ark:/67531/metadc279352/.
Full textAbstract:
As electronic devices became smaller, interest in quantum-confined semiconductor nanostructures increased. Self-assembled mesoscale semiconductor structures of II-VI nanocrystals are an especially exciting subject because of their controllable band gap and unique photophysical properties. Several preparative methods to synthesize and control the sizes of the individual nanocrystallites and the electronic and optical properties have been intensively studied. Fabrication of patterned nanostructures composed of quantum-confined nanoparticles is the next step toward practical applications. We have developed an innovative method to fabricate diverse nanostructures which relies on the size and a shape of a chosen deoxyribonucleic acid (DNA) template.
48
Benhsaien, Abdessamad. "Self-assembled quantum dot semiconductor nanostructures modeling: Photonic device applications." Thesis, University of Ottawa (Canada), 2006. http://hdl.handle.net/10393/27225.
Full textAbstract:
A microscopic analysis of a vertical stack of self-assembled InAs/GaAs lens-shaped quantum dot nanostructures is presented. The analysis revolves around a rigorous Hamiltonian formulation of an eight-band k.p. perturbation to account for the lattice-mismatch strain endured by the islands. The numerical implementation yields the effective bandgap energy and electronic structure of an InAs/GaAs quantum dot. Within the framework of a resonant two-level energy system, material gain and absorption spectra are calculated up to a third-order susceptibility to include nonlinearity. The material gain polarization dependence is expressed in the dipole transition strength. Polarization-dependent anisotropy factors corresponding to different interband transitions are derived and shown to satisfy a momentum conservation rule.
Modal analysis of a rectangular core waveguide realized by imbedding the active quantum dot layer(s) into a cladding medium with lower refractive index is presented. Polarization-independent modal gain is achieved by optimizing the width of the rectangular core waveguide. In illustration of a quantum dot device, a realistic semiconductor optical amplifier model accounting for both stimulated and spontaneous emission is considered. The calculated carrier density longitudinal profile yields other parameters characterizing the amplifier performance.
49
Ngo, Anh T. "Spin-orbit Effects and Electronic Transport in Nanostructures." Ohio University / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1292260134.
Full text
50
Hakanen, Jani. "Modeling of nanostructures with complex source and drain." Thesis, Linköping University, The Department of Physics, Chemistry and Biology, 2004. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-4285.
Full textAbstract:
In this thesis we report on calculations for open quantum mechanical and certain microwave systems. The models refer to a quantum point contact and an electron cavity. We model this open system with an imaginary potential as source and drain, and use the finite difference method to make our calculations. We report on general features of the model we have found, and compare our calculations with measurements made on microwave cavities.