Dissertations / Theses: 'Quantum confinement'

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Tsegaye, Takele Dessie. "Confinement Mechanisms in Quantum Chromodynamics." University of Cincinnati / OhioLINK, 2003. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1051373650.

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Takele, Tsegaye. "Confinement mechanisms in quantum cherodynamics." Cincinnati, Ohio : University of Cincinnati, 2002. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=ucin1051373650.

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Downing, Charles Andrew. "Quantum confinement in low-dimensional Dirac materials." Thesis, University of Exeter, 2015. http://hdl.handle.net/10871/17215.

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This thesis is devoted to quantum confinement effects in low-dimensional Dirac materials. We propose a variety of schemes in which massless Dirac fermions, which are notoriously diffcult to manipulate, can be trapped in a bound state. Primarily we appeal for the use of external electromagnetic fields. As a consequence of this endeavor, we find several interesting condensed matter analogues to effects from relativistic quantum mechanics, as well as entirely new effects and a possible novel state of matter. For example, in our study of the effective Coulomb interaction in one dimension, we demonstrate how atomic collapse may arise in carbon nanotubes or graphene nanoribbons, and describe the critical importance of the size of the band gap. Meanwhile, inspired by groundbreaking experiments investigating the effects of strain, we propose how to confine the elusive charge carriers in so-called velocity barriers, which arise due to a spatially inhomogeneous Fermi velocity triggered by a strained lattice. We also present a new and beautiful quasi-exactly solvable model of quantum mechanics, showing the possibilities for confinement in magnetic quantum dots are not as stringent as previously thought. We also reveal that Klein tunnelling is not as pernicious as widely believed, as we show bound states can arise from purely electrostatic means at the Dirac point energy. Finally, we show from an analytical solution to the quasi-relativistic two-body problem, how an exotic same-particle paring can occur and speculate on its implications if found in the laboratory.

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Wesslén, Carl. "Confinement Sensitivity in Quantum Dot Spin Relaxation." Doctoral thesis, Stockholms universitet, Fysikum, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-142133.

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Quantum dots, also known as artificial atoms, are created by tightly confining electrons, and thereby quantizing their energies. They are important components in the emerging fields of nanotechnology where their potential uses vary from dyes to quantum computing qubits. Interesting properties to investigate are e.g. the existence of atom-like shell structures and lifetimes of prepared states. Stability and controllability are important properties in finding applications to quantum dots. The ability to prepare a state and change it in a controlled manner without it loosing coherence is very useful, and in some semiconductor quantum dots, lifetimes of up to several milliseconds have been realized. Here we focus on dots in semiconductor materials and investigate how the confined electrons are effected by their experienced potential. The shape of the dot will effect its properties, and is important when considering a suitable model. Structures elongated in one dimension, often called nanowires, or shaped as rings have more one-dimensional characteristics than completely round or square dots. The two-dimensional dots investigated here are usually modeled as harmonic oscillators, however we will also consider circular well models. The effective potential confining the electrons is investigated both in regard to how elliptical it is, as well as how results differ when using a harmonic oscillator or a circular well potential. By mixing spin states through spin-orbit interaction transitioning between singlet and triplet states becomes possible with spin independent processes such as phonon relaxation. We solve the spin-mixing two-electron problem numerically for some confinement, and calculate the phonon transition rate between the lowest energy singlet and triplet states using Fermi's golden rule. The strength of the spin-orbit interaction is varied both by changing the coupling constants, and by applying an external, tilted, magnetic field. The relation between magnetic field parameters and dot parameters are used to maximize state lifetimes, and to model experimental results.

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 3: Manuscript.

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Abdelrahman, Ahmed M. "Magnetic micro-confinement of quantum degenerate gases." Thesis, Edith Cowan University, Research Online, Perth, Western Australia, 2011. https://ro.ecu.edu.au/theses/411.

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In this dissertation we explore the basic principles of the magnetic micro-confinement of the quantum degenerate gases where the approach of the so-called two-dimensional magnetic lattices has been theoretically and experimentally investigated. In this research a new generation of two-dimensional magnetic lattice has been proposed and considered as a developing phase for the previous approaches. Its advantage relies on introducing a simplified method to create single or multiple micro-traps of magnetic field local minima distributed, at a certain working distance, above the surface of a thin film of permanent magnetic material. The simplicity in creating the magnetic field local minima at the micro-scale manifests itself as a result of imprinting specific patterns through the thin film using suitable and available micro-fabrication techniques. In this approach, to create multiple micro-traps, patterned square holes of size αh X αh spaced by αs are periodically distributed across the x/y plane taking a two-dimensional grid configuration. These magnetic field local minima are recognized by their ability to trap and confine quantum single-particles and quantum degenerate gases at various levels of distribution in their phase spaces, such as ultracold atoms and virtual quantum particles. Based on the nature of the interaction between the external confining potential fields and the different types of quantum particles, this research is conducted through two separate but not different phases. We performed theoretical and/or experimental investigations, for both phases, at the vicinity of the magnetic micro-confinement and its suitability for trapping quantum particles. A special attention is paid to inspect the coherence in such systems defined in terms of providing an accessible coupling to the internal quantum states of the magnetically trapped particles. Such coherence is considered as one of the important ingredients for simulating condensed matter systems and processing quantum information.

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Harankahage, Dulanjan Padmajith Dharmasena. "Quantum Confinement Beyond the Exciton Bhor Radius in Quantum Dot Nanoshells." Bowling Green State University / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1593955468720583.

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李德豪 and Tak-ho Alex Li. "Stripe quantum well waveguides using implantation induced optical confinement." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1997. http://hub.hku.hk/bib/B31237381.

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Koulentianos, Dimitrios. "Quantum confinement effect in materials for solar cell applications." Thesis, Uppsala universitet, Materialteori, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-237189.

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Li, Tak-ho Alex. "Stripe quantum well waveguides using implantation induced optical confinement /." Hong Kong : University of Hong Kong, 1997. http://sunzi.lib.hku.hk/hkuto/record.jsp?B19145421.

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Hart, A. "Magnetic monopoles and confinement in lattice gauge theory." Thesis, University of Oxford, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.337718.

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Büttner, Kirsten. "Confinement and the infrared behaviour of the gluon propagator." Thesis, Durham University, 1996. http://etheses.dur.ac.uk/5298/.

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We investigate the infrared behaviour of the gluon propagator in Quantum Chromo- dynamics (QCD). A natural framework for such a non-perturbative study is the complex of Schwinger-Dyson equations (SDE).The possible infrared behaviour of the gluon, found by self-consistently solving the approximate boson SDE, is studied analytically. We find that only an infrared enhanced gluon propagator, as singular as 1/p(^4) as p(^2) → 0, is consistent and demonstrate why softer solutions, that others have found, are not allowed. Reassuringly the consistent, enhanced infrared behaviour is indicative of the confinement of quarks and gluons, implying, for example, area-law behaviour of the Wilson loop operator and forbidding a Kāllen-Lehmann spectral representation of both quark and gluon propagators. We then briefly consider the implications of these results for models of the pomeron. The enhancement of the gluon propagator does however introduce infrared divergences in the SDE and these need to be regularised. So far model forms of the enhanced gluon propagator have been used in studies of dynamical chiral symmetry breaking and hadron phenomenology. Though very encouraging results have been obtained, one might hope to use the gluon propagator obtained directly from non-perturbative QCD to calculate hadron observables. We therefore attempt to eliminate the infrared divergences in the SDEs in a self- consistent way, entirely within the context of the calculational scheme. To do this we introduce an infrared regulator λ in the truncated gluon SDE in quenched QCD. We find that this regulator is indeed determined by the equation and bounded by the QCD-scale Aqcd- Thus it is possible to perform the regularisation within the SDEs. However, we have not been able to choose λ < Aqcd.

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Worrall, Anthony Duncan. "The Schwinger-Dyson equations and confinement in quantum chromo-dynamics." Thesis, Durham University, 1985. http://etheses.dur.ac.uk/7024/.

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The Schwinger-Dyson equations for the gluon and quark propagators are investigated in the covariant gauge. The renormalization functions are approximated suitably and the value of the parameters are determined by requiring that the functions be numerically self-consistent solutions over appropriate ranges of momenta. In the case of the gluon the Schwinger-Dyson equation is truncated by neglecting the the two loop contributions and the triple gluon vertex is approximated by a form proposed by Mandelstam which has the same behaviour as the more complicated longitudinal vertex determined from the Slavnov-Taylor identity. The equation is then closed and the integrals are calculated by dimensional regularization and renormalized to remove a mass term. In the quark case the dominant part of the quark-gluon vertex is determined from the Ward-Takahashi identity to give, with the gluon, a closed equation. The angular integrals are then calculated by an appropriate choice of coordinate frame. The quark function is approximated by a power series in the non-perturbative regime and the usual perturbative result elsewhere. The radial integrals are then calculated with appropriate regularization and renormalization. It is found that the gluon propagator has approximately a singularity of the form 1/q(^4) which leads to a roughly linear confining potential. The effect of this enhanced singularity on the quark propagator is to suppress the propagation of quarks at low momenta.

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Weiss, Stephan. "Nonequilibrium quantum transport and confinement effects in interacting nanoscale conductors." Aachen Shaker, 2008. http://d-nb.info/990088294/04.

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Patel, Sailesh. "Magneto-optical studies of 2D, 1D and 0D electron systems." Thesis, University of Exeter, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.337804.

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Amin, Victor. "Ligand-Mediated Control of the Confinement Potential in Semiconductor Quantum Dots." Thesis, Northwestern University, 2015. http://pqdtopen.proquest.com/#viewpdf?dispub=3741337.

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This thesis describes the mechanisms by which organic surfactants, particularly thiophenols and phenyldithiocarbamates, reduce the confinement potential experienced by the exciton of semiconductor quantum dots (QDs). The reduction of the confinement potential is enabled by the creation of interfacial electronic states near the band edge of the QD upon ligand adsorption. In the case of thiophenols, we find that this ligand adsorbs in two distinct binding modes, (i) a tightly bound mode capable of exciton delocalization, and (ii) a more weakly bound mode that has no discernable effect on exciton confinement. Both the adsorption constant and reduction in confinement potential are tunable by para substitution and are generally anticorrelated. For tightly bound thiophenols and other moderately delocalizing ligands, the degree of delocalization induced in the QD is approximately linearly proportional to the fractional surface area occupied by the ligand for all sizes of QDs. In the case of phenyldithiocarbamates, the reduction in the confinement potential is much greater, and ligand adjacency must be accounted for to model exciton delocalization. We find that at high surface coverages, exciton delocalization by phenyldithiocarbamates and other highly delocalizing ligands is dominated by ligand packing effects. Finally, we construct a database of electronic structure calculations on organic molecules and propose an algorithm that combines experimental and computational screening to find novel delocalizing ligands.

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Renken, Volker. "Electron confinement and quantum well states in two-dimensional magnetic systems." [S.l.] : [s.n.], 2007. http://deposit.ddb.de/cgi-bin/dokserv?idn=985573546.

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Sun, Xiangzhong 1968. "The effect of quantum confinement on the thermoelectric figure of merit." Thesis, Massachusetts Institute of Technology, 1999. http://hdl.handle.net/1721.1/9308.

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Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 1999.
Includes bibliographical references (p. 161-165).
The thermoelectric figure of merit (Z) determines the usefulness of a material for thermoelectric energy conversion applications. Since the 1960's, the best thermoelectric material has been Bi2Te3 alloys, with a ZT of 1.0 at a temperature ofT = 300 K. The advancement of nano-scale technologies has opened up the possibility of engineering materials at nano-scale dimensions to achieve low-dimensional thermoelectric structures which may be superior to their bulk forms. In this thesis, I established the basis of the low dimensional thermoelectric transport principle in the Si/Si1-xGex quantum well superlattice (two-dimensional) system and in the Bi quantum wire (one-dimensional) system. In bulk form, Si1_xGex is a promising thermoelectric material for high temperature applications. The Si/Si1 _xGex quantum well superlattice structures are studied based on their electronic band structures using semiclassical transport theory. Detailed subband structures are considered in an infinite series of finite height quantum wells and barriers. A significant enhancement of the thermoelectric figure of merit is expected. Based on my calculations, experimental studies are designed and performed on MBE grown Si/Sii -xGex quantum well superlattice structures. The experimental results are found to be consistent with theoretical predictions and indicate a significant enhancement of Z within the quantum wells over bulk values. The bismuth quantum wire system is a one-dimensional (ID) thermoelectric system. Bismuth as a semimetal is not a good thermoelectric material in bulk form becamm of the approximate cancellation between the electron and hole contributions to the Seebeck coefficient. However, quantum confinement can be introduced by making Bi nanowires to yield a ID semiconductor. ID transport properties are calculated along the principal crystallographic directions. By carefully tailoring the Bi wire size and carrier concentration, substantial enhancement in Z is expected. A preliminary experimental study of Bi nanowire arrays is also presented.
by Xiangzhong Sun.
Ph.D.

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Andrew), Patterson Alex A. (Alex. "An analytical framework for field electron emission, incorporating quantum- confinement effects." Thesis, Massachusetts Institute of Technology, 2013. http://hdl.handle.net/1721.1/84863.

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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2013.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 141-151).
As field electron emitters shrink to nanoscale dimensions, the effects of quantum confinement of the electron supply and electric field enhancement at the emitter tip play a significant role in determining the emitted current density (ECD). Consequently, the Fowler-Nordheim (FN) equation, which primarily applies to field emission from the planar surface of a bulk metal may not be valid for nanoscale emitters. While much effort has focused on studying emitter tip electrostatics, not much attention has been paid to the consequences of a quantum-confined electron supply. This work builds an analytical framework from which ECD equations for quantum-confined emitters of various geometries and materials can be generated and the effects of quantum confinement of the electron supply on the ECD can be studied. ECD equations were derived for metal emitters from the elementary model and for silicon emitters via a more physically-complete version of the elementary model. In the absence of field enhancement at the emitter tip, decreasing an emitter's dimensions is found to decrease the total ECD. When the effects of field enhancement are incorporated, the ECD increases with decreasing transverse emitter dimensions until a critical dimension dpeak, below which the reduced electron supply becomes the limiting factor for emission and the ECD decreases. Based on the forms of the ECD equations, alternate analytical methods to Fowler-Nordheim plots are introduced for parameter extraction from experimental field emission data. Analysis shows that the FN equation and standard analysis procedures over-predict the ECD from quantum-confined emitters. As a result, the ECD equations and methods introduced in this thesis are intended to replace the Fowler-Nordheim equation and related analysis procedures when treating field emission from suitably small field electron emitters.
by Alex A. Patterson.
S.M.

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Riley, James R. "A Systematic Investigation of Quantum Confinement Effects in Bismuth Nanowire Arrays." Thesis, Boston College, 2009. http://hdl.handle.net/2345/693.

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Thesis advisor: Michael Graf
Bismuth is an interesting element to study because the low effective mass of its charge carriers makes the material sensitive to quantum confinement effects. When bismuth is reduced to the nanoscale two interesting phenomena may occur: it may transition from a semimetal to a semiconductor, or charge carriers in special surface states may begin to dominate the behavior of the material. Arrays of bismuth nanowires of various diameters were studied to investigate these possibilities. The magnetoresistance of the arrays was measured and the period of Shubnikov-de Haas oscillations suggested an increase in the effective mass and density of the material’s charge carriers for small nanowire diameters. These increases suggested that electrons were present in surface states and strongly influenced the material’s behavior when its dimensions were sufficiently reduced. The magnetization of the nanowire arrays was also measured and the lack of de Haas-van Alphen oscillations for certain diameter nanowires suggested that electrons were not present in surface states and that instead the material was transitioning from a semimetal to a semiconductor. Heat capacity measurements were planned to reconcile the two experiments. My detailed calculations demonstrated that heat capacity measurements were feasible to determine the presence, or absence, of surface charge carriers. Because the electronic contribution to the material’s heat capacity is small a calorimeter platform was constructed with ultra-low heat capacity components
Thesis (BS) — Boston College, 2009
Submitted to: Boston College. College of Arts and Sciences
Discipline: College Honors Program
Discipline: Physics

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Ford, Frank R. "Loop study of Gribov-Zwanzigar confinement and mass operators in quantum chromodynamics." Thesis, University of Liverpool, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.526882.

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Weiss, Stephan [Verfasser]. "Nonequilibrium quantum transport and confinement effects in interacting nanoscale conductors / Stephan Weiss." Aachen : Shaker, 2008. http://d-nb.info/1162793899/34.

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Eddie, Iain Mackenzie. "Carrier confinement in vertical-cavity surface-emitting lasers by quantum well intermixing." Thesis, University of Glasgow, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.433190.

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Raciti, Rosario. "Quantum confinement effects on light absorption in Germanium for solar energy conversion." Doctoral thesis, Università di Catania, 2017. http://hdl.handle.net/10761/3689.

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The world demand for energy is continuously increasing with a rate that will soon become unsustainable given the current exploitation of energy sources (such as fossil fuels). In addition, it should be figured out that most of commonly used energy resource are limited and that humankind has liberated a quantity of carbon (as CO2) in the past 250 years that it took our planet about 250 million of years to sequester. In this context, a wide and exciting range of possible solutions to provide enough and cleaner energy is represented by nanotechnologies offering innovative materials with interesting effects exploitable for energy production, distribution and saving. Among other materials, Group-IV semiconductors have been deeply investigated since they allow the fabrication of abundant, non-toxic, mono-elemental nanostructures (as Si quantum dots, C nanotubes, Ge nanowires, et al.) thanks to high purity and mature technology. Moreover, fascinating effects due to quantum confinement in this nanostructures can be effectively exploited for energy production in photovoltaics devices. Among them, Ge reveals interesting optical properties due to its quasi-direct bandgap, higher absorption coefficient and larger exciton Bohr radius with respect to Si, giving the chance to easily tune the optical properties by exploiting quantum confinement effect (QCE). However, the properties of Ge quantum dots (QDs) depends not only on the size as many other parameters can concur in controlling their optical behavior, especially for what concerns the optical bandgap. For this reason, the aim of this thesis is devoted to a detailed investigation of the optical properties of Ge QD, with particular emphasis on the light absorption properties and its modulation by QCE.

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Almalki, Shaimaa. "Nano-engineering of High Harmonic Generation in Solid State Systems." Thesis, Université d'Ottawa / University of Ottawa, 2019. http://hdl.handle.net/10393/39308.

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High harmonic generation (HHG) in solids has two main applications. First, HHG is an all-solid-state source of coherent attosecond very ultraviolet (VUV) radiation. As such, it presents a promising source for attosecond science. The ultimate goal of attosecond science is to make spatially and temporally resolved movies of microscopic processes, such as the making and breaking of molecular bonds. Second, the HHG process itself can be used to spatially and temporally resolve fast processes in the condensed matter phase, such as charge shielding, multi-electron interactions, and the dynamics and decay of collective excitations. The main obstacles to realize these goals are: the very low efficiency of HHG in solids and incomplete understanding of the ultrafast dynamics of the complex many-body processes occurring in the condensed matter phase. The theoretical analysis developed in this thesis promises progress along both directions. First, it is demonstrated that nanoengineering by using lower-dimensional solids can drastically enhance the efficiency of HHG. The effect of quantum confinement on HHG in semiconductor materials is studied by systematically varying the confinement width along one and two directions transverse to the laser polarization. Our analysis shows growth in high harmonic efficiency concurrent with a reduction of ionization. This decrease in ionization comes as a consequence of an increased band gap resulting from the confinement. The increase in harmonic efficiency results from a restriction of wave packet spreading, leading to greater re-collision probability. Consequently, nanoengineering of one and two-dimensional nanosystems may prove to be a viable means to increase harmonic yield and photon energy in semiconductor materials driven by intense laser fields. Thus, it will contribute towards the development of reliable, all-solid-state, small-scale, and laboratory attosecond pulse sources. Second, it is shown that HHG from impurities can be used to tomographically reconstruct impurity orbitals. A quasi-classical three-step model is developed that builds a basis for impurity tomography. HHG from impurities is found to be similar to the high harmonic generation in atomic and molecular gases with the main difference coming from the non-parabolic nature of the bands. This opens a new avenue for strong field atomic and molecular physics in the condensed matter phase and allows many of the processes developed for gas-phase attosecond science to be applied to the condensed matter phase. As a first application, my conceptual study demonstrates the feasibility of tomographic measurement of impurity orbitals. Ultimately, this could result in temporally and spatially resolved measurements of electronic processes in impurities with potential relevance to quantum information sciences, where impurities are prime candidates for realizing qubits and single photon sources. Although scanning tunneling microscope (STMs) can measure electron charge distributions in impurities, measurements are limited to the first few surface layers and ultrafast time resolution is not possible yet. As a result, HHG tomography can add complementary capacities to the study of impurities.

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Werwa, Eric 1970. "The role of quantum confinement effects in the visible photoluminescence from silicon nanoparticles." Thesis, Massachusetts Institute of Technology, 1997. http://hdl.handle.net/1721.1/43547.

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Kempf, James G. "Probing quantum confinement at the atomic scale with optically detected nuclear magnetic resonance." Diss., Pasadena, Calif. : California Institute of Technology, 2001. http://resolver.caltech.edu/CaltechETD:etd-08282001-123851.

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Shi, Teng. "Confined States in GaAs-based Semiconducting Nanowires." University of Cincinnati / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1460447182.

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Reynolds, Bryan. "Electronic Transport Properties of Nanonstructured Semiconductors: Temperature Dependence and Size Effects." University of Cincinnati / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1463130513.

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Maelger, Jan. "Perturbative perspectives on the Phase diagram of Quantum ChromoDynamics." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLX050/document.

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L'étude du diagramme des phases de la Chromodynamique Quantique (QCD) et des transitons associées (déconfinement et restoration de la symétrie chirale) représentent des défis majeurs de la Physique moderne et nombreuses sont les approches théoriques qui visent à en sonder les multiples facettes. Du fait de l'intensité de l'interaction forte dans les régimes d’énergie pertinents pour les transitions susmentionnées, ces approches sont en général de nature non-perturbative, la théorie des perturbations étant réputée inapplicable à ces échelles. Il est, cependant, bien établi que le point de départ de la théorie usuelle des perturbations, basée sur la procédure de fixation de jauge de Faddeev-Popov, est ambigu à ces échelles (ambiguïté de Gribov). Dans ce contexte, une approche perturbative modifiée, basée sur le Lagrangien de Curci et Ferrari, a été proposée, via l’ajout phénoménologique d'un terme de masse effectif pour le gluon en jauge de Landau. Cette approche a été testée avec succès, notamment dans sa capacité à reproduire les fonctions de corrélation de la théorie Yang-Mills (et QCD dans la limite de quarks lourds) et la thermodynamique à temperature et potentiel chimique non nuls.Dans cette thèse, nous avons testé la robustesse de ces résultats en évaluant la structure de phase de la QCD avec quarks lourds au deuxième ordre de la théorie des perturbations dans le modèle de Curci-Ferrari et en comparant nos résultats à ceux d'approches nonperturbatives. Nos résultats indiquent que, dans ce régime de quarks lourds, le diagramme de phases est contrôlée perturbativement. Nous avons égalementétendu notre étude au cas de la QCD avec quarks légers en utilisant un schéma de resommation qui exploite la présence de petits paramètres dans le régime infrarouge de la QCD. Dans le secteur des quarks, cette démarche donne lieu à la resommation des fameux diagrammes dits "arc-en-ciel”. Ici, nous généralisons ce formalisme à temperature et densité non nulles et en presence d'un champ de fond gluonique. Nous réalisons une toute première étude qualitative des prédictions du modèle CF concernant l’existence possible d’un point critique dans le diagramme de phases de QCD sur la base d’une version simplifiée des équations générales ainsi obtenues
Unravelling the structure of the QCD phase diagram and its many aspects such as (de)confinementand chiral symmetry breaking, is one of the big challenges of modern theoretical physics, and manyapproaches have been devised to this aim. Since perturbation theory is believed to cease feasibilityat low energy scales, these approaches treat the relevant order parameters, the quark condensate andthe Polyakov loop, non-perturbatively. However, it is also well-established that the starting point forperturbation theory, the Fadeev-Popov gauge-fixing procedure, is inherently ill-defined in the infrareddue to the presence of Gribov ambiguities. In this context, a modified perturbative approach based onthe Curci-Ferrari Lagrangian has been introduced, where a phenomenologically motivated effective gluonmass term is added to the Landau gauge-fixed action. Prior to the beginning of the thesis, this approach hasproven extremely fruitful in its descriptions of (unquenched) Yang-Mills correlation functions and thermodynamics at (non)zero temperature and density.Throughout the thesis we extend this analysis to the entire phase structure of QCD and QCD-liketheories and test the validity of the model in various regimes of interest. For instance, to further aprevious one-loop study in the regime of heavy quark masses, we have computed the two-loop quarksunset diagram in the presence of a non-trivial gluon background in a finite temperature and densitysetting. We come to the conclusion that the physics underlying center symmetry is well-described by our perturbative model with a seemingly robust weak-coupling expansion scheme. Furthermore, we study the regime of light quarks by means of a recently proposed resummation scheme which exploits the presence of actual small parameters in the Curci-Ferrari description of infrared QCD. In the quark sector, this leads to the renown rainbow equations. We extend this first-principle setup to nonzero temperature, chemical potential, and gluon background. We perform a first qualitative analysis of the prediction of the model concerning the possible existence of a critical endpoint in the QCD phase diagram by using a simplified version of these general equations

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Mondelo-Martell, Manel. "Quantum Confinement of Gaseous Molecules in Nanostructures: Effects on the Dynamics and Internal Structure." Doctoral thesis, Universitat de Barcelona, 2018. http://hdl.handle.net/10803/586179.

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Quantum confinement effects, understood as the changes on the structure and dynamics of a molecule when it goes from a free environment to a cavity with some characteristic length of the order of the nanometer, represent both a challenge and an opportunity. A challenge, because there is still work to be done in order to be able to understand and model them properly. An opportunity, because they offer the means to tune molecular properties such as adsorption, diffusion, or even reactivity. The present Doctoral Thesis is focused on the theoretical and computational study of the system consisting on a single H2 (or D2) molecule trapped in the hollow cavity of a narrow Single--walled Carbon Nanotube. Since Dillon and coworkers suggested in 1997 the existence of quantum confinement effects as an explanation for the unexpectedly high H2 uptake in carbon nanotubes, this particular system has received much attention from theoretical and experimental points of view. Here we intend to gain more insight on it by developing new analysis tools for high dimensional eigenstates, and by improving the model with respect to previous works. The former has been achieved through the use of overlap and partial overlap functions, which has provided with an intuitive way to understand the coupling between the different degrees of freedom by comparison of the actual eigenstates of the system with a separable model. Regarding the improvement of the model, we have worked on it from two perspectives: first, we have included new molecular degrees of freedom to the system, namely the motion of the center of mass of the molecule along the axis of the nanotube. This has allowed us to obtain diffusion rates for H2 and D2 inside the nanotube in a full quantum mechanics framework, which to the best of our knowledge had not been achieved before. The study of the diffusion dynamics has also allowed us to define an adiabatic representation of the Hamiltonian, taking advantage of the quasi separability of the diffusion coordinate and the remaining degrees of freedom, to increase the efficiency of the propagations with high accuracy. As a second means to improve the model, we have developed a system--bath coupling Hamiltonian in order to see how the phonons of the nanostructure affect the dynamics of the confined molecule. We have seen that both sets of degrees of freedom (molecular and phonons) are strongly coupled due to the linear momentum exchange between them. Time--dependent Perturbation Theory calculations have determined that the characteristic time for the momentum exchange is shorter than that for diffusion, which suggests that the friction with the nanotube may have a relevant effect on the transport properties of the confined molecule.
Els efectes de confinament quàntic, entesos com els canvis en l’estructura i la dinàmica d’una molècula quan va des d’un entorn lliure a una cavitat amb alguna longitud característica de l’ordre del nanòmetre, representen un repte i una oportunitat. Un repte, perquè encara hi ha feina per poder comprendre-les i modelar-les correctament. Una oportunitat, perquè ofereixen els mitjans per ajustar les propietats moleculars, com adsorció, difusió o fins i tot reactivitat. La present tesi doctoral es centra en l’estudi teòric i computacional del sistema consistent en una sola molècula de H2 (o bé de D2) atrapada a la cavitat interna d’un nanotub de carboni estrets d’una sola paret. Des de que Dillon i coautors van suggerir al 1997 l’existència d’efectes de confinament quàntic com a explicació de la inesperadament alta adsorció de H2 en nanotubs de carboni, aquest tema ha rebut molta atenció des de punts de vista teòrics i experimentals. La intenció d’aquesta Tesi és obtenir més informació sobre aquest fenomen mitjançant el desenvolupament de noves eines d’anàlisi per a estats propis d’alta dimensionalitat, i la millora del model respecte a treballs anteriors. El primer s’ha aconseguit mitjançant l’ús de funcions de solapament i solapaments parcial, que han proporcionat una manera intuïtiva d’entendre l’acoblament entre els diferents graus de llibertat per comparació amb estats propis reals d’un model separable del sistema. Pel que fa a la millora del model, hem treballat des de dues perspectives: en primer lloc, hem inclòs nous graus de llibertat moleculars al sistema, concretament el moviment del centre de massa de la molècula al llarg de l’eix del nanotub. Això ens ha permès obtenir coeficients de difusió per al H2 i el D2 dins del nanotub utilitzant un formalisme totalment mecànic–quàntic, cosa que no s’havia fet prèviament. L’estudi de la dinàmica de difusió també ens ha permès definir una representació adiabàtica de l’Hamiltonià del sistema, aprofitant la quasi separabilitat entre la coordenada de difusió i la resta de graus de llibertat, per tal d’augmentar l’eficàcia de les propagacions amb gran precisió. Com a segon mitjà per millorar el model, hem desenvolupat un Hamiltonià d’acoblament sistema–bany per tal de veure com els fonons de la nanoestructura afecten la dinàmica de la molècula confinada. Hem vist que ambdós conjunts de graus de llibertat (moleculars i fonons) estan fortament acoblats a causa de l’intercanvi de moment lineal entre ells. Càlculs de Teoria de Perturbacions Dependents del Temps han determinat que el temps característic de l’intercanvi de moment és més curt que el de la difusió, cosa que suggereix que la fricció amb el nanotub pot tenir un efecte rellevant sobre les propietats del transport de la molècula confinada.

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31

Capdevilla, Roldan Rodolfo Maia [UNESP]. "Dynamical chiral symmetry breaking: the fermionic gap equation with dynamical gluon mass and confinement." Universidade Estadual Paulista (UNESP), 2013. http://hdl.handle.net/11449/92026.

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Alguns aspectos da quebra de simetria quiral para quarks na representação fundamental são discutidos no contexto das equações de Schwinger-Dyson. Estudamos a equação de gap fermionica incluindo o efeito de uma massa dinêmica para os gluons. Ao estudar esta equação de gap verificamos que a intenção não é forte o suficiente para gerar uma massa dinâmica dos quarks compatível com os dados experimentais. Também discutimos como a introdução de um propagador confinante pode mudar este cenário, exatamente como foi proposto por Cornwall [1] recentemente, desta forma estudamos uma equação de gap completa, composta pela troca de um gluon massivo e por um termo confinante; M('p POT 2') = 'M IND. c('p POT 2') + 'M IND. 1g'('p POT 2'). Encontramos soluções assintótica desta equação de gap nos casos de constante de acoplamento constante e corredora. Este último caso corresponde a um aprimoramento do cálculo com constante de acoplamento constante feito por Doff, Machado e Natale [2]
Some aspects of chiral symmetry breaking for quarks in the fundamental representation are discussed in the framework of the Schwinger-Dyson equations. We study the fermionic gap equation including effects of dynamical gluon mass. Studying the bifurcation equation of this gap equation we verify that the interaction is not strong enough to generate a satisfactory dynamical quark mass. We also discuss how the introduction of a confining propagator may change this scenario as recently pointed out by Cornwall [1], so we study a complete gap equation composed by the one-dressed-gluon exchange term and a confining term: M('p POT 2') = 'M IND. c('p POT 2') + 'M IND. 1g'('p POT 2'). We find asymptotic solutions for this gap equation in the cases of constant coupling and running coupling constant. This last case is an improvement of the constant coupling calculation of Doff, Machado and Natale [2]

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32

Capdevilla, Roldan Rodolfo Maia. "Dynamical chiral symmetry breaking : the fermionic gap equation with dynamical gluon mass and confinement /." São Paulo, 2013. http://hdl.handle.net/11449/92026.

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Orientador: Adriano Antonio Natale
Banca: Adriano Doff Sotta Gomes
Banca: Alex Gomes Dias
Resumo: Alguns aspectos da quebra de simetria quiral para quarks na representação fundamental são discutidos no contexto das equações de Schwinger-Dyson. Estudamos a equação de gap fermionica incluindo o efeito de uma massa dinêmica para os gluons. Ao estudar esta equação de gap verificamos que a intenção não é forte o suficiente para gerar uma massa dinâmica dos quarks compatível com os dados experimentais. Também discutimos como a introdução de um propagador confinante pode mudar este cenário, exatamente como foi proposto por Cornwall [1] recentemente, desta forma estudamos uma equação de gap "completa", composta pela troca de um gluon massivo e por um termo confinante; M('p POT 2') = 'M IND. c('p POT 2') + 'M IND. 1g'('p POT 2'). Encontramos soluções assintótica desta equação de gap nos casos de constante de acoplamento "constante" e "corredora". Este último caso corresponde a um aprimoramento do cálculo com constante de acoplamento "constante" feito por Doff, Machado e Natale [2]
Abstract: Some aspects of chiral symmetry breaking for quarks in the fundamental representation are discussed in the framework of the Schwinger-Dyson equations. We study the fermionic gap equation including effects of dynamical gluon mass. Studying the bifurcation equation of this gap equation we verify that the interaction is not strong enough to generate a satisfactory dynamical quark mass. We also discuss how the introduction of a confining propagator may change this scenario as recently pointed out by Cornwall [1], so we study a "complete" gap equation composed by the one-dressed-gluon exchange term and a confining term: M('p POT 2') = 'M IND. c('p POT 2') + 'M IND. 1g'('p POT 2'). We find asymptotic solutions for this gap equation in the cases of "constant coupling" and "running coupling constant". This last case is an improvement of the constant coupling calculation of Doff, Machado and Natale [2]
Mestre

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33

Williams, Owen Leyton. "A theoretical study of charge confinement in quantum dots : modelling the SnO2 charge writing process." Thesis, Swansea University, 2007. https://cronfa.swan.ac.uk/Record/cronfa42429.

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A suite of models is constructed to facilitate the simulation of the SnO[2] charge writing process. In particular, at dimensions where the semiconductor band bending does not fully evolve, this entails the self-consistent solution of the non-linear Poisson equation and the Kohn-Sham equations at non-zero temperature, with the charge in the occupied surface states also self-consistently reconciled with the fundamental electron density generating the confining potential. In this way, a full quantum mechanical treatment of the discrete eigenstates of the quantum dot, inclusive of electron-electron effects, is made, and a Tip-QD-Substrate tunnelling model developed. This work favourably conforms with observed experimental measurements, not only satisfying the recorded data on the ratios of surface state densities far better than existing models, but also offers a tentative explanation for some of the hitherto unsatisfactorily explained sensitivity behaviour of poly crystal line gas sensors on the decrease of the grain radii. It models the charging of a spherical 4nm radius nanocrystal well, with the calculated I-V characteristic clearly exhibiting indications of the Coulomb blockade effect in good agreement with experiment. The calculated maximum electron complement of one nanocrystal of between 81 and 87 injected electrons with a modal potential difference interval between charge transfer events of 0.065V, is in excellent concordance with the experimentally inferred population of 86 elections, charge storage events occurring at intervals of 0.07V.

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34

Shahid, Robina. "Green Chemical Synthesis of II-VI Semiconductor Quantum Dots." Doctoral thesis, KTH, Funktionella material, FNM, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-104980.

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Nanotechnology is the science and technology of manipulating materials at atomic and molecular scale with properties different from bulk. Semiconductor QDs are important class of nanomaterials with unique physical and chemical properties owing to the quantum confinement effect. Size dependent optical properties make research on semiconductor QDs more attractive in the field of nanotechnology. Semiconductor QDs are usually composed of combination of elements from groups II–VI, III–V, or IV–VI of the periodic table. Group II-VI semiconductor QDs (ZnS, ZnSe, ZnO, CdSe, CdS) are most extensively studied systems, having bandgap which can be engineered through the variation of the material composition and size. Most common QDs are made of CdE (E=S, Se, Te) which are toxic. Recent environmental regulations restrict the use of toxic metals and therefore QDs containing nontoxic metals such as Zn are of great importance. The chemical synthesis of QDs involves different methods. Usually high temperature thermal decomposition of organometallic compounds in high boiling point organic solvents is used which needs long reaction time and involves complex synthesis procedures. New simpler and efficient synthetic routes with alternative solvents are required. Recently the synthesis of non-toxic QDs using green chemical routes is a promising approach receiving increasing attention. The aim of this Thesis is to develop novel routes for synthesis of semiconductor QDs employing green nanomaterial synthesis techniques. Therefore, in this work, we developed different green chemical routes mainly for the synthesis Zn-based QDs. Low temperature synthesis routes were developed for the synthesis of ZnS and ZnO QDs. Microwave irradiation was also used as efficient heating source which creates numerous nucleation sites in the solution, leading to the formation of homogeneous nanoparticles with small size and narrow size distribution. Different polar solvents with high MW absorption were used for synthesis of ZnS QDs. We also introduced ionic liquids as solvents in the synthesis of ZnS QDs using microwave heating. ILs are excellent reaction media for absorbing microwaves and are recognized as ‘green’ alternative to volatile and toxic organic solvents. For ZnS systems, the QDs produced by different methods were less than 5 nm in size as characterized by high-resolution transmission electron microscopy (HR-TEM). Selected area electron diffraction (SAED) patterns revealed that ZnS QDs synthesized by low temperature synthesis technique using conventional heating are of cubic crystalline phase while the QDs synthesized by using MW heating are of wurtzite phase. The optical properties were investigated by UV-Vis absorption spectrum and show a blue shift in absorption as compared to bulk due to quantum confinement effect. The photoluminescence (PL) spectra of ZnS QDs show different defect states related emission peaks and depend on different synthesis methods, high bandedge related emission is observed for ZnS QDs synthesized by using ionic liquids. ZnO QDs synthesized by low temperature route were found to be less than 4 nm in size and also show a blue shift in their absorption. The PL spectrum show bandedge related emission which is blue shifted compared with bulk with no emission originating from surface defect levels. The results show that QDs are of high crystalline quality with narrow size distribution. A comparative study of using conventional and MW heating in the synthesis of CdSe QDs was performed. The reactions involving microwave heating showed enhanced rates and higher yields. The developed methods involve all principles for green nanomaterials synthesis i.e. design of safer nanomaterials, reduced environmental impact, waste reduction, process safety, materials and energy efficiency.

QC 20121115

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Khattak, Shaukat Ali. "Exciton confinement in strain-engineered InAs quantum dots in metamorphic In_{x}Ga_{1-x}As." Thesis, Lancaster University, 2015. http://eprints.lancs.ac.uk/77217/.

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In this work, magneto-photoluminescence at low temperature, 4.2 K, is used to probe the exciton confinement in strain-engineered InAs/In_{x}Ga_{1-x}As/GaAs metamorphic quantum dots (QDs), emitting at telecom wavelengths (1.3 µm - 1.6 µm). The emission wavelength can be tuned by changing two independent parameters, i.e.,indium content, x, in In_{x}Ga_{1-x}As upper and lower confining layers and thickness of lower confining layer (LCL), d. Varying x changes the band offset and QD-CLmismatch (strain inside the QD), while varying d changes only QD-CL mismatch. We investigate the dependence of confinement on the QD-CL mismatch and band offset. Zero-magnetic-field spectra showed that wavelength (PL energy) increases(decreases) with increasing x, for a constant d, and with increasing d, for a constant x, which was attributed to be due to relaxation of strain inside the QD that is, in turn,a function of x and d. No correlation between wavelength and intensity was observed. Magneto-photoluminescence results revealed that for a subset of samples, the exciton effective mass increases linearly, more or less, with increasing QD-CL mismatch (QD strain), while its Bohr radius has no correlation with mismatch. The diamagneticshift coefficient increases 12-fold with decreasing mismatch from ∼ 7.5 % to 4.5 %, which is attributed to low effective mass, which in turn, is due to low QD strain. For high mismatch ( > 5.8 %), the Bohr radius is not determined, implying that it is less than10 nm, smaller than the dot radius. For indium composition x = 0.28 and 0.31, and for d > 1000˚A, the wave-function spills over out of the dot. For x = 0.35, the Bohr radii are, counter intuitively, found to be smaller than for samples with larger band offset (x = 0.31). Initially, it was explained as a spilling of the wave-function over vertically resulting in strong lateral confinement of exciton, but this explanation is not supported by our model calculations. Another explanation is, therefore, presented by carrying out temperature dependence and magnetic field dependence, at various temperatures, of PL energy: there are different dots, at x = 0.35, with different size where thermal escape of carriers from smaller dots to bigger ones occurs with increasing temperature, and the PL energy, in magnetic field, is contributed more by smaller dots than the bigger ones.

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36

Thierry, François. "Etude des propriétés de nanoparticules semiconductrices pour les cellules solaires hybrides." Thesis, Aix-Marseille, 2015. http://www.theses.fr/2015AIXM4381.

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Cette thèse, réalisée dans l'équipe OPTO-PV du laboratoire IM2NP, porte sur l'étude des propriétés particulières des nanostructures de petites dimensions pour des application optoélectroniques. Pour le solaire photovoltaïque, leur utilisation permet d'augmenter l'efficacité et de réduire les coûts. Après avoir étudié les différentes technologies et phénomènes photovoltaïques, nous avons choisi les cellules hybrides organiques - nanosphères semiconductrices comme structures d'étude. Nous avons alors développé une approche numérique de détermination des propriétés intrinsèques des boîtes quantiques. Notre méthode est rapide et nécessite peu de paramètres pour une utilisation à la fois prédictive et explicative. Nous déterminons les propriétés électronique avec l'approximation de la masse effective en la modifiant pour tenir compte de la non-parabolicité des bandes électroniques. Nous utilisons ces résultats pour évaluer les propriétés optiques, particulièrement l'absorption qui joue un rôle important dans le processus photovoltaïque. Nous prenons en compte des effets de couplages diélectriques sur ces propriétés ainsi que des aspects thermodynamiques. Ces outils nous permettent d'étudier l'effet du confinement quantique des charges sur le comportement optoélectroniques de nanostructures de différents types: multipuits couplés, fils de section circulaire et boîtes sphériques. La réalisation et la caractérisation de couches minces de PMMA incorporant des nanosphères homogènes et (cœur)coquille composées de différents semiconducteurs valident notre approche et posent les bases de l'étude de couches actives hybrides pour la réalisation de cellules solaires performantes
This thesis was conducted in the OPTO-PV team of the IM2NP laboratory. Its aim is to study the peculiar properties of low-dimensional nanostructures for use in optoelectronic applications. For photovoltaics in particular, they can be used for the realization of innovative devices with theoretical hight efficiencies at low costs. After we evaluated the various technologies and phenomena that can be used in nanostructured photovoltaics, we decided to choose an hybrid organic polymer - inorganic quantum dots solar cell as study structure. We then developed a numerical approach to determine the intrinsic properties of quantum dots. Our method is fast and requires few parameters so that we can conduct predictive and explicative studies. We start with the evaluation of the electronic properties under the effective mass approximation that we modify to take into account the non-parabolicity of the energy bands. We use the results to derive the optical properties with emphasis on absorption that plays an important role in the photovoltaic process. We take dielectric coupling effects and also thermodynamic effects into account. Those tools allow the study of the effect of quantum confinement on the optoelectronic behavior of various nanostructures: coupled quantum wells, circular cross-section quantum wires and spherical dots. The fabrication and characterization of PMMA thin-films containing homogeneous and (core)shell quantum dots of different semiconductors, validate our approach and constitute the first step towards the study of hybrid active layers for efficient solar cells

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37

DEL, GOBBO SILVANO. "Cadmium sulfide quantum dots: growth and optical properties." Doctoral thesis, Università degli Studi di Roma "Tor Vergata", 2009. http://hdl.handle.net/2108/873.

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Negli ultimi anni, c’è stato un rapido sviluppo delle tecniche di crescita dei materiali nanostrutturati, e un forte impulso è stato dato dall’introduzione delle tecniche di crescita colloidale. Tali tecniche consentono di crescere un ampia gamma di materiali nanostrutturati, metalli e semiconduttori, con elevata cristallinità, dimensioni ridotte (< 5 nm) e con una distribuzione delle dimensioni molto stretta. Il solfuro di cadmio (CdS) nanostrutturato ha promettenti future applicazioni tecnologiche, come ad esempio nei dispositivi optoelettronici, celle solari ad alta efficienza e come tracciante fluorescente in biologia. Tuttavia, per poter sfruttare al meglio le proprietà fisiche a favore delle citate applicazioni è di fondamentale importanza una conoscenza approfondita delle proprietà fisiche. In questa tesi, sono state studiate le proprietà optovibrazionali e optoelettroniche dei quantum dots (QDs) di solfuro di cadmio cresciuti tramite un metodo di crescita colloidale. Tramite i metodi di crescita colloidale è possibile produrre QDs con dimensione ridotta e una distribuzione della dimensione molto stretta. La sintesi dei CdS-QDs consiste nella termolisi a circa 260 °C dello stearato di cadmio in presenza di solfuro di idrogeno in un solvente organico altobollente (1-ottadecene). La velocità della crescita e la dimensione finale dei QDs sono regolate dalla presenza di una molecola surfattante, l’ossido di triottilfosfina (TOPO). In particolare, QDs con una determinata dimensione e con una sua distribuzione molto stretta possono essere ottenuti regolando opportunamente la temperatura di crescita, la concentrazione dei precursori e principalmente la concentrazione del surfattante e del tempo di reazione (crescita arrestata). La morfologia, la dimensione (diametro) e la distribuzione dei diametri sono state determinate tramite TEM. Tramite spettroscopia di assorbimento, si ottengono informazioni sugli stati elettronici, inoltre, sfruttando la relazione esistente tra la band gap e il diametro, si può determinare il diametro medio di un campione di QDs. Le proprietà emissive dei QDs sono state studiate tramite spettroscopia di fotoluminescenza (PL) e dall’energia della banda di PL si può ottenere una stima del diametro medio dei QDs. Dalla larghezza di banda degli spettri di assorbimento e di PL si può ottenere anche una stima sulla distribuzione del diametro dei QDs. Un estesa parte del lavoro riguarda lo studio delle proprietà vibrazionali dei CdS-QDs, tramite spettroscopia Raman. Queste indagini sono state effettuate su campioni di CdS-QDs cresciuti appositamente con diversi diametri. Per eseguire misure micro-Raman, i campioni di CdS-QDs coordinati da molecole di TOPO che hanno una consistenza gelatinosa, sono stati trattati con acido tioglicolico (TGA). Questo trattamento è necessario per avere CdS-QDs in forma di polvere, la quale è più adatta per essere studiata tramite spettroscopia Raman. Per evitare effetti termici negli spettri o il danneggiamento del campione, le misure micro-Raman devono essere effettuate con potenze del laser molto basse. Negli spettri Raman di CdS-QDs si osserva uno spostamento del picco del fonone LO verso frequenze più basse , in particolare, tale spostamento è più marcato per i QDs più piccoli, mentre, al crescere del diametro, la frequenza si avvicina progressivamente a quella del bulk. Questa diminuzione di frequenza è causata dall’espansione del cristallo che avviene nei QDs, con il conseguente indebolimento dei legami per i quali diminuisce la frequenza di risonanza. Oltre a questo, il confinamento quantistico dei fononi è visibile come un allargamento asimmetrico della linea dei fononi, e come l’apparizione di un nuovo picco a circa 270 cm-1. Alcune pubblicazioni assegnano questo picco ai modi di superficie, mentre altri lo descivono come la conseguenza delle nuove regole di selezione dovute dalla bassa dimensionalità. Lo studio ha anche lo scopo di comparare le previsioni teoriche basate sia sul modello “dielectric continuum” che sui fononi di superficie con i risultati sperimentali. È stata trovata una relazione tra i valori di frequenza dei fononi predetti teoricamente e i risultati sperimentali, in particolare, le frequenze dei fononi di superficie sono in accordo con i risultati sperimentali. In conclusione, lo scopo di questo studio consiste nello sviluppo di un metodo per crescere CdS-QDs con le caratteristiche fisiche desiderate (diamtero voluto e distribuzione di diametro stretta) per poterne poi effettuare uno studio sistematico delle proprietà vibrazionali ed elettroniche.
In recent years, there has been a rapid development of the growth techniques of nanostructured materials, and a particular breakthrough was given by the introduction of colloidal growth techniques. These techniques allow to grow by affordable facilities, a wide range of nanostructured materials, metals and semiconductors, with high crystallinity, reduced size, narrow size distribution. Nanostructured cadmium sulfide (CdS) has promising future applications as in the realization of optoelectronic devices, high efficiency solar cells as well as fluorescent biological probe. However, in order to fully exploit the potential technological applications, the study of the physical properties of such materials is of crucial importance. In this thesis, the optoelectronic and optovibrational properties of cadmium sulfide quantum dots (QDs) grown by colloidal chemical method are studied. By the means of colloidal growth, it is possible to grow QDs with reduced size and narrow size distribution. The synthesis of CdS-QDs consists in the thermolysis (T=260 °C) of cadmium stearate in presence of hydrogen sulfide in a high temperature boiling point solvent (1-octadecene). The growth rate and final QDs size are regulated by the presence of the surfactating molecule trioctylphosphine oxide (TOPO). QDs with a determined size and a narrow size distribution can be obtained properly adjusting the growth parameters such as temperature, precursors concentrations, and principally the surfactant concentration and reaction time (arrested growth). The QDs morphology, their size and their size distribution is determined by TEM imaging. By absorption spectroscopy, information regarding the electronic states in QDs are obtained, and exploiting the relation existing between band gap and QD diameter, the mean diameter of the QDS is determined. The emissive properties of the QDs are probed by photoluminescence spectroscopy (PL). From the energy of PL band, an estimation of the QDs diameter can be obtained. Based on the width of absorbance and PL bands, the width of QDs size distributions can be estimated. A large part of the work is concerned with the study of vibrational properties of CdS-QDs by Raman spectroscopy. These investigations are carried out on the CdS-QDs samples purposely grown with different average sizes. In order to perform micro-Raman measurements, the gel-like TOPO-coated CdS-QDs are treated to replace the TOPO layer by thioglycolic acid (TGA). This treatment is necessary in order to have powder-like CdS-QDs being more suitable to a Raman scattering study. To avoid thermal effects or damage to the sample, the micro-Raman measurements must to be performed using very low laser powers (on the sample). In the Raman spectra of CdS-QDs, a decrease of the phonon frequency (red-shift) with respect to the bulk CdS frequency is observed. In particular, the red-shift is expected to be more pronounced for the smallest QDs, while at the increasing of QDs size, the phonon frequency will approach progressively to the bulk value. This red-shift is caused by the lattice expansion and by a subsequent weakening of the bonds which causes a reduction of the resonance frequency. Beyond the red-shift, the quantum confinement is visible also as an asymmetric broadening of the phonon line and by the apparition of a new peak a circa 270 cm-1. Some reports assign this peak to surface modes, while other reports describe this mode as a consequence of new selection rules arising from the reduced dimensionality. The study has also the aim to cross check the theoretical prediction based on the dielectric continuum model and on the surface modes with the experimental results. A relation between the theory and the experiment has been found, in particular, the predicted surface frequencies are in good agreement with the experiments. In conclusion, the goal of this thesis work is to develop a method to grow CdS-QDs with the desired physical characteristics (narrow size distribution) suitable for a systematic study of optical properties (vibrational and electronic).

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38

Palacios-Berraquero, Carmen. "Quantum-confined excitons in 2-dimensional materials." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/275721.

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The 2-dimensional semiconductor family of materials called transition metal dichalcogenides (2d-TMDs) offers many technological advantages: low power consumption, atomically-precise interfaces, lack of nuclear spins and ease of functional integration with other 2d materials are just a few. In this work we harness the potential of these materials as a platform for quantum devices: develop a method by which we can deterministically create single-photon emitting sites in 2d-TMDs, in large-scale arrays. These we call quantum dots (QDs): quantum confinement potentials within semiconductor materials which can trap single-excitons. The single excitons recombine radiatively to emit single-photons. Single-photon sources are a crucial requirement for many quantum information technology (QIT) applications such as quantum cryptography and quantum communication. The QDs are formed by placing the flakes over substrates nano-patterned with protru- sions which induce local strain and provoke the quantum confinement of excitons at low temperatures. This method has been successfully tested in several TMD materials, hence achieving quantum light at different wavelengths. We present one of the very few systems where quantum confinement sites have been shown to be deterministically engineered in a scalable way. Moreover, we have demonstrated how the 2d-based QDs can be embedded within 2d- heterostructures to form functional quantum devices: we have used TMD monolayers along with other 2d-materials - graphene and hexagonal boron nitride - to create quan- tum light-emitting diodes that produce electrically-driven single-photons. Again, very few single-photon sources can be triggered electrically, and this provides a great ad- vantage when considering on-chip quantum technologies. Finally, we present experimental steps towards using our architecture as quantum bits: capturing single-spins inside the QDs, using field-effect type 2d-heterostructures. We are able to controllably charge the QDs with single-electrons and single-holes – a key breakthrough towards the use of spin and valley pseudospin of confined carriers in 2d-materials as a new kind of optically addressable matter qubit. This work presents the successful marriage of 2d-semiconductor technology with QIT, paving the way for 2-dimensional materials as platforms for scalable, on-chip quantum photonics.

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Schneider, Philipp-Immanuel. "Theoretical description of strongly correlated ultracold atoms in external confinement." Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät I, 2013. http://dx.doi.org/10.18452/16829.

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Heutzutage können ultrakalte Atome in unterschiedlichsten optischen Fallenpotenzialen eingefangen werden, während sich ihre Wechselwirkung durch die Ausnutzung von magnetischen Feshbachresonanzen kontrollieren lässt. Der Einschluss und die resonante Wechselwirkung können zu einer starken Korrelation der Atome führen, welche es erlaubt, mit ihnen physikalische Phänomene zu simulieren, deren Simulation mit heutigen Computern nicht durchführbar wäre. Eine maßgeschneiderte Kontrolle der Korrelationen könnte es schließlich ermöglichen, mit ultrakalten Atomen einen Quantencomputer zu implementieren. Um die Flexibilität und gute Kontrollierbarkeit ultrakalter Atome voll ausnutzen zu können, ist das Ziel dieser Dissertation die präzise theoretische Beschreibung stark korrelierter, eingeschlossener Atome an einer Feshbachresonanz. Das Wechselspiel zwischen dem Einschluss der Atome und einer Feshbachresonanz wird in dieser Arbeit zunächst anhand eines von Grund auf hergeleiteten analytischen Modells einer Feshbachresonanz zwischen Atomen in einer harmonischen Falle untersucht. Basierend auf diesem Modell wird ein Ansatz entwickelt, wechselwirkende Atome an einer Feshbachresonanz in einem optischen Gitter über ein Bose-Hubbard-Modell zu beschreiben. Im Gegensatz zu aufwendigeren numerischen Methoden erlaubt das Bose-Hubbard-Modell mit der Einbeziehung nur weniger Blochbänder die präzise Vorhersage der Eigenenergien und des dynamischen Verhaltens der Atome im optischen Gitter. Weiterhin wird eine Methode zur Lösung der zeitabhängingen Schrödingergleiung für zwei wechselwirkende Atome in einem dynamischen optischen Gitter entwickelt. Schließlich wird ein Ansatz vorgestellt, wie sich mit ultrakalten Atomen in einem dynamischen optischen Gitter ein Quantencomputer implementieren ließe. Als Quantenregister dient der korrelierte Mott-Zustand von repulsiv wechselwirkenden Atomen. Quantenoperationen werden durch periodisches Wackeln des optischen Gitters getrieben.
Today, ultracold atoms can be confined in various optical trapping potentials, while their mutual interaction can be controlled by magnetic Feshbach resonances. The confinement and resonant interaction can lead to a strong correlation of the atoms, which allows for the quantum simulation of physical phenomena whose classical simulation is computationally intractable. A tailored control of these correlations might eventually enable the implementation of a quantum computer with ultracold atoms. In order to take advantage of the flexibility and precise control of ultracold atoms, this thesis aims to provide a precise theoretical description of strongly correlated, confined atoms at a magnetic Feshbach resonance. The interplay between the confinement of the atoms and the Feshbach resonance is investigated by deriving from first principles a model that enables the complete analytic description of harmonically trapped ultracold atoms at a Feshbach resonance. This model is subsequently used to develop a Bose-Hubbard model of atoms in an optical lattice at a Feshbach resonance. In contrast to more elaborate numerical calculations, the model can predict the eigenenergies and the dynamical behavior of atoms in an optical lattice with high accuracy including only a small number of Bloch bands. Furthermore, a method id developed that solves the time-dependent Schrödinger equation for two interacting atoms in a dynamic optical lattice. Finally, a proposal for the implementation of a quantum computer with ultracold atoms in a dynamic optical lattice is presented. It utilizes the correlated Mott-insulator state of repulsively interacting atoms as a quantum register. Quantum operations are driven by a periodic shaking of the optical lattice.

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PANTALEI, CLAUDIA. "Single-particle dynamics of helium mixtures and 4He in nanometric confinement." Doctoral thesis, Università degli Studi di Roma "Tor Vergata", 2008. http://hdl.handle.net/2108/473.

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Scopo di questa tesi e' lo studio, tramite Deep Inelastic Neutron Scattering, della dinamica microscopica di due differenti sistemi di elio, a bassa temperatura (circa 2 K): una miscela isotopica (nella fase fluida e vicino al punto di melting) e 4He in in confinamento nanometrico. L'interesse per l'elio, gia' dai primi decenni del `900, nasce dalla sua unica proprieta': e' l'unico elemento in natura a non avere una fase solida allo zero assoluto. A basse temperature, quindi, presenta effetti quantistici, normalmente trascurabili in altri sistemi fisici, che nelle stesse condizioni solidificano. l'elio e' quindi l'unico banco di prova per i modelli teorici quantistici, in particolare per lo studio di bosoni e fermioni interagenti. In questo contesto, molti esperimenti sono stati effettuati sull'elio, sia nella fase liquida che solida. Misure su 3He e 4He hanno mostrato che l'energia cinetica dei liquidi puri dipende dalla densita' del sistema, crescendo con una diminuzione del volume molare. D'altra parte, la dinamica microscopica delle miscele mostra un differente comportamento rispetto al 3He e 4He puri: l'energia cinetica media dell'isotopo piu' leggero, a volumi molari maggiori di 25cm3/mole, sembra essere indipendente dal volume molare e dalla concentrazione. Questo andamento potrebbe essere spiegato da effetti quantistici, come gli effetti di scambio. Nella prima parte del presente lavoro si e' investigata la dinamica delle miscele per volumi molari tra 22cm3/mole e 25cm3/mole, e gli esperimenti compiuti hanno mostrato che in questo range di volumi molari l'energia cinetica media degli atomi di 3He risulta dipendente dal volume molare, aumentando fino ad avere un valore corrispondente a quello del 3He puro. Recentemente sono state compiute anche misure per studiare l'influenza di un confinamento sull'elio. Esperimenti su 4He, adsorbito in superfici piane o substrati porosi, hanno rivelato un elevato aumento nel valore dell'energia cinetica. Questo comportamento e' stato attribuito alla localizzazione degli atomi di He dovuta al potenziale di interazione He-substrato, che influenza fortemente i primi due o tre layers. Questi tipi di effetti possono essere studiati confinando 4He in pori cilindrici con differenti diametri dei pori. Scopo della seconda parte di questa tesi e' stata appunto quella di determinare l'energia cinetica media degli atomi di 4He adsorbiti in sistemi nanoporosi a geometria cilindrica (Xerogel) con due diametri medi dei pori, di 24ºA 160ºA, per valutare la dipendenza della dinamica a singola particella con la dimensione dei pori e con il tipo di geometria. Le misure sono state effettuate a T=2.5K, a pressione di vapor saturo e con un riempimento dei pori del 95%. L'esperimento ha mostrato che l'energia cinetica del 4He nei pori e' maggiore rispetto a quella del 4He liquido nelle stesse condizioni. I risultati sono stati interpretati tramite un modello teorico, secondo il quale gli atomi si posizionano in anelli concentrici, stratificando layer per layer, e con un'energia cinetica dipendente dal rapporto tra il diametro del poro e quello dell'atomo di elio.
The aim of this thesis work is the study, by means of Deep Inelastic Neutron Scatter- ing, of the microscopic dynamics of two different helium systems at low temperature (T=2K): an isotopic helium mixture (in the fluid phase and near the melting point) and a system of 4He in nanometric confinement. The interest in the helium, from the first decades of 1900, is due to its unique features: it is the only element in nature that doesn't have a solid phase at absolute zero. Thus, at low temperatures it presents quantum effects, usually negligible in other physical systems that in this condition crystallise. The helium is thus the unique test-bed for theoretical quantum models, in particular for studying the interacting boson (4He) and fermion (3He) systems. Moreover, if in 4He are added some atoms of 3He it is possible to derive important information about the interplay of these two statistics. In this context, several experiments on liquid and solid helium have been performed. Measurements on pure 3He and 4He have shown that the mean kinetic energy of pure liquids depends on the density of the system and increases decreasing the molar volume. On the other hand, the microscopic dynamics of helium mixtures reveals quite a different picture with respect to pure 3He and 4He: the mean kinetic energy of the light isotope, above a molar volume of 25cm3/mole, shows a remarkable independence from molar volume and concentration. This behaviour could be explained by quantum effects, such as exchange effects. The first part of the present work deals with the experiments performed to investigate the dynamics of the mixtures from 22cm3/mole to 25cm3/mole and shows how, at these low molar volumes, the mean kinetic energy of 3He starts again to be strongly dependent on the molar volume, increasing until reaching, at 22.7cm3/mole, the corresponding value of pure helium. Recent measurements have been also performed to investigate the influence of confinement on helium. Experiments on 4He, adsorbed in flat surface or slit geometry porous substrates, have shown a large increase in helium mean kinetic energy. This has been attributed to the strong localisation effects induced by the helium-substrate interaction potential, which mainly influence the firsts two or three adsorbed layers. Such effects can be also investigated by confining 4He atoms in cylindrical pore geometries and by studying their dynamics as function of pore size. Aim of the second part of the thesis has been the determination of the single particle mean kinetic energy of 4He adsorbed in cylindrical silica nanopores (Xerogel) having two different pore diameters, namely, 24 ºAand 160 ºA, and to evaluate the dependence of single- particle dynamics on pore sizes, layer coverage, and confining system geometry. The measurements have been performed at a temperature of T=2.5K, saturated vapour pressure, and 95% volume filling. Significant changes in the values of the single particle mean kinetic energy are found: they are remarkably higher than the value of normal liquid 4He at the same conditions. The results are interpreted in terms of a model in which 4He atoms are arranged in concentric annuli along the cylindrical pore axis, growing layer-by-layer and with the mean kinetic energy mainly dependent on the ratio between the atomic diameter and the pore diameter.

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Lapitski, Denis. "Development of the Quantum Lattice Boltzmann method for simulation of quantum electrodynamics with applications to graphene." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:e89cd11b-da2c-4c34-be9f-7b3d711e2e64.

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We investigate the simulations of the the Schrödinger equation using the onedimensional quantum lattice Boltzmann (QLB) scheme and the irregular behaviour of solution. We isolate error due to approximation of the Schrödinger solution with the non-relativistic limit of the Dirac equation and numerical error in solving the Dirac equation. Detailed analysis of the original scheme showed it to be first order accurate. By discretizing the Dirac equation consistently on both sides we derive a second order accurate QLB scheme with the same evolution algorithm as the original and requiring only a one-time unitary transformation of the initial conditions and final output. We show that initializing the scheme in a way that is consistent with the non-relativistic limit supresses the oscillations around the Schrödinger solution. However, we find the QLB scheme better suited to simulation of relativistic quantum systems governed by the Dirac equation and apply it to the Klein paradox. We reproduce the quantum tunnelling results of previous research and show second order convergence to the theoretical wave packet transmission probability. After identifying and correcting the error in the multidimensional extension of the original QLB scheme that produced asymmetric solutions, we expand our second order QLB scheme to multiple dimensions. Next we use the QLB scheme to simulate Klein tunnelling of massless charge carriers in graphene, compare with theoretical solutions and study the dependence of charge transmission on the incidence angle, wave packet and potential barrier shape. To do this we derive a representation of the Dirac-like equation governing charge carriers in graphene for the one-dimensional QLB scheme, and derive a two-dimensional second order graphene QLB scheme for more accurate simulation of wave packets. We demonstrate charge confinement in a graphene device using a configuration of multiple smooth potential barriers, thereby achieving a high ratio of on/off current with potential application in graphene field effect transistors for logic devices. To allow simulation in magnetic or pseudo-magnetic fields created by deformation of graphene, we expand the scheme to include vector potentials. In addition, we derive QLB schemes for bilayer graphene and the non-linear Dirac equation governing Bose-Einstein condensates in hexagonal optical lattices.

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Galvani, Benoit. "Modélisation du transport électronique quantique : effet du confinement et des collisions dans les cellules solaires." Electronic Thesis or Diss., Aix-Marseille, 2019. http://www.theses.fr/2019AIXM0402.

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La limite de Shockley-Queisser représente le compromis entre la non-exploitation des photons d’énergie insuffisante et les pertes par thermalisation des porteurs photo-générés à hautes énergies. Il existe des dispositifs photovoltaïques permettant de dépasser cette limite, basés sur les propriétés quantiques des porteurs et de leur transport. La compréhension des phénomènes physiques quantiques est essentiel pour l’élaboration de nouvelles solutions. L’objectif de cette thèse consiste à réaliser une étude numérique des effets liés au confinement et aux collisions dans des cellules solaires. Dans une première partie consacrée au modèle théorique implémenté, nous détaillons le formalisme des fonctions de Green hors-équilibre et son utilisation dans le cadre de notre étude. Nous proposons un modèle numérique permettant de prendre en compte les interactions electron-phonon et électron-photon au moyen de self-energies. Les deux parties suivantes présentent l'application du formalisme dans le cas de deux dispositifs. Le premier système est une cellule à base de puits quantiques. Le calcul de la densité d’états locale a permis de mettre en évidence le phénomène de minibandes survenant dans les systèmes périodiques quantiques. Le second système est une cellule à base de matériaux pérovskites hybrides. Déjà utilisé pour la conception de cellules tandem, il subsiste toutefois des incertitudes concernant les mécanismes de transport des porteurs dans de tels matériaux hybrides. Notre travail a permis d’apporter des éléments de compréhension sur les effets de l’interaction électron-phonon dans ce matériau, notamment sur les caractéristiques optiques et électriques du dispositif
The Shockley-Queisser limit represents the compromise between the non-exploitation of low energy photons andthe thermalization losses of high-energy photo-generated carriers. There are devices that can overcome this limit, based on the quantum properties and transport of carriers. The understanding of the physical phenomena occurring at these nanoscales is a key component to the development of new solutions. The goal of this thesis is to conduct a numerical study of the effects of confinement and scattering in solar cells. In a first part dedicated to the theoretical model, we detail the non-equilibrium Green’s functions formalism and its use in the context of our study. We give details on the numerical model of electron-phonon and electron-photon scatterings with interaction self-energies. The two following parts show examples of application of the Green’s function formalism in the case of two devices. The first system is a multi quantum wells solar cell. Calculations of the local density of states permit to highlight the phenomenon of minibands occurring in such quantum periodic systems. The second system is a solar cell based on perovskite hydrid materials. Already used for the design of tandem cells, there is still uncertainties concerning carrier transport mechanisms in such organic-inorganic materials. Our work has provided information about the effects of electron-phonon scattering in such materials, in particular on the opti-cal and electrical characteristics of the device

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Li, Li. "Study of Metal-Insulator-Metal Diodes for Photodetection." University of Dayton / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=dayton1367319217.

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Briosne, frejaville Clémence. "Transport et confinement optique d'atomes de strontium pour une expérience de microscope à gaz quantique." Thesis, université Paris-Saclay, 2020. http://www.theses.fr/2020UPASP037.

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Les travaux présentés dans ce manuscrit de thèse portent sur la construction d’un nouveau dispositif pour atomes froids de strontium 84. Les expériences réalisées sur ce montage porteront sur la dynamique de relaxation de gaz quantiques hors équilibre. Cette dynamique est étudiée pour des gaz bidimensionnels sur réseau. Ce manuscrit de thèse s'attache à décrire la conception des systèmes optiques utilisés pour piéger et manipuler les gaz lors des expériences. En particulier, le montage optique utilisé pour le transport des atomes entre deux positions de l'enceinte à vide est présenté. La solution choisie pour la réalisation du piège bidimensionnel est elle aussi détaillée. Enfin, un microscope à gaz quantiques est mis en place afin de mesurer in situ les fonctions de corrélation spatiales à partir de la répartition des atomes dans le piège bidimensionnel. Une caractérisation du microscope est présentée dans ce manuscrit. Bien que la conception des différents systèmes optiques soit terminée dans sa première version, il nous reste quelques étapes de construction avant d'achever le montage de notre dispositif expérimental
This manuscript presents the construction of a new quantum ultracold atom experiment using strontium 84. The aim of this experiment is to study the relaxation dynamics of quantum gases initially prepared in an out-of-equilibrium state. We will investigate bidimensional gases on a lattice. This manuscript aims to describe the optical systems designed for trapping and manipulating the atoms during the experiment. Specifically, we present our optical solution to transport the atoms between locations in the vacuum chamber. We also discuss the choices we made to create the bidimensional lattice. Lastly, a quantum gas microscope is implemented to measure the spatial correlation functions from the atoms’ distribution in the lattice. A characterization of the microscope is laid out in this manuscript. Though we determined a first version of our optical systems, there are still a few steps needed to complete the experimental setup

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Issac, Abey. "Photoluminescence Intermittency of Semiconductor Quantum Dots in Dielectric Environments." Doctoral thesis, Universitätsbibliothek Chemnitz, 2006. http://nbn-resolving.de/urn:nbn:de:swb:ch1-200601267.

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The experimental studies presented in this thesis deal with the photoluminescence intermittency of semiconductor quantum dots in different dielectric environments. Detailed analysis of intermittency statistics from single capped CdSe/ZnS, uncapped CdSe and water dispersed CdSe/ZnS QDs in different matrices provide experimental evidence for the model of photoionization with a charge ejected into the surrounding matrix as the source of PL intermittency phenomenon. The distribution of the dark state lifetimes can be described by a power law over a wide range while that of bright state can be described by a power law at shorter times followed by an exponential decay. The lifetimes of the bright and dark states are influenced by the dielectric properties of the surrounding environment. Our experimental results show that the lifetime of the dark state increases with the dielectric constant of the matrix. This is very clear from the linear correlation between αoff and f (ε). We propose a self-trapping model to explain the increase of dark state lifetimes with the dielectric constant of the matrix. A charge will be more stabilized in a medium with high dielectric constant. An energetically more favourable state for an electron in a high dielectric medium decreases the return probability which eventually increases the duration of the off-time. Moreover, the self-trapping model establishes a general model for distribution of states in a matrix. We like to mention, that in the case of bright states, a qualitative observation is the cross over of the on-time power law behavior to an exponential one. The power law part of the decay is nearly matrix independent while the exponential decay, which limits the maximum on-time, strongly depends on dielectric properties of the environment. The exponential part of the on-time probability decays much faster in a high dielectric medium and there exists a linear relation between the time constant of the exponential decay and f (ε). Theoretical background has been provided for the observed results using the recently published DCET model which correlates PL intermittency of QDs with properties of the environment. This supports our previous conjecture of a general model for matrix controlled blinking process. The disagreement between experimentally observed dependence of αoff and f (ε) for different matrices with that of the static tunnelling model proposed by Verberk is due to the fact that the tunneling model considers only an electron transfer between a QD and spatially distributed trap states in vacuum. These states are already stabilized states. It does not assume any medium in between. Therefore, matrix dependent blinking kinetics can not be explained quantitatively by tunneling model even though tunneling between a QD and spatially distributed trap states gives a power law distribution for the blinking kinetics. DCET is a more general (dynamic) model. The bright and dark state parabolas contain QD, charge and the matrix. Therefore, this model could in principle explain matrix dependent blinking kinetics in a better way, for example, the energy difference between the minima of the bright and dark state parabolas (-ΔG0) is defined by the stabilization energy of the system provided by the matrix. However, due to lack of the relevant intrinsic parameters we did not compare this relationship and dependence qualitatively
Betrachtet man die Fluoreszenz einzelner Farbstoffmoleküle oder Halbleiternanokristalle bei kontinuierlicher Anregung, so stellt man fest, dass die im Zeitverlauf beobachtete Intensität einer stochastischen Variation unterliegt, d. h. dass das Chromophor zwischen emittierenden und nicht emittierenden Zuständen, auch Hell- und Dunkelzuständen genannt, hin- und herschaltet. Dieses als Blinken bekannte Phänomen ist physikalisch wie auch technologisch herausfordernd, lässt es doch einerseits die Realisierbarkeit einer Reihe von quantenoptischen Anwendungen, so z. B. auf dem Gebiet der Quantenkryptographie, dem Quantum Computing oder der optischen Schaltungstechnik auf Basis einzelner Quantenobjekte, in naher Zukunft möglich erscheinen. Andererseits setzt es gewissen Anwendungen, die auf die permanente Sichtbarkeit des Chromophors aufbauen, Grenzen, so zum Beispiel der Verwendung als Lumineszenzmarker in der medizinischen Diagnostik. Weiterhin ist festzustellen, dass das Blinken kritisch von den äußeren Bedingungen und von den Umgebungsparametern abhängt. Aus diesen und anderen Gründen ist ein fundamentales Verständnis der physikalischen Ursachen und der Wechselwirkungsprozesse unerlässlich. Die Forschung dazu steckt noch in den Kinderschuhen. Basierend auf umfangreiche Messungen der Fluoreszenzzeitreihen einzelner Nanokristalle aus CdSe und CdSe/ZnS in verschiedenen Umgebungen, zeigt diese Dissertation exemplarisch den Einfluss der Dielektrizitätsparameter auf das Blinken. Zur Erklärung des Sachverhalts wird ein so genanntes Self-Trapping-Modell zu Rate gezogen. Demnach kommt es zu einer Ionisation des Quantenobjekts und anschließender Ladungstrennung, woraufhin die abgetrennte Ladung für eine gewisse Zeit in der Umgebung lokalisiert bleibt. Die Dauer der Lokalisierung und damit der emittierenden und nicht emittierenden Perioden hängt von der dielektrischen Funktion des umgebenden Materials ab. Dies ist als direkter Nachweis für den photoinduzierten Ladungstransfer als Ursache des Fluoreszenzblinkens zu deuten. Die Arbeit demonstriert, dass die experimentellen Zeitreihen die charakteristischen Merkmale eines diffusionsgesteuerten Ladungstransferprozesses besitzen und nimmt dabei den gegenwärtigen wissenschaftlichen Diskurs über geeignete theoretische Modelle des Fluoreszenzblinkens auf

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Gongyo, Shinya. "The Gribov problem beyond Landau gauge Yang-Mills theory." 京都大学 (Kyoto University), 2015. http://hdl.handle.net/2433/199098.

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Morfonios, Christian [Verfasser], and Peter [Akademischer Betreuer] Schmelcher. "Control of coherent transport by the interplay of confinement and magnetic fields in open quantum billiards / Christian Morfonios. Betreuer: Peter Schmelcher." Hamburg : Staats- und Universitätsbibliothek Hamburg, 2014. http://d-nb.info/1064077153/34.

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Scardera, Giuseppe ARC Centre of Excellence in Advanced Silicon Photovoltaics &amp Photonics Faculty of Engineering UNSW. "Correlating structural and optical properties of silicon nanocrystals embedded in silicon nitride: An experimental study of quantum confinement for photovoltaic applications." Publisher:University of New South Wales. ARC Centre of Excellence in Advanced Silicon Photovoltaics & Photonics, 2008. http://handle.unsw.edu.au/1959.4/41472.

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Silicon nanocrystals embedded in silicon nitride have received attention as promising materials for optoelectronic applications. More specifically, band gap engineering of novel materials based on silicon nanocrystals has been proposed for possible application in an all-silicon tandem solar cell within the field of `third generation' photovoltaics. Such an application would require nanocrystals to exhibit quantum confinement whereby the optical and electrical properties of a film could be tuned by controlling the size of these `quantum dots'. This thesis investigates the correlation between the structural and optical properties of silicon nanocrystals grown in silicon nitride multilayer structures via solid phase crystallisation, as part of an experimental investigation into quantum confinement. A study of the relevant processing parameters for the solid phase crystallization of silicon nanocrystals in amorphous silicon nitride is presented and the effectiveness of the multilayer approach for controlling nanocrystal size is demonstrated. Structural characterisation using transmission electron microscopy and glancing incidence x-ray diffraction is complemented with a new application of Fourier transform infrared spectroscopy for the detection of silicon nanocrystals. A case study on the effects of annealing temperature on the photoluminescence from silicon nitride multilayers is presented. While a clear correlation between the structural, molecular and optical properties is demonstrated, evidence of quantum confinement remains ambiguous. The investigation into the limits of parameter space for the formation of silicon nanocrystals in silicon nitride multilayers also leads to the formation of a novel Si-Si3N4 nanocomposite material. A comprehensive study of the photoluminescence from silicon nanocrystals embedded in nitride is presented in the context of homogeneous and multilayer nitride films. Size dependent PL and absorption is demonstrated for silicon nitride multilayers with silicon-rich silicon nitride layer thicknesses varying from 1 to 4.5 nm, indicating the formation of quantum wells. These same structures are annealed to form arrays of silicon nanocrystals. Although the PL and absorption spectra suggest quantum effects, inherent ambiguities remain. The findings in this thesis provide greater insight into the nature of confinement and indicate the need for further research if the successful implementation of these structures into an all silicon tandem cell is to be achieved.

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Rasin, Ahmed Tasnim. "High efficiency quantum dot-sensitised solar cells by material science and device architecture." Thesis, Queensland University of Technology, 2014. https://eprints.qut.edu.au/78822/1/Ahmed%20Tasnim_Rasin_Thesis.pdf.

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This thesis studied cadmium sulfide and cadmium selenide quantum dots and their performance as light absorbers in quantum dot-sensitised solar cells. This research has made contributions to the understanding of size dependent photodegradation, passivation and particle growth mechanism of cadmium sulfide quantum dots using SILAR method and the role of ZnSe shell coatings on solar cell performance improvement.

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Zieliński, Marcin. "Nanoscale engineering of semiconductor heterostructures for quadratic nonlinear optics and multiphoton imaging." Phd thesis, École normale supérieure de Cachan - ENS Cachan, 2011. http://tel.archives-ouvertes.fr/tel-00585601.

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Nonlinear coherent scattering phenomena from single nanoparticles have been recently proposed as alternative processes for fluorescence in multiphoton microscopy staining. Commonly applied nanoscale materials, however, have reached a certain limit in size dependent detection efficiency of weak nonlinear optical signals. None of the recent efforts in detection of second-harmonic generation (SHG), the lowest order nonlinear process, have been able to cross a ~40 nm size barrier for nanoparticles (NPs), thus remaining at the level of "large" nanoscatterers, even when resorting to the most sensitive detection techniques such as single-photon counting technology. As we realize now, this size limitation can be significantly lowered when replacing dielectric insulators or wide gap semiconductors by direct-gap semiconducting quantum dots (QDs). Herein, a new type of highly nonlinear nanoprobes is engineered in order to surpass above mentioned size barrier at the single nanoparticle scale. We consider two-photon resonant excitation in individual zinc-blende CdTe QDs of about 12.5 nm diameter, which provide efficient coherent SHG radiation, as high as 105 Hz, furthermore exhibiting remarkable sensitivity to spatial orientation of their octupolar crystalline lattice. Moreover, quantum confinement effects have been found to strongly contribute to the second-order nonlinear optical susceptibility χ(2) features. Quantitative characterization of the χ(2) of QDs by way of their spectral dispersion and size dependence is therefore undertaken by single particle spectroscopy and ensemble Hyper-Rayleigh Scattering (HRS) studies. We prove that under appropriate conditions, χ(2) of quantum confined semiconducting structures can significantly exceed that of bulk. Furthermore, a novel type of semiconducting hybrid rod-on-dot (RD) QDs is developed by building up on crystalline moieties of different symmetries, in order to increase their effective quadratic nonlinearity while maintaining their size close to a strong quantum confinement regime. The new complex hybrid χ(2) tensor is analyzed by interfering the susceptibilities from each component, considering different shape and point group symmetries associated to octupolar and dipolar crystalline structures. Significant SHG enhancement is consequently observed, exceeding that of mono-compound QDs, due to a coupling between two nonlinear materials and slower decoherence, which we attribute to the induced spatial charge separation upon photoexcitation.

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Dissertations / Theses on the topic 'Quantum confinement'

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