Dissertations / Theses on the topic 'Quantum material'
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1
Zietal, Robert J. "Quantum elecrodynamics near material boundaries." Thesis, University of Sussex, 2010. http://sro.sussex.ac.uk/id/eprint/2520/.
Abstract:
Quantum electrodynamics in free-space is a well-understood and a very successful theory. This is not the case when polarizable boundaries are present, which is a common scenario. The presence of reflective surfaces affects the photon field. Thereby the quantummechanical vacuum fluctuations of the electromagnetic field are constrained leading to changes in the interaction energies of charged particles which are directly measurable. One of the most famous examples of such an effect is the Lamb shift of an atom in front of a perfectly reflecting mirror, which depends on the distance of the atom from the mirror, thus giving rise to an attractive force - the so-called Casimir-Polder force. This thesis touches upon current challenges of quantum electrodynamics with externally applied boundary conditions, which is of increasing importance for nanotechnology and its applications in physics, chemistry and biology. When studying the abovementioned vacuum effects one can use models of various degrees of sophistication for the material properties that need to be taken into account. The simplest is to assume perfect reflectivity. This leads to simple boundary conditions on the electromagnetic field and thereby its quantum fluctuations. The difficulty of such calculations then lies only in the possibly complex geometry of the macroscopic body. The next possible level of sophistication is to allow imperfect reflectivity. The simplest way to achieve this is by considering a material with constant and frequency-independent refractive index. However, for all real material surfaces the reflectivity is frequency-dependent. Causality then requires that dispersion is accompanied by absorption. The aim of this project was twofold: (i) to construct, using well-understood tools of theoretical physics, the microscopic theory of quantum systems, like atoms, interacting with macroscopic polarizable media, which would facilitate relatively simple perturbative calculations of QED corrections due to the presence of boundaries, (ii) to apply the developed formalism to the calculation of the Casimir-Polder force between an atom and a realistic material.
2
Matloob, Mohammad Reza. "Theory of electromagnetic field quantization in material media." Thesis, University of Essex, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.282572.
3
Wang, Qi. "Study of InGaN based quantum dot material and devices." Thesis, University of Sheffield, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.522509.
4
Wong, Huei Ching. "Investigation of quantum dot based material systems for metro-access network." Thesis, University of Bristol, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.437270.
5
Blay, Claire. "Characterisation of intermixed quantum well material by measurements of spontaneous emission." Thesis, University of Bath, 2000. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.323571.
6
BRUNI, FRANCESCO. "NOVEL MATERIAL DESIGN AND MANIPULATION STRATEGIES FOR ADVANCED OPTOELECTRONIC APPLICATIONS." Doctoral thesis, Università degli Studi di Milano-Bicocca, 2017. http://hdl.handle.net/10281/151660.
Abstract:
Il mio progetto di dottorato è stato focalizzato sui semiconduttori organici per applicazioni fotovoltaiche e di fotorivelazione. Inizialmente ho lavorato sul controllo morfologico di blende binarie di molecole organiche e fullereni usando la cosiddetta strategia dei pigmenti latenti. In particolare ho lavorato sull'ingenierizzazione dello strato attivo di celle solari organiche a eterogiunzione. Ho dimostrato una nuova strategia per controllare la segregazione di fase in film sottili di molecole elettron donatrici e fullereni, introducendo nel sistema un network di legami di idrogeno attivato termicamente. Successivamente ho studiato i processi di accumulazione di carica all’interfaccia tra acqua e un semiconduttore polimerico per applicazioni biomediche per mezzo di nanocristalli colloidali biemissivi con alta sensibilità verso agenti elettronattrattori. In fine, ho dedicato l’ultima parte del mio lavoro all’approfondimento delle possibili applicazioni di questa classe di nanocristalli come sensori raziometrici di pH intracellulare e come vernici per il monitoraggio ottico della pressione.My PhD has been focused on organic semiconductors for photovoltaics and photodetecting applications. Initially, I worked on the control of the morphology in binary blends of small organic molecules and fullerenes using the so called latent pigment approach. Subsequently, I investigated the charge accumulation and polarization effect occurring at the interface between water and a polymeric semiconductor used as optical component in retinal prosthesis by means of inorganic colloidal nanocrystals featuring a ratiometric sensing ability for electron withdrawing agents. As a last part of the work, I focalized on the applications of these nanocrystals as ratiometric sensors for intracellular pH probing and pressure optical monitoring. Specifically, during the first part of my PhD, I worked in the field of organic photovoltaics on the morphology engineering of the active layer of small molecules bulk-heterojunction solar cells. I demonstrated a new strategy to fine tune the phase-segregation in thin films of a suitably functionalized electron donor blended with fullerene derivatives by introducing in the system a post-deposition thermally activated network of hydrogen bonds that leads to improved stability and high crystallinity. Moreover, this process increases the carrier mobility of the donor species and allows for controlling the size of segregated domains resulting in an improved efficiency of the photovoltaic devices. This work revealed the great potential of the latent hydrogen bonding strategy that I subsequently exploited to fabricate nanometric semiconductive features on the film surface by using a very simple maskless lithographic technique. To do so, I focalized a UV laser into a confocal microscope and used the objective as a “brush” to thermically induce a localized hydrogen bonding driven crystallization with diffraction limited resolution. My work on organic semiconductors continued with a study on the surface polarization driven charge separation at the P3HT/water interfaces in optoelectronic devices for biologic applications. In this work, I probed the local accumulation of positive charges on the P3HT surface in aqueous environment by exploiting the ratiometric sensing capabilities of particular engineered core/shell heterostuctures called dot-in-bulk nanocrystals (DiB-NCs). These structures feature two-colour emission due to the simultaneous recombination of their core and shell localized excitons. Importantly, the two emissions are differently affected by the external chemical environment, making DiB-NCs ideal optical ratiometric sensors. In the second part of my PhD, I, therefore, focalized on the single particle sensing application of DiB-NCs. Specifically, I used them to ratiometrically probe intracellular pH in living cells. With this aim, I studied their ratiometric response in solution by titration with an acid and a base. Subsequently, I internalized them into living human embryonic kidney (HEK) cells and monitored an externally induced alteration of the intracellular pH. Importantly, viability test on DiB-NCs revealed no cytotoxicity demonstrating their great potential as ratiometric pH probes for biologic application. Finally, I used DiB-NCs as a proof-of-concept single particle ratiometric pressure sensitive paint (r-PSP). In this application, the emission ratio between the core and the shell emission is used to determine the oxygen partial pressure and therefore the atmospheric pressure of the NC environment.
7
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.
Abstract:
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.
8
Pillar-Little, Timothy J. Jr. "CARBON QUANTUM DOTS: BRIDGING THE GAP BETWEEN CHEMICAL STRUCTURE AND MATERIAL PROPERTIES." UKnowledge, 2018. https://uknowledge.uky.edu/chemistry_etds/94.
Abstract:
Carbon quantum dots (CQDs) are the latest generation of carbon nanomaterials in applications where fullerenes, carbon nanotubes, and graphene are abundantly used. With several attractive properties such as tunable optical property, edge-functionalization, and defect-rich chemical structure, CQDs have the potential to revolutionize optoelectronics, electro- and photocatalysis, and biomedical applications. Chemical modifications through the addition of heteroatoms, chemical reduction, and surface passivation are found to alter the band gap, spectral position, and emission pathways of CQDs. Despite extensive studies, fundamental understanding of structure-property relationship remains unclear due to the inhomogeneity in chemical structure and a complex emission mechanism for CQDs.
This dissertation outlines a series of works that investigate the structure-property relationship of CQDs and its impact in a variety of applications. First, this relationship was explored by modifying specific chemical functionalities of CQDs and relating them to differences observed in optical, catalytic, and pharmacological performance. While a number of scientific articles reported that top-down or bottom-up synthesized CQDs yielded similar properties, the results herein present dissimilar chemical structures as well as photoluminescent and metal sensing properties. Second, the role of nitrogen heteroatoms in top-down synthesized CQD was studied. The effect of nitrogen atoms on spectral position and fluorescence quantum yield was considerably studied in past reports; however, thorough investigation to differentiate various nitrogen related chemical states was rarely reported. By finely tuning both the quantity of nitrogen doping and the distribution of nitrogen-related chemical states, we found that primary amine and pyridine induce a red-shift in emission while pyrrolic and graphitic nitrogen produced a blue-shift in emission. The investigation of nitrogen chemical states was extended to bottom-up synthesized CQDs with similar results. Finally, top-down, bottom-up, nitrogen-doped and chemically reduced CQDs were separately tested for their ability to act as photodynamic anti-cancer agents. This series of experiments uncovered the distribution of reactive oxygen species produced during light exposure which elucidated the photodynamic mechanisms of cancer cytotoxicity. The results presented in this dissertation provide key insight into engineering finely-tailored CQDs as the ideal nanomaterial for a broad range of applications.
9
Hatami, Soheil, Christian Würth, Martin Kaiser, Susanne Leubner, Stefanie Gabriel, Lydia Bahrig, Vladimir Lesnyak, et al. "Absolute photoluminescence quantum yields of IR26 and IR-emissive Cd₁₋ₓHgₓTe and PbS quantum dots: method- and material-inherent challenges." Royal Society of Chemistry, 2015. https://tud.qucosa.de/id/qucosa%3A36307.
Abstract:
Bright emitters with photoluminescence in the spectral region of 800–1600 nm are increasingly important as optical reporters for molecular imaging, sensing, and telecommunication and as active components in electrooptical and photovoltaic devices. Their rational design is directly linked to suitable methods for the characterization of their signal-relevant properties, especially their photoluminescence quantum yield (Φf ). Aiming at the development of bright semiconductor nanocrystals with emission >1000 nm, we designed a new NIR/IR integrating sphere setup for the wavelength region of 600–1600 nm. We assessed the performance of this setup by acquiring the corrected emission spectra and Φf of the organic dyes |trybe, IR140, and IR26 and several infrared (IR)-emissive Cd₁₋ₓHgₓTe and PbS semiconductor nanocrystals and comparing them to data obtained with two independently calibrated fluorescence instruments absolutely or relative to previously evaluated reference dyes. Our results highlight special challenges of photoluminescence studies in the IR ranging from solvent absorption to the lack of spectral and intensity standards together with quantum dot-specific challenges like photobrightening and photodarkening and the size-dependent air stability and photostability of differently sized oleate-capped PbS colloids. These effects can be representative of lead chalcogenides. Moreover, we redetermined the Φf of IR26, the most frequently used IR reference dye, to 1.1 × 10⁻³ in 1,2-dichloroethane DCE with a thorough sample reabsorption and solvent absorption correction. Our results indicate the need for a critical reevaluation of Φf values of IR-emissive nanomaterials and offer guidelines for improved Φf measurements.
10
Stavrinou, Paul Nicholas. "A study of InP-based strained layer heterostructures." Thesis, University College London (University of London), 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.261711.
11
Yong, Jennifer Chai Lin. "Investigation of quantum well material systems for 1300nm wavelength high-speed uncooled lasers." Thesis, University of Bristol, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.399933.
12
Farrokhi, M. Javad. "ELECTRONIC PROPERTIES OF ATOMICALLY THIN MATERIAL HETEROSTRUCTURES." UKnowledge, 2019. https://uknowledge.uky.edu/physastron_etds/67.
Abstract:
There is a movement in the electronic industry toward building electronic devices with dimensions smaller than is currently possible. Atomically thin 2D material, such as graphene, bilayer graphene, hBN and MoS2 are great candidate for this goal and they have a potential set of novel electronic properties compare to their bulk counterparts due to the exhibition of quantum confinement effects. To this goal, we have investigated the electric field screening of multilayer 2D materials due to the presence of impurity charge in the interface and vertical electric fifield from back gate. Our result shows a dramatic difference of screening behavior in high and low charging limit, which depends on the number of layers as well. We also have an extensive study on quantum tunneling effect in graphene and bilayer graphene heterojunctions. The peculiar electronic properties of graphene lead to an unusual scattering effect of electron in graphene n-p junction. We implement the cohesive tunneling effect to explain the nonlinear electron transport in ultrashort channel graphene devices. This nonlinear behavior could make them tremendously useful for ultra-fast electronic applications.
13
Moreira, Manuel Viotr A. "Fabrication and characterisation of surface grating DFB lasers using AlGaAs/GaAs quantum well material." Thesis, University of Glasgow, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.361730.
14
Young, Amber Lynn. "Characterization of Immobilized Aqueous Quantum Dots: Efforts in High-Resolution Microscopy." Diss., The University of Arizona, 2011. http://hdl.handle.net/10150/145169.
Abstract:
Semiconductor quantum dots (QDs), particles several nanometers in diameter, exhibit a range of interesting properties that arise as a result of quantum confinement. Among these characteristics is photoluminescence, and unlike traditional fluorophores, the fluorescence emission of QDs is characterized by broad absorption and narrow emission that is a function of the particle diameter. This allows high spatial resolution to be achieved using spectral discrimination of closely spaced QDs.We propose applying QD fluorescence as a tool to sense the local environment of the QD to achieve wide-field sensing at high-resolution. Many factors influence QD fluorescence from the growth parameters and choice of ligand to the local environment of the QD post-fabrication. Nano-materials in the local QD environment influence the spectral or temporal characteristics of the QD fluorescence and detecting these changes enables identification of the location and motion of these nanoparticles with resolution on the order of a few nanometers.We have fabricated aqueous colloidal cadmium telluride QDs, experimenting with the choice of thiol-based ligand to influence the chemistry in post-processing and application. A wide range of tools have been used to characterize the spectral and physical properties of the QDs. We have successfully immobilized QDs on a variety of substrates including glass coverslips, silicon and indium tin oxide coated glass. Immobilization is achieved with even and consistent distributions of QDs on the substrate by using self-assembly of the colloidal particles onto substrates functionalized with N1-(3-Trimethoxysilylpropyl)diethylenetriamine (DETA) silane.Using fluorescence microscopy we have successfully demonstrated the detection of interactions between QDs and other nano-materials including green fluorescent protein and gold seed particles, demonstrating that QDs may, in principle, be used in a wide field microscopy technique to sense nano-materials with high resolution.
15
Menzel, Jan Philipp. "Wavelength-dependent photoreactivity for macromolecular material design." Thesis, Queensland University of Technology, 2021. https://eprints.qut.edu.au/210196/1/Jan%20Philipp_Menzel_Thesis.pdf.
Abstract:
This thesis is a study of light-induced chemical reactions and the dependence of their reactivity and selectivity on the wavelength of light. Both experimental methods using tunable laser systems and light emitting diodes as well as computational simulation methods are developed that establish an understanding of light-induced bond-forming reactions. Information on wavelength-dependent reactivity is used to predict the rate of LED light induced reactions. The design of systems with chemical reaction pathways that are fully controllable by the wavelength of light paves the way to advanced 3D micro- and nano-printing of macromolecular materials through direct laser writing.
16
Iyengar, Ananth Shalvapulle. "Synthesis and characterization of micro/nano material for thermoelectric applications." Case Western Reserve University School of Graduate Studies / OhioLINK, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=case1276182370.
17
Laskin, Gennadii [Verfasser], and Jochen [Akademischer Betreuer] Mannhart. "Fabrication and analysis of nanostructures in the quantum material SrRuO3 / Gennadii Laskin ; Betreuer: Jochen Mannhart." Stuttgart : Universitätsbibliothek der Universität Stuttgart, 2019. http://d-nb.info/1201646308/34.
18
Höglund, Linda. "Growth and characterisation of InGaAs-based quantum dots-in-a-well infrared photodetectors." Doctoral thesis, Linköpings universitet, Materiefysik, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-15774.
Abstract:
This thesis presents results from the development of quantum dot (QD) based infrared photodetectors (IPs). The studies include epitaxial growth of QDs, investigations of the structural, optical and electronic properties of QD-based material as well as characterisation of the resulting components. Metal-organic vapour phase epitaxy is used for growth of self-assembled indium arsenide (InAs) QDs on gallium arsenide (GaAs) substrates. Through characterisation by atomic force microscopy, the correlation between size distribution and density of quantum dots and different growth parameters, such as temperature, InAs deposition time and V/III-ratio (ratio between group V and group III species) is achieved. The V/III-ratio is identified as the most important parameter in finding the right growth conditions for QDs. A route towards optimisation of the dot size distribution through successive variations of the growth parameters is presented. The QD layers are inserted in In0.15Ga0.85As/GaAs quantum wells (QWs), forming so-called dots-in-a-well (DWELL) structures. These structures are used to fabricate IPs, primarily for detection in the long wavelength infrared region (LWIR, 8-14 μm). The electron energy level schemes of the DWELL structures are revealed by a combination of different experimental techniques. From Fourier transform photoluminescence (FTPL) and FTPL excitation (FTPLE) measurements the energy level schemes of the DWELL structures are deduced. Additional information on the energy level schemes is obtained from tunneling capacitance measurements and the polarization dependence studies of the interband transitions. From tunneling capacitance measurements, the QD electron energy level separation is confirmed to be 40-50 meV and from the polarization dependence measurements, the heavy hole character of the upper hole states are revealed. Further characterisation of the IPs, by interband and intersubband photocurrent measurements as well as dark current measurements, is performed. By comparing the deduced energy level scheme of the DWELL structure and the results of the intersubband photocurrent measurements, the origin of the photocurrent is determined. The main intersubband transition contributing to the photocurrent is identified as the QD ground state to a QW excited state transition. Optical pumping is employed to gain information on the origin of an additional photocurrent peak observed only at temperatures below 60 K. By pumping resonantly with transitions associated with certain quantum dot energy levels, this photocurrent peak is identified as an intersubband transition emanating from the quantum dot excited state. Furthermore, the detector response is increased by a factor of 10, when using simultaneous optical pumping into the quantum dots states, due to the increasing electron population created by the pumping. In this way, the potentially achievable responsivity of the detector is predicted to be 250 mA/W. Significant variations of photocurrent and dark currents are observed, when bias and temperature are used as variable parameters. The strong bias and temperature dependence of the photocurrent is attributed to the escape route from the final state in the QW, which is limited by tunneling through the triangular barrier. Also the significant bias and temperature dependence of the dark current could be explained in terms of the strong variation of the escape probability from different energy states in the DWELL structure, as revealed by interband photocurrent measurements. These results are important for the future optimisation of the DWELL IP. Tuning of the detection wavelength within the LWIR region is achieved by means of a varying bias across the DWELL structure. By positioning the InAs quantum dot layer asymmetrically in a 8 nm wide In0.15Ga0.85As/GaAs quantum well, a step-wise shift in the detection wavelength from 8.4 to 10.3 μm could be achieved by varying the magnitude and polarity of the applied bias. These tuning properties could be essential for applications such as odulators and dual-colour infrared detection.On the day of the defence date the status on article IV was: Accepted.
19
Souza, de Almeida Jailton. "Designing and Tuning the Properties of Materials by Quantum Mechanical Calculations." Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-6923.
20
Akram, Nadeem. "Photonic devices with MQW active material and waveguide gratings : modelling and characterisation." Doctoral thesis, KTH, Mikroelektronik och Informationsteknik, IMIT, 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-433.
Abstract:
The research work presented in this thesis deals with modelling, design and characterisation of passive and active optical waveguide devices. The rst part of the thesis is related to algorithm development and numerical modelling of planar optical waveguides and gratings using the Method of Lines (MoL). The basic three-point central-di erence approximation of the δ2=δx2 operator used in the Helmholtz equation is extended to a new ve-point and seven-point approximation with appropriate interface conditions for the TE and TM elds. Di erent structures such as a high-contrast waveguide and a TM surface plasmon mode waveguide are simulated, and improved numerical accuracy for calculating the optical mode and propagation constant is demonstrated. A new fast and stable non-paraxial bi-directional beam propagation method, called Cascading and Doubling algorithm, is derived to model deep gratings with many periods. This algorithm is applied to model a quasi-guided multi-layer anti-resonant reecting optical waveguide (ARROW) grating polarizing structure. In the second part of the thesis, our focus is on active optical devices such as vertical-cavity and edge-emitting lasers. With a view to improve the bandwidth of directly modulated laser, an InGaAsP quantum well with InGaAlAs barrier is studied due to its favorable band o set for hole injection as well as for electron con nement. Quantum wells with di erent barrier bandgap are grown and direct carrier transport measurements are done using time and wavelength resolved photoluminescence upconversion. Semi-insulating regrown Fabry-Perot lasers are manufactured and experimentally evaluated for light-current, optical gain, chirp and small-signal performance. It is shown that the lasers having MQW with shallow bandgap InGaAlAs barrier have improved carrier transport properties, better T0, higher di erential gain and lower chirp. For lateral current injection laser scheme, it is shown that a narrow mesa is important for gain uniformity across the active region. High speed directly modulated DBR lasers are evaluated for analog performance and a record high spurious free dynamic range of 103 dB Hz2=3 for frequencies in the range of 1-19 GHz is demonstrated. Large signal transmission experiment is performed at 40 Gb/s and error free transmission for back-to-back and through 1 km standard single mode ber is achieved.QC 20100827
21
Wigblad, Dan. "Structural and optical characterization of Si/Ge quantum dots." Thesis, Linköping University, The Department of Physics, Chemistry and Biology, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-11672.
Abstract:
In this study silicon-germanium quantum dots grown on silicon have been investigated. The aim of the work was to find quantum dots suitable for use as a thermistor material. The quantum dots were produced at KTH, Stockholm, using a RPCVD reactor that is designed for industrial production.
The techniques used to study the quantum dots were: HRSEM, AFM, HRXRD, FTPL, and Raman spectroscopy. Quantum dots have been produced in single and multilayer structures.
As a result of this work a multilayer structure with 5 layers of quantum dots was produced with a theoretical temperature coefficient of resistance of 4.1 %/K.
22
Peli, S. "UNFOLDING THE ULTRAFAST INTERPLAY BETWEEN DELOCALIZED WAVEFUNCTIONS AND LOCALIZED ELECTRONIC INTERACTIONS IN QUANTUM CORRELATED MATERIALS." Doctoral thesis, Università degli Studi di Milano, 2015. http://hdl.handle.net/2434/259322.
Abstract:
The complex phase diagram of strongly correlated materials is regulated by the delicate interplay between the low-energy electronic excitations and those involving higher energy scales. Here we combine time-resolved optical spectroscopy, conventional laser photoemission (tr-ARPES) and XUV-laser photoemission (XUV-ARPES) to investigate, with an out-of-equilibrium approach, the high-energy electron dynamics in two families of superconducting copper oxides: the single-layer Bi2Sr2-xLaxCuO6+d (Bi2201) and double-layer Bi2Sr2Ca0.92Y0.08Cu2O8+d (Bi2212). We focused on the pump induced modification of the charge-transfer (CT) transition at >2 eV, that strongly reflects the correlation degree of the electronic wavefunction. We observe a qualitative change of the dynamics of the CT transition at T=300 K and hole doping p_cr=0.17+-0.02. We argue that the observed change at p_cr is intimately connected to the quantum critical point at T=0, from which different charge and spin ordering tendencies emerge. Furthermore, time-resolved XUV angle-resolved-photoemission experiments allowed us to track the transient occupation of both the conduction and the oxygen bands during the relaxation dynamics. Our results unveiled the different nature (bonding, non-bonding) of the oxygen bands at 1.5 eV binding energy. This is reflected in a strong bottleneck in the relaxation of the holes photoexcited in the O-2p-pi band at (pi,pi) which is non-bonding with the 3dx^2-y^2 Cu states.These results challenge the state-of-the-art models that describe the relaxation dynamics in copper oxides.
23
Zafarullah, Ijaz. "Thulium ions in a yttrium aluminum garnet host for quantum computing applications material analysis and single qubit operations /." Thesis, Montana State University, 2008. http://etd.lib.montana.edu/etd/2008/zafarullah/ZafarullahI0508.pdf.
Abstract:
Rare-earth-doped crystals have been used for optical signal processing and storage applications. In this dissertation, their potential for quantum computing applications is explored. In one quantum computing scheme, information is stored in nuclear spin states and this information is then processed by using optical pulses through the coupling of these nuclear spin states to a common electronic level. To implement this scheme, nuclear spin states and coupling of these nuclear spin states to a common electronic level is required. Preliminary work in rare-earth materials like Pr3+ and Eu3+ has shown promising results regarding their suitability for quantum computing applications. One particular problem with these materials is that their transition wavelengths are only accessible with dye lasers. These lasers are inherently unstable, and currently few available systems exhibit the stability required for quantum computing applications. An alternative choice was to investigate other rare-earth ions like thulium. Thulium has a transition wavelength that can be accessed with diode lasers, which are commercially available, easy to stabilize, and compact. This dissertation is based on our investigations of Tm3+:YAG for quantum computing applications. Investigations involved a detailed characterization of the material. Nuclear spin states, in Tm3+:YAG, were obtained by applying an external magnetic field to the sample. First, interaction of an external magnetic field with the thulium ions at various sites in the crystal was analyzed. This analysis was used to measure the magnetic anisotropy in the material. These results show that it is possible, with the suitable choice of the magnetic orientation and the site in the crystal, to build a working 3-level quantum system. In the demonstration of single qubit operations in Tm3+:YAG, we first theoretically studied the effect of Gaussian spatial beam on the single qubit operations. Later on, we experimentally prepared a single isolated ensemble of ions in the inhomogeneously broadened absorption profile of the medium. This single isolated ensemble of ions was used as a test-bed to implement the single qubit operations. We also isolated two ensembles of ions in the inhomogeneous absorption profile of the medium. The interaction between these two isolated ensembles of ions was also studied.
24
Frisk, Andreas. "Growth and characterization of Ge quantum dots on SiGe-based multilayer structures." Thesis, Linköping University, Department of Physics, Chemistry and Biology, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-16674.
Abstract:
Thermistor material can be used to fabricate un-cooled IR detectors their figure of merit is the Temperature Coefficient of Resistance (TCR). Ge dots in Si can act as a thermistor material and they have a theoretical TCR higher than for SiGe layers but they suffer from intermixing of Si into the Ge dots. Ge dots were grown on unstrained or strained Si layers and relaxed or strained SiGe layers at temperatures of 550 and 600°C by reduced pressure chemical vapor deposition (RPCVD). Both single and multilayer structures where grown and characterized. To achieve a strong signal in a thermal detector a uniform shape and size distribution of the dots is desired. In this thesis work, an endeavor has been to grow uniform Ge dots with small standard deviation of their size. Scanning electron microscopy (SEM) and Atomic force microscopy (AFM) have been used to characterize the size and shape distribution of Ge dots. Ge contents measured with Raman spectroscopy are higher at lower growth temperatures. Simulation of TCR for the most uniform sample grown at 600°C give 4.43%/K compared to 3.85%/K for samples grown at 650°C in a previous thesis work.
Strained surfaces increases dot sizes and make dots align in crosshatched pattern resulting in smaller density, this effect increases with increasing strain.
Strain from buried layers of Ge dots in a multilayer structure make dots align vertically. This alignment of Ge dots was very sensitive to the thickness of the Si barrier layer. The diameter of dots increase for each period in a multilayer structure. When dots are capped by a Si layer at the temperature of 600°C intermixing of Si into the Ge dot occurs and the dot height decrease.
25
Pal, Singh Amrit [Verfasser], and Roman [Akademischer Betreuer] Schnabel. "Intensity-dependent phase shifts in optical materials for quantum state preparation and absorption measurements in thin film coatings and bulk material / Amrit Pal Singh ; Betreuer: Roman Schnabel." Hamburg : Staats- und Universitätsbibliothek Hamburg, 2018. http://d-nb.info/1161530266/34.
26
Yang, Hung-Yu. "Novel Electromagnetic Responses in Topological Semimetals: Case Studies of Rare-Earth Monopnictides and RAlX Material Family." Thesis, Boston College, 2021. http://hdl.handle.net/2345/bc-ir:109188.
Abstract:
Thesis advisor: Fazel TaftiSince the idea of topology was realized in real materials, the hunt is on for new candidates of topological semimetals with novel electromagnetic responses. For example, topological states can be highly conductive due to a topological protection, which can be destroyed in a magnetic field and lead to an extremely high magnetoresistance. In Weyl semimetals, a transverse current that would usually require a magnetic field to emerge, can be generated by intrinsic Berry curvature without a magnetic field -- the celebrated anomalous Hall effect. In this dissertation, both phenomena mentioned above are studied in rare-earth monopnictides and RAlX material family (R=rare-earths, X=Ge/Si), respectively. The monopnictides are ideal for the study of extreme magnetoresistance because of their topological transitions and abundant magnetic phases. In LaAs, we untied the connection between topological states and the extreme magnetoresistance, the origin of which is clarified. In HoBi, we found an unusual onset of extreme magnetoresistance controlled by a magnetic phase dome. On the other hand, RAlX material family is a new class of Weyl semimetals breaking both inversion and time-reversal symmetries. In particular, in PrAlGeₓSi₁₋ₓ (x=0-1), we unveiled the first transition from intrinsic to extrinsic anomalous Hall effect in ferromagnetic Weyl semimetals, and the role of topology is discussed. In CeAlSi, we found that the Fermi level can be tuned as close as 1 meV away from the Weyl nodes; moreover, a novel anomalous Hall response appears only when the Fermi level is tuned to be near the Weyl nodes. Thus, we established a new transport response solely induced by Weyl nodes
Thesis (PhD) — Boston College, 2021
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Physics
27
Ji, Botao. "Synthèse et propriétés optiques de quantum dots fluorescents plasmoniques." Phd thesis, Université Pierre et Marie Curie - Paris VI, 2014. http://pastel.archives-ouvertes.fr/pastel-01065068.
Abstract:
Grâce aux plasmons de surface des nanoparticules métalliques et aux propriétés optiques et électroniques exceptionnelles des quantum dots (QDs), les nanostructures hybrides QD/métal ont suscité beaucoup d'intérêt depuis une dizaine d'années en raison de leurs nombreuses applications potentielles. Cependant, les hybrides QD/or colloïdaux n'ont été que rarement obtenus bien que ces structures soient particulièrement prometteuses du point de vue optique et qu'elles puissent être manipulées très facilement, car dispersées en solution. Dans cette étude, nous avons réussi à mettre au point pour la première fois une méthode de synthèse généralisée permettant d'obtenir des structures hybrides cœur/coque/coque QD/SiO2/Au (autrement appelés QDs dorés). Tout d'abord, les QDs hydrophobes ont été encapsulés individuellement dans des billes de silice par une méthode d'émulsion inverse. Les nanoparticules QDs/SiO2 ainsi obtenues ont ensuite été recouvertes d'une nanocoque continue d'or via un processus de dépôt en solution. Les épaisseurs de la silice et de la couche d'or - deux paramètres importants pour les QDs dorés - peuvent être ajustées indépendamment afin d'obtenir les dimensions souhaitées. Nous avons montré que les QDs dorés individuels à base de QDs CdSe/CdS à coque épaisse possèdent une émission stable et poissonienne à température ambiante et sont très photostables. Cette nouvelle structure de QDs dorés se comporte comme un résonateur plasmonique avec un facteur de Purcell élevé (~6), en très bon accord avec les simulations. Nous présentons également des auto-assemblages de QDs hydrophobes en superparticules (SPs). Un choix judicieux de QDs donne aux SPs des propriétés exceptionnelles telles qu'une émission de fluorescence intense, non-clignotante et multicolore. Des SPs multifonctionnelles peuvent aussi être obtenues en associant des nanocristaux magnétiques et fluorescents. La croissance d'une coque de silice sur les SPs a permis d'augmenter leur stabilité et nous avons démontré que cette couche de silice pouvait elle-même être recouverte d'une nanocoque d'or pour améliorer la photostabilité et la biocompatibilité de ces SPs.
28
Wingen, Georg. "Vertikale Wanderwellenmodulatoren auf Quantenfilmbasis für die optische Informationsverarbeitung - Vertical travelling-wave modulators using quantum-well material for optical information processing." Gerhard-Mercator-Universitaet Duisburg, 2001. http://www.ub.uni-duisburg.de/ETD-db/theses/available/duett-10122001-113233/.
Abstract:
The vertical optical travelling-wave modulator represents a new high-frequency element for optical data processing. The modulator is designed for a wavelength of 1200 nm and combines the layered structure of vertical optical modulators based on pseudomorphic InGaAs/GaAs quantum-wells with microwave lines from the MMIC technology. The quantum confined Stark effect is used to modulate the optical signal. The multiple quantum-well structure is embedded into an asymmetrical Fabry-Perot resonator built from a Bragg reflector and an aluminum layer to increase the electrooptical effect. The modulators produces a reflectivity change of 0,28. Large scale travelling-wave modulators are capable of critical frequencies higher than 40 GHz. By varying the width of the central conductor of the microwave line, the slow-wave factor could be varied experimentally between 4.8 and 12,7. The applied microwave produces a modulation lattice along the microwave line which can be used in a high frequency beam-steering element for optical information processing
29
Silva, Carlos Jacinto da. "Caracterização termo-óptica de materiais lasers usando a técnica de lente térmica." Universidade de São Paulo, 2006. http://www.teses.usp.br/teses/disponiveis/76/76132/tde-09042008-094626/.
Abstract:
Neste trabalho investigamos as propriedades térmicas (difusividade, condutividade, variação do caminho óptico com a temperatura, etc) e processos de perdas devido a interações entre íons em materiais lasers vítreos e cristalinos. Desde que a eficiência quântica de fluorescência, , está diretamente relacionada com esses mecanismos que levam a supressão da luminescência, os estudos foram realizados principalmente observando os efeitos desses agentes sobre . Espectros de lente térmica (LT) foram usados para determinar , eficiência de transferência de energia matriz-íon, e para analisar efeitos de sítios de defeitos sobre . Um novo método usando a técnica de LT foi proposto para determinar e o coeficiente de temperatura do caminho óptico. Com esta nova abordagem analisamos em materiais vítreos os mecanismos de supressão da luminescência em função da concentração de íons de Nd. Também a usamos para estudar efeitos de radicais de OH e outras impurezas em matrizes vítreas de fosfato dopadas com Yb3+. Processos de conversão ascendente Auger, os quais são importantes em sistemas lasers de alta potência, foram investigados em vidros e cristais dopados com Nd3+. Nos vidros o estudo foi realizado em função da concentração de íons dopantes. Neste estudo a técnica de LT mostrou ser muito sensível, apresentando resultados com erros bem menores que os existentes na literatura. Seguindo o estudo de perdas, investigamos distorção óptica induzida pela luz em função da temperatura, potência de excitação e polarização no cristal ferroelétrico SBN. Usando a técnica de LT, investigamos as propriedades térmicas através da transição de fase ferroelétrica-paraelétrica em cristais de SBN. Complementando, esta tese contribui significativamente para a caracterização de materiais laser, considerando que vários dos mecanismos de perdas estudados propriamente devem ser levados em conta em projetos de laser. Este trabalho também apresenta a técnica de lente térmica como uma ferramenta valiosa para tal estudo.In this work we investigate the thermal properties (diffusivity, conductivity, temperature coefficient of the optical path length change, etc) and loss processes owing to ion-ion interactions in laser glassy and crystalline materials. Since the fluorescence quantum efficiency, , is directly related to these mechanisms that lead to luminescence quenching, the studies were performed mainly observing the effects of these processes on . Thermal lens (TL) spectra were used to determine , energy transfer efficiency between matrix-ion, and to analyze effects of \"dead site\" on . A new method based on the TL technique was proposed to determine q and the temperature coefficient of the optical path length change. This new approach was used to investigate concentration quenching mechanisms in glassy materials. It was also used to study effects of OH radicals and other impurities in Yb3+ doped phosphate glasses. Upconversion Auger processes, which are very important for high power laser systems, were investigated in Nd3+ -doped glasses and crystals. In glasses the study was performed as a function of doping ions. In this study the TL showed to be very sensitive, presenting results with uncertainties much smaller than previous literatures. Following the study of losses, we investigate light induced optical distortion as a function of temperature, excitation power, and polarization in SNB ferroelectric crystal. Using the TL technique, the thermal properties through the phase transition ferroelectric-paraelectric in SBN crystals were investigated. In addition, this thesis significantly contributes for characterization of laser materials with potential applications, considering that severa1 of the loss mechanisms studied here must be considered in laser design. This work also presents the TL technique as a valuable to01 for the present study.
30
Diac, Andreea Petronela. "Design, synthesis and supramolecular architectures of new heterocyclic compounds with potential applications in material chemistry and photovoltaic conversion." Thesis, Angers, 2015. http://www.theses.fr/2015ANGE0019/document.
Abstract:
La thèse intitulée «Design, Synthesis and SupramolecularArchitectures of New Heterocyclic Compounds with PotentialApplications in Material Chemistry and Photovoltaic Conversion” eststructurée en cinq chapitres traitant de nouveaux: a)cyclopenta[c]pyrannes hétérocyclique; b)des propriétés fluorescentes; d) potentiels dispositifs de l'électroniquemoléculaire; d) donneurs moléculaires pour les photovoltaïquesorganiques et e) carbon‘quantum’dots électroluminescents.Le premier chapitre présente une étude des dérivéspseudoazulenique ayant une unité cyclopenta[porte sur leur synthèse, l'analyse structurale et leur comportement dansdes réactions de substitution électrophile pour obtenir des composésayant des propriétés fluorescentes.Le deuxième chapitre présentediastéréoisomères et l'étude de propriétés de fluorescencedérivés d’indenopyrone.Le troisième chapitre décrit la synthèse des nouvellesarchitectures basées sur l’unité cyclopenta[être modifiés structurellement par l'influence d'un stimulus chimiqueou électrochimique afin d'élaborer des potentiels dispositifs del'électronique moléculaire.Dans le quatrième chapitre, la synthèsedes propriétés électroniques des nouvelles molécucellules solaires organiques (OSC) ontLe cinquième et dernier chapitre décrit la passivation desdéfauts de surface des nanoparticules de carbone avec desmolécules organiques ou des polymères pour obtenir desnanoparticules de carbone photoluminescentse surnommé ‘quantum dotsThe thesis entitled “Design, Synthesis and SupramolecularArchitectures of New Heterocyclic Compounds with PotentialApplications in Material Chemistry and Photovoltaic Conversion” isstructured into five chapters concerning new: a) heterocycliccyclopenta[c]pyrans; b) indenopyrone derivatives with fluorescentproperties; c) potential devices of molecular electronics; d)donors for organic photovoltaics and e) electroluminescent carbon‘quantum’ dots.The first chapter presents a study of pseudoazulenederivatives having a cyclopenta[c]pyran unit. The survey comprises thesynthesis, structural analysis and reactivity towards electrophilicsubstitution in order to obtain fluorescent compounds.The second chapter deals with the separation odiastereoisomers and the study of fluorescent propertiesindenopyrone derivatives.The third chapter describes the synthesis of newarchitectures based on cyclopenta[c]pyran unit that can be structurallymodified by the influence of a chemical or electrochemical stimulus inorder to work as potential devices in molecular electronics.In the fourth chapter, the synthesis andelectronic properties of new molecular donors for organic solar cellswas described.The fifth and last chapter outlines the passivation of surfacedefects on carbon nanoparticles using small organic molecules orpolymers in order to obtain photoluminescent carbon nanoparticlesdubbed as carbon‘quantum’dots
31
Schober, Giulio Albert Heinrich [Verfasser], and Manfred [Akademischer Betreuer] Salmhofer. "Quantum Field Theory of Material Properties: Its Application to Models of Rashba Spin Splitting / Giulio Albert Heinrich Schober ; Betreuer: Manfred Salmhofer." Heidelberg : Universitätsbibliothek Heidelberg, 2016. http://d-nb.info/1180738195/34.
32
Zhang, Kuanshou. "Intracavity optical nonlinear devices using X(2) quasi-phase-matched material : classical and quantum properties and application to all-optical regeneration." Paris 6, 2002. http://www.theses.fr/2002PA066553.
33
Khatib, Maher Al. "EPR Spectroscopy for the investigation of materials of technological and industrial interest." Doctoral thesis, Università di Siena, 2019. http://hdl.handle.net/11365/1070360.
Abstract:
The research presented in this Doctoral Thesis discusses mainly the use of Electron Paramagnetic Spectroscopy for the characterization of materials of technological interest. The longitudinal relaxation properties of a vanadyl porphyrin complex have been investigated using pulse EPR experiments at Q (34 GHz) and J-band (263 GHz) frequencies, and the molecule proposed as suitable candidate for quantum processors engineering. The experimental knowledge developed through these relaxation studies, have been transferred to the field of melanins biopigments characterization. The interest for this class of biopigments was derived from the vast amount of applications melanin can cover in the electrochemical and optoelectronic field (e.g. low immunoresponse coating for medical electroanalytical devices, or UV-Vis radiation absorber for solar energy harvesting devices). A novel bacterial melanin from Streptomyces cyaneofuscatus bacteria, and melanin pigments of enzymatic origin, were first studied through S (4 GHz), X (9 GHz) and Q-band (34 GHz) multifrequency EPR. The composition of the bacterial and enzymatic pigments was described, with the support of computer simulation and existing literature in the field. The relaxation properties of these melanin pigments were investigated by means of X and Q-band continuous wave EPR, as well as with Q-band pulse EPR experiments. Differences in terms of longitudinal relaxation times were observed for the melanin pigments of different origin, so that pulse EPR could be proposed either as a tool to distinguish among different melanin species, as well as probe to investigate the structure and dynamics of the radical species present in these natural pigments. A last chapter on the use of computer simulations for the modeling of the electrochemical devices that could be designed to host melanin coated electrodes is presented. In that context, a general model for the evaluation of electrodic currents generated under different geometrical and physical parameters of the systems has been proposed. The physical description was carried out using a dimensionless form of the governing equations, so that the findings of that research can be adapted to particular cases of study. The diverse content of the thesis is thought to reflect the multidisciplinary nature of materials research.
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Doñate, Buendía Carlos. "Synthesis of nanomaterials by high throughput pulsed-laser based systems: Application in biomedicine and material processing." Doctoral thesis, Universitat Jaume I, 2019. http://hdl.handle.net/10803/668339.
Abstract:
The work developed in this thesis is based on the study of systems for the generation of nanoparticles in liquids by means of pulsed lasers, improving their production and using the generated nanoparticles in applications of great technological relevance such as biomedical imaging or additive manufacturing. For the femtosecond laser production improvement, the novel implementation of a spatio temporal focusing system is proposed which, by varying the temporal pulse duration out-of-focus, suppresses nonlinear effects in the liquid medium and the associated energy losses. For the colloidal size reduction by fragmentation, a continuous flow system is proposed that allows to increase the control over the irradiation parameters while ensuring homogeneous irradiation. Finally, the carbon quantum dots generated are used as fluorescent markers and different metal and oxide nanoparticles are synthesized for their later application as bactericides and in the improvement of materials used in additive manufacturing.El trabajo desarrollado en esta tesis se basa en el estudio de sistemas de generación de nanopartículas en líquidos mediante láser pulsado, mejorando su producción y empleando los nanomateriales generados en aplicaciones como imagen biomédica o additive manufacturing. Para la mejora en la producción mediante láser de femtosegundo se propone la implementación de un sistema de focalización espacio-temporal que, mediante la variación de la duración temporal de los pulsos fuera de foco, consigue suprimir los efectos no lineales en el medio líquido. Para la mejora en el proceso de reducción del tamaño de coloides, se propone un sistema de flujo continuo que aumenta el control sobre los parámetros de irradiación. Finalmente, los puntos cuánticos de carbono generados se utilizan como marcadores fluorescentes y se sintetizan distintas nanopartículas metálicas y óxidos para su posterior aplicación como bactericidas y en la mejora de materiales utilizados en fabricación aditiva.
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Ferreira, Danilo Cardoso [UNESP]. "Elaboração de um material didático aplicado ao ensino de física para utilização do experimento virtual da dupla fenda." Universidade Estadual Paulista (UNESP), 2015. http://hdl.handle.net/11449/132892.
Abstract:
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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
A mecânica quântica é uma das áreas da Física que surgiu em meados de 1900 e permanece em desenvolvimento até os dias atuais. Diversos aparatos tecnológicos são consequência deste importante ramo da Física que também contribui com a Medicina, Matemática, Filosofia, Literatura e Biologia. Logo, é imprescindível que o contato com esta ciência ocorra no contexto do ensino médio. Para inserir o estudante no mundo da mecânica quântica, isto é, na física do infinitamente pequeno, o aluno deve abandonar o pensamento clássico e pensar em termos do comportamento quântico e do indeterminismo no processo de medida, isto é, desenvolver a capacidade de abstração. Sendo este, o objetivo deste trabalho. Para tanto, foi escolhido o experimento da dupla fenda que permite trabalhar com a dualidade onda-partícula do elétron e do fóton. Por meio deste experimento, o aluno pode ser inserido paulatinamente numa trajetória rumo a descrição quântica necessária para o exame dos fenômenos subatômicos. A análise experimental é cuidadosamente realizada com o auxílio de laboratórios virtuais, disponibilizados gratuitamente em sítios eletrônicos, os quais representam um recurso que permite realizar procedimentos experimentais que necessitariam de grande aparato laboratorial. O experimento da dupla fenda é analisado em três etapas, relatadas a seguir: (i) a dupla fenda com partículas clássicas; (ii) a dupla fenda com ondas clássicas e; (iii) a dupla fenda com objetos quânticos como elétrons e fótons. O objetivo é demonstrar o comportamento dual do elétron. Posteriormente, para concluir de forma precisa e justificar o comportamento quântico do elétron é apresentado o princípio da indeterminação de Heisenberg e suas implicações filosóficas. Sendo assim, o objetivo central desta pesquisa é buscar integrar o comportamento quântico, que acontece na escala atômica, principalmente no contexto do ensino médio. Algumas orientações sobre como aplicar este trabalho em outros níveis de ensino aparecem no decorrer do texto e nos apêndices. Apresentando o comportamento dual, onda–partícula, do elétron, a interpretação probabilística e o princípio de incerteza. Acreditamos que o aluno será capaz de compreender um grande número de fenômenos que acontece em escalas que não são do domínio da mecânica clássica quando, em contato com estes temas. Este tema faz parte do conteúdo de física moderna contemporânea que vem sendo abordado em livros textos e vestibulares. Além disso, algumas das novas tecnologias utilizam a física quântica, desde microscópios eletrônicos, nanotecnologia, computação quântica, semicondutores, diodos (incluindo o LED), transistores, computadores, tablets, GPS, satélites, radares, aviões, lasers, scanners de código de barras, sistemas militares de defesa, CD e Blu-Ray players, criptografia, células fotoelétricas, sensores diversos, basicamente, tudo que é eletrônico. Um dos objetivos do trabalho é verificar quais os conhecimentos prévios o corpo discente possui, antes do contato com o conteúdo de física quântica, ou seja, o que faz parte do senso comum sobre este tema. Além disso, pretendemos verificar se o aluno consegue: i) distinguir, no final da aplicação desta pesquisa, que as leis da física em escalas atômicas são diferentes das leis da física clássica, ii) a importância da mecânica quântica na tecnologia e na sociedade.
Quantum mechanics is one of the areas of physics that emerged in mid-1900 and remains in development to the current day. Several technological devices are a result of this important branch of physics that also helps to Medicine, Mathematics, Philosophy, Literature and Biology. Therefore, it is essential that contact with this science occurs at the high school level, what actually occurs in a limited way, when it happens. To place the student in the world of quantum mechanics, that is, the infinitely small of physics, the student must leave the classical thought and think in terms of the quantum behavior and indeterminacy in the measurement process, namely to develop the capacity for abstraction. This is accurately the aim of this work. Thus, the double-slit experiment that lets you work with the wave-particle duality of the electron and the photon was chosen. Through this experiment, students can be gradually inserted on a path toward quantum description necessary for the examination of subatomic phenomena. The experimental analysis is carefully performed with the aid of virtual laboratories, available for free in electronic sites, which represent a resource to perform experimental procedures that would require large laboratory apparatus. The double slit experiment is analyzed in the following three steps, reported: (i) the slit paired with classical particles; (ii) the double slit and with classical waves; (iii) the double slit with quantum objects such as electrons and photons. The goal is to demonstrate the electron dual behavior. Later to complete accurately and justify the electron quantum behavior shows the principle of indeterminacy of Heisenberg and its philosophical implications. Thus, the main objective of this research is to seek to integrate quantum behavior, which takes place at the atomic scale, especially in the high school level. Some guidance on how to apply this work in other levels of education appear throughout the text and in the appendices. Introducing the dual behavior wave-particle, the electron, the probabilistic interpretation and the uncertainty principle. We believe that students will be able to understand a number of phenomena that occurs on scales that are not the classical mechanics of the domain when in contact with these topics. This topic is part of the contemporary modern physics content that is being addressed in texts and entrance exam books. In addition, some of the new technologies using quantum physics, from electronic microscopes, nanotechnology, quantum computing, semiconductors, diodes (including LED), transistors, computers, tablets, GPS, satellites, radar, aircraft, lasers, code scanners bars, military defense systems, CD and Blu-Ray players, encryption, photoelectric cells, various sensors, basically, everything is electronic. One of the goals of the work is to check what prior knowledge the student body has, before contact with quantum physics content, so the part of common sense on this issue. In addition, we intend to verify that the student is able to: i) distinguish, at the end of the application of this research, that the laws of physics at atomic scales are different from the laws of classical physics, ii) the importance of quantum mechanics in technology and society.
36
Ariyawansa, Gamini. "Semiconductor Quantum Structures for Ultraviolet-to-Infrared Multi-Band Radiation Detection." Digital Archive @ GSU, 2007. http://digitalarchive.gsu.edu/phy_astr_diss/17.
Abstract:
In this work, multi-band (multi-color) detector structures considering different semiconductor device concepts and architectures are presented. Results on detectors operating in ultraviolet-to-infrared regions (UV-to-IR) are discussed. Multi-band detectors are based on quantum dot (QD) structures; which include quantum-dots-in-a-well (DWELL), tunneling quantum dot infrared photodetectors (T-QDIPs), and bi-layer quantum dot infrared photodetectors (Bi-QDIPs); and homo-/heterojunction interfacial workfunction internal photoemission (HIWIP/HEIWIP) structures. QD-based detectors show multi-color characteristics in mid- and far-infrared (MIR/FIR) regions, where as HIWIP/HEIWIP detectors show responses in UV or near-infrared (NIR) regions, and MIR-to-FIR regions. In DWELL structures, InAs QDs are placed in an InGaAs/GaAs quantum well (QW) to introduce photon induced electronic transitions from energy states in the QD to that in QW, leading to multi-color response peaks. One of the DWELL detectors shows response peaks at ∼ 6.25, ∼ 10.5 and ∼ 23.3 µm. In T-QDIP structures, photoexcited carriers are selectively collected from InGaAs QDs through resonant tunneling, while the dark current is blocked using AlGaAs/InGaAsAlGaAs/ blocking barriers placed in the structure. A two-color T-QDIP with photoresponse peaks at 6 and 17 µm operating at room temperature and a 6 THz detector operating at 150 K are presented. Bi-QDIPs consist of two layers of InAs QDs with different QD sizes. The detector exhibits three distinct peaks at 5.6, 8.0, and 23.0 µm. A typical HIWIP/HEIWIP detector structure consists of a single (or series of) doped emitter(s) and undoped barrier(s), which are placed between two highly doped contact layers. The dual-band response arises from interband transitions of carriers in the undoped barrier and intraband transitions in the doped emitter. Two HIWIP detectors, p-GaAs/GaAs and p-Si/Si, showing interband responses with wavelength thresholds at 0.82 and 1.05 µm, and intraband responses with zero response thresholds at 70 and 32 µm, respectively, are presented. HEIWIP detectors based on n-GaN/AlGaN show an interband response in the UV region and intraband response in the 2-14 µm region. A GaN/AlGaN detector structure consisting of three electrical contacts for separate UV and IR active regions is proposed for simultaneous measurements of the two components of the photocurrent generated by UV and IR radiation.
37
Fu, Kai. "Growth Dynamics of Semiconductor Nanostructures by MOCVD." Doctoral thesis, KTH, Teoretisk kemi (stängd 20110512), 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-11447.
Abstract:
Semiconductors and related low-dimensional nanostructures are extremely important in the modern world. They have been extensively studied and applied in industry/military areas such as ultraviolet optoelectronics, light emitting diodes, quantum-dot photodetectors and lasers. The knowledge of growth dynamics of semiconductor nanostructures by metalorganic chemical vapour deposition (MOCVD) is very important then. MOCVD, which is widely applied in industry, is a kind of chemical vapour deposition method of epitaxial growth for compound semiconductors. In this method, one or several of the precursors are metalorganics which contain the required elements for the deposit materials. Theoretical studies of growth mechanism by MOCVD from a realistic reactor dimension down to atomic dimensions can give fundamental guidelines to the experiment, optimize the growth conditions and improve the quality of the semiconductor-nanostructure-based devices. Two main types of study methods are applied in the present thesis in order to understand the growth dynamics of semiconductor nanostructures at the atomic level: (1) Kinetic Monte Carlo method which was adopted to simulate film growths such as diamond, Si, GaAs and InP using the chemical vapor deposition method; (2) Computational fluid dynamics method to study the distribution of species and temperature in the reactor dimension. The strain energy is introduced by short-range valence-force-field method in order to study the growth process of the hetero epitaxy. The Monte Carlo studies show that the GaN film grows on GaN substrate in a two-dimensional step mode because there is no strain over the surface during homoepitaxial growth. However, the growth of self-assembled GaSb quantum dots (QDs) on GaAs substrate follows strain-induced Stranski-Krastanov mode. The formation of GaSb nanostructures such as nanostrips and nanorings could be determined by the geometries of the initial seeds on the surface. Furthermore, the growth rate and aspect ratio of the GaSb QD are largely determined by the strain field distribution on the growth surface.QC 20100713
38
Kraft, Marco. "Spectroscopic characterization of upconversion nanomaterials with systematically varied material composition and surface chemistry." Doctoral thesis, Humboldt-Universität zu Berlin, 2019. http://dx.doi.org/10.18452/19657.
Abstract:
Ziel dieser Doktorarbeit war es, den Einfluss von verschiedenen Parametern auf die spektroskopischen Eigenschaften von Lanthanid-basierten Aufkonversions-Materialien zu erforschen. Ein besonderer Fokus lag dabei auf hexagonalen Natrium-Yttrium-Tetrafluorid Kristallen, die mit dreifachgeladenen Yb und Er oder Tm Ionen kodotiert wurden. Eine wesentliche Voraussetzung für mögliche Anwendungen dieser Kristalle ist ein Verständnis aller ihrer wichtigen photophysikalischen Besonderheiten.
Die erste Studie dieser Doktorarbeit untersuchte daher, wieso Nanokristalle viel weniger absorbierte in ausgesendete Photonen umwandeln als mikrokristalline Teilchen. Die Ergebnisse zeigten, dass man ungeschalte Kristalle aufgrund von Oberflächen-Lösch-Effekten in zwei Teile unterteilen kann, einen strahlenden Kern und eine Schale aus stark oder vollständig gelöschten oberflächennahen Lanthanid-Ionen, welche für Kristalle abnehmender Größe einen immer größeren Volumenanteil einnimmt. Die zweite Studie untersuchte exemplarisch, ob eine kompliziertere Partikelarchitektur, bestehend aus einem einfach-dotierten Er Kern und Yb als Schalenmaterial, diesen Effizienzverlust der Lumineszenz reduzieren kann. Die Ergebnisse zeigten jedoch, dass dies nicht der Fall ist. Eine weitere Studie untersuchte den Einfluss der Konzentration der Tm Ionen in Yb, Tm kodotierten Nanokristallen auf die spektroskopischen Eigenschaften dieser Materialien und zeigte, dass für eine maximale Emission im Lichtwellenbereich über 700 nm andere Tm Konzentrationen benötigt werden als für maximale Lichtemissionen in den unteren Lichtwellenbereichen. Die letzte Studie untersuchte den Einfluss eines zuvor berichteten Zersetzungsprozesses von exemplarisch ausgewählten Yb, Tm kodotierte Nanokristallen in wässrigen Dispersionen auf deren spektroskopische Eigenschaften. Mithilfe dieser Ergebnisse war es möglich, mehrere Emissionsbanden als Parameter für das Langzeit-Stabilitäts-Monitoring dieser Materialien zu identifizieren.This PhD thesis investigated the influence of various parameters on the spectroscopic properties of so-called upconversion nanoparticles (UCNPs). A special emphasis was dedicated to hexagonal-phase sodium yttrium tetrafluoride crystals that were codoped with trivalent Yb and either Er or Tm ions. Such UCNPs can, however, experience no breakthrough in the field of UC nanotechnology before all of their important photophysical features are understood. The first study of this PhD thesis therefore investigated, why nanocrystalline upconverters with different surface chemistries convert less absorbed to emitted photons than their microcrystalline counterparts. The results revealed that upconverting crystals apparently have to be subdivided into two parts, with one being the luminescent core and the other being a completely dark shell that is quenched by surface effects and assumes an ever increasing volumetric content for small UCNPs. The second study exemplarily investigated, if a more complex particle nanostructure that consisted of a Er doped core, surrounded by a Yb doped shell, could overcome these efficiency losses, however, it concluded that it does not. Another study explored the influence of Tm doping concentrations of Yb, Tm codoped nanocrystals on their spectroscopic properties and concluded that different Tm doping concentrations are required for a maximum upconversion luminescence in the wavelength regions above 700 nm, than for the wavelength regions below that. The last study of this PhD thesis investigated the influence of a previously reported dissolution process of UCNPs in aqueous solutions on the spectroscopic properties of exemplarily chosen Yb, Tm codoped nanocrystals. These results were then utilized to identify several upconversion emission bands that can be used as a screening parameter for the long-term stability monitoring of UCNPs.
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Cozzarini, Luca. "Nanomaterials based on II-VI Semiconductors." Doctoral thesis, Università degli studi di Trieste, 2012. http://hdl.handle.net/10077/7359.
Abstract:
2010/2011This thesis describes: (i) synthesis and characterization of colloidal nanocrystals of II-VI semiconductor compounds; (II) development of two novel materials using such nanocrystals as “building blocks”: (IIa) a nanocrystals/polymer composite, to be used as phosphor in LED-based lighting devices; (IIb) an inorganic, nano-structured multiphase material, showing a promising geometry as an electronic intermediate band material. Different typologies of nanocrystals (single-phase, alloyed or core-shells) were successfully synthesized using air-stable, safe reagents. Their optical properties (absorption spectrum, fluorescence wavelength and fluorescence quantum yield) were mapped as function of different parameters. Good results in engineering optical properties were achieved by: (a) changing size and/or composition in single-phase nanocrystals; (b) tuning shell composition and thickness and/or mutually diffusing one material into the other in multi-phase nanocrystals. The influence of different surface ligands on optical properties and on solubility in different media was also studied. Nanocrystal/polymer composite lenses were obtained from nanocrystals with desired fluorescence wavelength and quantum yield, mixed in an appropriate solvent with polymer pellets. The mixture was drop casted or tape casted on a solid substrate, obtaining solid, transparent lenses after solvent evaporation. A nano-structured, all-inorganic material (composed of semiconducor nanocrystals embedded into a wider bandgap semiconductor) was obtained through self-assembly and densification of colloidal core-shells nanocrystals. The realization of this composite supracrystal was achieved via a multi-step process: (i) colloidal synthesis of core-shell nanocrystals; (ii) surface ligands exchange; (iii) assembly; (iv) heat treatment. Evolution of the optical properties during heat treatment suggests that it is possible to sinter the shell material without altering the internal nano-heterostructure, if temperature and time of the treatment are controlled properly.
In questa tesi sono descritti: (I) la sintesi colloidale e la caratterizzazione di nanocristalli di semiconduttori II-VI; (II) lo sviluppo, utilizzando i suddetti nanocristalli quali “unità da costruzione”, di due materiali innovativi: (IIa) un composito nanocristalli/polimero, da usare come fosforo in dispositivi per illuminazione basati su LED; (IIb) un materiale inorganico nano-strutturato multifase, con una geometria promettente quale materiale a banda elettronica intermedia. Differenti semiconduttori II-VI sono stati sintetizzati in forma di nanocristalli (monofasici, in forma di lega o in struttura di tipo “core-shell”) usando reagenti sicuri e stabili in atmosfera. Le loro proprietà ottiche (spettro di assorbimento, lunghezza d’onda di fluorescenze e resa quantica di fluorescenza) sono state mappate in funzione di numerosi parametri. Sono stati raggiunti ottimi risultati nel controllo delle proprietà ottiche sia in nanocristalli a fase singola (modificandone le dimensioni o la composizione chimica) che in nanocristalli multifase (regolandone la composizione e lo spessore della “shell”, nonché mutualmente diffondendo un materiale nell’altro). È stata anche studiata l’influenza di differenti leganti superficiali sulle proprietà ottiche e sulla solubilità dei nanocristalli in differenti solventi. Lenti composite di nanocristalli/polimero sono state ottenute a partire da nanocristalli aventi la lunghezza d’onda e la resa quantica di fluorescenza desiderate, mescolandoli con pellet di polimero in solventi appropriati. La miscela è stata depositata su un supporto, tramite drop casting o tape casting, ottenendo lenti solide trasparenti dopo l’evaporazione del solvente. Un materiale inorganico nano strutturato (costituito da nanocristalli di semiconduttore racchiusi all’interno di un secondo materiale semiconduttore a bandgap maggiore) è stato ottenuto tramite l’autoassemblaggio e la densificazione di nanocristalli core-shell sintetizzati con procedure di chimica colloidale. La realizzazione di suddetto sovra-cristallo si è svolta in più fasi: (i) sintesi colloidale; (ii) sostituzione dei leganti superficiali; (iii) assemblaggio; (iv) trattamento termico. I risultati derivanti dallo studio dell’evoluzione delle proprietà ottiche durante il trattamento termico suggeriscono che sia possibile sinterizzare il materiale della shell senza alterare la nano-eterostruttura interna, se la temperatura e il tempo del trattamento sono scelti opportunamente.
XXIV Ciclo
1983
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Hoffman, Amy Jo Gray Harry B. Hoffmann Michael R. "Photocatalytic reactions on quantum-sized semiconductor colloids : photoinitiated polymerization of vinylic monomers, formation of hydrogen peroxide and organic peroxides, oxidation of carboxylic acids, and synthesis of humic-like material /." Diss., Pasadena, Calif. : California Institute of Technology, 1993. http://resolver.caltech.edu/CaltechETD:etd-09272005-134829.
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Ainsebaa, Abdelmalek. "Diodes électroluminescentes hybrides organiques inorganiques : Mécanismes aux interfaces, courant et lumière." Phd thesis, Université Paris-Diderot - Paris VII, 2010. http://tel.archives-ouvertes.fr/tel-00526529.
Abstract:
Les diodes électroluminescentes hybrides organiques-inorganiques ou Quantum Dot- Light-Emitting Diodes (QD-LED) sont le parangon de dispositifs qui associeraient les propriétés semi-conductrices des matériaux organiques conjugués, ainsi que leur facilité de mise en oeuvre en couche mince, aux propriétés exceptionnelles (couleur accordable par la taille, bon rendement quantique de photoluminescence) des nanoparticules nanométriques de semi-conducteurs inorganiques, telles que CdSe/ZnS (TOPO). Diverses approches ont été explorées pour optimiser les QD-LEDs ; elles reposent sur la réalisation d'architectures diverses (uni, bi, tri couches) combinant matériaux organiques et QDs (en couche compacte ou dispersés dans une matrice), obtenues par diverses méthodes de dépôt (spin coating, tampon, impression jet d'encre). Dans ce travail de thèse, nous avons d'abord réalisé une structure originale sous forme de diodes comportant une couche hybride nanocomposite, obtenue en incorporant les QDs dans une matrice diélectrique de PMMA. Le but était de contrôler les flux de porteurs pour maximiser leur recombinaison sur les QDs. Les mesures des caractéristiques électriques, d'électro- et photoluminescence sont discutées, conjointement à des études de la morphologie des dépôts par AFM qui ont montré comment la microstructure dépendait des caractéristiques du, ou des, solvant(s) utilisé(s) pour le spin coating. D'une façon générale, les nanoparticules sont agrégées et la couche apparait inhomogène et rugueuse, ce qui permet des contacts entre les couches de transport. La faible électroluminescence résulterait de transferts d'excitation à partir d'états excités produits par les recombinaisons à ces endroits. Dans une seconde partie, nous avons déposé les QDs par spin coating à partir de solutions dans l'heptane, un solvant qui ne perturbe pas les couches organiques préalablement déposées. Les morphologies observées sont celles d'ilots compacts, avec des taux de couverture allant de 0 à 100%. La diminution, puis la disparition, de l'électroluminescence aux fortes couvertures confirme que l'émission des QDs provient exclusivement de transferts d'excitation à partir d'espèces excitées générées à l'interface organique-organique, en fonction de la nature des matériaux organiques. Les mécanismes d'injection des porteurs à la cathode, dépendant de la nature de celle-ci et de la couche de transport d'électrons, sont variables eux-aussi.
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Pinto, Melissa Fabíola Siqueira. "Estudo químico-quântico de propriedades estruturais e eletrônicas de oligofluorenos com potenciais aplicações tecnológicas." Universidade de São Paulo, 2007. http://www.teses.usp.br/teses/disponiveis/75/75131/tde-14042008-104809/.
Abstract:
Nesta tese foi analisada a evolução das propriedades eletrônicas e estruturais de oligômeros do fluoreno e de dois derivados, relacionada com a inserção gradativa de unidades monoméricas, utilizando-se os métodos de química quântica semi-empríricos, ab initio Hartree-Fock e Teoria do Funcional da Densidade (DFT). Para tal finalidade, foram realizadas análises metodológicas assaz detalhadas com os monômeros e dímeros (fluoreno, FF e TFFF), cujos resultados mostraram que os métodos semi-empíricos AM1 e PM3 são inadequados para descrever as distribuições das cargas derivadas do potencial eletrostático e, por conseguinte, as propriedades que dependem das mesmas. Além disso, as análises conformacionais realizadas para as três espécies de dímeros mostraram que ambas as estruturas estáveis do TFFF apresentaram o mesmo momento de dipolo total, provavelmente devido à influência dos átomos de flúor sobre o sistema. Ademais, com as investigações concernentes ao crescimento gradual das cadeias oligoméricas foi constatado que os espectros de UV e as energias dos orbitais fronteira calculados para os oligômeros seguiram a mesma tendência exibida por seus polímeros correspondentes, segundo os resultados de voltametria cíclica e de UV divulgados na literatura. Enquanto que os ângulos diedros adotados pelos oligômeros indicaram que suas conformações não são planares. Quanto à energia total, verificou-se que o decaimento linear relativo aos oligômeros do TFFF sucedeu aproximadamente três vezes mais rápido que o observado para os do fluoreno. Portanto, as aplicações dos fundamentos da Química Quântica e a correlação entre esses resultados teóricos e medidas experimentais reportadas na literatura, evidenciaram que com estas informações é factível estimar algumas propriedades de polímeros orgânicos ?-conjugados a partir de seus oligômeros, utilizando-se métodos de química quântica apropriados.In this thesis, the evolution of electronic and structural properties of fluorene oligomers and two derivatives were accomplished relating to the gradual insertion of monomeric units using the quantum chemical methods semiempirical, ab initio Hartree-Fock and Density Functional Theory (DFT). For this purpose, detailed methodological analyses were performed with their monomers and dimers (fluorene, FF and TFFF). These results showed that the semiempirical methods AM1 and PM3 are inadequate to describe the atomic charges derived from electrostatic potential distributions and, hence, for the properties depending of them. In addition, the conformational analysis achieved for these three dimers showed that both stable structures found for the TFFF dimer possess the same total dipole moment, this is probably due to the influence of the fluorine atoms on the system. Moreover, with investigations regarding to the gradual increase in the oligomeric chain, it was verified that the UV spectra and the frontier orbital energies calculated for the oligomers provided the same tendency bespoken by their relative polymers from cyclic voltametry and UV measurements shown in the literature. Besides, the dihedral angles found in the oligomers indicated that their conformations are not planar. Furthermore, the total energy linear decay of the TFFF oligomers behaves approximately three times faster than that exhibited by the fluorene oligomers. In this way, the applications of the Quantum Chemistry principles and the correlation between these theoretical results and experimental data reported in the literature evidenced that using these information is feasible to estimate some properties of polymers from their oligomers by means of appropriate methods of quantum chemistry.
43
Diestel, André Luiz Cosenza. "Uma análise de produtos educacionais para o ensino de física quântica desenvolvidos no âmbito de um Mestrado profissional em ensino de física." reponame:Biblioteca Digital de Teses e Dissertações da UFRGS, 2017. http://hdl.handle.net/10183/169546.
Abstract:
Observa-se uma expressiva expansão de mestrados profissionais (MP) na área de Ensino desde a homologação pela Capes, em 2001. Esta dissertação de mestrado analisa trabalhos e produtos educacionais criados no âmbito dos Mestrados Profissionais em Ensino de Física, na área do Ensino de Mecânica Quântica, em uma específica universidade federal. A pesquisa apoia-se em uma análise do discurso, tratando o Trabalho de Conclusão (dissertação+produto educacional) como um enunciado no sentido Bakhtiniano. A partir desta análise, destacamos uma série de inconsistências que permeiam desde o referencial teórico, em muitos casos usado apenas de forma a cumprir exigências sem que seja articulado significativamente à proposta metodológica, até referências que aparecem apenas para contemplar determinados grupos onde o autor está inserido. Por fim destacamos a pouca relevância dos Produtos Educacionais produzidos, não atendendo às demandas escolares e, em muitos casos, trilhando um sentido oposto às pesquisas na área de Ensino de Física.A significant expansion of professional master’s degree (PM) in the field of Science Education is observed, since its approval in 2001. This dissertation analyses works and educational products created in the scope of professional master’s degree in Physics Teaching, focusing the topic of Quantum Physics Teaching, developed in a federal university. The research relies on a bakhtinian analysis, treating the conclusion work (dissertation + educational product) as an utterance, according to Bakhtin’s theory. From this analysis, it is possible to emphasize a series of inconsistencies that permeate the theoretical framework, in many cases used as a way of fulfilling demands, not significantly articulated to the methodological proposal, also including references that arise only to contemplate certain groups in which the author is inserted. Finally, it is highlighted the low relevance of educational products produced, which do not meet the schools’ demands, and, in many cases, follow an opposite way if compared to researches in the area of Physics Education.
44
Timothy, H. Hsieh Timothy (Timothy Hwa-wei). "Topological materials and quantum entanglement." Thesis, Massachusetts Institute of Technology, 2015. http://hdl.handle.net/1721.1/103228.
Abstract:
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2015.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 83-91).
As the title implies, this thesis consists of two main topics: materials which realize topological phases of matter and applications of the concept of entanglement in understanding topological phases and their transitions. The first part will focus on a particular class of materials called topological crystalline insulators (TCI), which are bulk insulators with metallic boundary states protected by crystal mirror symmetries. The realization of TCIs in the SnTe class of materials and the anti-perovskite family will be described. The second part will focus on using entanglement notions to probe a topological phase transition, based on a single topological wavefunction. This is achieved by performing extensive partitions of the wavefunction, such as a checkerboard partition. Implementing this technique in one dimension naturally involves the use of tensor networks, which will be reviewed and then utilized.
by Timothy H. Hsieh.
Ph. D.
45
Rival, Olivier. "Organic materials for quantum computation." Thesis, University of Oxford, 2009. http://ora.ox.ac.uk/objects/uuid:3674b9ce-c284-47b5-ab0d-76d094c849f0.
Abstract:
Quantum mechanics has a long history of helping computer science. For a long time, it provided help only at the hardware level by giving a better understanding of the properties of matter and thus allowing the design of ever smaller and ever more efficient components. For the last few decades, much research has been dedicated to finding whether one can change computer science even more radically by using the principles of quantum mechanics at both the hardware and algorithm levels. This field of research called Quantum Information Processing (QIP) has rapidly seen interesting theoretical developments: it was in particular shown that using superposition of states leads to computers that could outperform classical ones. The experimental side of QIP however lags far behind as it requires an unprecedented amount of control and understanding of quantum systems. Much effort is spent on finding which particular systems would provide the best physical implementation of QIP concepts. Because of their nearly endless versatility and the high degree of control over their synthesis, organic materials deserve to be assessed as a possible route to quantum computers. This thesis studies the QIP potential of spin degrees of freedom in several such organic compounds. Firstly, a study on low-spin antiferromagnetic rings is presented. It is shown that in this class of molecular nanomagnets the relaxation times are much longer than previously expected and are in particular long enough for up to a few hundred quantum operations to be performed. A detailed study of the relaxation mechanisms is presented and, with it, routes to increasing the phase coherence time further by choosing the suitable temperature, isotopic and chemical substitution or solvent. A study of higher-spin systems is also presented and it is shown that the relaxation mechanisms are essentially the same as in low-spin compounds. The route to multi-qubit system is also investigated: the magnetic properties of several supermolecular assemblies, in particular dimers, are investigated. Coupling between neighbouring nanomagnets is demonstrated and experimental issues are raised concerning the study of the coherent dynamics of dimers. Finally a study of the purely organic compound phenanthrene is reported. In this molecule the magnetic moment does not result from the interactions between several transition metal ions as in molecular nanomagnets but from the photoexcitation of an otherwise diamagnetic molecule. The interest of such a system in terms of QIP is presented and relaxation times and coupling to relevant nuclei are identified.
46
Kozii, Vladyslav. "Exotic superconductivity in quantum materials." Thesis, Massachusetts Institute of Technology, 2019. https://hdl.handle.net/1721.1/127702.
Abstract:
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2019
Cataloged from the official PDF of thesis.
Includes bibliographical references (pages 345-342).
The theory of superconductivity developed by Bardeen, Cooper, and Schrieffer has proven to correctly describe a wide class of metals, where the effective attraction between electrons is mediated by phonons. Despite huge success, this theory fails to explain certain types of superconductivity, which includes but not limited to topological superconductivity and superconductivity in systems with low carrier density. We study new exciting properties of these materials and discuss possible microscopic mechanisms for exotic superconductivity. In Part I of this thesis, we explore the properties of two-component superconductors with strong spin-orbit coupling. Our study is motivated by the experiments on a topological superconductor candidate material, Bi2Se3 doped with Cu, Sn, or Nb atoms. Generally, superconductivity in such systems comes in two flavors: nematic, which breaks rotational symmetry of the crystal, and time-reversal breaking chiral.
We study the relative energetics and different features specific to each of these flavors. We find that, in three dimensions, the nematic superconductors generically possess full pairing gap on the Fermi surface, thus representing a solid-state realization of a time-reversal-invariant topological superconductor. On the contrary, chiral superconductors host non-degenerate point nodes on the Fermi surface and represent the superconducting analog of topological Weyl semimetals; the low-energy excitations in these materials are itinerant Majorana fermions. In Part II, we suggest possible microscopic mechanisms for unconventional superconductivity. We show that strong fluctuations of the inversion-breaking order parameter induce instability in an odd-parity superconducting channel, suggesting a route towards topological superconductivity. Using bosonization, we generalize this result to one-dimensional systems.
We apply our findings to study superconductivity in three-dimensional Dirac materials with extremely low density of carriers. Finally, we discuss the mechanism for nematic superconductivity from density wave fluctuations in two-dimensional systems, with possible application to twisted bilayer graphene. The results presented in this thesis are mainly based on Refs. [1, 2, 3, 4, 5, 6, 7].
by Vladyslav Kozii.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Physics
47
Vallés, Pelarda Marta. "Different approaches to improve perovskite-based photovoltaic and optoelectronic devices." Doctoral thesis, Universitat Jaume I, 2022. http://dx.doi.org/10.6035/14104.2022.368259.
Abstract:
Halide perovskites have had a huge impact on different fields in the last decade due to their versatility and amazing optoelectronic properties. However, there are still some issues to improve towards their future utilization and commercialization. In this thesis, several topics associated with perovskite-based photovoltaic and optoelectronic devices are addressed. The different works contribute to enhance the optoelectronic properties of perovskite quantum dots, to replace the most employed hole transporting material in photovoltaic devices (spiro-OMeTAD) and, moreover, to study a methodology which can be incorporated in up-scaling procedures towards their future commercialization as solar cells.Programa de Doctorat en Ciències
48
Palacios-Berraquero, Carmen. "Quantum-confined excitons in 2-dimensional materials." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/275721.
Abstract:
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.
49
Earnshaw, Mark Peter. "Quantum well electrorefraction materials and devices." Thesis, University of York, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.298387.
50
Babkevich, Peter. "Quantum materials explored by neutron scattering." Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:5f6ef05d-e846-47e1-b59f-864ea4fa2f3f.
Abstract:
This thesis describes neutron scattering experiments on strongly correlated systems exhibiting a range of emergent phenomena: antiferromagnetism, charge order, superconductivity and multiferroicity. I have examined the La_{2}CoO_{4} compound which is a Mott insulator and orders antiferromagnetically near room temperature. The La_{2}CoO_{4} sample was studied using spherical neutron polarimetry and I present magnetic structure models to describe the two antiferromagnetic phases of the compound. Furthermore, the magnetic fluctuations have been investigated using neutron time-of-flight technique. This has allowed us to extract the dominant exchange interactions in the system. More interestingly, the work on La_{2}CoO_{4} presented in this thesis provides a basis for the experimental evidence of an hourglass dispersion in La_{5/3}Sr_{1/3}CoO_{4}, previously only observed in the copper oxide based superconductors. This dispersion has been understood in terms of a stripe ordered magnetic phase and was found to be well described by a linear spin-wave model. Neutron scattering experiments were also carried out on the new iron-based high-temperature superconductors, FeSe_{x}Te_{1−x}. A range of compositions were studied, including both antiferromagnetically ordered and superconducting. Below the superconducting phase transition temperature, a spin resonance mode was found centred on the antiferromagnetic wavevector. This is an important feature shared by many unconventional superconductors. The spin resonance intensity was found to reflect the order parameter of the superconducting state. Polarised inelastic neutron scattering experiments have revealed a small anisotropy between the in-plane and out-of-plane magnetic fluctuations at the resonance. This anisotropy cannot be readily explained by the usual anisotropic terms in the Hamiltonian. This could be evidence of new physics in the FeSe_{x}Te_{1−x} superconductors. Finally, I have studied CuO – a high-temperature multiferroic. Analysis of polarised neutron diffraction experiments shows that the magnetic domain population can be varied using an externally applied electric field. This unambiguously demonstrates coupling between the magnetic and ferroelectric degrees of freedom. Using representation analysis I derive the incommensurate magnetic structure in the multiferroic phase. The origin of the magnetoelectric coupling is consistent with models based on the inverse Dzyaloshinskii-Moriya interaction.