Dissertations / Theses: 'Superlattices'

1

Deans, Mark Edward. "Phonons in superlattices." Thesis, University of Cambridge, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.254406.

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2

Hadizad, M. Reza. "Lattice dynamics of superlattices." Thesis, University of Essex, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.292758.

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3

Rajakarunanayake, Yasantha Nirmal McGill T. C. McGill T. C. "Optical properties of Si-Ge superlattices and wide band gap II-VI superlattices /." Diss., Pasadena, Calif. : California Institute of Technology, 1991. http://resolver.caltech.edu/CaltechETD:etd-07122007-074702.

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4

Müggenburg, Jan. "Ion beam analysis of metallic vanadium superlattices : Ion beam analysis of metallic vanadium superlattices." Thesis, Uppsala universitet, Tillämpad kärnfysik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-328067.

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5

Evans, S. D. "Langmuir-Blodgett superlattices incorporating porphyrins." Thesis, Lancaster University, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.235169.

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6

Pulsford, Nicolas J. "Optical studies of semicondutor superlattices." Thesis, University of Oxford, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.257905.

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7

Chen, Peixuan. "Thermal transport through SiGe superlattices." Doctoral thesis, Universitätsbibliothek Chemnitz, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:ch1-qucosa-159170.

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Understanding thermal transport in nanoscale is important for developing nanostructured thermolelectric materials and for heat management in nanoelectronic devices. This dissertation is devoted to understand thermal transport through SiGe based superlattices. First, we systematically studied the cross-plane thermal conductivity of SiGe superlattices by varying the thickness of Si(Ge) spacers thickness. The observed additive character of thermal resistance of the SiGe nanodot/planar layers allows us to engineer the thermal conductivity by varying the interface distance down to ~1.5 nm. Si-Ge intermixing driven by Ge surface segregation is crucial for achieving highly diffusive phonon scattering at the interfaces. By comparing the thermal conductivity of nanodot Ge/Si superlattices with variable nanodot density and superlattices with only wetting layers, we find that the effect of nanodots is comparable with that produced by planar wetting layers. This is attributed to the shallow morphology and further flattening of SiGe nanodots during overgrowth with Si. Finally, the experiments show that the interface effect on phonon transport can be weakened and even eliminated by reducing the interface distance or by enhancing Si-Ge intermixing around the interfaces by post-growth annealing. The results presented in this dissertation are expected to be relevant to applications requiring optimization of thermal transport for heat management and for the development of thermoelectric materials and devices based on superlattice structures
Verständnis des thermischen Transport auf Nanoskala ist sowohl grundlegend für die Entwicklung nanostrukturierter Materialien, als auch für Temperaturkontrolle in nanoelektronischen Bauteilen. Diese Dissertation widmet sich der Erforschung des thermischen Transports durch SiGe basierenden Übergittern. Variationen, der Si(Ge) Schichtdicken, wurden zur systematischen Untersuchung der Normalkomponente zur Wachstumsrichtung der Wärmeleitfähigkeit, von SiGe Übergittern, genutzt. Die Beobachtung des additiven Charakters, des thermischen Widerstands, der SiGe Schichten, mit oder ohne Inselwachstum, ermöglicht die Erstellung von Strukturen mit bestimmter Wärmeleitfähigkeiten durch die Variation der Schichtdicken bis zu einer Minimaldistanz zweier Schichtübergänge von ~1.5nm. Die Ge Segregation führt zu einer Vermischung, von Si und Ge, welche eine essentielle Rolle zur diffusen Phononenstreuung spielt. Unsere Untersuchungen, von planaren Übergittern und Übergittern mit variabler Inseldichte, zeigen, dass Inseln und planare Schichten zu einer vergleichbaren Reduktion, der Wärmeleitfähigkeit, führen. Diese Beobachtung lässt sich, sowohl auf die flache Morphologie als auch die Abplattung der SiGe Inseln, aufgrund der Überwachsung mit Si, zurückführen. Die Experimente zeigen außerdem, dass sich der Barriereneffekt, der Schichtgrenzen, durch Reduktion der Schichtabstände und durch verstärkte Vermischung im Bereich der Schichtgrenzen, durch Erhitzung, eliminieren lässt. Die präsentierten Messungen sind sowohl, für die Entwicklung jener Bauteile, die eine Optimierung des thermischen Transports oder Temperaturmanagment erfordern, als auch von thermoelektrischen Matieralien und Bauteilen, basierend auf Übergittern, relevant

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8

Chen, Peixuan. "Thermal transport through SiGe superlattices." Doctoral thesis, Universitätsverlag der Technischen Universität Chemnitz, 2014. https://monarch.qucosa.de/id/qucosa%3A20177.

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Understanding thermal transport in nanoscale is important for developing nanostructured thermolelectric materials and for heat management in nanoelectronic devices. This dissertation is devoted to understand thermal transport through SiGe based superlattices. First, we systematically studied the cross-plane thermal conductivity of SiGe superlattices by varying the thickness of Si(Ge) spacers thickness. The observed additive character of thermal resistance of the SiGe nanodot/planar layers allows us to engineer the thermal conductivity by varying the interface distance down to ~1.5 nm. Si-Ge intermixing driven by Ge surface segregation is crucial for achieving highly diffusive phonon scattering at the interfaces. By comparing the thermal conductivity of nanodot Ge/Si superlattices with variable nanodot density and superlattices with only wetting layers, we find that the effect of nanodots is comparable with that produced by planar wetting layers. This is attributed to the shallow morphology and further flattening of SiGe nanodots during overgrowth with Si. Finally, the experiments show that the interface effect on phonon transport can be weakened and even eliminated by reducing the interface distance or by enhancing Si-Ge intermixing around the interfaces by post-growth annealing. The results presented in this dissertation are expected to be relevant to applications requiring optimization of thermal transport for heat management and for the development of thermoelectric materials and devices based on superlattice structures.
Verständnis des thermischen Transport auf Nanoskala ist sowohl grundlegend für die Entwicklung nanostrukturierter Materialien, als auch für Temperaturkontrolle in nanoelektronischen Bauteilen. Diese Dissertation widmet sich der Erforschung des thermischen Transports durch SiGe basierenden Übergittern. Variationen, der Si(Ge) Schichtdicken, wurden zur systematischen Untersuchung der Normalkomponente zur Wachstumsrichtung der Wärmeleitfähigkeit, von SiGe Übergittern, genutzt. Die Beobachtung des additiven Charakters, des thermischen Widerstands, der SiGe Schichten, mit oder ohne Inselwachstum, ermöglicht die Erstellung von Strukturen mit bestimmter Wärmeleitfähigkeiten durch die Variation der Schichtdicken bis zu einer Minimaldistanz zweier Schichtübergänge von ~1.5nm. Die Ge Segregation führt zu einer Vermischung, von Si und Ge, welche eine essentielle Rolle zur diffusen Phononenstreuung spielt. Unsere Untersuchungen, von planaren Übergittern und Übergittern mit variabler Inseldichte, zeigen, dass Inseln und planare Schichten zu einer vergleichbaren Reduktion, der Wärmeleitfähigkeit, führen. Diese Beobachtung lässt sich, sowohl auf die flache Morphologie als auch die Abplattung der SiGe Inseln, aufgrund der Überwachsung mit Si, zurückführen. Die Experimente zeigen außerdem, dass sich der Barriereneffekt, der Schichtgrenzen, durch Reduktion der Schichtabstände und durch verstärkte Vermischung im Bereich der Schichtgrenzen, durch Erhitzung, eliminieren lässt. Die präsentierten Messungen sind sowohl, für die Entwicklung jener Bauteile, die eine Optimierung des thermischen Transports oder Temperaturmanagment erfordern, als auch von thermoelektrischen Matieralien und Bauteilen, basierend auf Übergittern, relevant.

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9

BELL, JOHN A. "BRILLOUIN SCATTERING FROM METAL SUPERLATTICES." Diss., The University of Arizona, 1987. http://hdl.handle.net/10150/184045.

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Acoustic modes guided by thin-film metal superlattices have been investigated using Brillouin spectroscopy. Samples were grown on both single-crystal sapphire and fused silica substrates by alternately sputtering two different metals to yield a total thickness in the range 0.3 - 0.5 μm. Structural and chemical characterization of the polycrystalline films was performed using x-ray diffraction. Rutherford backscattering and optical interferometry. Thermally excited acoustic waves in the metal film create a surface ripple which weakly interacts with light incident from a single mode argon laser. A tandem Fabry-Perot consisting of two synchronized 3-pass cavities is used to measure the frequency shift of light which is inelastically scattered from acoustic waves. The contrast ratio of this interferometer exceeds 10¹⁰ and provides sufficient stray light rejection to detect the surface Rayleigh wave and as many as 13 higher order acoustic modes. The elastic stiffness constants of the anisotropic superlattices were estimated by fitting the measured acoustic mode velocities to a parameterized acoustic model. A comparison is made between these elastic constants and those predicted from the properties of the separate bulk constituents. The dependence of bilayer wavelength on the elastic properties of both Cu/Nb and Mo/Ta superlattices over the range of roughly 10 to 200 Å was determined. The unexpected softening of Cu/Nb superlattices within a range of bilayer wavelengths near 20 Å which was reported previously is qualitatively similar to the measurements reported here. It is shown that the elastic stiffness coefficient with the largest variation is c₄₄. The stiffness variations determined for the Mo/Ta samples are much smaller than for Cu/Nb. It is suggested that this is due to either structural differences (Cu/Nb is fcc-bcc and Mo/Ta is bcc-bcc) or the smaller interfacial lattice mismatch for Mo/Ta. Interfacial strain is found to be strongly correlated with the stiffness variations of the Mo/Ta samples. However, the underlying cause of these variations in stiffness remains anomalous. This dissertation also reports the first observations of Love waves and Stoneley waves by Brillouin scattering. The purely transverse Love waves guided by Cu/Nb films were detected by elasto-optic scattering from the evanescent acoustic strain in the sapphire substrate. The stiffness coefficient c₁₂ of the hexagonally symmetric metal film cannot be determined by the other guided acoustic waves which ripple the surface. Molybdenum in contact with fused silica is predicted to support a Stoneley wave which is guided by the interface. The lowest order Sezawa made guided by a molybdenum film was found to evolve to the Stoneley wave as the film becomes thicker. These measurements together with measurements of the surface Rayleigh wave show that the stiffness of the sputtered metal films is quite homogeneous and independent of film thickness.

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10

Boufelfel, Ahmed. "Iron-based magnetic metallic superlattices." Diss., The University of Arizona, 1988. http://hdl.handle.net/10150/184340.

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For the first time we prepared and investigated the structural, magnetic, and electrical transport properties of Fe/W, Fe/Mo, and Fe/Pd metallic superlattices. We made a theoretical attempt to explain the induced increase or decrease of the magnetization at the magnetic superlattice interfaces. We used several x-ray diffraction techniques to determine the structural properties of our superlattices. Mossbauer spectroscopy and neutron scattering were used to determine the induced microscopic magnetic effects due to the superlattice structure. Brillouin scattering spectroscopy was used to determine the elastic and magnetic properties of our samples. We investigated the electrical transport properties over a wide range of temperatures of Fe/Pd and Fe/W superlattices.

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11

Zirngibl, Martin. "High speed photodetectors on strained superlattices /." [S.l.] : [s.n.], 1990. http://library.epfl.ch/theses/?nr=854.

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12

Deacon, Russell. "Magnetotransport studies of InAs/GaSb superlattices." Thesis, University of Oxford, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.427872.

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13

Beardsley, Ryan. "Coherent phonon processes in semiconductor superlattices." Thesis, University of Nottingham, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.546547.

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14

Turton, Richard John. "Optical spectra of silicon - germanium superlattices." Thesis, University of Newcastle Upon Tyne, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.264483.

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15

Adderley, B. M. "Nonlinear optical properties of semiconductor superlattices." Thesis, University of Newcastle Upon Tyne, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239137.

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16

Wong, K. B. "Electronic properties of GaAs-GaAlAs superlattices." Thesis, University of Newcastle Upon Tyne, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.356158.

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17

Hosseini-Khorasgani, Sayed-Ali. "Third harmonic generation in semiconductor superlattices." Thesis, University of Newcastle Upon Tyne, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.318603.

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18

Liu, Yong. "Phonons in heterojunctions and semiconductor superlattices." Thesis, University of Exeter, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.335015.

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19

Raj, N. "Dielectric and magnetic properties of superlattices." Thesis, University of Essex, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.381931.

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20

Constantinou, N. C. "Theory of plasmon-polaritions in superlattices." Thesis, University of Essex, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.381926.

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21

Wilkins, Caroline Jane Theresa. "Magnetic properties of rare earth superlattices." Thesis, University of Southampton, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.368119.

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22

Huang, Wujie. "Spin-obit coupling in optical superlattices." Thesis, Massachusetts Institute of Technology, 2016. http://hdl.handle.net/1721.1/103235.

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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2016.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 173-176).
Quantum simulation is emerging as an exciting and active frontier in atomic physics. It allows us not only to verify existing models with high precision, but also to engineer novel systems with strong correlations and exotic topologies. Recent efforts have been made to include synthetic gauge fields and spin-orbit couplings into ultracold quantum gas experiments, which would enable us to study the quantum Hall effect, topological insulators as well as topological superfluids. This thesis will describe the experimental implementation of a new spin-orbit coupled system using pseudospin-1/2 in an optical superlattices, as well as progress towards detecting the stripe phase in this system. The first part of this thesis describes the development of a new apparatus for performing quantum simulations with sodium and lithium in optical lattices. A quantum simulation program is challenging itself, therefore having a stable platform for preparing quantum gases is essential for this task. We'll describe our development in reliable and efficient production of sodium Bose-Einstein condensates and lithium degenerate Fermi gases, as well as the characterization of our optical lattice system in a superfluid to a Mott-insulator quantum phase transition. The dynamics of a Bloch oscillation in a tilted lattice has also been studied as an important step towards the implementation of synthetic magnetic fields in our system. The second part of this thesis describes the experimental realization of spin-orbit coupling in a pseudospin-1/2 system using an optical superlattice. This new scheme uses orbital states in a tilted double-well as the pseudospins, therefore does not require near-resonant Raman light to flip the spins and promise longer lifetimes compared to earlier spin-orbit coupling experiments in atomic gases. It also features a robust miscible ground state with stationary density stripes, which is closely related to the concept of supersolidity in condensed matter systems. We'll present our experimental implementation of this new system, signatures of the resonant spin-orbit coupling, as well as progress toward experimental detection of the stripe phase via Bragg scattering. This pseudospin-1/2 system could also be used for simulating quantum magnetism,and potentially novel models with topological properties and Majorana excitations.
by Wujie Huang.
Ph. D.

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23

Huesmann, Hannah [Verfasser]. "Artificial Nanoparticle-Polymer Superlattices / Hannah Huesmann." Mainz : Universitätsbibliothek der Johannes Gutenberg-Universität Mainz, 2021. http://d-nb.info/1229616853/34.

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24

Matharu, Satpal. "Charge dynamics in coupled semiconductor superlattices." Thesis, Loughborough University, 2015. https://dspace.lboro.ac.uk/2134/17158.

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In this thesis, we investigate the collective electron dynamics in single and coupled superlattice systems under the influence of a DC electric field. Firstly, we illustrate that Bloch oscillations suppress electron transport and the resulting charge domains form self-sustained current oscillations. Upon the application of a tilted magnetic field, stochastic web structures are shown to form in the phase space of the electron trajectory. This occurs only when the Bloch and cyclotron frequencies are commensurate allowing the electrons to demonstrate chaotic unbounded trajectories, leading to an increase in transport. The charge domain dynamics also present additional peaks during such resonances. The rapid changes in the dynamical states found is an example of non-KAM chaos. We show then the amplitude and frequency of current oscillations in a single superlattice can be controlled. Secondly, two models are designed to mutually couple two semiconductor superlattices by a common resistive load. We examine the effects of coupling strength and frequency detuning on the collective current dynamics. The devices are considered to be arranged together on a single substrate as well as on individual substrates. Large AC power is witnessed during anti-phase and in-phase synchronization between current oscillations. Finally, two superlattices are coupled through a resonance circuit incorporating single mode resonances from external influences in the circuit. In this system, chaotic current dynamics are induced with regions of chaos separating different regions of synchronization. High frequency oscillations with minimal phase difference cause the largest power generation. In all three coupling models high frequency components are found in the Fourier power spectra. The power generated in the coupled systems is found greater and at times more than double the power generated in the autonomous superlattice. Thus this thesis provides innovative methods of enhancing and controlling powerful high-frequency signals. This effectively gives manipulation over the intensity of the electromagnetic radiation produced by the superlattice.

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25

Hugi, Johannes. "Ultrafast MSM photodetectors on InGaAs/GaAs superlattices /." [S.l.] : [s.n.], 1994. http://library.epfl.ch/theses/?nr=1229.

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26

Rieder, Bernhard. "Semiclassical transport in semiconductor superlattices with boundaries." [S.l. : s.n.], 2004. http://deposit.ddb.de/cgi-bin/dokserv?idn=972880070.

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27

Zheng, Liqiu. "Spin density wave phases in semiconductor superlattices." Connect to this title online, 2007. http://etd.lib.clemson.edu/documents/1202500635/.

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28

P'ng, M. Y. "Strength and deformation of coherently strained superlattices." Thesis, Queen Mary, University of London, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.413463.

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29

Sarthour, Roberto S. "Magnetism in rare-earth superlattices and alloys." Thesis, University of Oxford, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.312552.

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30

Warnatz, Tobias. "Synthesis and Characterization of Fe/MgO Superlattices." Thesis, Uppsala universitet, Materialfysik, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-247094.

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31

Milton, Brian E. "Lateral surface superlattices in strained InGaAs layers." Thesis, University of Glasgow, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.340295.

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32

White, Julian Darryn. "Phonons in (Si)_m/(Ge)_n superlattices." Thesis, University of Cambridge, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239759.

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33

Gilbert, Timothy George. "Electronic states of ultrathin GaAs/AlAs superlattices." Thesis, University of Leicester, 1988. http://hdl.handle.net/2381/35893.

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The continuing refinement of crystal growth techniques has made possible the fabrication of semiconductor superlattices where the period can be as small as one lattice constant. Prediction of many of the properties of such systems requires a detailed description of their electronic structure. In this thesis, a self-consistent pseudopotential method which includes a parametrization scheme has been used to calculate the electronic properties of (GaAs)n(AlAs)n superlattices with n ranging from 1 to 4. The parametrization scheme is used to reproduce energy gaps at the principal symmetry points for the bulk constituents and the resulting parameter set is employed in all subsequent calculations. The n=l superlattice is found to be indirect with the conduction band minimum at R (equivalent to the zincblende L point) and all the thicker systems are pseudodirect in good agreement with experimental results. The lowest conduction band state at the zone centre for all systems is found to be mainly X-derived reflecting the importance of zone translating effects here. By analysing the states near to the band edges, the observed pattern of confinement in states of the n=l superlattice shows the band offsets to have at most a small role, in contrast to the thicker systems where a definite relationship was established. Moreover, the results suggest that Dingle's "15% rule" is consistently violated and that a valence band offset of about 30-40% is obtained which changes little with layer thickness. Attempts to study the effects of hydrostatic pressure on the n=3 superlattice were in part successful and predicted quite complex behaviour for the electronic states. Much of the discrepancy between the results obtained and the experimental data was attributed to the inadequacies of the empty-core pseudopotential to model the ions.

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34

Ferre, Llin Lourdes. "Thermoelectric properties on Ge/Si1−xGex superlattices." Thesis, University of Glasgow, 2014. http://theses.gla.ac.uk/4861/.

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Thermoelectric generation has been found to be a potential field which can be exploited in a wide range of applications. Presently the highest performances at room temperature have been using telluride-based devices, but these tech- nologies are not compatible with MEMs and CMOS processing. In this work Silicon and Germanium 2D superlattices have been studied using micro fabri- cated devices, which have been designed specifically to complete the thermal and electrical characterization of the different structures. Suspended 6-contact Hall bars with integrated heaters, thermometers and ohmic contacts, have been micro-fabricated to test the in-plane thermoelectric properties of p-type superlattices. The impact of quantum well thickness on the two thermoelectric figures of merit, for two heterostructures with different Ge content has been studied. On the other hand, etch mesa structures have been presented to study the cross-plane thermoelectric properties of p and n-type superlattices. In these experiments are presented: the impact of doping level on the two figures of merit, the impact of quantum well width on the two figures of merit, and the more efficient reduction of the thermal conductivity by blocking phonons with different wavelengths. The n-type results showed the highest figures of merit values reported in the literature for Te-free materials, presenting power factors of 12 mW/K2 · m, which exceeded by a factor of 3 the highest values reported in the literature. The results showed, that Si and Ge superlattices could compete with the current materials used to commercialise thermoelectric modules. In addi- tion, these materials have the advantage of being compatible with MEMs and CMOS processing, so that they could be integrated as energy harvesters to create complete autonomous sensors.

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35

Santos, Peter J. (Peter Jeffries). "Self-assembling nanocomposite Tectons for ordered superlattices." Thesis, Massachusetts Institute of Technology, 2020. https://hdl.handle.net/1721.1/127907.

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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, May, 2020
Cataloged from the official PDF of thesis.
Includes bibliographical references (pages 260-280).
Nanocomposites, materials of heterogeneous composition with at least one of the phases having dimensions between 1-100 nm, can be produced with unique properties dependent on their composition and geometric configuration. However, it is a major challenge to precisely and simultaneously design the structure of synthetic nanocomposites at the nanoscale, microscale, and macroscale. To create advanced nanocomposites in which both structure and composition can be programmed across these disparate size regimes, we have developed a new nanoparticle-based building block, the Nanocomposite Tecton (NCT). An NCT consists of an inorganic nanoparticle core and a polymeric shell, with each chain terminating in a supramolecular binding group at the periphery of the NCT.
As each NCT contains both an inorganic nanoparticle and a polymer phase, each building block is itself a nanocomposite, and the incorporation of supramolecular binding groups allows for the directed assembly of NCTs that contain complementary binding groups. These reversible supramolecular interactions enable the assembly of NCTs into ordered arrays, and the collective behavior of the binding groups can be regulated by the dynamics of the polymer chains. The NCTs are capable of rapidly self-assembling into several different crystalline phases that are determined by the design of the building block, and are resilient against dispersity in the molecular weight of the polymer brush and the diameter of the nanoparticle cores. NCTs have been synthesized with both gold and iron oxide nanoparticle cores, indicating the ability to produce NCTs at reasonable scales.
Moreover, the incorporation of multiple nanoparticle compositions allows for the synthesis of NCT-based materials with plasmonic and magnetic properties that can affect, as well as be affected by, the assembly process. We further demonstrate that the crystallization kinetics can be modulated to induce the assembly of NCTs into faceted crystallites with micron-sized diameters, and the resulting NCT crystallites can be post-processed into bulk solids with arbitrary macroscopic shape and controlled grain size. The NCT design concept is therefore a highly modular and versatile building block capable of fabricating materials with controlled structures at the levels of atomic composition and molecular geometry, nanoscale organization, microstructure, and macroscopic form.
by Peter J. Santos.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Materials Science and Engineering

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36

Apostolakis, Apostolos. "High-frequency acoustoelectronic phenomena in miniband superlattices." Thesis, Loughborough University, 2017. https://dspace.lboro.ac.uk/2134/25277.

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The motion of a quantum particle in a periodic potential can generate rich dynamics in the presence of a driving field. Such systems include, but are not limited to, semiconductor superlattices which exhibit a very anisotropic band structure that results into pronounced nonlinearities and high carrier mobility. In this thesis, we investigate the semiclassical dynamics and electron transport in a spatially periodic potential driven by a propagating wave. Firstly, we examine the transport features of an electron in a single miniband superlattice driven by a high-frequency acoustic plane wave. In this system, the nonlinear electron dynamics crucially depends on the amplitude of the acoustic wave. The transport characteristics are studied by means of a non-linearised kinetic model. In particular, to provide a realistic description of the directed transport, we employ the exact path-integral solutions of the Boltzmann transport equation. The calculated electron drift velocity and the time-averaged velocity show a nonmonotonic dependence upon the amplitude of the acoustic wave with multiple pronounced extrema. We found out that the changes in the velocity-amplitude characteristics are directly associated with a series of global bifurcations due to topological rearrangements of the phase space of the system. These dramatic transformations are connected with superlattice intraminiband transitions, and accompanied by inelastic emission (absorption) of the quantum particle. The bifurcations also signify the transitions between different dynamical regimes, involving unconfined electron motion, wave-dragging and phonon-assisted Bloch oscillations. Each regime has a characteristic spectral fingerprint, which manifests itself in appearance of specific high-frequency components in the spectra of the corresponding averaging trajectory. Secondly, we consider to use the acoustically pumped superlattices for an amplification of THz electromagnetic waves, involving the mechanisms similar to the Bloch gain in electrically biased superlattices. In particular, we predict the tunable THz gain due to nonlinear oscillations which are associated with the localised motion of electrons confined by a propagating potential wave. Traditionally, one of the key issues which emerges from considering different schemes for achieving small signal gain in superlattices, is the control of electric stability. Here, it is shown that for our case of the fast miniband electrons driven by an acoustic wave, terahertz gain can occur without the electric instability. Additionally, we find that the characteristic changes in the averaged velocities are connected to the shape of gain profiles. Consequently, the analytic findings, which determine the transitions between different dynamical regimes at the bifurcations, hold up for the behaviour of amplification of high-frequency electromagnetic waves. The increase of the miniband width, results in an enhancement of the effect of phase space restructuring on the drift velocity and high-frequency gain. Finally, we analyse the case for a superlattice device utilising acoustic waves with a very slow propagation speed. Benefiting from a simple solution of the Boltzmann equation, here we clarify the role of spatial nonlinearity both in miniband electron dynamics and in amplification of an electromagnetic wave. We show that nonlinear Bloch oscillations occur at a single critical value of the wave amplitude, inducing high negative differential drift velocity. Within this model, we also explain how the amplification of a high-frequency signal can arise below the threshold for an excitation of Bloch oscillations.

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37

Maher, Christopher andrew. "NMR study of paramagnetic nano-checkerboard superlattices." W&M ScholarWorks, 2012. https://scholarworks.wm.edu/etd/1539623592.

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With the ubiquity of electronic devices, finding ways to improve quality or fabrication methods of components is an important area of study. This dissertation looks at two sets of materials that may be used to address this need. The first is a series of disordered perovskites of the form Nd⅔xLi3 xTiO3. These materials are notable for the way the lithium becomes spontaneously patterned during synthesis into square planar regions, the dimensions of which are only dependent upon the initial concentration of lithium. Through the use of point-charge calculations, the paramagnetic and first-order quadrupole interaction tensors for each of the 28 unique lithium sites of the x = 0.083 concentration were calculated and used to accurately simulate the experimental spectra. From this, it was observed that the 28 crystallographically distinct sites present in that particular concentration could be grouped into three sets based on the principal values of the paramagnetic interaction tensors. Qualitative analysis of spectra from the other concentrations suggests that this grouping holds for other concentrations, with only the relative number of sites in each group changing. Additionally, jump dynamics were incorporated into the simulations of one of the sites in order to explain the broadening that occurs at lower temperatures.;The second study included in this dissertation is focused on lithium in a pair of high-dielectric microwave ceramics, Ca(Li1/3Nb 2/3)O3 and (Ca2/3La1/3)(Li1/3 Nb2/3)O3. Experimental results are reported for the temperature-dependence of both the spin-lattice relaxation rate and the isotropic chemical shift for each material. For both samples, the isotropic shift was linear with temperature, with the isotropic shift of Ca(Li 1/3Nb2/3)O3 having a stronger temperature dependence (3.53 Hz??K-1 compared to 2.65 Hz??K -1). The spin-lattice relaxation rates of both samples follow an Arrhenius relationship with temperature, with Ca(Li1/3Nb 2/3)O3 sample having an activation energy of 5.08 kJ ?? (mol ?? K)-1 and (Ca2/3La1/3)(Li 1/3Nb2/3)O3 having an activation energy of 2.21kJ ?? (mol ?? K)-1. In addition to the lithium study, there were also spectra acquired that observed the niobium nucleus in each material, which has a noticeably more complex spectrum. For the (Ca2/3 La1/3)(Li1/3Nb2/3)O3 sample, a double-quantum satellite-transition magic angle spinning pulse sequence was used to determine the isotropic chemical shift as well as the quadrupole product of each of the five resolved sites.

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Leung, H. T. "The incommensurate phases in intermediate plagioclase feldspars." Thesis, University of Cambridge, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.360914.

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Tank, Rashmi Winfried. "A theoretical study of semiconductor nanometer structures." Thesis, University of Oxford, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.240473.

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Fyath, Raad Sami. "Advanced avalanche photodiode receivers in optical communications." Thesis, Bangor University, 1990. https://research.bangor.ac.uk/portal/en/theses/advanced-avalanche-photodiode-receivers-in-optical-communications(7774537f-4772-4a52-b216-d04db73b3781).html.

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This thesis is concerned with a detailed study of the performance of superlattice avalanche photodiodes (SAPDs) and the implications for high bit rate direct-detection optical fibre communication systems. In these advanced detectors the electron to hole ionisation rate ratio is artificially enhanced through selective heating of the electron distribution to reduce the excess noise associated with the randomness of the avalanche multiplication and to ensure high gain-bandwidth product. Thus SAPDs are suitable for long wavelength applications (1.3-1.6 pm) where most compound semiconductor materials otherwise have comparable electron and hole ionisation rates. A comprehensive discrete ionisation model is developed to assess the performance of SAPDs; emphasis being placed on the gain, excess noise factor, gain moment generating function (MGF), and gain-bandwidth product. The model is quite flexible and it is found that other device impairments such as dark current and the number of ionisations per stage caused by the injected carrier can be readily incorporated into the formulation. The performance of optical receivers employing SAPDs is examined using a Gaussian approximation (GA) and taking into account the influence of various device impairments. To assess the accuracy of GA a rigorous statistical analysis is developed using a MGF formulation. New signal designs for optical communications devised specifically for APD receivers are described. These signals achieve simultaneously both zero intersymbol interference and zero telegraph distortion with respect to a depressed optimum threshold and are thus well suited to untimed transmission. Importantly, they also offer improved tolerance to alignment jitter when they used in conventional fully retimed receivers.

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Blair, John L. "Optical properties of the square superlattice photonic crystal structure and optical invisibility cloaking." Diss., Georgia Institute of Technology, 2010. http://hdl.handle.net/1853/37112.

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The refraction properties of photonic crystal lattices offers methods to control the beam steering of light through use of non-linear dispersion contours. In this thesis new photonic crystal structures, such as the square and triangular superlattices, that provide novel refractive properties are analyzed. The property difference between rows in these structures is the hole radius Δr. The difference in hole sizes leads to observation of the superlattice effect, that is, a change in the refractive index Δn between opposite rows of holes. The index difference becomes a function of the size of the smaller r2 hole area or volume due to the addition of the higher index background material compared to the larger r1 holes. The difference in hole radii Δr = r1 - r2 is referred to as the static superlattice strength and is designated by the ratio of r2/r1. The superlattice strength increases as the ratio of r2/r1 decreases. The hole size modulation creates modified dispersion contours that can be used to fabricate advanced beam steering devices through the introduction of electro-optical materials and a controlled bias. A discussion of these superlattice structures and their optical properties will be covered, followed by both static and dynamic tunable device constructions utilizing these designs. Also, static tuning of the devices through the use of atomic layer deposition, as well as active tuning methods utilizing liquid crystal (LC) infiltration, sealed LC cells, and the addition of electro-optic material will be discussed. Also in this thesis we present designs to implement a simpler demonstration of cloaking, the carpet cloak, in which a curved reflective surface is compressed into a flat reflective surface, effectively shielding objects behind the curve from view with respect to the incoming radiation source. This approach eliminates the need for metallic resonant elements. These structures can now be fabricated using only high index dielectric materials by the use of electron beam lithography and standard cleanroom technologies. The design method, simulation analysis, device fabrication, and near field optical microscopy (NSOM) characterization results are presented for devices designed to operate in the 1400-1600nm wavelength range. Improvements to device performance by the deposition/infiltration of linear, and potentially non-linear optical materials, were investigated.

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Amann, Andreas. "Nonlinear and chaotic front dynamics in semiconductor superlattices." [S.l.] : [s.n.], 2004. http://deposit.ddb.de/cgi-bin/dokserv?idn=970075081.

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Sun, Zhouzhou. "Self-sustained current oscillations in weakly coupled superlattices /." View abstract or full-text, 2005. http://library.ust.hk/cgi/db/thesis.pl?PHYS%202005%20SUN.

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Broddefalk, Arvid. "Magnetic properties of transition metal compounds and superlattices." Doctoral thesis, Uppsala University, Department of Materials Science, 2000. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-535.

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Magnetic properties of selected compounds and superlattices have been experimentally studied using SQUID (superconducting quantum interference device) and VSM (vibrating sample magnetometer) magnetometry, neutron diffraction and Mössbauer spectroscopy measurements combined with theoretical ab initio calculations.

The magnetic compounds (Fe_1-xM_x)₃P, M=Co or Mn have been studied extensively. It was found that Co can substitute Fe up to x=0.37. Increasing the Co content leads to a reduction of the Curie temperature and the magnetic moment per metal atom. Mn can substitute Fe up to x=0.25 while Fe can be substituted into Mn₃P to 1-x=0.33. On the iron rich side, the drop in Curie temperature and magnetic moment when increasing the Mn content is more rapid than for Co substitution. On the manganese rich side an antiferromagnetic arrangement with small magnetic moments was found.

The interlayer exchange coupling and the magnetocrystalline anisotropy energy of Fe/V superlattices were studied. The coupling strength was found to vary with the thickness of the iron layers. To describe the in-plane four-fold anisotropy, the inclusion of surface terms proved necessary.

The in-plane four fold anisotropy was also studied in a series of Fe/Co superlattices, where the thickness of the Co layers was kept thin so that the bcc structure could be stabilized. Only for samples with a large amount of iron, the easy axis was found to be [100]. The easy axis of bulk bcc Co was therefor suggested to be [111].

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Yip, Sui Yu. "Electronic properties of superlattices and the RKKY interaction." Thesis, Lancaster University, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.315466.

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Warburton, Richard John. "High magnetic field studies of strained layer superlattices." Thesis, University of Oxford, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.293428.

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Jehan, David Antony. "Magnetic structures in rare earth metals and superlattices." Thesis, University of Oxford, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.357569.

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Simpson, John Andrew. "Magnetic properties of rare-earth elements and superlattices." Thesis, University of Oxford, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.308539.

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Springett, Roger James. "Raman scattering studies of III-V semiconductor superlattices." Thesis, University of Oxford, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.260178.

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Hughes, D. T. "Electronic states in semiconductor superlattices and quantum wells." Thesis, Durham University, 1989. http://etheses.dur.ac.uk/6519/.

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The aim of this thesis is two-fold. Firstly to show how a complex bandstructure matching technique has been developed which allows detailed investigations to be made of various low dimensional structures. Secondly the method will be used to investigate interesting physical properties of quantum wells and superlattices. Consequently the thesis will begin with an exposition of the matching technique, giving an indication of the numerical methods used for computational calculations. Evidence will then be given, in the form of a comparison between the present work and the experimental and theoretical results to be found in the literature, of the efficacy of the method. A detailed description of results of calculations carried out on the valence band of AlAs/GaAs and AlGaAs/GaAs quantum wells will show how energy levels and wavefunctions have been calculated as well as giving an explanation of hybridisation and anti-crossing effects. In order to extend the method to strained systems it will be shown how strain can be incorporated into the model by simple empirical fits of calculated bulk properties to experiment. This method will be used to model two particular Si/Ge structures: a Ge(_0.25)Si(_0.75)/Si/Ge(_0.25)Si(_0.75) quantum well and a (Si)(_4)(Ge)(_4) superlattice. To allow a better comparison with experiment for the superlattice a description is given of a method for calculating optical matrix elements between superlattice states; and the results of such calculations are discussed.

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Dissertations / Theses on the topic 'Superlattices'

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