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Optical lattices make it possible to trap and coherently control large ensembles of ultracold atoms. They provide the possibility to create lattice potentials that mimic the structure of solid-state systems, and to control these potentials dynamically. In this thesis, we study how dynamical manipulations of the lattice geometry can be used to perform different tasks, ranging from quantum information processing to the creation of diatomic molecules. We first examine the dynamical properties of ultracold atoms trapped in a lattice whose periodicity is dynamically doubled. We derive a model describing the dynamics of the atoms during this process, and compute the different interaction parameters of this model. We investigate different ways of using this lattice manipulation to optimise the initialisation time of a Mott-insulating state with one atom per site, and provide a scaling law related to the interaction parameters of the system. We go on to show that entangling operations between the spin of adjacent atoms are realisable with optical lattices forming arrays of double-well potentials. We study the creation of a lattice containing a spin-encoded Bell-pair in each double-well, and show that resilient, highly-entangled many-body states are realisable using lattice manipulations. We show that the creation of cluster-like states encoded on Bell-pairs can be achieved using these systems, and we provide measurement networks that allow the execution of quantum algorithms while maintaining intact the resilience of the system. Finally, we investigate the possibility to create a diatomic molecular state and simulate Fermi systems via the excitation to Rydberg levels of ground-state atoms trapped in optical lattices. We develop a method based on symbolical manipulations to compute the interaction parameters between highly-excited electrons, and evaluate them for different electronic configurations. We use these parameters to investigate the existence of diatomic molecular states with equilibrium distances comparable to typical lattice spacings. Considering the possibility to excite atoms trapped in an optical lattice to Rydberg levels such that the electronic cloud of neighbouring atoms overlap, we propose a model describing their interactions and compute its parameters. If such systems were realised, they would allow the simulation of Fermi systems at a temperature much below the Fermi temperature, thus enabling the observation of quantum phenomena hitherto inaccessible with current technology.

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by Lo Wai Hung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2002. / Includes bibliographical references. / Text in English; abstracts in English and Chinese. / by Lo Wai Hung. / Acknowledgements --- p.1 / Abstract / 論文摘要 --- p.ii / Table of Contents --- p.iv / List of Figures --- p.vi / List of Tables --- p.viii / Chapter Chapter 1. --- Introduction / Chapter 1.1. --- Magnetoresistance --- p.1 -1 / Chapter 1.1.1. --- Giant magnetoresistance (GMR) --- p.1 -2 / Chapter 1.1.2. --- Colossal Magnetoresistace (CMR) --- p.1 -2 / Chapter 1.2. --- Doping effects in La1-xCaxMn03 --- p.1-4 / Chapter 1.3. --- Metal-Insulator transition in CMR materials --- p.1 -8 / Chapter 1.3.1. --- The sharpness in Metal-Insulator transition --- p.1 -9 / Chapter 1.3.2. --- Possible model to explain CMR in rare-earth manganites --- p.1-12 / Chapter 1.4. --- Low field magnetoresistance --- p.1-14 / Chapter 1.4.1.1. --- Single crystal and polycrystalline perovskite manganites --- p.1-14 / Chapter 1.4.1.2. --- Manganite trilayer junctions --- p.1-15 / Chapter 1.4.2. --- Possible mechanism of low field MR --- p.1-16 / Chapter 1.5. --- Our motivation --- p.1-17 / Chapter 1.5.1. --- Brief review of several manganite superlattices systems --- p.1-18 / Chapter 1.5.2. --- Scope of this thesis work --- p.1-20 / References --- p.1-21 / Chapter Chapter 2. --- Epitaxial growth of LCMO thin films / Chapter 2.1. --- Deposition techniques --- p.2-1 / Chapter 2.1.1. --- Induction --- p.2-1 / Chapter 2.1.2. --- Facing-target sputtering (FTS) --- p.2-1 / Chapter 2.1.3. --- Vacuum system --- p.2-3 / Chapter 2.2. --- Fabrication and characterization of LCMO and PCMO targets --- p.2-4 / Chapter 2.3. --- Epitaxial growth of LCMO thin films --- p.2-9 / Chapter 2.3.1. --- Substrate materials --- p.2-9 / Chapter 2.3.2 --- Deposition --- p.2-10 / Chapter 2.3.2.1. --- Sample preparation --- p.2-10 / Chapter 2.3.2.2. --- Deposition procedure --- p.2-10 / Chapter 2.3.2.3. --- Inter-target distance --- p.2-11 / Chapter 2.3.2.4. --- Deposition Rate --- p.2-15 / Chapter 2.4. --- Substrate temperature effect --- p.2-17 / Chapter 2.4.1. --- Crystal Structure --- p.2-17 / Chapter 2.4.2. --- Transport properties --- p.2-20 / Chapter 2.4.2.1. --- Sharpness of M-I transport properties --- p.2-24 / Chapter 2.4.2.2. --- Magnetoresistance of LCMO/NGO films --- p.2-27 / Chapter 2.5. --- Thickness of LCMO thin film --- p.2-28 / Chapter 2.5.1. --- Crystal Structure --- p.2-29 / Chapter 2.5.2. --- M-I transition properties --- p.2-31 / Chapter 2.5.2.1. --- Sharpness of M-I transport properties --- p.2-35 / Chapter 2.5.2.2. --- Magnetoresistance of LCMO/NGO films --- p.2-36 / Chapter 2.5.2.3. --- Surface Morphology --- p.2-38 / Chapter 2.6. --- Epitaxial growth of PCMO thin films --- p.2-40 / Chapter 2.7. --- Conclusion --- p.2-42 / References --- p.2-43 / Chapter Chapter 3. --- LCMO/PCMO superlattices --- p.3-1 / Chapter 3.1. --- Variation of the PCMO thickness in LCMO/PCMO superlattices --- p.3-2 / Chapter 3.1.1. --- Sample Preparation --- p.3-2 / Chapter 3.1.2. --- Structure characterization by XRD --- p.3-3 / Chapter 3.1.3. --- Transport properties --- p.3-10 / Chapter 3.1.3.1. --- Sharpness of M-I transport properties --- p.3-14 / Chapter 3.1.3.2. --- Magnetoresistance of LCMO/PCMO superlattices --- p.3-16 / Chapter 3.2. --- Variation of the number of LCMO/PCMO bilayer --- p.3-19 / Chapter 3.2.1. --- Sample Preparation --- p.3-19 / Chapter 3.2.2. --- Structure characterization by XRD --- p.3-21 / Chapter 3.2.3. --- Transport properties --- p.3-23 / Chapter 3.2.3.1. --- Sharpness of M-I transport properties --- p.3-27 / Chapter 3.2.3.2. --- Magnetoresistance of LCMO/PCMO superlattices --- p.3-28 / Chapter 3.3. --- Fine adjusting the thickness of PCMO around 10Ain LCMO/PCMO superlattices / Chapter 3.3.1. --- Sample Preparation --- p.3-31 / Chapter 3.3.2. --- Characterization ofLCMO/PCMO superlattices by XRD --- p.3-32 / Chapter 3.3.3. --- Transport properties --- p.3-35 / Chapter 3.3.3.1. --- Sharpness of M-I transport properties --- p.3-39 / Chapter 3.3.3.2. --- Magnetoresistance of LCMO/PCMO superlattices --- p.3-41 / Chapter 3.4. --- Conclusion --- p.3-43 / References --- p.3-44 / Chapter Chapter 4. --- Low-field magnetoresistance (LFMR) / Chapter 4.1. --- Low-field magnetoresistance --- p.4-1 / Chapter 4.2. --- Conclusion --- p.4-5 / References --- p.4-6 / Chapter Chapter 5. --- Structure characterization of LCMO/PCMO superlatticess by crater edge profiling --- p.5-1 / Chapter 5.1. --- Sample preparation --- p.5-2 / Chapter 5.2. --- Structure Characterization --- p.5-2 / Chapter 5.2.1. --- X-ray diffraction (XRD) --- p.5-2 / Chapter 5.2.2. --- The crater edge profiling --- p.5-5 / Chapter 5.2.2.1. --- SEM --- p.5-5 / Chapter 5.2.2.2. --- AES line scan --- p.5-10 / Chapter 5.3. --- Crater edge profiling of P1OO/STO --- p.5-12 / Chapter 5.4. --- Conclusion --- p.5-15 / References --- p.5-16 / Chapter Chapter 6. --- Conclusion --- p.6-1

Semiconductor devices based on III-V materials have been the focus of intense research due to their superior electron mobility and favorable energy direct bandgap which are applicable in infrared wavelength range optoelectronics and high speed electronic systems. The thesis presented here consists of two thrusts; the first focusing on infrared applications, and the second focusing on InP-based heterojunction bipolar transistors (HBTs). In the first thrust, we investigate type-II InAs/GaSb superlattice IR detector devices and the effect of substrate orientation on InSb and InAs nanostructure morphology. In the second thrust, we study InP-based high frequency HBTs. A low resistance InAs ohmic contact is demonstrated, and we presented along with a study of the crystalline qualities in GaAs0.5Sb0.5 films grown on tilted- axis InP substrates. Chapter 2 presents fabrication and characterization of two type-II superlattice structures with 15 monolayer (ML) InAs/12ML GaSb and 17ML InAs/7ML GaSb grown on GaSb (100) substrates by solid-source molecular beam epitaxy (MBE). The X-ray diffraction (XRD) measurements of both the 15ML InAs/12ML GaSb and 17MLInAs/7ML GaSb superlattices indicated excellent material and interface qualities. The cutoff wavelengths of 15ML InAs/12ML GaSb and 17ML InAs/7ML GaSb superlattices photodetectors were measured to be 6.6μm and 10.2μm, respectively. These different spectral ranges were achieved by growing alternating layers of varying thicknesses which allowed for bandgap engineering of the superlattices of InAs and GaSb. Lastly, a mid-IR type-II superlattice photodiode was demonstrated at 80K with a cutoff wavelength at 6.6µm. The device exhibited a near background limited performance (BLIP) detectivity at 80K and higher temperature operation up to 280K. In Chapter 3, we show that the (411) orientation, though not a naturally occurring surface, is a favorable orientation to develop a buffer layer into a super flat surface at a certain high growth temperature. The (411) surface is a combination of localized (311) and (511) surfaces but at a high growth temperature, adatoms can obtain enough energy to overcome the energy barrier between these localized (311) and (511) surfaces and form a uniform (411) surface with potential minima. This results in a super flat surface which is promising for high-density nanostructure growth. In this work, this is the first time that the highest InSb and InAs nanostructures density can be achieved on the (411) surface which is in comparison with the (100), (311), and (511) surfaces. Chapter 4 of this thesis addresses the use of an InAs layer as a low-resistance ohmic contact to InP-based heterostructure devices. Selective area crystal growth of InAs on a dielectric (Benzocyclobutene, BCB polymer) covered InP (100) substrate and direct growth of InAs on InP substrate were performed by MBE. Heavy doping of InAs using Te was carried out to determine the lowest sheet resistance. Based on scanning electron microscope (SEM) and XRD measurements, increasing substrate temperature from 210 ℃ to 350 ℃, led to an improvement in crystallinity from a polycrystalline layer to a single crystal layer with a corresponding improvement of surface morphology. Moreover, a narrow X-ray diffraction peak indicated full-relaxation of the inherent 3.3% lattice-mismatch in InAs/InP layers. Furthermore, around 290 ℃ a tradeoff was reached between crystallinity and optimized dopant incorporation of Te into InAs for the lowest sheet resistance. Lastly, Chapter 5 discusses the effect of substrate tilting on the material properties of MBE grown GaAsSb alloys closely lattice-matched to an InP substrate. InP(100) substrates tilted 0°off-(on-axis), 2°off-, 3°off-, and 4°off-axis were used for MBE growth; then the material qualities of GaAsSb epitaxial layers were compared using various techniques, including high resolution XRD, photoluminescence (PL) and transmission-line measurements (TLM). Substrate tilting improved the crystalline quality of the GaAsSb alloys, as shown by a narrower XRD linewidth and enhanced optical quality as evidenced by a strong PL peak. The results of TLM show that the lowest sheet resistance was achieved at a 2° off-axis tilt.

O objetivo principal deste projeto foi o estudo das propriedades ópticas de estruturas semicondutoras do tipo super-redes, formadas a partir da heteroestrutura de GaAs/AlAs, crescida através da técnica de Epitaxia por Feixe Molecular. No trabalho apresentamos estudos feitos em super-redes do tipo (GaAs)n/(AlAs)n, crescidas em substratos semi-isolantes e orientados nas direções (100) e (nl1) com n=1,2,3,5,7 e nas polaridades A e B. Para cada periodicidade (n x n), as estruturas foram crescidas simultaneamente num mesmo porta amostra e sob as mesmas condições. As amostras foram estudadas através das técnicas de fotoluminescência a baixa temperatura e em função da temperatura. São apresentados também resultados preliminares de um estudo feito com a técnica de fotoluminescência de excitação. A técnica de difração de elétrons de alta energia foi utilizada durante o crescimento epitaxial para aferição da periodicidade da estrutura. A eficiência quântica, a posição do pico de luminescência estão fortemente correlacionados com a direção de crescimento. As medidas de fotoluminescência em função da temperatura mostram também um decréscimo anômalo da largura de linha. A partir dos resultados ópticos foi proposta a formação de microestruturas de mais baixa diemnsionalidade nos poços, formadas por flutuações nas interfaces (microrugosidades), e originárias do modo de crescimento adotado (sem interrupção nas interfaces). O comportamento óptico observado é semelhante ao de estruturas de mais baixa dimensionalidade (pontos quânticos). Este efeito é acentuado nas direções (311) e (21l), devido a própria morfologia da superfície / The main objective of this work was the study of the optical properties of semiconductors superlattices, formed by the (GaAs)n,/(AlAs)n, heterostructure, and grown by technique of Molecular Beam Epitaxy. In the work, we presented studies in (GaAs)n/(AlAs)n, superlattices, grown on semi-insulating substrates oriented in planes (100) and (n11) with n=l, 2, 3, 5, 7 and in the polarities A and B. For each periodicity (n x n), the structures were simultaneously grown in a same sample holder, and under the same conditions. The samples were studied by the photoluminescence techniques at low temperature and in function of the temperature. Preliminares results of a study done with the technique of excitation photoluminescence are also presented. The technique of high energy eletron difraction was used during the epitaxial growth for the monitoring of the periodicity of the structure. The quantum efficiency and the positions of the luminescence peak are strongly correlated with the growth direction. The photoluminescence measures in function of the temperature also show an anomalous decrease in linewidth. The analyses of the optical results shown the possibility of low dimension microstructures formation in the wells, due to the interfaces fluctuations, and related with the growth mode (without interruption at the interface). The observed optical behavior is similar to the observed in the structures of lower dimensionality (quantum dots). This effect is accentuated in the plans (311) and (211), due to the morphology of the surface

The development and characterization of lithographically patterned dielectric superlattice systems are presented, which have enabled the first clear realization of fully developed fractal mini-gaps owing to the interplay between a quantizing magnetic field and a lithographically defined spatial superlattice potential. Following a history of lateral superlattice gating on 2-D electron gas systems, we present patterned dielectric superlattice graphene systems of unmatched quality, allowing for the characterization of Hofstadter fractal band structure under triangular and square lattice geometries. Hexagonal boron nitride, graphene heterostructures are uniquely suited to integration with patterned gating structures, due to their high mobility and thin encapsulating dielectric environment. These systems have already been utilized for the observation of Hofstadter’s fractal spectrum through the moiré superlattice effect, but such systems are limited in their tunability. The patterned dielectric superlattice allows for control of the superlattice geometry, polarity, and strength. Utilizing this control, we compare the resultant fractal spectra from both triangular and square superlattice potentials, which confer unique gap structures in agreement with their lattice symmetry. More generally, patterned dielectric superlattices can be used to generate a variety of spatially dependent scalar potentials onto van der Waals heterostructures with length scales of order 10nm, while maintaining low disorder.

Electron transport and relaxation may be substantially different in low-dimensional systems compared to that observed in bulk material. In the present work, Monte Carlo models are used for the solution to the Boltzmann transport equation, with scattering rates calculated quantum mechanically for superlattice and quantum wells. Carrier relaxation following optical excitation is examined in multiple quantum well systems. Simulated results of the carrier relaxation process in coupled asymmetric wells and modulation doped wells are in good agreement with published experimental results on similar structures. Scattering rates in superlattices due to polar optical phonons, intervalley phonons, ionized impurities and carrier-carrier scattering are derived. Carrier transport through high energy superlattice minibands is examined in superlattice base hot electron transistors. Additionally, transport in the ballistic limit in periodic quantum wire structures, including geometric superlattices, is examined utilizing a mode matching method. / Graduation date: 1992

Thesis (Ph. D.)--West Virginia University, 2001. / Title from document title page. Document formatted into pages; contains x, 73 p. : ill. (some col.). Vita. Includes abstract. Includes bibliographical references (p. 69-72).