This thesis explores transport properties of lateral, gate defined quantum dots in GaAs/AlGaAs heterostructures. The term "quantum dot" as defined in this thesis refers to small regions of charge carriers within a 2-dimensional electron gas (2DEG), established via electrically biased surface gates used to isolate the charge carriers from the rest of the 2DEG, which are confined to lengths scales on the order of nanometers. Several other forms of quantum dots exist in the research community, including colloidal and self-assembled dots. In this thesis, however, we consider only gate defined quantum dots and nanostructures. Recent advancements in the research areas of quantum dot (QD) and single electron transistors (SET) have opened up an exciting opportunity for the development of nanostructure devices. Of the various devices, our attention is drawn in particular to detectors, which can respond to a single photon over a broad frequency spectrum, namely, microwave to infrared (IR) frequencies. Here, we report in chapter 5 transport measurements of parallel quantum dots, fabricated on a GaAs/AlGaAs 2-dimensional electron gas material, under the influence of external fields associated with 110GHz signals. In this experiment, transport measurements are presented for coupled quantum dots in parallel in the strong-tunneling Coulomb blockade (CB) regime. From this experiment we present experimental results and discuss the dependence on quantum dot size, fabrication techniques, as well as the limitations in developing a QD photon detector for microwave and IR frequencies, whose noise equivalent power (NEP) can be as sensitive as 10-22 W/Hz1/2. The charging energy EC of a quantum dot is the dominant term in the Hamiltonian and is inversely related to the self capacitance of the dot Cdot according to EC = e2/Cdot. The temperature of the charge carriers within the 2DEG must be kept below a certain value, namely KBT, so that the thermal energy of the electrons does not exceed the charging energy EC of the dot. Keeping the temperature below the KBT limit prevents electrons from entering or leaving the dot at random, thereby allowing one to precisely control the number of electrons in the dot. In order to raise the operating temperature T of the single photon detector we must also raise the charging energy EC, which is accomplished by decreasing Cdot. Since Cdot is directly related to the dimensions of the quantum dot our focus was directed at decreasing the overall size of the quantum dots. For smaller gate defined quantum dots the inclusion of shallower 2DEG's is necessary. However, the experiments that we carried out to determine the effect of 2DEG depth on lateral gate geometries, described in Chapter 6, indicate that leakage currents within a GaAs/AlGaAs heterostructure increased dramatically as the 2DEG depth became shallower. At this moment the leakage current in shallower 2DEG materials is one of the most significant technical challenges in achieving higher operating temperatures of the single photon detector. / Physics

In this thesis, we have investigated the lowest-energy members of the quantized intrinsically localized modes of vibration (ILMs) of the monatomic ß Fermi-Pasta-Ulam Hamiltonian in three-dimensions. We analytically find the excitation of different center of mass momenta. Using the Ladder Approximation, we find that the ILMs occur preferentially for centre of mass momenta at which the van-Hove singularities in the two-phonon density of states coalesce. When the ILMs first form they split off from the top of the two-phonon continuum. The ILMs can be categorized as having a spin of either S=2 or S=0 and have other internal quantum numbers. Moreover, the S=0 ILMs form for lower values of the interaction than the S=2 ILMs. We also focus on the temperature dependence of the ILMs. At zero temperature, the ILMs can form in three-dimensions, but only if the interaction exceeds a minimum value. As the temperature is raised, the magnitude of the minimal interaction required to stabilize the ILM is reduced. This is in a qualitative agreement with the experiments of Manley {\it et al.}, which only found the ILMs of NaI at elevated temperatures. We have also examined the ILM many-body wave functions and find that the relative coordinate part of the wave functions has symmetries associated with internal quantum numbers. According to our numerical results, the localization length increases with decreasing values of the strength of interaction. The results are presented in D. Kanbur and P. S. Riseborough, Phil. Mag. Letts, 94, 424-432 (2014) and D. Kanbur and P. S. Riseborough, Phys. Rev. B, 90, 134301 (2014). This work was supported by the US Department of Energy, Office of Basic Energy Science, Materials Science and Engineering through the award DEFG02-84ER45872. / Physics

The focus of this thesis work is the study of the nanoscale electronic properties of magnetically coupled superconductor/ferromagnet hybrid structures using low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) under ultra-high vacuum conditions. There are a number of novel effects that can occur due to the non-homogenous magnetic field from the ferromagnet, which directly influence the global and local superconducting properties. These effects include the generation of vortices/anti-vortices by the non-uniform magnetic stray field, local modulations in the critical temperature, filamentary superconductivity close to the transition temperature, and superconducting channels that can be controlled by external magnetic fields. Prior to this dissertation the subject of superconductor/ferromagnet hybrid structures has been mainly studied using global measurements (such as transport and magnetization) or scanning probe techniques that are sensitive to the magnetic field. Scanning tunneling microscopy probes the local electronic density of states with atomic resolution, and therefore is the only technique that can study the emergence of superconductivity on the length scale of the coherence length. The novel results presented in this dissertation show that magnetically coupled superconductor/ferromagnet heterostructures offer the possibility to control and tune the strength and location of superconductivity and superconducting vortices, which has potential for promising technological breakthroughs in computing and power applications. / Physics

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Master of Science / Department of Physics / Bruce M. Law / Interest in the properties of room-temperature ionic liquids is rapidly expanding. Although there have been numerous studies concerning their preparation, their use as a reaction medium and their physical properties, Ionic Liquids (ILs) are so new that many of their bulk physicochemical properties, optical properties, surface properties, toxicities etc. are unknown or only just beginning to be characterized. The highly polar nature of the ILs causes the surfaces of the liquids to become highly ordered in comparison with the surfaces of many other types of organic liquids. Surface structuring at the liquid-vapor interface of the imidazolium based ILs can be examined by using Brewster Angle Ellipsometry and Contact Angle Measurement. The preliminary observation of Ellipsometric measurement shows that there is an interfacial order-disorder transition at temperature Tc=385 K. This result is not analyzed yet but the initial thought behind this is an indication of a ferroelectric transition at the liquid-vapor interface of dipole moment of ILs. From the contact angle measurement it is shown that there is a remarkable change in the contact angle of the imidazolium based ILs over short interval of time ([similar to] ten minutes). Also study of the spreading of the ILs on hard surface shows that there is some definite structural dependence

<p> This dissertation explores the effects of pressure and light on chalcogenide based materials. In ZnSe, ZnTe, and CdSe the surprising precipitation of the constituent anion under hydrostatic pressure and moderate laser exposure in high quality bulk and MBE film samples is explored in detail. In ZnSe the anomalous broadening in the TO(&Gamma;) phonon region is explored by careful low laser power pressure cycling experiments. The experimental results are supported with density functional theory calculations of the phonon band structure. Finally, the photo-induced crystallization onset of amorphous selenium films is explored as a function of temperature and substrate structure. The morphology of the photocrystallized spots is also explored using Raman mapping, optical microscopy, and atomic force microscopy.</p>

The work in this dissertation has focused on the optical properties of semiconductor microcavities containing one or more high-quality, narrow-linewidth quantum wells, and how the appropriate design and growth of such structures can result in a nonperturbative coupling of light and matter. We apply the term Normal-Mode Coupling to describe this interaction, as it can be ascribed to the dipole interaction lifting the degeneracy between field and emitter resonances, resulting in a strongly coupled two- (or more-) oscillator system. Linear reflection, transmission, and photoluminescence measurements for the two-oscillator systems show two dips or peaks near zero detuning, whereas samples with two nonidentical quantum wells coupled to the microcavity display a three-resonance behavior. It is demonstrated that the linewidths of these samples are not only functions of the uncoupled cavity and exciton lineshapes, but are also sensitive to the local variations of the index of refraction and optical absorption. To this end, absorption measurements of multiple-quantum-well samples lead to a phenomenological derivation of the optical susceptibility inclusive of the influence of structural disorder. Use of this susceptibility in a transfer-matrix calculation then gives good agreement with experiment. The ability to see well-resolved normal-mode coupling peaks at room temperature is also demonstrated. Here, the distributed Bragg reflector mirror layers are created through oxidation of the AlAs mirror layers, resulting in increased field confinement and larger splitting. The superlative splitting-to-linewidth ratios at resonance for these samples make them ideal candidates for nonlinear studies. Pump-probe transmission and photoluminescence studies utilizing both resonant and nonresonant pump excitation are presented. Nonlinear saturation of the quantum-well excitonic resonance leads to increased absorption at the normal-mode coupling transmission peaks, which reduces their amplitude. For relatively small positive cavity-exciton detunings, there is a good correspondence between photoluminescence crossover and the opening up of probe transmission at the uncoupled cavity-mode resonance. It is demonstrated that this occurs when the exciton absorption is bleached, and the coupling undergoes a transition from nonperturbative to weak. In this case, nonlinear absorption measurements on a cavityless multiple-quantum-well sample provide the nonlinear optical susceptibility for use in a transfer-matrix simulation for the off-resonant pumping experiments.

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This study presents the use of Feynman's Quantum Path Integral (QPI) approach in the Molecular Dynamic Simulation of two electrons in molten KCl. In this research, we have successfully implemented an original technique to tackle the questions of spin dependent quantum exchange phenomenon between two electrons. It was found that two electrons with antiparallel spins form a stable bipolaronic complex and those with parallel spins repel each other and form two dissociated or singlet states. Calculations of the average energies compare well with previous computational findings by Selloni et al. who used a direct integration of the time dependent Schrodinger equation. The radial distribution function illustrated clearly that the triplet state nests itself among the cations, namely K+. The electron-electron separation distance was found to be &sim;3.5 A for the triplet state and the singlet case showed the electrons being repelled as far as possible; namely half the size of the simulation cell &sim;7 A.