Performance of Equal Phase-Shift Search for One Iteration

Grover presented the phase-shift search by replacing the selective inversions by selective phase shifts of $\pi /3$. In this paper, we investigate the phase-shift search with general equal phase shifts. We show that for small uncertainties, the failure probability of the Phase-$\pi /3$ search is smaller than the general phase-shift search and for large uncertainties, the success probability of the large phase-shift search is larger than the Phase-$\pi /3$ search. Therefore, the large phase-shift search is suitable for large-size of databases.Comment: 10 pages, 4 figure

Modeling and analyzing the evolution of cellular networks using stochastic geometry

The increasing complexity of cellular network due to its continuous evolution has made the conventional system level simulations time consuming and cost prohibitive. By modeling base station (BS) and user locations as spatial point processes, stochastic geometry has recently been recognized as a tractable and efficient analytical tool to quantify key performance metrics. The goal of this dissertation is to leverage stochastic geometry to develop an accurate spatial point process model for the conventional homogeneous macro cellular network, and to address the design and analysis challenges for the emerging cellular networks that will explore new spectrum for cellular communications. First, this dissertation proposes to use the repulsive determinantal point processes (DPPs) as an accurate model for macro BS locations in a cellular network. Based on three unique computational properties of the DPPs, the exact expressions of several fundamental performance metrics for cellular networks with DPP configured BSs are analytically derived and numerically evaluated. Using hypothesis testing for various performance metrics of interest, the DPPs are validated to be more accurate than the Poisson point process (PPP) or the deterministic grid model. Then the focus of this dissertation shifts to emerging networks that exploit new spectrum for cellular communications. One promising option is to allow the centrally scheduled cellular system to also access the unlicensed spectrum, wherein a carrier sensing multiple access with collision avoidance (CSMA/CA) protocol is usually used, as in Wi-Fi. A stochastic geometry-based analytical framework is developed to characterize the performance metrics for neighboring Wi-Fi and cellular networks under various coexistence mechanisms. In order to guarantee fair coexistence with Wi-Fi, it is shown that the cellular network needs to adopt either a discontinuous transmission pattern or its own CSMA/CA like mechanisms. Next, this dissertation considers cellular networks operating in the millimeter wave (mmWave) band, where directional beamforming is required to establish viable connections. Therefore, a major design challenge is to learn the necessary beamforming directions through the procedures that establish the initial connection between the mobile user and the network. These procedures are referred to as initial access, wherein cell search on the downlink and random access on the uplink are the two major steps. Stochastic geometry is again utilized to develop a unified analytical framework for three directional initial access protocols under a high mobility scenario where users and random blockers are moving with high speed. The expected delay for a user to succeed in initial access, and the average user-perceived downlink throughput that accounts for the initial access overhead, are derived for all three protocols. In particular, the protocol that has low beam-sweeping overhead during cell search is found to achieve a good trade-off between the initial access delay and user-perceived throughput performance. Finally, in contrast to the high mobility scenario for initial access, the directional cell search delay in a slow mobile network is analyzed. Specifically, the BS and user locations are fixed for long period of time, and therefore a strong temporal correlation for SINR is experienced. A closed-form expression for the expected cell search delay is derived, indicating that the expected cell search delay is infinite for noise-limited networks (e.g., mmWave) whenever the non-line-of-sight path loss exponent is larger than 2. By contrast, the expected cell search delay for interference-limited networks is proved to be infinite when the number of beams to search at the BS is smaller than a certain threshold, and finite otherwise.Electrical and Computer Engineerin

High efficiency III/V thin film solar cells : light trapping, antireflection, and band structure engineering

textPhoton management via submicron and subwavelength nanostructures has been extensively studied over the last decade, and has become one of the most important approaches of boosting the energy conversion efficiency for thin-film photovoltaic devices. The incorporation of low dimensional nanostructures, such as GaAs/InGaAs quantum wells, into typical GaAs single-junction cells will extend the cell absorption further into the sub-GaAs bandgap region but usually results in reduced cell open-circuit voltage. As a consequence, various bandgap engineering techniques for improving the energy conversion efficiency for quantum well solar cells have been reported. This dissertation will describe studies of light trapping in multiple GaAs/InGaAs quantum well solar cells via nanostructured front side dielectric coating and back side metal/dielectric contacts, photovoltaic performance enhancement for bulk and flexible thin-film GaAs solar cells through subwavelength nanostructured antireflection coating, and bandgap engineering techniques for GaAs/InGaAs multiple quantum well solar cells. In the study of nanostructured dielectric antireflection coatings, a 5.8% increase in short-circuit current density is observed for the GaAs/In₀.₃Ga₀.₇As multiple quantum well cell coated with TiO₂ nanostructured coating compared to the cell coated with conventional Si₃N₄ single-layer antireflection coating even in the presence of high surface recombination. Numerical simulation shows that as high as 13% increase in short-circuit current density can be achieved without surface recombination. In the study of GaAs/In₀.₃Ga₀.₇As multiple quantum well solar cells integrated with nanostructured back side metal/dielectric contacts, as high as 2.9% per quantum well external quantum efficiency is achieved, significantly surpassing the 1% per quantum well external quantum efficiency typically observed in quantum well solar cells. In both studies, two major mechanisms contributing to the increased longer wavelength quantum well absorption have been elucidated: Fabry-Perot resonances and scattering into guided optical modes. In application of subwavelength-scale optical nanostructures on bulk and flexible epitaxial lift-off GaAs solar cells for broadband, omnidirectional improvement of photovoltaic performance, 1.1× increase in short-circuit current density is observed for the bulk GaAs cell fully integrated with optical nanostructures compared to the unpatterned cell (1.09× increase in short-circuit current density for flexible epitaxial lift-off GaAs cell) at normal incidence, while 1.67× increase in short-circuit current density is observed (1.52× increase in short-circuit current density is observed for flexible epitaxial lift-off GaAs cell) at 80° angle of incidence. In the study of bandgap engineering strategies for improving the photovoltaic performance for GaAs/InGaAs multiple quantum well solar cells, a quantum well solar cell with graded quantum well depths, which has an average 18% indium concentration in quantum wells, is shown to yield improvements in both open-circuit voltage and short-circuit current density compared to a GaAs/In₀.₁₈Ga₀.₈₂As quantum well solar cell with constant quantum well depths across the intrinsic region. The results of this study suggest that such an approach can also be implemented in quantum well solar cells with more complex quantum well structures, such as ternary or quaternary quantum wells, where the conduction and valence band offsets of each quantum well can be simultaneously engineered.Electrical and Computer Engineerin

Computational investigation of functional perovskites

Functional perovskites have been investigated extensively for many years. Thousands of new perovskites are synthesized and studied every year. Many functional perovskites have been widely employed in industry. Density functional theory (DFT) calculations have been used to obtain a better understanding of functional perovskites, especially their electronic and structural properties. During my graduate study, I investigated perovskite’s properties on ionic transport, magnetic ordering, ferroelectricity, physical property and phase transition using DFT calculations. In the first case, I simulated the ionic transport process in several Ruddlesden- Popper (RP) phases. Climbing image nudged elastic band (CI-NEB) calculation was used to get accurate oxygen interstitial migration barrier. I established a linkage between interstitial migration barrier and perovskite’s octahedral rotation with symmetry mode approach. Two factors, including A-site atom radius and epitaxial strain, were used to reduce interstitial migration barrier in my simulation. My study on ionic transport in RP phases provides guidance on the design of fast ionic transport in perovskite oxides. In the second case, DFT calculation was employed to investigate a double perovskite’s magnetic and electronic properties. A new ferroelectric mechanism in perovskite, associated with the displacement of coplanar Mn²⁺, was discovered experimentally. My DFT calculation explained the origin of coplanar displacement from an orbital point of view. In addition, DFT simulations were used in the design of ferroelectricity enhancement perovskite. In the last case, I simulated structural behaviors under pressure of several double perovskites. The results show that these double perovskites can be divided into two groups based on their octahedral rotations under pressure. The origin of their distinct volume reduction mechanisms was studied through DFT simulations. The difference between the two mechanisms and their influence on bulk modulus were discussed based on my computational results.Materials Science and Engineerin

Cycling in small suburban communities : a case study of Georgetown, TX

Small communities have natural advantages in promoting cycling: Smaller geographic scale, major destinations within biking distances, and relatively low volumes of vehicle traffic. Davis, CA and Boulder, CO are known exemplary cycling-friendly communities in the United States. In Texas, however, cycling in small communities remains rare as a transportation means. This study aimed at understanding the driving factors for cycling in Texas suburban communities. The report presents a case study of Georgetown, a suburban city in the Austin Metropolitan Area in Texas. Georgetown is contemplating a Bike Master Plan to address the growing interest in and concerns over cycling in the community. A survey on cycling in Georgetown was conducted in fall 2016, for which this author was a member of the survey team. The survey included two parts, an online version of questionnaire and an onsite version for environmental audits, covering the following main topics: public opinions on cycling, cycling behavioral characteristics, environmental/infrastructure conditions for cycling. The report analyzes survey results and discusses opportunities and challenges facing Georgetown to cycling. The study findings help inform the Bike Master Plan effort by Georgetown, TX planners. Lessons learned from the Georgetown study are also valuable to the state-wide endeavor to promote cycling, particularly in small communities in Texas.Community and Regional Plannin

Paper-based electrochemical platforms for separation, enrichment, and detection

Paper based analytical devices (PADs) have great potential in the application of point-of-care diagnosis. This dissertation focuses on the design and application of PADs, especially ones that integrate with electrochemical systems, to tackle various problems in analytical chemistry, such as multi-analyte separation, sample enrichment, and sensitive detection. Four types of PADs are described in this dissertation. The first PAD (oPAD-Ep) is designed for multi-analyte separation. The oPAD-Ep is fabricated using the principle of origami to create a stack of connected paper layers as an electrophoresis channel. Due to the thinness of paper, a high electric field can be achieved with low voltage supply. Serum proteins can be separated and the device can be unfolded for post-analysis. The second PAD (oPAD-ITP) is designed on a similar principle as the oPAD-Ep, but it is applied for sample enrichment. The major modification is to adjust electrolyte conditions to enable isotachophoretic enrichment of analytes. DNA with various lengths can be enriched within a few minutes, and can be collected on one of the paper folds. The third PAD (hyPAD) also focuses on sample enrichment. The device is assembled with two different paper materials, nitrocellulose and cellulose. The hyPAD can perform faradaic ion concentration polarization experiments. This technique uses faradaic electrochemistry to create a local electric field gradient in the paper channel and can enrich charged analytes including: DNA, proteins, and nanoparticles. The fourth PAD (oSlip-DNA) focuses on sensitive electrochemical detection of DNA hybridization assays. This method integrates magnetic enrichment and electrochemical signal amplification via silver nanoparticles. Using voltammetry, sensitive detection of Hepatitis B Virus DNA is achieved on the low-cost device.Chemistr

Cosmology in a universe with Bose-Einstein-condensed scalar field dark matter

We consider an alternative cold dark matter candidate, ultralight bosons (m>10^{-22} eV/c^2) described by a complex scalar field (SFDM) with global U(1) symmetry, with comoving particle number density conserved after particle production during standard reheating. We allow for repulsive self-interaction. In a Lambda-SFDM universe, SFDM starts relativistic, evolving from stiff (w=1) to radiation-like (w=1/3), becoming nonrelativistic (w=0) at late times. Thus, a stiff-SFDM-dominated era precedes the familiar radiation-dominated era. SFDM particle mass m and quartic self-interaction strength lambda, are therefore constrained by cosmological observables, N_{eff}, the effective number of neutrino species during BBN, and z_{eq}, the matter-radiation equality redshift. Since the stochastic gravitational wave background (SGWB) from inflation is amplified during the stiff-SFDM-dominated era, it can also contribute a radiation-like component large enough to affect these observables. Remarkably, this amplification makes this SGWB detectable by current GW experiments, e.g., aLIGO/Virgo and LISA, for Lambda-SFDM models satisfying cosmological constraints, for a range of reheat temperatures T_{re} and currently allowed values of tensor-to-scalar ratio r. For given r and lambda/(mc^2)^2, the marginally-allowed Lambda-SFDM model for each T_{re} has the smallest m that satisfies cosmological constraints. For example, for marginally-allowed models with r=0.01 and lambda/(mc^2)^2=10^{-18} eV^{-1} cm^3, null detection by the aLIGO O1 run excludes 8.75*10^3<T_{re} (GeV)<1.7*10^5 at 95% confidence, demonstrating that GW experiments already place a new kind of cosmological constraint on SFDM. A wider parameter range should be accessible to aLIGO/Virgo O5, with potential to detect this signature of Lambda-SFDM. For this same illustrative family, 3-sigma detection is predicted for 600<T_{re} (GeV)<10^7.Astronom

Scanning probe microscopy study of thin film solar cells

textThin film solar cells, such as CdTe, CuIn [subscript x] Ga [subscript 1-x] Se₂ (CIGS), Cu₂ZnSnS₄ (CZTS) and Cu₂ZnSnSe₄ (CZTSe), have been intensively studied for their unique features and excellent prospect of mass production in industry. The p-n junction is the most critical part of the thin film solar cell and greatly influences the performance. In this thesis work, the p-n junctions and the device layers of multiple kinds of thin film solar cells have been studied by using scanning probe microscopy based techniques. The scanning spreading resistance microscopy (SSRM) has been developed on the cross-section of CdTe solar cells to study the resistance and carrier concentration distribution in different layers of the device. The CdTe sample was cleaved and milled with the argon ion beam to get a flat cross-section. The multiple device layers of the device were identified by the resistance mapping. A high-resistance region around the junction on the CdTe side due to carrier depletion was measured. With the AFM laser illumination, the resistance in the deep depletion region dropped and the resistance across the entire CdTe layer became relatively uniform due to domination of photo-excited carriers. With carriers injected by applying a forward-bias voltage to the working device, the resistance in the deep depletion region decreased and the region moved toward the CdS/CdTe interface. These observed trends and observations are consistent with device physics. We also measured the surface potential and the electric field across the junction using scanning Kelvin probe force microscopy (SKPFM) in the cross-section of the standard CIGS, ZnS(O,OH)/CIGS and the standard CZTSe devices. Both the heterojunction and homojunction situations of the three solar cells were simulated using the PC1D software. The simulation results were compared with the experimental results to analyze the properties of the junction. The comparison results provided the possible ranges of the thickness and carrier concentration of n-CIGS/n-CZTSe layer.Physic