In the Kane silicon-based electron-mediated nuclear spin quantum computer architecture, phosphorus is doped at precise positions in a silicon lattice, and the P donor nuclear spins act as qubits. Logical operations on the nuclear spins are performed using externally applied magnetic and electric fields. There are two important interactions: the hyperfine and exchange interactions, crucial for logical qubit operations. Single qubit operations are performed by applying radio frequency magnetic fields resonant with targeted nuclear spin transition frequencies, tuned by the gate-controlled hyperfine interaction. Two qubit operations are mediated through the exchange interaction between adjacent donor electrons. It is important to examine how these two interactions vary as functions of experimental parameters. Here we provide such an investigation. First, we examine the effects of varying several experimental parameters: gate voltage, magnetic field strength, inter donor separation, donor depth below the silicon oxide interface and back gate depth, to explore how these variables affect the donor electron density. Second, we calculate the hyperfine interaction and the exchange coupling as a function of these parameters. These calculations were performed using various levels of effective mass theory. In the first part of this thesis we use a multi-valley effective mass approach where we incorporate the full Si crystal Bloch structure in calculating the donor electron energy in the bulk silicon. Including the detailed Bloch structure is very computationally intensive, thus when we considered the effect of the externally applied fields in the second and third part, we employ an approach where we focus on the smooth donor-modulated envelope function to determine the response of the donor electron to the applied electric and magnetic fields and qubit position in the lattice. The electric field potential was obtained using Technology Computer Aided Design software, and the interfaces were modelled as a barrier using a step function. One of the critical results of this theoretical study was finding that there exist two regimes for the behaviour of the donor electron in response to the applied gate voltage, dependent on donor distance from the gate. When the qubit is in close proximity to the gate the electron transfer to the gate is gradual. However if the qubit is located far enough from the gate, we found that the donor electron is ionised toward the gate for gate voltages above a certain threshold. Another significant development we have made is in our calculations of the exchange coupling between two adjacent donor electrons. We extended our original Heitler-London basis to describe the two-electron system, and adopted a molecular orbital method where we included a a basis of 78 singlet and 66 triplet two-electron states. In addition to calculating a more accurate exchange coupling, we also evaluated the energy spectrum of the two electron double donor system. We aim to provide relevant information for the experimental design of these devices and highlight the significance of environmental factors other than gate potential that affect the donor electron.

Thesis (Ph.D.); Submitted to Iowa State Univ., Ames, IA (US); 12 Dec 2003. / Published through the Information Bridge: DOE Scientific and Technical Information. "IS-T 2605" Chang-Hwan Kim. 12/12/2003. Report is also available in paper and microfiche from NTIS.

This thesis investigates a number of novel magnetic materials and high temperature superconductors using high-field magnetometry and muon spin spectroscopy (μSR). The main measurement techniques are briefly described and a study of the dimer material [Cu(pyrazine)(glycine)]ClO₄ is presented to demonstrate the use of the proximity detector oscillator as a susceptometer in high magnetic fields. μSR is a highly effective tool for probing magnetic order, spin freezing and spin dynamics. However, in some circumstances its performance may be impaired by the extent to which it perturbs the material under study. Using μSR, density functional theory and crystal field calculations, I identify an experimental situation in the family of candidate quantum spin ices Pr₂B₂O₇ (B = Sn, Zr, and Hf), in which the measured response is dominated by a muon-induced distortion of the local structure. This issue is also addressed in a study of the spin dynamics in the canonical spin ice Ho2Ti2O7. Although computational work indicates a similar muon-induced effect in both Ho₂Ti₂O₇ and Dy₂Ti₂O₇, the μSR data is not dominated by this perturbation. The remainder of this thesis is concerned with studying the superconducting properties of a number of Fe-based materials, including LiFeP which is found to have an enhanced superfluid stiffness in relation to its transition temperature. Also reported is the effect of structural disorder on the superconducting state in recently discovered Sr_0.3(NH₂)_y(NH₃)_1-yFe₂Se₂. Pulsed magnetic field measurements are used to probe the temperature dependence of the upper critical field, giving a maximum value of μ₀H_c2(0)≈33(2) T. I also investigate the effect of intercalating additional ammonia, via reversible adsorption and desorption in the related superconductor Li_x[(NH₂)_y(NH₃)_1-y]_zFe₂Se₂ (z = 1, 2). These reactions were carried out in situ on the muon beamline so that the superfluid stiffness could be measured using transverse-field μSR on a single sample.

abstract: As the world energy demand increases, semiconductor devices with high energy conversion efficiency become more and more desirable. The energy conversion consists of two distinct processes, namely energy generation and usage. In this dissertation, novel multi-junction solar cells and light emitting diodes (LEDs) are proposed and studied for high energy conversion efficiency in both processes, respectively. The first half of this dissertation discusses the practically achievable energy conversion efficiency limit of solar cells. Since the demonstration of the Si solar cell in 1954, the performance of solar cells has been improved tremendously and recently reached 41.6% energy conversion efficiency. However, it seems rather challenging to further increase the solar cell efficiency. The state-of-the-art triple junction solar cells are analyzed to help understand the limiting factors. To address these issues, the monolithically integrated II-VI and III-V material system is proposed for solar cell applications. This material system covers the entire solar spectrum with a continuous selection of energy bandgaps and can be grown lattice matched on a GaSb substrate. Moreover, six four-junction solar cells are designed for AM0 and AM1.5D solar spectra based on this material system, and new design rules are proposed. The achievable conversion efficiencies for these designs are calculated using the commercial software package Silvaco with real material parameters. The second half of this dissertation studies the semiconductor luminescence refrigeration, which corresponds to over 100% energy usage efficiency. Although cooling has been realized in rare-earth doped glass by laser pumping, semiconductor based cooling is yet to be realized. In this work, a device structure that monolithically integrates a GaAs hemisphere with an InGaAs/GaAs quantum-well thin slab LED is proposed to realize cooling in semiconductor. The device electrical and optical performance is calculated. The proposed device then is fabricated using nine times photolithography and eight masks. The critical process steps, such as photoresist reflow and dry etch, are simulated to insure successful processing. Optical testing is done with the devices at various laser injection levels and the internal quantum efficiency, external quantum efficiency and extraction efficiency are measured. / Dissertation/Thesis / Ph.D. Physics 2010

abstract: The research of this dissertation involved quantitative characterization of electrostatic potential and charge distribution of semiconductor nanostructures using off-axis electron holography, as well as other electron microscopy techniques. The investigated nanostructures included Ge quantum dots, Ge/Si core/shell nanowires, and polytype heterostructures in ZnSe nanobelts. Hole densities were calculated for the first two systems, and the spontaneous polarization for wurtzite ZnSe was determined. Epitaxial Ge quantum dots (QDs) embedded in boron-doped silicon were studied. Reconstructed phase images showed extra phase shifts near the base of the QDs, which was attributed to hole accumulation in these regions. The resulting charge density was (0.03±0.003) holes/nm3, which corresponded to about 30 holes localized to a pyramidal, 25-nm-wide Ge QD. This value was in reasonable agreement with the average number of holes confined to each Ge dot determined using a capacitance-voltage measurement. Hole accumulation in Ge/Si core/shell nanowires was observed and quantified using off-axis electron holography and other electron microscopy techniques. High-angle annular-dark-field scanning transmission electron microscopy images and electron holograms were obtained from specific nanowires. The intensities of the former were utilized to calculate the projected thicknesses for both the Ge core and the Si shell. The excess phase shifts measured by electron holography across the nanowires indicated the presence of holes inside the Ge cores. The hole density in the core regions was calculated to be (0.4±0.2) /nm3 based on a simplified coaxial cylindrical model. Homogeneous zincblende/wurtzite heterostructure junctions in ZnSe nanobelts were studied. The observed electrostatic fields and charge accumulation were attributed to spontaneous polarization present in the wurtzite regions since the contributions from piezoelectric polarization were shown to be insignificant based on geometric phase analysis. The spontaneous polarization for the wurtzite ZnSe was calculated to be psp = -(0.0029±0.00013) C/m2, whereas a first principles' calculation gave psp = -0.0063 C/m2. The atomic arrangements and polarity continuity at the zincblende/wurtzite interface were determined through aberration-corrected high-angle annular-dark-field imaging, which revealed no polarity reversal across the interface. Overall, the successful outcomes of these studies confirmed the capability of off-axis electron holography to provide quantitative electrostatic information for nanostructured materials. / Dissertation/Thesis / Ph.D. Physics 2011

abstract: This thesis focuses on the theoretical work done to determine thermodynamic properties of a chalcopyrite thin-film material for use as a photovoltaic material in a tandem device. The material of main focus here is ZnGeAs2, which was chosen for the relative abundance of constituents, favorable photovoltaic properties, and good lattice matching with ZnSnP2, the other component in this tandem device. This work is divided into two main chapters, which will cover: calculations and method to determine the formation energy and abundance of native point defects, and a model to calculate the vapor pressure over a ternary material from first-principles. The purpose of this work is to guide experimental work being done in tandem to synthesize ZnGeAs2 in thin-film form with high enough quality such that it can be used as a photovoltaic. Since properties of photovoltaic depend greatly on defect concentrations and film quality, a theoretical understanding of how laboratory conditions affect these properties is very valuable. The work done here is from first-principles and utilizes density functional theory using the local density approximation. Results from the native point defect study show that the zinc vacancy (VZn) and the germanium antisite (GeZn) are the more prominent defects; which most likely produce non-stoichiometric films. The vapor pressure model for a ternary system is validated using known vapor pressure for monatomic and binary test systems. With a valid ternary system vapor pressure model, results show there is a kinetic barrier to decomposition for ZnGeAs2. / Dissertation/Thesis / M.S. Materials Science and Engineering 2011

We present a comprehensive study of a low-dimensional spin-half quantum antiferromagnet, the J-Q model, in the presence of an external (Zeeman) magnetic field using numerical methods, chiefly stochastic series expansion quantum Monte Carlo with directed loop updates and quantum replica exchange. The J-Q model is a many-body Hamiltonian acting on a lattice of localized spin-half degrees of freedom; it augments the Heisenberg exchange with a four-spin interaction of strength Q. This model has been extensively studied at zero field, where the Q term drives a quantum phase transition from a Néel-like state to a valence-bond solid (a nonmagnetic state consisting of a long-range-ordered arrangement of local singlet bonds between sites). This transition is believed to be an example of deconfined quantum criticality, where the excitations are spinons—exotic spin-half bosons. We study the J-Q model in the presence of a magnetic field in both one and two dimensions. In one dimension, there is metamagnetism above a critical coupling ratio (Q/J)min. Metamagnetism is a first-order quantum phase transition characterized by discontinuities in the magnetization as a function of field (magnetization jumps). We derive an exact expression for (Q/J)min = 2/9, and show that the metamagnetism is caused by the onset of attractive interactions between magnons (flipped spins on a polarized background). We predict that the same mechanisms will produce metamagnetism in the unfrustrated antiferromagnetic J1-J2 model with anisotropy. Below (Q/J)min, the saturation transition is continuous and we show that it is governed by the expected zero-scale-factor universality. In two dimensions, we also find metamagnetism above a critical coupling ratio (Q/J)min=0.417, caused by the same mechanism as in the one-dimensional case. In two dimensions we also show evidence of an anomalous temperature dependence of specific heat arising from field-induced Bose-Einstein condensation of spinons at the deconfined quantum critical point. / 2019-11-06T00:00:00Z

For the past three decades, newly discovered carbon nanostructures such as fullerenes, graphene and carbon nanotubes (CNTs) have revolutionized the field of nanoscience, introducing many practical and potential applications pertaining to their exceptional structural, mechanical, thermal, and optoelectronic properties. Raman spectroscopy has been an instrumental technique for characterizing these materials due to its non-destructive nature and high sensitivity to the material responses. While Raman spectroscopy is broadly used for identifying specific material types and quality, it has also been increasingly useful as a tool for probing the electronic and excitonic properties, as well as their interplay with the vibrational properties in the aforementioned carbon nanomaterials. In this dissertation, we present our Raman-related research on carbon nanotubes and a new member of the nano-carbon family - carbon nanohoops (cycloparaphenylenes, or CPPs). We discuss our new findings on the resonance Raman spectroscopy (RRS) of various semiconducting CNTs, with the focus on the Raman excitation profiles (REPs) for the G-band. The asymmetric lineshapes observed in the G-band REPs for the second excitonic (E22) transition of these CNTs contradict a long-held approximation, the Franck-Condon principle, for the vibronic properties of the carbon nanotubes. In addition, the G-band REPs from the closely spaced E33 and E44 transitions are investigated, and we demonstrate that these excitonic levels exhibit significant quantum interference effects between each other. We also present the first comprehensive study of Raman spectroscopy of CPPs. Analogously to CNTs, we show that Raman spectroscopy can be used to identify CPPs of different sizes. A plethora of Raman modes are observed in these spectra, including modes that are comparable to those of CNTs, such as the G-band, as well as Raman peaks that are unique for CPPs. Calculated Raman spectra using density functional theory (DFT) are compared with the experimental results for the assignment of different modes. Furthermore, we refine our knowledge of the CPP Raman modes by concentrating on the even-numbered CPPs. By taking advantage of the symmetry arguments in the even [n]CPPs, we are able to utilize group theory and accurately identify the size dependences of different Raman-active modes.

Research on two-dimensional (2D) materials currently occupies a sizeable fraction of the materials science community, which has led to the development of a comprehensive body of knowledge on such layered structures. However, the goal of this thesis is to deepen the understanding of the comparatively unknown heterostructures composed of different stacked layers. First, we utilise linear-scaling density functional theory (LS-DFT) to simulate intricate interfaces between the most promising layered materials, such as transition metal dichalcogenides (TMDC) or black phosphorus (BP) and hexagonal boron nitride (hBN). We show that hBN can protect BP from external influences, while also preventing the band-gap reduction in BP stacks, and enabling the use of BP heterostructures as tunnelling field effect transistors. Moreover, our simulations of the electronic structure of TMDC interfaces have reproduced photoemission spectroscopy observations, and have also provided an explanation for the coexistence of commensurate and incommensurate phases within the same crystal. Secondly, we have developed new functionality to be used in the future study of 2D heterostructures, in the form of a linear-response phonon formalism for LS-DFT. As part of its implementation, we have solved multiple implementation and theoretical issues through the use of novel algorithms.

This Dissertation is devoted to computational study of the solidification, dynamics and mechanics of model semiflexible polymers with variable chain flexibility as well as a computational investigation of the clogging phenomena observed in granular materials. Chain stiffness is an intrinsic factor that governs single-chain flexibility. It plays a critical role in the physics of polymeric materials. In this work, we employ a coarse-grained polymer model in which chain stiffness can be tuned by a single parameter (bending stiffness kb) that yields chain shape ranging from coil-like to rod-like in the flexible and very stiff limit respectively. In chapter 2, we focus on how chain stiffness affects how polymer melts solidify under thermal cooling. We observe a strong dependence of the solid-state morphology (formed after cooling) upon chain flexibility. In the flexible limit, we find that monomers possess crystalline order while chains retain random-walk like structure. In higher stiffness regime glass formation is obtained while nematic ordering typical of lamellar precursors coexists with close-packing in the rod-like limit. Surprisingly we observe various structures ranging from spiral, to multi-domain nematic phases in the intermediate values of kb. In chapter 3 we go a step further to relate the solidification behaviors of chains discussed in chapter 2 to their melt dynamics. We probe the microstructure and the dynamics of flexible, intermediate-stiffness and rod-like chains. We find that melts of flexible and stiff chains that crystallize under cooling show simple and fast dynamics with Arrhenius temperature dependence. Interestingly, the intermediate-stiffness chains exhibit Vogel-Fulcher dynamical relaxation typical of fragile glass-formers even though their ground states is a nematic-close-packed crystal. There is no compelling argument based on static micro-structure change explaining this dynamical arrest to be found. However, we find that the dynamics of intermediate-stiffness chains is dominated by the stringlike cooperative motion that correlates along their chain backbones. This cooperative rearrangement which is absent in other systems appears to be the main cause of the dynamical arrest observed for intermediate-stiffness chains. In chapter 4, we turn to another class of materials where the negligible contribution of thermal fluctuations gives rise to an interesting phenomenon, i.e. the clogging transition. Clogging is a probabilistic event that occurs through a transition from a homogeneous flowing state to a heterogeneous or phase separated jammed state. The granular system under study is an assemble of bidisperse disks externally driven through a two dimensional periodic substrate. We find that the probability for clogging strongly depend on particle packing, obstacle number and the driving direction. Surprisingly, under relevant conditions we observe a size-specific clogging transition in which the smaller species get trapped while the larger species keep flowing. Chapter 5 returns to discuss the polymer solidification in the context of isostaticity. Results from the simulations of semiflexible polymers described in chapter 2 allow us to derive a generalized isostaticity criterion that can be applied to finite-stiffness chains. The new criterion is based on the characteristic ratio C which characterizes the slow freezing out of configurational freedom of chains as chain stiffness increases. The results of the average coordination number at solidification Z(Ts) suggest a link between jamming in athermal systems and solidification in their thermal counterparts. Finally, in chapter 6 we study the effect of chain stiffness on the mechanical response of glassy polymers. We investigate shear deformation of three systems with a different degree of entanglement. We find that loosely entangled chains display strong shear banding and undergo fracture via chain pullout. In contrast, tightly entangled chains fail at high enough strain along a well-defined plane via chain scission shortly after chains are pulled taut. We explain these chain-stiffness-dependent behaviors qualitatively using the segmental packing efficiency argument and quantitatively using modern plasticity measures