Browsing by Subject "Tunneling"
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Item Aldol Reactions - Isotope Effects, Mechanism and Dynamic Effects(2011-02-22) Vetticatt, Mathew J.The mechanism of three important aldol reactions and a biomimetic transamination is investigated using a combination of experimental kinetic isotope effects (KIEs), standard theoretical calculations and dynamics trajectory simulations. This powerful mechanistic probe is found to be invaluable in understanding intricate details of the mechanism of these reactions. The successful application of variational transition state theory including multidimensional tunneling to theoretically predict isotope effects, described in this dissertation, represents a significant advance in our research methodology. The role of dynamic effects in aldol reactions is examined in great detail. The study of the proline catalyzed aldol reaction has revealed an intriguing new dynamic effect - quasiclassical corner cutting - where reactive trajectories cut the corner between reactant and product valleys and avoid the saddle point. This phenomenon affects the KIEs observed in this reaction in a way that is not predictable by transition state theory. The study of the Roush allylboration of aldehydes presents an example where recrossing affects experimental observations. The comparative study of the allylboration of two electronically different aldehydes, which are predicted to have different amounts of recrossing, suggests a complex interplay of tunneling and recrossing affecting the observed KIEs. The Mukaiyama aldol reaction has been investigated and the results unequivocally rule out the key carbon-carbon bond forming step as rate-limiting. This raises several interesting mechanistic scenarios - an electron transfer mechanism with two different rate-limiting steps for the two components, emerges as the most probable possibility. Finally, labeling studies of the base catalyzed 1,3- proton transfer reaction of fluorinated imines point to a stepwise process involving an azomethine ylide intermediate. It is found that dynamic effects play a role in determining the product ratio in this reaction.Item Modeling and constraining inflationary and pre-inflationary eras(2016-08) Aravind, Aditya; Paban, Sonia; Fischler, Willy; Distler, Jacques; Kilic, Can; Shapiro, Paul RThe paradigm of cosmic inflation has had great success in explaining the statistical properties of fluctuations in the Cosmic Microwave Background (CMB). In this dissertation we discuss a few avenues for modeling and constraining the inflationary universe - constraints on excited states of inflationary fluctuations, some aspects of multi-field tunneling and also constraints on and predictions from a specific model of inflation connecting Higgs physics and dark matter. First, we show that in standard single field slow roll inflation, Bogoliubov excitations of the fluctuation spectrum are tightly constrained by observations. These constraints ensure that the squeezed limit non-gaussianity obtained from such excited states cannot be large. They also rule out any significant imprints in the CMB coming from a sudden transition from kinetic energy domination to inflation. We then explore tunneling in the context of field theory, a scenario that has potential relevance to the pre-inflationary universe. We discuss subtleties involved in choosing the trajectory for tunneling out of a metastable vacuum in a multi-field potential. In particular, we use exact solutions and scaling relations to show that tunneling may happen along directions with large barriers, thus making the common intuition coming from quantum mechanical tunneling unreliable in estimating the tunneling trajectory and therefore, the bounce action. We then explore a specific model of inflation that involves the addition of a scalar singlet and fermionic dark matter to the standard Higgs inflation scenario. We show that dark matter constraints and the requirement to support successful inflation significantly constrain the available parameter space for this model. We also find that the model generically predicts a small value of the tensor-to-scalar ratio r, similar to standard Higgs inflation, though it allows for a larger range of values for the scalar spectral tilt nS.Item Remote USB Ports(2013-10-01) Roshan, RakeshSimplicity, easy to install, plug & play, high bandwidth, low latency and source of power are features of USB devices. Due to these features, many sensors and actuators are manufactured with USB interfaces for use in industries. The sensors and actuators need to be installed in fields. A computer system with USB interfaces is required to be present at the location of USB device for its working. In industry, these sensors and actuators are scattered over a large geographical area. The computers connected to them expose a large attack surface. These computers can be consolidated using virtualization and networking to reduce the attack surface. In order to consolidate computers, we need solution to extend USB port over networks so that, a USB sensor or actuator placed in fields can be accessed by a system remotely and securely. In this thesis, we propose a remote USB port, which is an abstraction of a USB port. In the USB core driver of the server machine, with the hub information, port status of all the ports is stored in a port status table. On the client machine a virtual host driver is created to manage proxy USB ports. When a device is inserted or removed from the USB port on the server, the client gets notified and corresponding device driver is loaded or unloaded respectively. To secure URBs, URB headers are encrypted before sending them over networks. We have implemented our solution in the Linux 3.5 kernel. We tested our solution on two machines connected over a 100 Mbps network. Various different types of USB devices were connected in the server machine and tested from the client machine. We found our solution to be device, device driver and USB protocol independent and transparent to network and device failures.Item Risk analysis in tunneling with imprecise probabilities(2010-08) You, Xiaomin; Tonon, Fulvio; Rathje, Ellen M.; Gilbert, Robert B.; Manuel, Lance; Smirnoff, Timothy P.Due to the inherent uncertainties in ground and groundwater conditions, tunnel projects often have to face potential risks of cost overrun or schedule delay. Risk analysis has become a required tool (by insurers, Federal Transit Administration, etc.) to identify and quantify risk, as well as visualize causes and effects, and the course (chain) of events. Various efforts have been made to risk assessment and analysis by using conventional methodologies with precise probabilities. However, because of limited information or experience in similar tunnel projects, available evidence in risk assessment and analysis usually relies on judgments from experienced engineers and experts. As a result, imprecision is involved in probability evaluations. The intention of this study is to explore the use of the theory of imprecise probability as applied to risk analysis in tunneling. The goal of the methodologies proposed in this study is to deal with imprecise information without forcing the experts to commit to assessments that they do not feel comfortable with or the analyst to pick a single distribution when the available data does not warrant such precision. After a brief introduction to the theory of imprecise probability, different types of interaction between variables are studied, including unknown interaction, different types of independence, and correlated variables. Various algorithms aiming at achieving upper and lower bounds on previsions and conditional probabilities with assumed interaction type are proposed. Then, methodologies have been developed for risk registers, event trees, fault trees, and decision trees, i.e. the standard tools in risk assessment for underground projects. Corresponding algorithms are developed and illustrated by examples. Finally, several case histories of risk analysis in tunneling are revisited by using the methodologies developed in this study. All results obtained based on imprecise probabilities are compared with the results from precise probabilities.Item Tunnel MOS Heterostructure Field Effect Transistor for RF Switching Applications(2013-06-06) Rezanezhad Gatabi, ImanGaN RF switches are widely used in today?s communication systems. With digital communications getting more and more popular nowadays, the need for improving the performance of involved RF switches is inevitable. Designing low ON-state resistance GaN switches are exceedingly important to improve the switch insertion loss, isolation and power loss. Moreover, considerations need to be taken into account to improve the switching speed of the involved GaN HEMTs. In this dissertation, a new GaN HEMT structure called ?Tunnel MOS Heterostructure FET (TMOSHFET)? is introduced which has lower ON-state resistance and faster switching speed compared to conventional AlGaN/GaN HEMTs. In the switch ON process, the channel of this device is charged up by electron tunneling from a layer underneath the channel as opposed to typical AlGaN/GaN HEMTs in which electron injection from the source is charging up the channel. The tunneling nature of this process together with the shorter travel distance of electrons in TMOSHFET provide for a faster switching speed. In order to understand the tunneling mechanisms in TMOSHFET, the fabrication of AlGaN/GaN Schottky Barrier Diodes (SBDs) with various AlGaN thicknesses is demonstrated on Si (111) substrate. The impacts of SF6 dry etching on the trap density and trap state energy of AlGaN surface are investigated using the GP/w- w method. Various tunneling mechanisms at different biases are then characterized in samples and compared with each other. To improve the source and drain resistances in TMOSHFET, a model is generated to optimize the 2DEG density and electric field in AlGaN/GaN heterostructure based on Al mole fraction, AlGaN thickness and the thickness of SiN passivation layer and it is experimentally verified by non-contact Hall 2DEG density measurements. The spontaneous and piezoelectric polarizations together with strain relaxation have been implemented into the model, taking into account the annealing effects. From the experimental data on obtained parameters, the operation and device parameterization of the TMOSHFET is outlined and design considerations to improve the device R_(ON)-V_(BR) figure of merit are discussed.