12023 research outputs found
Sort by
Economic Fluctuations and Capitalistic Production: A Case Study in Robustness Constraints
Get PDFA central pursuit of macroeconomic research is to understand the source of short run variations in aggregate economic variables. To this end, the branch of macroeconomics known as Real Business Cycle (RBC) theory emphasizes the role of disturbances to the real economy while abstracting from nominal variables (e.g. money). According to RBC theory, business cycles are the result of optimal responses to exogenous stochastic disturbances on technology in a structure of capitalistic production. In this report, we contend that the structure of capitalistic production per se constrains the ability of the economy to absorb shocks. That is, even if the feedback behavior in the model is designed to mitigate fluctuation (and is not necessarily optimal relative to some inter-temporal utility), the resulting sensitivity is nevertheless constrained by a lower bound. Moreover, we show that this lower bound is exacerbated with increasing steady state consumption, capital and investment. Concretely, we show that the Ramsey model, linearized about its steady state equilibrium, has a non-minimum phase structure and therefore its sensitivity is constrained by the control theoretic design limits. Moreover, the non-minimum phase zero is given by the inverse of the discount factor. As the discount factor approaches unity, steady state consumption approaches optimal steady state consumption, but the non-minimum phase zero approaches the closed unit circle exacerbating the sensitivity constraints
Insights into Neural Crest Evolution
Get PDFNeural crest cells are unique to vertebrates and essential to the development and evolution of the craniofacial skeleton. Using a combination of DiI cell lineage tracing, transcriptomics, and analysis of key transcription factors of the Sox Family, I examined neural crest development in the sea lamprey, Petromyzon marinus, as the most basal extant vertebrate from which it is possible to get embryos. The results have uncovered distinct cranial and trunk neural crest subpopulations along the anterior-posterior axis of the lamprey embryo, with a clear separation between the two. However, no evidence of the presence of an intermediate vagal neural crest population was uncovered. Comparing cranial neural crest genes between lamprey and chick, either by examining individual candidate genes or whole genome transcriptome analysis, reveals significant changes in the cranial neural crest gene regulatory network of lamprey compared with chick. In particular, the lamprey cranial neural crest is "missing" several gnathostome cranial crest genes. We speculate that these may underlie the evolutionary divergence of craniofacial development between jawed and jawless vertebrates. Despite the absence of vagal neural crest, DiI-labeling shows that trunk neural crest-derived cells, likely homologous to mammalian Schwann cell precursors, contribute to the lamprey enteric nervous system, potentially representing the most primitive form of neural crest cells contribution to the ENS. Finally, I characterized key members of the Sox Family (Sox B-F) due to their importance in neural crest specification in other species. In comparative studies of the SoxC genes (Sox4, Sox11, and Sox12) in both lamprey and Xenopus, I found similar expression patterns and a novel key role in early neural crest specification, suggesting a conserved role of the SoxC genes amongst vertebrates. Taken together, this work represents important progress in characterizing the early evolution of the neural crest in vertebrates and its role in the transition from jawless to jawed vertebrates
Investigation of Capacitive Discharge Heating of Metallic Glasses
Get PDFIn recent years, the discovery of bulk metallic glasses with exceptional properties has generated much interest. One of their most intriguing features is their capacity for viscous flow above the glass transition temperature. This characteristic allows metallic glasses to be formed like plastics at modest temperatures. However, crystallization of supercooled metallic liquids in the best bulk metallic glass-formers is much more rapid than in most polymers and silicate glass-forming liquids. The short times to crystallization impairs experimentation on and processing of supercooled glass-forming metallic liquids. A technique to rapidly and uniformly heat metallic glasses at rates of 105 to 106 kelvin per second is presented. A capacitive discharge is used to ohmically heat metallic glasses to temperatures in the super cooled liquid region in millisecond time-scales. By heating samples rapidly, the most time-consuming step in experiments on supercooled metallic liquids is reduced orders of magnitude in length. This allows for experimentation on and processing of metallic liquids in temperature ranges that were previously inaccessible because of crystallization.
A variety of forming techniques, including injection molding and forging, were coupled with capacitive discharge heating to produce near net-shaped metallic glass parts. In addition, a new forming technique, which combines a magnetic field with the heating current to produce a forming force, was developed. Viscosities were measured in previously inaccessible temperature ranges using parallel plate rheometry combined with capacitive discharge heating. Lastly, a rapid pulse calorimeter was developed with this technique to investigate the thermophysical behavior of metallic glasses at these rapid heating rates.</p
I. Synthetic Studies Toward the Total Synthesis of Polycyclic Natural Products – Communesin F, Perophoramidine and Ineleganolide. II. Nickel Catalyzed Intramolecular C–O Bond Formation
Get PDFExpedient synthetic approaches to the highly functionalized polycyclic alkaloids communesin F and perophoramidine are described using a unified approach featuring a key decarboxylative allylic alkylation to access a crucial and highly congested 3,3-disubstituted oxindole. Described are two distinct, stereoselective alkylations that produce structures in divergent diastereomeric series possessing the critical vicinal all-carbon quaternary centers needed for each synthesis. Synthetic studies toward these challenging core structures have revealed a number of unanticipated modes of reactivity inherent to these complex alkaloid scaffolds. Finally, a previously unknown mild and efficient deprotection protocol for the o-nitrobenzyl group is disclosed – this serendipitous discovery permitted a concise endgame for the formal syntheses of both communesin F and perophoramidine.
In addition, the atroposelective synthesis of PINAP ligands has been accomplished via a palladium-catalyzed C–P coupling process through dynamic kinetic resolution. These catalytic conditions allow access to a wide variety of alkoxy- and benzyloxy-substituted PINAP ligands in high enantiomeric excess.
An efficient and exceptionally mild intramolecular nickel-catalyzed carbon–oxygen bond-forming reaction between vinyl halides and primary, secondary, and tertiary alcohols has been achieved. This operationally simple method allows direct access to cyclic vinyl ethers in high yields in a single step.
Finally, synthetic studies toward polycyclic ineleganolide are described. The entire fragmented carbon framework has been constructed from this work. Highly (Z)-selective olefination was achieved by the method by the Ando group.</p
Design, Fabrication, and Mechanical Property Analysis of 3D Nanoarchitected Materials
Get PDFRecent developments in micro- and nanoscale 3D fabrication techniques have enabled the creation of materials with a controllable nanoarchitecture that can have structural features spanning 5 orders of magnitude from tens of nanometers to millimeters. These fabrication methods in conjunction with nanomaterial processing techniques permit a nearly unbounded design space through which new combinations of nanomaterials and architecture can be realized. In the course of this work, we designed, fabricated, and mechanically analyzed a wide range of nanoarchitected materials in the form of nanolattices made from polymer, composite, and hollow ceramic beams. Using a combination of two-photon lithography and atomic layer deposition, we fabricated samples with periodic and hierarchical architectures spanning densities over 4 orders of magnitude from ρ=0.3-300kg/m3 and with features as small as 5nm. Uniaxial compression and cyclic loading tests performed on different nanolattice topologies revealed a range of novel mechanical properties: the constituent nanoceramics used here have size-enhanced strengths that approach the theoretical limit of materials strength; hollow aluminum oxide (Al2O3) nanolattices exhibited ductile-like deformation and recovered nearly completely after compression to 50% strain when their wall thicknesses were reduced below 20nm due to the activation of shell buckling; hierarchical nanolattices exhibited enhanced recoverability and a near linear scaling of strength and stiffness with relative density, with E∝ρ1.04 and σy∝ρ1.17 for hollow Al2O3 samples; periodic rigid and non-rigid nanolattice topologies were tested and showed a nearly uniform scaling of strength and stiffness with relative density, marking a significant deviation from traditional theories on “bending” and “stretching” dominated cellular solids; and the mechanical behavior across all topologies was highly tunable and was observed to strongly correlate with the slenderness λ and the wall thickness-to-radius ratio t/a of the beams. These results demonstrate the potential of nanoarchitected materials to create new highly tunable mechanical metamaterials with previously unattainable properties
Earth-Abundant Zinc-IV-Nitride Semiconductors
Get PDFThis investigation is motivated by the need for new visible frequency direct bandgap semiconductor materials that are abundant and low-cost to meet the increasing demand for optoelectronic devices in applications such as solid state lighting and solar energy conversion. Proposed here is the utilization of zinc-IV-nitride materials, where group IV elements include silicon, germanium, and tin, as earth-abundant alternatives to the more common III-nitrides in optoelectronic devices. These compound semiconductors were synthesized under optimized conditions using reactive radio frequency magnetron sputter deposition. Single phase ZnSnN2, having limited experimental accounts in literature, is validated by identification of the wurtzite-derived crystalline structure predicted by theory through X-ray and electron diffraction studies. With the addition of germanium, bandgap tunability of ZnSnxGe1-xN2 alloys is demonstrated without observation of phase separation, giving these materials a distinct advantage over InxGa1-xN alloys. The accessible bandgaps range from 1.8 to 3.1 eV, which spans the majority of the visible spectrum. Electron densities, measured using the Hall effect, were found to be as high as 1022 cm−3 and indicate that the compounds are unintentionally degenerately doped. Given these high carrier concentrations, a Burstein-Moss shift is likely affecting the optical bandgap measurements. The discoveries made in this thesis suggest that with some improvements in material quality, zinc-IV-nitrides have the potential to enable cost-effective and scalable optoelectronic devices
Essays in Behavioral Decision Theory
Get PDFMany different behavioral phenomena that cannot be rationalized by standard models in economics have been well-documented both in the real world and in lab experiments. Motivated by these behavioral phenomena, the purpose of this dissertation is three-fold. First, I develop axiomatic models of individual decision-making to explain these well-documented phenomena. Second, I derive the implications and predictions of these axiomatic models for intertemporal choice, asset pricing, and other economic contexts. Third, I provide connections between these seemingly separate behavioral phenomena and widely-used properties of preferences in economics and psychology. This dissertation consists of five chapters. The first chapter studies dynamic choice under uncertainty. The second and third chapters study choice over multi-attribute alternatives. The fourth and fifth chapters study stochastic choice.
The first chapter studies history-dependent risk aversion and focuses on a behavioral phenomenon called the reinforcement effect (RE), which states that people become less risk-averse after a good history than after a bad history. The RE is well-documented in consumer choices, financial markets, and lab experiments. I show that this seemingly anomalous behavior occurs whenever risk preferences are history-dependent (in a nontrivial way) and satisfy monotonicity with respect to first-order stochastic dominance. To study history-dependent risk aversion and the RE formally, I develop a behaviorally-founded model of dynamic choice under risk that generalizes standard discounted expected utility. To illustrate the usefulness of my model, I apply it to the Lucas tree model of asset pricing and draw implications of the RE for asset price dynamics. I find that, compared to history-independent models, assets are overpriced when the economy is in a good state and are underpriced in a bad state. Moreover, my model generates high, volatile, and predictable asset returns, and low and smooth bond returns, consistent with empirical evidence.
In the second chapter, I develop an axiomatic model of reference-dependent preferences in which reference points are endogenous. In particular, I focus on choices from menus of two-attribute alternatives, and the reference point for a given menu is a vector that consists of the minimums of each dimension of the menu. I characterize this model by two weakenings of the Weak Axiom of Revealed Preference (WARP) in addition to standard axioms. My model is not just consistent with the attraction effect and the compromise effect, well-known preference reversals, but it also provides a connection between these two effects and diminishing sensitivity, a widely used behavioral property in economics. The model also provides bounds on preference reversals. I apply the model to two different contexts, intertemporal choice and risky choice, and diminishing sensitivity has interesting implications. In intertemporal choice, the main implication of the model is that borrowing constraints produce a psychological pressure to move away from the constraints even if they are not binding. In risky choice, the model allows conflicting risk behaviors.
In the third chapter, I study choice over multidimensional alternatives. Making a choice between multidimensional alternatives is a difficult task. Therefore, a decision maker may adopt some procedure (heuristic) to simplify this task. I provide an axiomatic model of one such heuristic called the Intra-Dimensional Comparison (IDC) heuristic. The IDC heuristic is well-documented in the experimental literature on choice under risk. The IDC heuristic is a procedure in which a decision maker compares multidimensional alternatives dimension-by-dimension and makes a decision based on those comparisons. The model of the IDC heuristic provides a general framework applicable to many different contexts, including risky choice and social choice.
The fourth chapter is joint work with Federico Echenique and Kota Saito. We develop an axiomatic theory of random choice that builds on Luce's (1959) model to incorporate a role for perception. We capture the role of perception through perception priorities; priorities that determine whether an object or alternative is perceived sooner or later than other alternatives. We identify agents' perception priorities from their violations of Luce's axiom of independence from irrelevant alternatives (IIA). The direction of the violation of IIA implies an orientation of agents' priority rankings. We adjust choice probabilities to account for the effects of perception, and impose that adjusted choice probabilities satisfy IIA. So all violations of IIA are accounted for by the perception order. The theory can explain some very well-documented behavioral phenomena in individual choice. We can also explain the effects of forced choice and choice overload in experiments.
The fifth chapter studies how the ordering of alternatives (e.g., the location of products in a grocery store, the order of candidates on a ballot) affects a decision maker's choices. I develop an axiomatic model of random choice that builds on Luce's (1959) and incorporates the effect of the ordering of alternatives on choice frequencies. When the ordering of alternatives is observed, I characterize the model by two weakenings of IIA. When the ordering of alternatives is not observed, I can identify it from choice data. The model can accommodate the similarity, compromise, and attraction effects, violations of stochastic transitivity, and the choice overload, which are well-known behavioral phenomena in individual choice.</p
Ring/Chain versus Network: Architecture Induced by Self- versus Pairwise-Association of Telechelic Polymers
Get PDFNon-covalent associations, including hydrophobic interaction or ionic interaction for self-association, and metal coordination or hydrogen-bonding for complementary-association, have been widely used as key interactions in supramolecules formation with telechelic associative polymers. And a specific application of long associative telechelic polymers has been developed by our group for the mist-control and drag-reduction of liquid fuels. During the research on this project, self- and pairwise-associative telechelic polymers are able to be compared for the first time, and are shown to display distinct associative patterns. In order to design materials with the desired properties, it is imperative to understand the relationships between polymer chemical structure and their topology and dynamics.
In this thesis, self-associative telechelic polymer refers to α,ω-di(isophthalic acid) polycyclooctadiene (DA-PCOD), which can associate with itself through its acid ends. When tertiary amine-ended polymer is added into the mixture, isophthalic acid preferably associates pairwisely with tertiary amine due to the higher binding strength of charge-assisted hydrogen bond. And the 1:1 molar ratio mixture of α,ω-di(isophthalic acid) and α,ω-di(di(tertiary amine)) PCOD (DA/DB-PCOD) is named as pairwise-associative telechelic polymers. DA-PCOD is capable of multimeric association via directional hydrogen bonding due to the specific chemical structure of the isophthalic acid end, while DA/DB-PCOD exhibits dynamics that strikingly resembles that for linear covalent polymers. Temperature determines the binding strength of self- and pairwise- end association, and furthermore, the fraction of unbound ends and the distribution and topology of formed supramolecules/aggregates. Polymer length affects the dynamics of DA-PCOD mainly through determining the concentration of the end groups. And the net effect of chain length on the dynamics of DA/DB-PCOD is non-monotonic and varies with the specific temperature and concentration. The knowledge of structure-property relationships obtained from this work will enable future design of end group entities and other properties of these associative telechelic polymers for their specific applications.</p
Agonist Binding Studies at Two Subtypes of the Nicotinic Acetylcholine Receptor Involved in Parkinson’s Disease and Addiction
Get PDFNeuronal nicotinic acetylcholine receptors (nAChR) consist of pentameric ligand gated ion channels that typically regulate the release of neurotransmitter. This group of receptors is made of many subunits that combine into pentamers to form different subtypes, with each subtype having a unique pharmacology, function, and localization in the nervous system. The α6β2 subtype is found predominantly in the dopaminergic pathways in the brain, and is therefore a promising target for addiction and Parkinson’s disease. A major goal in treating these disorders is to develop subtype-selective agonists, and advanced knowledge of the binding site which sits at the α6-β2 subunit interface is critical. This thesis dissertation describes high precision, chemical scale structure-function studies designed to probe specific interactions between a variety of agonists and the amino acids which make up the α6β2 binding site.
Before these studies, which utilize nonsense suppression-based non-canonical amino acid mutagenesis, could be conducted, a heterologous expression system for α6β2 had to be developed.
Chapter 2 details four reporter mutations that allow high expression levels of α6β2 in Xenopus oocytes. Further work presented in this chapter characterizes a variety of compounds at this subtype including acetylcholine, the endogenous agonist, nicotine, and TC299423, a promising drug candidate designed to be α6-selective.
Chapters 3 and 4 discuss the structure-function studies used to probe for binding interactions of acetylcholine, nicotine, and TC299423 with the α6-β2 interface. Fluorination series were executed to probe for cation-π interactions with TrpB, TyrA, and TyrC2, all sites of the α6 face. Of the nine possible agonist-side chain interactions, the only functionally important cation-π interaction was found between acetylcholine and TrpB, suggesting the subtype has a unique pharmacology. Studies utilizing α-hydroxy acids were then performed to determine whether these agonists make a functional hydrogen bond between their amine NH and the backbone carbonyl associated with TrpB. Here, nicotine was found to make a strong hydrogen bond, whose energy was quantified via double-mutant cycle analysis, but TC299423 was not.
Chapter 5 further explores TC299423 at the α4β2 subtype. Experiments here showed that TC299423 makes a dual cation-π interaction with both TrpB and TyrC2. Further studies revealed this dual cation-π effect to be true for several secondary amines, and a structure-function study with nornicotine established this as a general feature for secondary amines.
Chapter 6 describes work done to probe for a hydrogen bond between the indole NH of α4 TrpB and a backbone carbonyl associated with L119 on the β2 subunit. This study required development of a new strategy to probe for hydrogen bonds as the amino acid sequence does not allow for α-hydroxy substitution. Instead, a fluorinated side chain strategy was used to inductively attenuate the hydrogen bond accepting ability of the carbonyl, and it proved the α4-β2 interfacial hydrogen prediction false.
Finally two appendices suggest possible avenues to explore with the new α6β2 expression system. Appendix A describes work done to determine whether there is cross-talk between α6β2 and P2X receptors. Appendix B details initial investigations on the effects of ethanol and other alcohols on the function of α6β2.</p
Toward Realizable Quantum Computers
Get PDFThe work in this thesis splits naturally into two parts: (1) experimentally oriented work consisting of experimental proposals for systems that could be used to implement quantum information tasks with current technology, and (2) theoretical work focusing on universal fault-tolerant quantum computers which we hope can be scaled as experimental capabilities continue to move forward