Bulk Aluminum at High Pressure: A First-Principles Study

The behavior of metals at high pressure is of great importance to the fields of shock physics, geophysics, astrophysics, and nuclear materials. In order to further understand the properties of metals at high pressures we studied the equation of state of aluminum using first-principles techniques up to 2500 GPa, pressures within reach of the planned L.L.N.L. National Ignition Facility. Our simulations use density-functional theory and density-functional perturbation theory in the generalized gradient approximation at 0K. We found core overlaps to become relevant beyond pressures of 1200 GPa. The equations of state for three phases (fcc, bcc, and hcp) were calculated predicting the fcc-hcp, fcc-bcc, and hcp-bcc transitions to occur at 215 GPa, 307 GPa, and 435 GPa respectively. From the phonon dispersions at increasing pressure, we predict a softening of the lowest transverse acoustic vibrational mode along the [110] direction, which corresponds to a Born instability of the fcc phase at 725 GPa.Comment: 4 pages, 5 figures, accepted to Phys. Rev. B as a Brief Report. This version has update many figures. Moreover we provided updated and more accurate numbers based on further in-depth analyses of potential computational error

Tracking disease outbreaks from sparse data with Bayesian inference

The COVID-19 pandemic provides new motivation for a classic problem in epidemiology: estimating the empirical rate of transmission during an outbreak (formally, the time-varying reproduction number) from case counts. While standard methods exist, they work best at coarse-grained national or state scales with abundant data, and struggle to accommodate the partial observability and sparse data common at finer scales (e.g., individual schools or towns). For example, case counts may be sparse when only a small fraction of infections are caught by a testing program. Or, whether an infected individual tests positive may depend on the kind of test and the point in time when they are tested. We propose a Bayesian framework which accommodates partial observability in a principled manner. Our model places a Gaussian process prior over the unknown reproduction number at each time step and models observations sampled from the distribution of a specific testing program. For example, our framework can accommodate a variety of kinds of tests (viral RNA, antibody, antigen, etc.) and sampling schemes (e.g., longitudinal or cross-sectional screening). Inference in this framework is complicated by the presence of tens or hundreds of thousands of discrete latent variables. To address this challenge, we propose an efficient stochastic variational inference method which relies on a novel gradient estimator for the variational objective. Experimental results for an example motivated by COVID-19 show that our method produces an accurate and well-calibrated posterior, while standard methods for estimating the reproduction number can fail badly

Controlled growth and doping of core-shell GaAs-based nanowires

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010.Includes bibliographical references (p. 151-158).The use of compound semiconductor heterostructures to create electron confinement has enabled the highest frequency and lowest noise semiconductor electronics in existence. Modem technology uses two-dimensional electron gasses and there is considerable interest to explore one-dimensional electron confinement. This thesis develops the materials science toolkit needed to fabricate, characterize, and control the compositional, structural and electronic properties of core-shell GaAs/AlGaAs nanowires towards studying quasi-one-dimensional confinement and developing high mobility electronics First, nanowire growth kinetics were studied to optimize nanowire morphology. Variations in nanowire diameter were eliminated by understanding the role Ga adatom diffusion on sidewall deposition and vertical growth was enabled by understanding the importance of Ga and As mass-transport to nanowire nucleation. These results demonstrate that arrays of vertically-aligned GaAs nanowires can be produced. Then, the deposition of epitaxial AlGaAs shells on GaAs nanowires was demonstrated. By reducing the nanowire aerial density the stability of the nanowire geometry was maintained. A variety of analytical electron microscopy techniques confirmed the shell deposition to be uniform, epitaxial, defect-free, and nearly atomic sharp. These results demonstrate that core-shell nanowires possess a core-shell interface free of many of the imperfections that lithographically-defined nanowires possess. Finally, the adverse effect of the Au seed nanoparticle during n-type doping was identified and n-type doping was achieved via the removal of the Au nanoparticle prior to doping. A combination of energy dispersive X-ray spectroscopy, current-voltage, capacitance-voltage, and Kelvin probe force microscopy demonstrated that if the Au seed nanoparticle is present during the shell deposition, Au diffuses from the seed nanoparticle and creates a rectifying IV behavior. A process was presented to remove the Au nanoparticle prior to shell deposition and was shown to produce uniform n-type doping. The conductivity of GaAs/n-GaAs nanowires was calculated as a function of donor concentration and geometric factors taking into account the effects of Fermi level pinning. The control demonstrated over all of these parameters is sufficient enough for core-shell nanowires to be considered candidates for high mobility electronics.by Michael Joseph Tambe.Ph.D

A Game-Theoretic Approach for Hierarchical Policy-Making

We present the design and analysis of a multi-level game-theoretic model of hierarchical policy-making, inspired by policy responses to the COVID-19 pandemic. Our model captures the potentially mismatched priorities among a hierarchy of policy-makers (e.g., federal, state, and local governments) with respect to two main cost components that have opposite dependence on the policy strength, such as post-intervention infection rates and the cost of policy implementation. Our model further includes a crucial third factor in decisions: a cost of non-compliance with the policy-maker immediately above in the hierarchy, such as non-compliance of state with federal policies. Our first contribution is a closed-form approximation of a recently published agent-based model to compute the number of infections for any implemented policy. Second, we present a novel equilibrium selection criterion that addresses common issues with equilibrium multiplicity in our setting. Third, we propose a hierarchical algorithm based on best response dynamics for computing an approximate equilibrium of the hierarchical policy-making game consistent with our solution concept. Finally, we present an empirical investigation of equilibrium policy strategies in this game in terms of the extent of free riding as well as fairness in the distribution of costs depending on game parameters such as the degree of centralization and disagreements about policy priorities among the agents

Preventing the preventable through effective surveillance: the case of diphtheria in a rural district of Maharashtra, India

Background: Epidemic diphtheria is still poorly understood and continues to challenge both developing and developed countries. In the backdrop of poor immunization coverage, non-existent adult boosters, weak case based surveillance and persistence of multiple foci, there is a heightened risk of re-emergence of the disease in epidemic forms in India. Investigating each outbreak to understand the epidemiology of the disease and its current status in the country is therefore necessary. Dhule a predominantly tribal and rural district in Northern Maharashtra has consistently recorded low vaccination coverages alongside sporaidic cases of diphtheria over the last years. Methods: This study reports the findings of an onsite survey conducted to assess a recent outbreak of diphtheria in Dhule district and the response mounted to it. Secondary data regarding outbreak detection and response were obtained from the district surveillance office. Clinical data were extracted from hospital records of eleven lab confirmed cases including one death case. Frequency distributions were calculated for each identified clinical and non- clinical variable using Microsoft™ Excel® 2010. Results: Our findings suggest a shift in the median age of disease to adolescents (10-15 years) without gender differences. Two cases (18%) reported disease despite immunization. Clinical symptoms included cough (82%), fever (73%), and throat congestion (64%). About 64% and 36% of the 11 confirmed cases presented with a well defined pseudomembrane and a tonsillar patch respectively. Drug resistance was observed in all three culture positive cases. One death occurred despite the administration of Anti-Diphtheric Serum in a partially immunized case (CFR 9%). Genotyping and toxigenicity of strain was not possible due to specimen contamination during transport as testing facilities were unavailable in the district. Conclusions: The outbreak raises several concerns regarding the epidemiology of diphtheria in Dhule. The reason for shift in the median age despite consistently poor immunization coverage (below 50%) remains unclear. Concomitant efforts should now focus on improving and monitoring primary immunization and booster coverages across all age groups. Gradually introducing adult immunization at ten year intervals may become necessary to prevent future vulnerabilities. Laboratory networks for genotyping and toxigenicity testing are urgently mandated at district level given the endemicity of the disease in the surrounding region and its recent introduction in remote Dhule. Contingency funds with pre- agreements to obtain ADS and DT/Td vaccines at short notice and developing standard case management protocols at district level are necessary. Monitoring the disease, emerging strains and mutations, alongside drug resistance through robust and effective surveillance is a pragmatic way forward

Test sensitivity is secondary to frequency and turnaround time for COVID-19 screening

The COVID-19 pandemic has created a public health crisis. Because SARS-CoV-2 can spread from individuals with presymptomatic, symptomatic, and asymptomatic infections, the reopening of societies and the control of virus spread will be facilitated by robust population screening, for which virus testing will often be central. After infection, individuals undergo a period of incubation during which viral titers are too low to detect, followed by exponential viral growth, leading to peak viral load and infectiousness and ending with declining titers and clearance. Given the pattern of viral load kinetics, we model the effectiveness of repeated population screening considering test sensitivities, frequency, and sample-to-answer reporting time. These results demonstrate that effective screening depends largely on frequency of testing and speed of reporting and is only marginally improved by high test sensitivity. We therefore conclude that screening should prioritize accessibility, frequency, and sample-to-answer time; analytical limits of detection should be secondary. &nbsp;</p

Novel Small Molecules Targeting the Intrinsically Disordered Structural Ensemble of a-Synuclein Protect Against Diverse a-Synuclein Mediated Dysfunctions

The over-expression and aggregation of α-synuclein (αSyn) are linked to the onset and pathology of Parkinson’s disease. Native monomeric αSyn exists in an intrinsically disordered ensemble of interconverting conformations, which has made its therapeutic targeting by small molecules highly challenging. Nonetheless, here we successfully target the monomeric structural ensemble of αSyn and thereby identify novel drug-like small molecules that impact multiple pathogenic processes. Using a surface plasmon resonance high-throughput screen, in which monomeric αSyn is incubated with microchips arrayed with tethered compounds, we identified novel αSyn interacting drug-like compounds. Because these small molecules could impact a variety of αSyn forms present in the ensemble, we tested representative hits for impact on multiple αSyn malfunctions in vitro and in cells including aggregation and perturbation of vesicular dynamics. We thereby identified a compound that inhibits αSyn misfolding and is neuroprotective, multiple compounds that restore phagocytosis impaired by αSyn overexpression, and a compound blocking cellular transmission of αSyn. Our studies demonstrate that drug-like small molecules that interact with native αSyn can impact a variety of its pathological processes. Thus, targeting the intrinsically disordered ensemble of αSyn offers a unique approach to the development of small molecule research tools and therapeutics for Parkinson’s disease

Research Directions for Service-Oriented Multiagent Systems

Today\u27s service-oriented systems realize many ideas from the research conducted a decade or so ago in multiagent systems. Because these two fields are so deeply connected, further advances in multiagent systems could feed into tomorrow\u27s successful service-oriented computing approaches. This article describes a 15-year roadmap for service-oriented multiagent system research