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Cover page of Magnetic electron collimation in three-dimensional semi-metals

Magnetic electron collimation in three-dimensional semi-metals

(2020)

© 2020, The Author(s). While electrons moving perpendicular to a magnetic field are confined to cyclotron orbits, they can move freely parallel to the field. This simple fact leads to complex current flow in clean, low carrier density semi-metals, such as long-ranged current jets forming along the magnetic field when currents pass through point-like constrictions. Occurring accidentally at imperfect current injection contacts, the phenomenon of "current jetting" plagues the research of longitudinal magneto-resistance, which is particularly important in topological conductors. Here we demonstrate the controlled generation of tightly focused electron beams in a new class of micro-devices machined from crystals of the Dirac semi-metal Cd3As2. The current beams can be guided by tilting a magnetic field and their range tuned by the field strength. Finite element simulations quantitatively capture the voltage induced at faraway contacts when the beams are steered towards them, supporting the picture of controlled electron jets. These experiments demonstrate direct control over the highly non-local signal propagation unique to 3D semi-metals in the current jetting regime, and may lead to applications akin to electron optics in free space.

Cover page of Developing scandium and yttrium coordination chemistry to advance theranostic radiopharmaceuticals

Developing scandium and yttrium coordination chemistry to advance theranostic radiopharmaceuticals

(2020)

© 2020, This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply. The octadentate siderophore analog 3,4,3-LI(1,2-HOPO), denoted 343-HOPO hereafter, is known to have high affinity for both trivalent and tetravalent lanthanide and actinide cations. Here we extend its coordination chemistry to the rare-earth cations Sc3+ and Y3+ and characterize fundamental metal–chelator binding interactions in solution via UV-Vis spectrophotometry, nuclear magnetic resonance spectroscopy, and spectrofluorimetric metal-competition titrations, as well as in the solid-state via single crystal X-ray diffraction. Sc3+ and Y3+ binding with 343-HOPO is found to be robust, with both high thermodynamic stability and fast room temperature radiolabeling, indicating that 343-HOPO is likely a promising chelator for in vivo applications with both metals. As a proof of concept, we prepared a 86Y-343-HOPO complex for in vivo PET imaging, and the results presented herein highlight the potential of 343-HOPO chelated trivalent metal cations for therapeutic and theranostic applications.

Cover page of A critical examination of compound stability predictions from machine-learned formation energies

A critical examination of compound stability predictions from machine-learned formation energies

(2020)

© 2020, The Author(s). Machine learning has emerged as a novel tool for the efficient prediction of material properties, and claims have been made that machine-learned models for the formation energy of compounds can approach the accuracy of Density Functional Theory (DFT). The models tested in this work include five recently published compositional models, a baseline model using stoichiometry alone, and a structural model. By testing seven machine learning models for formation energy on stability predictions using the Materials Project database of DFT calculations for 85,014 unique chemical compositions, we show that while formation energies can indeed be predicted well, all compositional models perform poorly on predicting the stability of compounds, making them considerably less useful than DFT for the discovery and design of new solids. Most critically, in sparse chemical spaces where few stoichiometries have stable compounds, only the structural model is capable of efficiently detecting which materials are stable. The nonincremental improvement of structural models compared with compositional models is noteworthy and encourages the use of structural models for materials discovery, with the constraint that for any new composition, the ground-state structure is not known a priori. This work demonstrates that accurate predictions of formation energy do not imply accurate predictions of stability, emphasizing the importance of assessing model performance on stability predictions, for which we provide a set of publicly available tests.

Cover page of Redistributive effects of different pension systems when longevity varies by socioeconomic status.

Redistributive effects of different pension systems when longevity varies by socioeconomic status.

(2020)

We propose a general analytical framework to model the redistributive features of alternative pension systems when individuals face ex ante differences in mortality. Differences in life expectancy between high and low socioeconomic groups are often large and have widened recently in many countries. Such longevity gaps affect the actuarial fairness and progressivity of public pension systems. However, behavioral responses to longevity and policy complicate analysis of possible reforms. Here we consider how various pension systems would perform in an OLG setting with heterogeneous longevity and ability. We evaluate redistributive effects of three Notional Defined Contribution plans and three Defined Benefit plans, calibrated on the US case. Compared to a benchmark non-redistributive plan that accounts for differences in mortality, US Social Security reduces regressivity from longevity differences, but would require group-specific life tables to achieve progressivity. Moreover, without separate life tables, despite apparent accounting gains, lower income groups would suffer welfare losses and higher income groups would enjoy welfare gains through indirect effects of pension systems on labor supply.

Cover page of Using agriculture for development: Supply- and demand-side approaches

Using agriculture for development: Supply- and demand-side approaches

(2020)

© 2020 Elsevier Ltd For most poor countries of today, using agriculture for development is widely recognized as a promising strategy. Yet, in these countries, investment in agriculture has mostly been lagging relative to international norms and recommendations. Current wisdom on how to use agriculture for development is that it requires asset building for smallholder farmers, productivity growth in staple foods, an agricultural transformation (diversification of farming systems toward high value crops), and a rural transformation (value addition through rural non-farm activities linked to agriculture). This sequence has too often been hampered by extensive market and government failures. We outline a theory of change where the removal of market and government failures to use this Agriculture for Development strategy can be addressed through two contrasted and complementary approaches. One is from the “supply-side” where public and social agents (governments, international and bilateral development agencies, NGOs, donors) intervene to help farmers overcome the major constraints to adoption: liquidity, risk, information, and access to markets. The other is from the “demand-side” where private agents (entrepreneurs, producer organizations) create incentives for smallholder farmers to modernize through contracting and vertical coordination in value chains. We review the extensive literature that has explored ways of using Agriculture for Development through these two approaches. We conclude by noting that the supply-side approach has benefited from extensive research but met with limited success. The demand-side approach has promise, but received insufficient attention and is in need of additional rigorous research which we outline.

How can airborne transmission of COVID-19 indoors be minimised?

(2020)

During the rapid rise in COVID-19 illnesses and deaths globally, and notwithstanding recommended precautions, questions are voiced about routes of transmission for this pandemic disease. Inhaling small airborne droplets is probable as a third route of infection, in addition to more widely recognized transmission via larger respiratory droplets and direct contact with infected people or contaminated surfaces. While uncertainties remain regarding the relative contributions of the different transmission pathways, we argue that existing evidence is sufficiently strong to warrant engineering controls targeting airborne transmission as part of an overall strategy to limit infection risk indoors. Appropriate building engineering controls include sufficient and effective ventilation, possibly enhanced by particle filtration and air disinfection, avoiding air recirculation and avoiding overcrowding. Often, such measures can be easily implemented and without much cost, but if only they are recognised as significant in contributing to infection control goals. We believe that the use of engineering controls in public buildings, including hospitals, shops, offices, schools, kindergartens, libraries, restaurants, cruise ships, elevators, conference rooms or public transport, in parallel with effective application of other controls (including isolation and quarantine, social distancing and hand hygiene), would be an additional important measure globally to reduce the likelihood of transmission and thereby protect healthcare workers, patients and the general public.

Cover page of A Metric for Linear Temporal Logic

A Metric for Linear Temporal Logic

(2020)

We propose a measure and a metric on the sets of infinite traces generated by a set of atomic propositions. To compute these quantities, we first map properties to subsets of the real numbers and then take the Lebesgue measure of the resulting sets. We analyze how this measure is computed for Linear Temporal Logic (LTL) formulas. An implementation for computing the measure of bounded LTL properties is provided and explained. This implementation leverages SAT model counting and effects independence checks on subexpressions to compute the measure and metric compositionally.