Phase equilibrium modeling for high temperature metallization on GaAs solar cells

Recent trends in performance specifications and functional requirements have brought about the need for high temperature metallization technology to be developed for survivable DOD space systems and to enhance solar cell reliability. The temperature constitution phase diagrams of selected binary and ternary systems were reviewed to determine the temperature and type of phase transformation present in the alloy systems. Of paramount interest are the liquid-solid and solid-solid transformations. Data are being utilized to aid in the selection of electrical contact materials to gallium arsenide solar cells. Published data on the phase diagrams for binary systems is readily available. However, information for ternary systems is limited. A computer model is being developed which will enable the phase equilibrium predictions for ternary systems where experimental data is lacking

How is Mathematics Education Philosophy Reflected in the Math Wars?

Throughout the duration of what has been termed the “math wars”, many overlygeneralized statements by both sides have detracted from the quest for a solution to the conflict between conceptual and procedural approaches to mathematics study. In terms of philosophies of mathematics education, the absolutist view posits that mathematical knowledge is certain and unchallengeable while the fallibilist view is that mathematical knowledge is never beyond revision and correction. We suggest that the major mathematics education reforms have been absolutist in focus and have not reflected the changing nature of the discipline. Thus we believe that true reform will reflect changing perceptions in mathematics education along with changes in American culture and its expectations of mathematics education

Prevalence and Patterns of Sugar-Sweetened Beverage Consumption in Canadian Youth: A Northern Focus

Regular consumption of sugar-sweetened beverages (SSBs) is a well-known risk factor for weight gain, tooth decay, and metabolic syndrome. Rates of SSB consumption in Nunavut specifically, have been noted to be exceptionally high. This study describes consumption rates of specific foods and beverages, with a focus on SSBs, among adolescents in Nunavut, northern Canada as a whole, and the Canadian provinces, using data from the 2010 and 2014 cycles of the Health Behaviour in School-aged Children (HBSC) study to investigate population characteristics and consumption patterns. Comparative analyses of consumption patterns for Nunavut, the three territories combined, and the southern provinces found that in 2010, those who consumed SSBs once a day or more comprised 53.1% of adolescents in Nunavut, 31.1% in the northern territories and 24% in the provinces. Comparable figures for 2014 were 55.0% in Nunavut, but only 27.0% in all the territories, and 19.1% in the provinces. The percentage of adolescents who consumed fruit and vegetables daily was also lower in Nunavut than in the provinces (65.5% vs. 85.3% in 2010, and 57.5% vs. 84.4% in 2014). More Nunavut adolescents consumed sweets and potato chips daily than provincial adolescents (42.6% vs. 27.6% in 2010, and 52.2% vs. 25.2% in 2014). A greater proportion of Nunavut adolescents reported high consumption of SSBs, as well as other energy-dense foods, when compared to adolescents in the three territories combined and in the provinces. These results confirm previous studies but provide a current and comprehensive analysis that can help inform future food and nutrition priorities and programing. La consommation régulière de boissons édulcorées est un facteur de risque bien connu pour le gain de poids, les caries et le syndrome métabolique. Au Nunavut plus particulièrement, la consommation de boissons édulcorées est exceptionnellement élevée. Cette étude décrit les taux de consommation d’aliments et de boissons spécifiques et met l’accent sur les boissons édulcorées chez les adolescents du Nunavut, du Nord canadien dans son ensemble, et des provinces canadiennes en s’appuyant sur les données des cycles de 2010 et de 2014 de l’enquête « Les comportements de santé des jeunes d’âge scolaire » (l’Enquête HBSC), enquête qui a pour but d’étudier les caractéristiques de la population et les habitudes de consommation. Les analyses comparatives des habitudes de consommation du Nunavut, des trois territoires dans leur ensemble et des provinces du sud du pays ont permis de remarquer qu’en 2010, 53,1 % des adolescents du Nunavut consommaient des boissons édulcorées une fois par jour ou plus, 31,1 % dans les territoires du Nord et 24 % dans les provinces. Pour 2014, les données comparables étaient de 55,0 % au Nunavut, mais de seulement 27,0 % dans l’ensemble des territoires et de 19,1 % dans les provinces. Le pourcentage d’adolescents qui consommait des fruits et des légumes au quotidien était moins élevé au Nunavut que dans les provinces (65,5 % par opposition à 85,3 % en 2010, et 57,5 % par opposition à 84,4 % en 2014). Au Nunavut, un plus grand nombre d’adolescents consommait des sucreries et des croustilles au quotidien par rapport aux adolescents des provinces (42,6 % par opposition à 27,6 % en 2010, et 52,2 % par opposition à 25,2 % en 2014). Une plus grande proportion d’adolescents du Nunavut a signalé une grande consommation de boissons édulcorées et d’autres aliments à forte teneur en énergie comparativement aux adolescents des trois territoires dans leur ensemble et aux adolescents des provinces. Ces résultats confirment les résultats d’études antérieures, en plus de présenter une analyse actuelle et exhaustive dont on peut se servir pour jeter de la lumière sur les priorités et la programmation futures en matière d’alimentation et de nutrition

Double non-equivalent chain structure on vicinal Si(557)-Au surface

We study electronic and topographic properties of the vicinal Si(557)-Au surface using scanning tunneling microscopy and reflection of high energy electron diffraction technique. STM data reveal double wire structures along terraces. Moreover behavior of the voltage dependent STM tip - surface distance is different in different chains. While the one chain shows oscillations of the distance which are sensitive to the sign of the voltage bias, the oscillations in the other chain remain unchanged with respect to the positive/negative biases. This suggests that one wire has metallic character while the other one - semiconducting. The experimental results are supplemented by theoretical calculations within tight binding model suggesting that the observed chains are made of different materials, one is gold and the other one is silicon chain.Comment: 9 pages, 12 figures, accepted for publication in Phys. Rev.

Shockley model description of surface states in topological insulators

We show that the surface states in topological insulators can be understood based on a well-known Shockley model, a one-dimensional tight-binding model with two atoms per elementary cell, connected via alternating tunneling amplitudes. We generalize the one-dimensional model to the three-dimensional case corresponding to the sequence of layers connected via the amplitudes, which depend on the in-plane momentum p = (p_x,p_y). The Hamiltonian of the model is described a (2 x 2) Hamiltonian with the off-diagonal element t(k,p) depending also on the out-of-plane momentum k. We show that the complex function t(k,p) defines the properties of the surface states. The surface states exist for the in-plane momenta p, where the winding number of the function t(k,p) is non-zero as k is changed from 0 to 2pi. The sign of the winding number defines the sublattice on which the surface states are localized. The equation t(k,p)=0 defines a vortex line in the three-dimensional momentum space. The projection of the vortex line on the two-dimensional momentum p space encircles the domain where the surface states exist. We illustrate how our approach works for a well-known TI model on a diamond lattice. We find that different configurations of the vortex lines are responsible for the "weak" and "strong" topological insulator phases. The phase transition occurs when the vortex lines reconnect from spiral to circular form. We discuss the Shockley model description of Bi_2Se_3 and the applicability of the continuous approximation for the description of the topological edge states. We conclude that the tight-binding model gives a better description of the surface states.Comment: 18 pages, 17 figures; version 3: Sections I-IV revised, Section VII added, Refs. [33]-[35] added; Corresponds to the published versio

Research Exchange - April 6, 2021 Anatomy of a Good Paper - Choosing Research Topics

Robert Davison and Alan Dennis discuss considerations in selecting a good and relevant research topics. Moderated by Amber Young, Davison and Dennis will focus on positioning and motivation to improve the overall contribution to IS research. Additionally, discussions focus on the limitations of gap-spotting and the position that research should be meaningful in its academic and practical implications

Response of the Shockley surface state to an external electrical field: A density-functional theory study of Cu(111)

The response of the Cu(111) Shockley surface state to an external electrical field is characterized by combining a density-functional theory calculation for a slab geometry with an analysis of the Kohn-Sham wavefunctions. Our analysis is facilitated by a decoupling of the Kohn-Sham states via a rotation in Hilbert space. We find that the surface state displays isotropic dispersion, quadratic until the Fermi wave vector but with a significant quartic contribution beyond. We calculate the shift in energetic position and effective mass of the surface state for an electrical field perpendicular to the Cu(111) surface; the response is linear over a broad range of field strengths. We find that charge transfer occurs beyond the outermost copper atoms and that accumulation of electrons is responsible for a quarter of the screening of the electrical field. This allows us to provide well-converged determinations of the field-induced changes in the surface state for a moderate number of layers in the slab geometry.Comment: 11 pages, 6 figures, 4 tables; accepted for publication by Phys. Rev. B; changes from v1 in response to referee comments, esp. to Sections I and V.B (inc. Table 4), with many added references, but no change in results or conclusion

Deferred and deterred: a review of literature on the impact of deferrals on blood donors

Background: Potential blood donors can be deferred due to concerns about the impact of the donation on their health or the safety of the blood supply. To date, we lack a comprehensive review of the impact of deferrals on donors and how to mitigate adverse effects. Aims: The aim of this review was to describe the available literature on deferrals, with a focus on the impact of deferrals on donors’ subsequent behaviour, potential reasons for impact and the effectiveness of strategies to improve deferral processes and facilitate donor return. Method: A narrative review of the literature on blood donation deferrals was undertaken. Results: Deferral rates vary widely across different contexts, with female, younger, first time and minority donors more likely to be ineligible to donate. There is clear evidence that deferrals impact on future donation behaviour, particularly for those deferred at their first donation attempt. Deferral has a negative emotional impact if the deferral is permanent or related to positive test results, while emotions experienced at the time of a temporary deferral are related to donors’ willingness to return. Conclusion: An understanding of the impact of deferrals from the donor perspective provides key information to improve the blood centre practices. There is preliminary evidence of the effectiveness of strategies to retain donors, including enabling the ineligible donor to make an alternative contribution, providing clear information about the deferral, notifying the donor when they can return to donate and addressing practical barriers to return

Glassy behaviour in an exactly solved spin system with a ferromagnetic transition

We show that applying simple dynamical rules to Baxter's eight-vertex model leads to a system which resembles a glass-forming liquid. There are analogies with liquid, supercooled liquid, glassy and crystalline states. The disordered phases exhibit strong dynamical heterogeneity at low temperatures, which may be described in terms of an emergent mobility field. Their dynamics are well-described by a simple model with trivial thermodynamics, but an emergent kinetic constraint. We show that the (second order) thermodynamic transition to the ordered phase may be interpreted in terms of confinement of the excitations in the mobility field. We also describe the aging of disordered states towards the ordered phase, in terms of simple rate equations.Comment: 11 page