Ferromagnetism and interlayer exchange coupling in short period (Ga,Mn)As/GaAs superlattices

Magnetic properties of (Ga,Mn)As/GaAs superlattices are investigated. The structures contain magnetic (Ga,Mn)As layers, separated by thin layers of non-magnetic GaAs spacer. The short period Ga$_{0.93}$Mn$_{0.07}$As/GaAs superlattices exhibit a paramagnetic-to-ferromagnetic phase transition close to 60K, for thicknesses of (Ga,Mn)As down to 23 \AA. For Ga$_{0.96}$Mn$_{0.04}$As/GaAs superlattices of similar dimensions, the Curie temperature associated with the ferromagnetic transition is found to oscillate with the thickness of non magnetic spacer. The observed oscillations are related to an interlayer exchange interaction mediated by the polarized holes of the (Ga,Mn)As layers.Comment: REVTeX 4 style; 4 pages, 2 figure

6-ft High-Power Electric Propulsion Test Port, EPL Tank 5 Installed

High-power electric propulsion is a critical component of NASA s proposed missions to the outer planets. Mission studies have shown that high-power, high-specific-impulse propulsion systems can deliver 2000 kg of scientific payload to Pluto with trip times on the order of 10 years. Of greater significance is the ability of these propulsion systems to place this science payload in orbit around the planet, rather than making the fast fly-bys associated with traditional chemical propulsion systems. Significant ground test programs are required to develop the new technologies needed for thrusters operating at power levels exceeding 20 kW, an order of magnitude above the state of the art

Structural and magnetic properties of GaMnAs layers with high Mn content grown by Migration Enhanced Epitaxy on GaAs(100) substrates

We have grown the ferromagnetic semiconductor GaMnAs containing up to 10% Mn by migration enhanced epitaxy at a substrate temperature of 150^oC. The alternate supply of As2 molecules and Ga and Mn atoms made it possible to grow single crystalline GaMnAs layers at very low substrate temperature, at which conventional molecular beam epitaxial growth under excess As supply is not possible due to As condensation. Secondary ion mass spectroscopy and X-ray diffraction measurements confirmed a higher Mn content in the films grown by this method in comparison to the GaMnAs layers grown by low temperature molecular beam epitaxy. The lattice constant of hypothetical zinc-blende structure MnAs is determined to be 5.9 \AA, which deviates somewhat from previously reported values. This deviation is ascribed to growth-condition dependent density of point defects. It is stressed that this effect must be taken into account for any assessment of Mn content from X-ray diffraction data. Magnetization measurements showed an onset of ferromagnetic ordering around 75 K for the GaMnAs layer with 10% Mn. This means that the trend of falling Curie temperatures with increasing Mn concentrations above 5.5% is broken. We tentatively assign this to the variation of the carrier concentration, including contributions from donor and acceptor centers formed by antisite defects and Mn doping, and increased density of magnetically active Mn ions.Comment: No LaTeX source; gzipped postscript text + 3 gzipped postscript figure

Mathematically Gifted Adolescents Have Deficiencies in Social Valuation and Mentalization

Many mathematically gifted adolescents are characterized as being indolent, underachieving and unsuccessful despite their high cognitive ability. This is often due to difficulties with social and emotional development. However, research on social and emotional interactions in gifted adolescents has been limited. The purpose of this study was to observe differences in complex social strategic behaviors between gifted and average adolescents of the same age using the repeated Ultimatum Game. Twenty-two gifted adolescents and 24 average adolescents participated in the Ultimatum Game. Two adolescents participate in the game, one as a proposer and the other as a responder. Because of its simplicity, the Ultimatum Game is an apt tool for investigating complex human emotional and cognitive decision-making in an empirical setting. We observed strategic but socially impaired offers from gifted proposers and lower acceptance rates from gifted responders, resulting in lower total earnings in the Ultimatum Game. Thus, our results indicate that mathematically gifted adolescents have deficiencies in social valuation and mentalization

Optimizing HVAC Control to Improve Building Comfort and Energy Performance

This paper demonstrates the benefits of optimal control in well-designed and operated buildings using a case study. The case study building was built in 2001. The HVAC and control systems have been installed with state-of-the-art equipment which include a terminal box temperature integrated minimum airflow reset. The building has been used and operated based on the design intents. This paper presents both the existing and the optimal control schedules, which include the VAV box operation schedule, AHUs optimal control, chiller and chilled water pump control, and boiler and hot water pump control. The measured hourly HVAC electricity consumption shows that annual savings of up to 40% can be achieved with an optimal control schedule

High-temperature magnetic anomalies in Sr-doped La manganite structures

The temperature dependence of the magnetization M, susceptibility {chi}, and magnetoresistance MR for 3 perovskite-variant manganite structures were studied: monoclinic (x=0.075) and orthorhombic (x=0. 125) La{sub 1-x}Sr{sub x}MnO{sub 3}, and tetragonal layered La{sub 2- 2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7} (x=0.4) with x also indicating the nominal fraction of Mn{sup 4+}. In each case, evidence is found for unusual magnetic states at temperatures T above their primary magnetic transitions. In the first case, the high-T {chi} deviates from Curie-Weiss expectations, in the second case the MR extends to high T, and in the last, M and {chi} exhibit short-range anomalies at high T. This suggests that a key feature of these systems is the existence of multiple magnetic energy scales, independent of structure, dimensionality, or doping levels

Impact of COVID-19 on Formal Education: An International Review of Practices and Potentials of Open Education at a Distance.

In terms of scale, shock, and disenfranchisement, the disruption to formal education arising from COVID-19 has been unprecedented. Anecdotally, responses from teachers and educators around the world range from heightened caution to being inspired by distance education as the 'new normal.' Of all the challenges, face-to-face and formal teaching have been most heavily affected. Despite some education systems demonstrating resilience, a major challenge is sustaining quality and inclusiveness in formal education suddenly delivered at a distance. In probing these issues, this article profiles international perspectives on the role of open education in responding to the impact on formal school and higher education caused by the COVID-19 pandemic. We proceed by highlighting and analysing practices and case studies from 13 countries representing all global regions, identifying and discussing the challenges and opportunities that have presented themselves. Reports cover the period from the beginning of 2020 until 11 March 2021, the first anniversary of the COVID-19 outbreak as declared by the World Health Organization. In our comparative study, we identify seven key aspects of which three (missing infrastructure and sharing OER, open education and access to OER, and urgent need for professional development and training for teachers) are directly related to open education at a distance. After comparing examples of existing practice, we make recommendations and offer insights into how open education strategies can lead to interventions that are effective and innovative¿to improve formal education at a distance in schools and universities in the future

Somatic Pairing of Chromosome 19 in Renal Oncocytoma Is Associated with Deregulated ELGN2-Mediated Oxygen-Sensing Response

Chromosomal abnormalities, such as structural and numerical abnormalities, are a common occurrence in cancer. The close association of homologous chromosomes during interphase, a phenomenon termed somatic chromosome pairing, has been observed in cancerous cells, but the functional consequences of somatic pairing have not been established. Gene expression profiling studies revealed that somatic pairing of chromosome 19 is a recurrent chromosomal abnormality in renal oncocytoma, a neoplasia of the adult kidney. Somatic pairing was associated with significant disruption of gene expression within the paired regions and resulted in the deregulation of the prolyl-hydroxylase ELGN2, a key protein that regulates the oxygen-dependent degradation of hypoxia-inducible factor (HIF). Overexpression of ELGN2 in renal oncocytoma increased ubiquitin-mediated destruction of HIF and concomitantly suppressed the expression of several HIF-target genes, including the pro-death BNIP3L gene. The transcriptional changes that are associated with somatic pairing of chromosome 19 mimic the transcriptional changes that occur following DNA amplification. Therefore, in addition to numerical and structural chromosomal abnormalities, alterations in chromosomal spatial dynamics should be considered as genomic events that are associated with tumorigenesis. The identification of EGLN2 as a significantly deregulated gene that maps within the paired chromosome region directly implicates defects in the oxygen-sensing network to the biology of renal oncocytoma