Giant Sunspots - August 12, 2024

The past few decades have made clear that the properties and performances of emerging functional and quantum materials can depend strongly on their local atomic and/or magnetic structure, particularly when details of the local structure deviate from the long-range structure averaged over space and time. Traditional methods of structural refinement (e.g., Rietveld) are typically sensitive only to the average structure, creating a need for more advanced structural probes suitable for extracting information about structural correlations on short length- and time-scales. In this Perspective, we describe the importance of local magnetic structure in several classes of emerging materials and present the magnetic pair distribution function (mPDF) technique as a powerful tool for studying short-range magnetism from neutron total-scattering data. We then provide a selection of examples of mPDF analysis applied to magnetic materials of recent technological and fundamental interest, including the antiferromagnetic semiconductor MnTe, geometrically frustrated magnets, and iron-oxide magnetic nanoparticles. The rapid development of mPDF analysis since its formalization a decade ago puts this technique in a strong position for making continued impact in the study of local magnetism in emerging materials.

A new deposition method developed by Goddard Space Flight Center fluorinates Al mirrors with XeF2 followed by a LiF coating to create what they term Al+XeLiF. This in-situ, room temperature process produces mirrors with high reflectivity in a broad spectral range, from the FUV to the IR, and is reported to be stable in relative humidities of 30% and lower. These mirrors are envisioned for missions requiring sensitivity down to 100 nm wavelength such as the habitable worlds observatory. Because most mission integration and testing campaigns require prolonged exposure to lab environments, and launch sites experience high relative humidities (RH) on average, some at 80% RH. We investigate Al+XeLiF stability in a wider range of temperatures and humidities along with employing additional characterization techniques including atomic force microscopy and x-ray photoelectron spectroscopy. We found that Al+XeLiF is stable in environments up to 82%RH when kept at cooler temperatures (3°C and 21°C). However, this material is unstable when stored at 60°C, experiencing roughening and loss in reflection from resulting Al surface plasmon excitation.

Although teachers have opportunities to learn about many things through teaching experience, we know little about how they develop science subject matter knowledge in this setting. With both limited opportunities to learn science subject matter knowledge before becoming teachers and minimal science professional development available while working as a teacher, it is important to understand the extent to which elementary teachers develop science subject matter knowledge in their regular classroom practice and the factors that influence that development. In this longitudinal, mixed methods study we collected both quantitative and qualitative data before and following their final field experience, which was their first opportunity to have significant teaching experience. Findings suggest two important factors for subject matter knowledge development: time for considering science subject matter and a learning setting that values science. In contrast, indicators of learner capacity (i.e., prior knowledge) and time teaching the topics were not associated with teacher subject matter knowledge development.

DBL is a novel pedagogical approach intended to improve students’ conditional knowledge and problem-solving skills by exposing them to a sequence of branching learning decisions. The DBL software provided students with ample opportunities to engage in the expert decision-making processes involved in complex problem-solving and to receive just-in-time instruction and scaffolds at each decision point. The purpose of this study was to examine the effects of decision-based learning (DBL) on undergraduate students’ learning performance in introductory physics courses as well as the mediating roles of cognitive load and self-testing for such effects. We used a quasi-experimental posttest design across two sections of an online introductory physics course including a total N = 390 participants. Contrary to our initial hypothesis, DBL instruction did not have a direct effect on cognitive load and had no indirect effect on student performance through cognitive load. Results also indicated that while DBL did not directly impact students’ physics performance, self-testing positively mediated the relationship between DBL and student performance. Our findings underscore the importance of students’ use of self-testing which plays a crucial role when engaging with DBL as it can influence effort input towards the domain task and thereby optimize learning performance.

The individual polarization components of nonlinear Thomson scattering arise from the separate dimensions of electron figure-8 motion caused by a linearly polarized laser field. We present the first measurements of nonlinear Thomson scattering in both emission hemispheres. In the electron average rest frame, the shape of the electron figure-8 path is symmetric about the laser polarization dimension. However, the periodic electron velocity is intrinsically asymmetric. The full scattering emission pattern reveals this asymmetry and the direction that electrons move around the figure-8 path.

We observed the Seyfert 1 galaxy Mrk 817 during an intensive multiwavelength reverberation mapping campaign for 16 months. Here, we examine the behavior of narrow UV absorption lines seen in the Hubble Space Telescope/Cosmic Origins Spectrograph spectra, both during the campaign and in other epochs extending over 14 yr. We conclude that, while the narrow absorption outflow system (at -3750 km s(-1) with FWHM = 177 km s(-1)) responds to the variations of the UV continuum as modified by the X-ray obscurer, its total column density (log N-H = 19.5 (+0.61)(-0.13) cm(-2)) did not change across all epochs. The adjusted ionization parameter (scaled with respect to the variations in the hydrogen-ionizing continuum flux) is log U-H = -1.0(-0.3)(+0.1) . The outflow is located at a distance smaller than 38 pc from the central source, which implies a hydrogen density of n(H) > 3000 cm(-3). The absorption outflow system only covers the continuum emission source and not the broad emission line region, which suggests that its transverse size is small (< 10(16) cm), with potential cloud geometries ranging from spherical to elongated along the line of sight.