89Y NMR Probe of Zn Induced Local Magnetism in YBa2(Cu(1-y)Zn(y))3O(6+x)

We present detailed data and analysis of the effects of Zn substitution on the planar Cu site in YBa$_2$Cu$_3$O$_{6+x}$ (YBCO$_{6+x}$) as evidenced from our $^{89}$Y NMR measurements on oriented powders. For $x<<1$ we find additional NMR lines which are associated with the Zn substitution. From our data on the intensities and temperature dependence of the shift, width, and spin-lattice relaxation rate of these resonances, we conclude that the spinless Zn 3$d$$^{10}$ state induces local moments on the near-neighbour ($$) Cu atoms. Additionally, we conjecture that the local moments actually extend to the farther Cu atoms with the magnetization alternating in sign at subsequent $nn$ sites. We show that this analysis is compatible with ESR data taken on dilute Gd doped (on the Y site) and on neutron scattering data reported recently on Zn substituted YBCO$_{6 + x}$. For optimally doped compounds $^{89}$Y $nn$ resonances are not detected, but a large $T$% -dependent contribution to the $^{89}$Y NMR linewidth is evidenced and is also attributed to the occurence of a weak induced local moment near the Zn. These results are compatible with macroscopic magnetic measurements performed on YBCO$_{6 + x}$ samples prepared specifically in order to minimize the content of impurity phases. We find significant differences between the present results on the underdoped YBCO$_{6 + x}$ samples and $$Al NMR data taken on Al$^{3+}$ substituted on the Cu site in optimally doped La$_2$CuO$_4$. Further experimental work is needed to clarify the detailed evolution of the impurity induced magnetism with hole content in the cuprates.Comment: To be published in EPJB 15 pages of text and figures in eps forma

PEER OBSERVATIONS AND FEEDBACK IN THE VIRTUAL CLASSROOM FOR SECONDARY TEACHERS

The COVID-19 pandemic resulted in several challenges for students and teachers in the United States. The introduction of virtual instruction was a challenge for many, especially teachers. As many educators attempted to determine how to best support their students and teachers, the latter looked for ways to create lessons and connect with students like they had not done before. A solution to help teachers navigate the time spent teaching virtually was peer observations during lessons, which allowed them to seek help from expert teachers as well as lend a hand to those who were struggling. Peer observations have been used in secondary education and other educational settings for over 50 years. During the pandemic, teachers relied upon one another for the same kind of support and expertise. For example, teachers in an urban eastern Texas city who faced the rapid implementation of virtual teaching during the pandemic asserted that peer observations as well as other forms of peer support helped many of them get through the struggles of virtual instruction. This interpretative phenomenological analysis employs Lev Vygotsky’s and Jenni Donohoo’s constructivist framework to answer the following questions: What are secondary teachers’ perceptions about leveraging virtual peer observations and feedback as a professional development tool? How do secondary teachers feel about the use of peer observations in virtual secondary classrooms? How do secondary teachers define peer observations for professional development? How do secondary teachers perceive the role of virtual peer observations in their professional practice? An analysis of six interview transcripts detailing the participating teachers’ experiences with virtual instruction and peer observations revealed five major themes: virtual peer observations, relationships, support and lack thereof, stress, and time. The study shows how teachers felt barely prepared to teach virtually in the early months of the pandemic, after March 2020, and how they almost fully relied on one another for support and direction regarding instruction. Recommendations for practice include continued professional development in digital learning and peer observations as well as expanded access to other teachers on other campuses to understand how they teach, manage their classrooms, and formulate lessons

The global structure of thin, stratified "alpha"-discs and the reliability of the one layer approximation

We report the results of a systematic comparison between the vertically averaged model and the vertically explicit model of steady state, Keplerian, optically thick "alpha"-discs. The simulations have concerned discs currently found in three different systems: dwarf novae, young stellar objects and active galactic nuclei. In each case, we have explored four decades of accretion rates and almost the whole disc area (except the narrow region where the vertically averaged model has degenerate solutions). We find that the one layer approach gives a remarkably good estimate of the main physical quantities in the disc, and specially the temperature at the equatorial plane which is accurate to within 30% for cases considered. The major deviations (by a factor < 4) are observed on the disc half-thickness. The sensitivity of the results to the "alpha"-parameter value has been tested for 0.001 < alpha < 0.1 and appears to be weak. This study suggests that the ``precision'' of the vertically averaged model which is easy to implement should be sufficient in practice for many astrophysical applications.Comment: 4 pages, PostScript. Accepted in Astronomy & Astrophysic

Disjunct Eastern Hemlock Populations of the Central Hardwood Forests: Ancient Relicts or Recent Long Distance Dispersal Events?

Eastern hemlock (Tsuga canadensis) is an evergreen conifer with a contiguous distribution extending from the southern Appalachian Mountains north to Nova Scotia and west across the Great Lakes region. Eastern hemlock is threatened with extirpation from much of this range by an introduced pest, the hemlock woolly adelgid (Orwig et al. 2002). In addition to the contiguous distribution, many small, isolated populations are located within the central hardwood forest region of Kentucky, Indiana, and Ohio (Braun 1928, Potzger and Friesner 1937, Van Stockum 1979). These disjunct populations form clearly delineated, often monospecific stands associated with unique natural features such as north facing cliff s and box canyons (Hart and Shankman 2005). Disjunct populations have long been of interest to biologists and two primary hypotheses for their origin have been proposed: 1) They are the product of rare long distance dispersal events (Gamache et al. 2003, Nathan 2006), or, 2) Remnants of what was once a portion of the contiguous distribution (Daubenmire 1931, Richardson et al. 2002). If long distance dispersal was responsible for the formation of these populations, we predict the resulting genetic bottlenecks to result in low within-population diversity, a correlation by distance to source populations and large between-population differences. Conversely, if these populations represent post-glacial relicts, we predict within population diversity to be dependent on population size, no correlation by distance to source populations, and low between-population differences