457 research outputs found
Stoichiometry control of sputtered CuCl thin films: Influence on ultraviolet emission properties
We demonstrate that the chemical composition of the sputtered CuCl thin films could be finely controlled by adjusting the bias to the substrate. The films deposited without any intentional bias were Cl rich (CuCl1+x), a bias of â22 V yielded stoichiometric CuCl, and a further increase in the negative bias resulted in Cl deficient films (CuCl1âx). The crystalline and optical properties were found to be associated with the chemical composition. Cl rich films showed a deep level green emission at around 515 nm in addition to ultraviolet (UV) excitonic emission. The stoichiometric films have higher optical quality, exhibiting a sharp UV emission at around 385 nm at room temperature, compared to nonstoichiometric samples. Visible luminescence related to deep level defects was not observed in the stoichiometric films. Changes in energy of the flux from the target and the subsequent ion bombardment on the substrate surface are correlated with the variations in chemical composition and their impact on the film microstructure and UV emission
Bringing exploratory learning online: problem-solving before instruction improves remote undergraduate physics learning
STEM undergraduate instructors teaching remote courses often use traditional lecture-based instruction, despite evidence that active learning methods improve student engagement and learning outcomes. One simple way to use active learning online is to incorporate exploratory learning. In exploratory learning, students explore a novel activity (e.g., problem solving) before a lecture on the underlying concepts and procedures. This method has been shown to improve learning outcomes during in-person courses, without requiring the entire course to be restructured. The current study examined whether the benefits of exploratory learning extend to a remote undergraduate physics lesson, taught synchronously online. Undergraduate physics students (Nâ=â78) completed a physics problem-solving activity either before instruction (explore-first condition) or after (instruct-first condition). Students then completed a learning assessment of the problem-solving procedures and underlying concepts. Despite lower accuracy on the learning activity, students in the explore-first condition demonstrated better understanding on the assessment, compared to students in the instruct-first condition. This finding suggests that exploratory learning can serve as productive failure in online courses, challenging students but improving learning, compared to the more widely-used lecture-then-practice method
OBSERVATIONS ON NAVICULA THALLODES (BACILLARIOPHYCEAE), A BLADE-FORMING DIATOM FROM THE BERING SEA 1
A thallus-forming diatom, Navicula thallodes Proschkina-Lavrenko, previously known only from the original collection at Bering Island (U.S.S.R.), has been found at Amchitka Island in the Aleutians, Alaska. The most remarkable observation of the present report is that N. thallodes may form blades up to 50 cm long, which to our knowledge is the greatest length reported for a colonial diatom. SEM observations of this diatom are presented for the first time.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65176/1/j.1529-8817.1988.tb04487.x.pd
A new method for short duration transient detection in radio images: Searching for transient sources in MeerKAT data of NGC 5068
Transient surveys are a vital tool in exploring the dynamic universe, with
radio transients acting as beacons for explosive and highly energetic
astrophysical phenomena. However, performing commensal transient surveys using
radio imaging can require a significant amount of computing power, data storage
and time. With the instrumentation available to us, and with new and exciting
radio interferometers in development, it is essential that we develop efficient
methods to probe the radio transient sky. In this paper, we present results
from an commensal short duration transient survey, on time scales of 8 seconds,
128 seconds and 1 hour, using data from the MeerKAT radio telescope. The
dataset used was obtained as part of a galaxy observing campaign, and we focus
on the field of NGC 5068. We present a quick, wide field imaging strategy to
enable fast imaging of large datasets, and develop methods to efficiently
filter detected transient candidates. No transient candidates were identified
on the time scales of 8 seconds, 128 seconds and 1 hour, leading to competitive
limits on the transient surface densities of deg,
deg, and deg at
sensitivities of 56.4 mJy, 19.2 mJy, and 3.9 mJy for the respective time
scales. We find one possible candidate that could be associated with a stellar
flare, that was rejected due to strict image quality control. Further short
time-scale radio observations of this candidate could give definite results to
its origin.Comment: 11 pages (9 main, 2 appendix), 8 figures, 2 tables. Submitted to
MNRA
Carbon States in Carbon-Encapsulated Nickel Nanoparticles Studied by Means of X-Ray Absorption, Emission, and Photoelectron Spectroscopies
Electronic structure of nickel nanoparticles encapsulated in carbon was
characterized by photoelectron, X-ray absorption, and X-ray emission
spectroscopies. Experimental spectra are compared with the density of states
calculated in the frame of the density functional theory. The carbon shell of
Ni nanoparticles has been found to be multilayer graphene with significant
(about 6%) amount of Stone--Wales defects. Results of the experiments evidence
protection of the metallic nanoparticles from the environmental degradation by
providing a barrier against oxidation at least for two years. Exposure in air
for 2 years leads to oxidation only of the carbon shell of Ni@C nanoparticles
with coverage of functional groups.Comment: 16 pages, 6 figures, accepted in J. Phys. Chem.
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Connecting research and practice in TESOL: a community of practice perspective
In line with a growing interest in teacher research engagement in second language education, this article is an attempt to shed light on teachersâ views on the relationship between teaching and practice. The data comprise semi-structured interviews with 20 teachers in England, examining their views about the divide between research and practice in their field, the reasons for the persistence of the divide between the two and their suggestions on how to bridge it. Wengerâs (1998) Community of Practice (CoP) is used as a conceptual framework to analyse and interpret the data. The analysis indicates that teacher experience, learning and ownership of knowledge emerging from participation in their CoP are key players in teachersâ professional practice and in the development of teacher identity. The participants construe the divide in the light of the differences they perceive between teaching and research as two different CoPs, and attribute the divide to the limited mutual engagement, absence of a joint enterprise and lack of a shared repertoire between them. Boundary encounters, institutionalised brokering and a more research-oriented teacher education provision are some of the suggestions for bringing the two communities together
Herschel/HIFI observations of spectrally resolved methylidyne signatures toward the high-mass star-forming core NGC6334I
In contrast to extensively studied dense star-forming cores, little is known
about diffuse gas surrounding star-forming regions. We study molecular gas in
the high-mass star-forming region NGC6334I, which contains diffuse, quiescent
components that are inconspicuous in widely used molecular tracers such as CO.
We present Herschel/HIFI observations of CH toward NGC6334I observed as part of
the CHESS key program. HIFI resolves the hyperfine components of its J=3/2-1/2
transition, observed in both emission and absorption. The CH emission appears
close to the systemic velocity of NGC6334I, while its measured linewidth of 3
km/s is smaller than previously observed in dense gas tracers such as NH3 and
SiO. The CH abundance in the hot core is 7 10^-11, two to three orders of
magnitude lower than in diffuse clouds. While other studies find distinct
outflows in, e.g., CO and H2O toward NGC6334I, we do not detect outflow
signatures in CH. To explain the absorption signatures, at least two absorbing
components are needed at -3.0 and +6.5 km/s with N(CH)=7 10^13 and 3 10^13
cm^-2. Two additional absorbing clouds are found at +8.0 and 0.0 km/s, both
with N(CH)=2 10^13 cm^-2. Turbulent linewidths for the four absorption
components vary between 1.5 and 5.0 km/s in FWHM. We constrain physical
properties of our CH clouds by matching our CH absorbers with other absorption
signatures. In the hot core, molecules such as H2O and CO trace gas that is
heated and dynamically influenced by outflow activity, whereas CH traces more
quiescent material. The four CH absorbers have column densities and turbulent
properties consistent with diffuse clouds: two are located near NGC6334, and
two are unrelated foreground clouds. Local density and dynamical effects
influence the chemical composition of physical components of NGC6334, causing
some components to be seen in CH but not in other tracers, and vice versa.Comment: Accepted by A&A Letters; 5 pages, 1 figure; v2: minor textual and
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Rapid spectral variability of a giant flare from a magnetar in NGC 253
Magnetars are neutron stars with extremely strong magnetic fields (1013 to 1015 gauss)1,2, which episodically emit X-ray bursts approximately 100 milliseconds long and with energies of 1040 to 1041 erg. Occasionally, they also produce extremely bright and energetic giant flares, which begin with a short (roughly 0.2 seconds), intense flash, followed by fainter, longer-lasting emission that is modulated by the spin period of the magnetar3,4 (typically 2 to 12 seconds). Over the past 40 years, only three such flares have been observed in our local group of galaxies3â6, and in all cases the extreme intensity of the flares caused the detectors to saturate. It has been proposed that extragalactic giant flares are probably a subset7â11 of short Îł-ray bursts, given that the sensitivity of current instrumentation prevents us from detecting the pulsating tail, whereas the initial bright flash is readily observable out to distances of around 10 to 20 million parsecs. Here we report X-ray and Îł-ray observations of the Îł-ray burst GRB 200415A, which has a rapid onset, very fast time variability, flat spectra and substantial sub-millisecond spectral evolution. These attributes match well with those expected for a giant flare from an extragalactic magnetar12, given that GRB 200415A is directionally associated13 with the galaxy NGC 253 (roughly 3.5 million parsecs away). The detection of three-megaelectronvolt photons provides evidence for the relativistic motion of the emitting plasma. Radiation from such rapidly moving gas around a rotating magnetar may have generated the rapid spectral evolution that we observe
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