1,694 research outputs found
Undoped Electron-Hole Bilayers in a GaAs/AlGaAs Double Quantum Well
We present the fabrication details of completely undoped electron-hole
bilayer devices in a GaAs/AlGaAs double quantum well heterostructure with a 30
nm barrier. These devices have independently tunable densities of the
two-dimensional electron gas and two-dimensional hole gas. We report
four-terminal transport measurements of the independently contacted electron
and hole layers with balanced densities from cm down
to cm at . The mobilities can exceed cm V s for electrons and
cm V s for holes.Comment: 3 pages, 3 figure
Measurement of electron-hole friction in an n-doped GaAs/AlGaAs quantum well using optical transient grating spectroscopy
We use phase-resolved transient grating spectroscopy to measure the drift and
diffusion of electron-hole density waves in a semiconductor quantum well. The
unique aspects of this optical probe allow us to determine the frictional force
between a two-dimensional Fermi liquid of electrons and a dilute gas of holes.
Knowledge of electron-hole friction enables prediction of ambipolar dynamics in
high-mobility electron systems.Comment: to appear in PR
Differing instructional needs for children of similar reading achievement grades two, four, and six
Thesis (Ed.M.)--Boston Universit
Assassins and apples: the environmental constraints of two snails that threaten Australian aquatic systems
Context: Alien freshwater snails pose a substantial risk to Australian native aquatic biota.Aims: This study aims to determine the thermal and salinity ranges of two introduced species within Australia, Pomacea sp. and Anentome sp., to facilitate predictions of their potential geographic range should they become widely established.Methods: Laboratory tests were conducted to assess behavioural responses of snails to altered temperature or salinity after different acclimation regimes.Key results: After acclimation at 25°C, Pomacea sp. had a median activity range of 13.5–38°C and Anentome sp. of 12–38.5°C. Higher acclimation temperatures produced observable effects, whereas lower acclimation temperatures did not. Salinity tolerances differed, with Pomacea sp. remaining active at up to 8 parts per thousand (ppt) (after acclimation at 25°C), with acclimation at 20°C resulting in a lower salinity tolerance. By contrast, Anentome sp. snails were active at up to 5 ppt after low salinity acclimation, demonstrating enhanced salinity tolerance compared with non-salinity acclimations.Conclusions: These results showed that both snails are capable of surviving temperatures and salinities that would allow invasion into subtropical and warm-temperate Australian aquatic systems.Implications: Free from the constraints of natural predators, competitors, and parasites, these snails should be of great concern to biosecurity agencies in Australia
An In-Depth Study of the Use of Eosin Y for the Solar Photocatalytic Oxidative Coupling of Benzylic Amines
The
direct utilization of solar light for synthetic photochemistry
is a sustainable and efficient technological goal. Herein we report
the first in-depth study on the use of the inexpensive organic photocatalyst
eosin Y for solar photocatalysis by demonstrating the oxidative coupling
of benzylic amines to form imines, a class of valuable intermediates
in chemical synthesis. By the use of a unique experimental setup with
a custom-built variable-intensity solar light simulator, replication
of a natural-sunlight environment was achieved. The relative significance
of different variables on the reaction rate constant was quantitatively
evaluated through comprehensive experimental design. Reaction kinetics
and mechanistic information were obtained using both a batch reactor
and a spinning-disc reactor. A maximum pseudo-first-order rate constant
of 1.59 × 10<sup>–3</sup> s<sup>–1</sup> was obtained
at a maximum turnover frequency of 192 h<sup>–1</sup> through
optimization of the reaction conditions. Experiments carried out using
a spinning-disc reactor confirmed that the reaction was not mass-transfer-limited
but rather photon-transfer-limited
A Study of the Formation of Single- and Double-Walled Carbon Nanotubes by a CVD Method
The reduction in H2/CH4 atmosphere of aluminum-iron oxides produces metal particles small enough to catalyze the formation of single-walled carbon nanotubes. Several experiments have been made using the same temperature profile and changing only the maximum temperature (800-1070 °C). Characterizations of the catalyst materials are performed using notably 57Fe Mo¨ssbauer spectroscopy. Electron microscopy and a macroscopical method are used to characterize the nanotubes. The nature of the iron species (Fe3+, R-Fe, ç-Fe-C, Fe3C) is correlated to their location in the material. The nature of the particles responsible for the high-temperature formation of the nanotubes is probably an Fe-C alloy which is, however, found as Fe3C by postreaction analysis. Increasing the reduction temperature increases the reduction yield and thus favors the formation of surface-metal particles, thus producing more nanotubes. The obtained carbon nanotubes are mostly single-walled and double-walled with an average diameter close to 2.5 nm. Several formation mechanisms are thought to be active. In particular, it is shown that the second wall can grow inside the first one but that subsequent ones are formed outside. It is also possible that under given experimental conditions, the smallest (<2 nm) catalyst particles preferentially produce double-walled rather than single-walled carbon nanotubes
One-carbon metabolism in cancer
Cells require one-carbon units for nucleotide synthesis, methylation and reductive metabolism, and these pathways support the high proliferative rate of cancer cells. As such, anti-folates, drugs that target one-carbon metabolism, have long been used in the treatment of cancer. Amino acids, such as serine are a major one-carbon source, and cancer cells are particularly susceptible to deprivation of one-carbon units by serine restriction or inhibition of de novo serine synthesis. Recent work has also begun to decipher the specific pathways and sub-cellular compartments that are important for one-carbon metabolism in cancer cells. In this review we summarise the historical understanding of one-carbon metabolism in cancer, describe the recent findings regarding the generation and usage of one-carbon units and explore possible future therapeutics that could exploit the dependency of cancer cells on one-carbon metabolism
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