1,110 research outputs found
Isotopic fractionation during soil uptake of atmospheric hydrogen
Soil uptake of atmospheric hydrogen (H<sub>2</sub>) and the associated hydrogen isotope effect were studied using soil chambers in a Western Washington second-growth coniferous forest. Chamber studies were conducted during both winter and summer seasons to account for large natural variability in soil moisture content (4β50%) and temperature (6β22 &deg;C). H<sub>2</sub> deposition velocities were found to range from 0.01β0.06 cm s<sup>&minus;1</sup> with an average of 0.033 &plusmn; 0.008 cm s<sup>&minus;1</sup> (95% confidence interval). Consistent with prior studies, deposition velocities were correlated with soil moisture below 20% soil moisture content during the summer season. During winter, there was considerable variability observed in deposition velocity that was not closely related to soil moisture. The hydrogen kinetic isotope effect with H<sub>2</sub> uptake was found to range from &minus;24&permil; to β109&permil;. Aggregate analysis of experimental data results in an average KIE of &minus;57 &plusmn; 5&permil; (95% CI). Some of the variability in KIE can be explained by larger isotope effects at lower (<10%) and higher (>30%) soil moisture contents. The measured KIE was also found to be correlated with deposition velocity, with smaller isotope effects occurring at higher deposition velocities. If correct, these findings will have an impact on the interpretation of atmospheric measurements and modeling of &delta;D of H<sub>2</sub>
Tubal ligation and risk of breast cancer
Although it has been demonstrated in previous studies that tubal ligation can have widespread effects on ovarian function, including a decrease in the risk of subsequent ovarian cancer, few studies have evaluated effects on breast cancer risk. In a population-based caseβcontrol study of breast cancer among women 20β54 years of age conducted in three geographic areas, previous tubal ligations were reported by 25.3% of the 2173 cases and 25.8% of the 1990 controls. Initially it appeared that tubal ligations might impart a slight reduction in risk, particularly among women undergoing the procedure at young ages (< 25 years). However, women were more likely to have had the procedure if they were black, less educated, young when they bore their first child, or multiparous. After accounting for these factors, tubal ligations were unrelated to breast cancer risk (relative risk (RR) = 1.09, 95% confidence interval (CI) 0.9β1.3), with no variation in risk by age at, interval since, or calendar year of the procedure. The relationship of tubal ligations to risk did not vary according to the presence of a number of other risk factors, including menopausal status or screening history. Furthermore, effects of tubal ligation were similar for all stages at breast cancer diagnosis. Further studies would be worthwhile given the biologic plausibility of an association. However, future investigations should include information on type of procedure performed (since this may relate to biologic effects) as well as other breast cancer risk factors. Β© 2000 Cancer Research Campaig
Raman Scattering Spectra of Elementary Electronic Excitations in Coupled Double-Quantum Well Structures
Using the time-dependent-local-density-approximation (TDLDA) within a
self-consistent linear response theory, we calculate the elementary excitation
energies and the associated inelastic light-scattering spectra of a strongly
coupled two-component plasma in a double-quantum well system with electron
occupation of symmetric and antisymmetric subbands. We find, consistent with
the results of a recent experimental Raman scattering study, that the
intersubband spin density excitations tend to merge with the single particle
excitations (i.e. the excitonic shift decreases monotonically) as the Fermi
energy increases beyond the symmetric-antisymmetric energy gap
. However, our TDLDA calculation does not show the abrupt
suppresion of the excitonic shift seen experimentally at a finite value of the
subband occupancy parameter .Comment: 9 pages, RevTeX, 5 figures available upon request, PIT-SDS-00
Lactose and benign ovarian tumours in a caseβcontrol study
We investigated the relation between benign ovarian tumours and lactose among 746 case women identified at seven New York metropolitan hospitals and 404 community controls, age and hospital frequency matched to the expected case distribution. No increase in risk was found for lactose (highest quartile versus lowest: adjusted odds ratio = 0.82 (95% CI 0.57β1.20) or for any other lactose foods. Β© 2000 Cancer Research Campaign http://www.bjcancer.co
Stimulated and spontaneous optical generation of electron spin coherence in charged GaAs quantum dots
We report on the coherent optical excitation of electron spin polarization in
the ground state of charged GaAs quantum dots via an intermediate charged
exciton (trion) state. Coherent optical fields are used for the creation and
detection of the Raman spin coherence between the spin ground states of the
charged quantum dot. The measured spin decoherence time, which is likely
limited by the nature of the spin ensemble, approaches 10 ns at zero field. We
also show that the Raman spin coherence in the quantum beats is caused not only
by the usual stimulated Raman interaction but also by simultaneous spontaneous
radiative decay of either excited trion state to a coherent combination of the
two spin states.Comment: 4 pages, 3 figures. Minor modification
A study of temperature-related non-linearity at the metal-silicon interface
In this paper, we investigate the temperature dependencies of metal-semiconductor interfaces in an effort to better reproduce the current-voltage-temperature (IVT) characteristics of any Schottky diode, regardless of homogeneity. Four silicon Schottky diodes were fabricated for this work, each displaying different degrees of inhomogeneity; a relatively homogeneous NiV/Si diode, a Ti/Si and Cr/Si diode with double bumps at only the lowest temperatures, and a Nb/Si diode displaying extensive non-linearity. The 77β300βK IVT responses are modelled using a semi-automated implementation of Tung's electron transport model, and each of the diodes are well reproduced. However, in achieving this, it is revealed that each of the three key fitting parameters within the model display a significant temperature dependency. In analysing these dependencies, we reveal how a rise in thermal energy βactivatesβ exponentially more interfacial patches, the activation rate being dependent on the carrier concentration at the patch saddle point (the patch's maximum barrier height), which in turn is linked to the relative homogeneity of each diode. Finally, in a review of Tung's model, problems in the divergence of the current paths at low temperature are explained to be inherent due to the simplification of an interface that will contain competing defects and inhomogeneities
Electrical control of optical orientation of neutral and negatively charged excitons in n-type semiconductor quantum well
We report a giant electric field induced increase of spin orientation of
excitons in n-type GaAs/AlGaAs quantum well. It correlates strongly with the
formation of negatively charged excitons (trions) in the photoluminescence
spectra. Under resonant excitation of neutral heavy-hole excitons, the
polarization of excitons and trions increases dramatically with electrical
injection of electrons within the narrow exciton-trion bias transition in the
PL spectra, implying a polarization sensitivity of 200 % per Volt. This effect
results from a very efficient trapping of neutral excitons by the quantum well
interfacial fluctuations (so-called "natural" quantum dots) containing resident
electrons.Comment: 18 pages, 4 figure
Duplex strand joining reactions catalyzed by vaccinia virus DNA polymerase
Vaccinia virus DNA polymerase catalyzes duplex-by-duplex DNA joining reactions in vitro and many features of these recombination reactions are reprised in vivo. This can explain the intimate linkage between virus replication and genetic recombination. However, it is unclear why these apparently ordinary polymerases exhibit this unusual catalytic capacity. In this study, we have used different substrates to perform a detailed investigation of the mechanism of duplex-by-duplex recombination catalyzed by vaccinia DNA polymerase. When homologous, blunt-ended linear duplex substrates are incubated with vaccinia polymerase, in the presence of Mg(2+) and dNTPs, the appearance of joint molecules is preceded by the exposure of complementary single-stranded sequences by the proofreading exonuclease. These intermediates anneal to form a population of joint molecules containing hybrid regions flanked by nicks, 1β5 nt gaps, and/or short overhangs. The products are relatively resistant to exonuclease (and polymerase) activity and thus accumulate in joining reactions. Surface plasmon resonance (SPR) measurements showed the enzyme has a relative binding affinity favoring blunt-ended duplexes over molecules bearing 3β²-recessed gaps. Recombinant duplexes are the least favored ligands. These data suggest that a particular combination of otherwise ordinary enzymatic and DNA-binding properties, enable poxvirus DNA polymerases to promote duplex joining reactions
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