318 research outputs found
Is manganese-doped diamond a ferromagnetic semiconductor?
We use density-functional theoretical methods to examine the recent
prediction, based on a mean-field solution of the Zener model, that diamond
doped by Mn (with spin S=5/2) would be a dilute magnetic semiconductor that
remains ferromagnetic well above room temperature. Our findings suggest this to
be unlikely, for four reasons: (1) substitutional Mn in diamond has a low-spin
S=1/2 ground state; (2) the substitutional site is energetically unfavorable
relative to the much larger "divacancy" site; 3) Mn in the divacancy site is an
acceptor, but with only hyperdeep levels, and hence the holes are likely to
remain localized; (4) the calculated Heisenberg couplings between Mn in nearby
divacancy sites are two orders of magnitude smaller than for substitutional Mn
in germanium.Comment: 5 pages, 5 figure
Isolated oxygen defects in 3C- and 4H-SiC: A theoretical study
Ab initio calculations in the local-density approximation have been carried out in SiC to determine the possible configurations of the isolated oxygen impurity. Equilibrium geometry and occupation levels were calculated. Substitutional oxygen in 3C-SiC is a relatively shallow effective mass like double donor on the carbon site (O-C) and a hyperdeep double donor on the Si site (O-Si). In 4H-SiC O-C is still a double donor but with a more localized electron state. In 3C-SiC O-C is substantially more stable under any condition than O-Si or interstitial oxygen (O-i). In 4H-SiC O-C is also the most stable one except for heavy n-type doping. We propose that O-C is at the core of the electrically active oxygen-related defect family found by deep level transient spectroscopy in 4H-SiC. The consequences of the site preference of oxygen on the SiC/SiO2 interface are discussed
“It's like giving him a piece of me.”: Exploring UK and Israeli women's accounts of motherhood and feeding
Objective The present study explored how Israeli and UK mothers integrate feeding into their conceptualisations of mothering 2–6 months post-partum. Background The nature and importance of motherhood is subject to differential contextual, cultural, political and historical influences. We set out to compare experiences of motherhood and feeding between these two countries using a qualitative approach. Methods Forty one women (mean age 36.4 ± 2.7 years) from Israel and the UK, mostly married or in a committed relationship were interviewed about their experience of pregnancy, motherhood and feeding. Data were analysed thematically. Results The experience of motherhood in the early postnatal period was dominated, for all mothers, by the experience of breastfeeding and clustered around three representations of mothering, namely; 1) a devoted mother who ignores her own needs; 2) a mother who is available for her infant but acknowledges her needs as well; and 3) a struggling mother for whom motherhood is a burden. Such representations existed within both cultural groups and sometimes coexisted within the same mothers. UK women described more struggles within motherhood whereas a tendency towards idealising motherhood was observed for Israeli women. Conclusion There are similarities in the ways that UK and Israeli women experienced motherhood and feeding. Where family life is strongly emphasized, mothers reported extremes of idealism and burden and associated an “ideal” mother with a breastfeeding mother. Where motherhood is represented as just one of many roles women take up, they are more likely to represent a “good enough” approach to mothering. Understanding the experience of motherhood and feeding in different cultural settings is important to provide the context for postnatal care specifically where mothers are reluctant to share problems or difficulties encountered
Measurement of the partial widths of the Z into up- and down-type quarks
Using the entire OPAL LEP1 on-peak Z hadronic decay sample, Z -> qbarq gamma
decays were selected by tagging hadronic final states with isolated photon
candidates in the electromagnetic calorimeter. Combining the measured rates of
Z -> qbarq gamma decays with the total rate of hadronic Z decays permits the
simultaneous determination of the widths of the Z into up- and down-type
quarks. The values obtained, with total errors, were Gamma u = 300 ^{+19}_{-18}
MeV and Gamma d = 381 ^{+12}_{-12} MeV. The results are in good agreement with
the Standard Model expectation.Comment: 22 pages, 5 figures, Submitted to Phys. Letts.
Search for R-Parity Violating Decays of Scalar Fermions at LEP
A search for pair-produced scalar fermions under the assumption that R-parity
is not conserved has been performed using data collected with the OPAL detector
at LEP. The data samples analysed correspond to an integrated luminosity of
about 610 pb-1 collected at centre-of-mass energies of sqrt(s) 189-209 GeV. An
important consequence of R-parity violation is that the lightest supersymmetric
particle is expected to be unstable. Searches of R-parity violating decays of
charged sleptons, sneutrinos and squarks have been performed under the
assumptions that the lightest supersymmetric particle decays promptly and that
only one of the R-parity violating couplings is dominant for each of the decay
modes considered. Such processes would yield final states consisting of
leptons, jets, or both with or without missing energy. No significant
single-like excess of events has been observed with respect to the Standard
Model expectations. Limits on the production cross- section of scalar fermions
in R-parity violating scenarios are obtained. Constraints on the supersymmetric
particle masses are also presented in an R-parity violating framework analogous
to the Constrained Minimal Supersymmetric Standard Model.Comment: 51 pages, 24 figures, Submitted to Eur. Phys. J.
Measurement of the Strong Coupling alpha s from Four-Jet Observables in e+e- Annihilation
Data from e+e- annihilation into hadrons at centre-of-mass energies between
91 GeV and 209 GeV collected with the OPAL detector at LEP, are used to study
the four-jet rate as a function of the Durham algorithm resolution parameter
ycut. The four-jet rate is compared to next-to-leading order calculations that
include the resummation of large logarithms. The strong coupling measured from
the four-jet rate is alphas(Mz0)=
0.1182+-0.0003(stat.)+-0.0015(exp.)+-0.0011(had.)+-0.0012(scale)+-0.0013(mass)
in agreement with the world average. Next-to-leading order fits to the
D-parameter and thrust minor event-shape observables are also performed for the
first time. We find consistent results, but with significantly larger
theoretical uncertainties.Comment: 25 pages, 15 figures, Submitted to Euro. Phys. J.
Measurement of the Hadronic Photon Structure Function F_2^gamma at LEP2
The hadronic structure function of the photon F_2^gamma is measured as a
function of Bjorken x and of the factorisation scale Q^2 using data taken by
the OPAL detector at LEP. Previous OPAL measurements of the x dependence of
F_2^gamma are extended to an average Q^2 of 767 GeV^2. The Q^2 evolution of
F_2^gamma is studied for average Q^2 between 11.9 and 1051 GeV^2. As predicted
by QCD, the data show positive scaling violations in F_2^gamma. Several
parameterisations of F_2^gamma are in agreement with the measurements whereas
the quark-parton model prediction fails to describe the data.Comment: 4 pages, 2 figures, to appear in the proceedings of Photon 2001,
Ascona, Switzerlan
A measurement of the tau mass and the first CPT test with tau leptons
We measure the mass of the tau lepton to be 1775.1+-1.6(stat)+-1.0(syst.) MeV
using tau pairs from Z0 decays. To test CPT invariance we compare the masses of
the positively and negatively charged tau leptons. The relative mass difference
is found to be smaller than 3.0 10^-3 at the 90% confidence level.Comment: 10 pages, 4 figures, Submitted to Phys. Letts.
Measurement of Rb in e+e- Collisions at 182 - 209 GeV
Measurements of Rb, the ratio of the bbbar cross-section to the qqbar cross-
section in e+e- collisions, are presented. The data were collected by the OPAL
experiment at LEP at centre-of-mass energies between 182 GeV and 209 GeV.
Lepton, lifetime and event shape information is used to tag events containing b
quarks with high efficiency. The data are compatible with the Standard Model
expectation. The mean ratio of the eight measurements reported here to the
Standard Model prediction is 1.055+-0.031+-0.037, where the first error is
statistical and the second systematic.Comment: 21 pages, 5 figures, Submitted to Phys. Letts
Measurement of the B0 Lifetime and Oscillation Frequency using B0->D*+l-v decays
The lifetime and oscillation frequency of the B0 meson has been measured
using B0->D*+l-v decays recorded on the Z0 peak with the OPAL detector at LEP.
The D*+ -> D0pi+ decays were reconstructed using an inclusive technique and the
production flavour of the B0 mesons was determined using a combination of tags
from the rest of the event. The results t_B0 = 1.541 +- 0.028 +- 0.023 ps, Dm_d
= 0.497 +- 0.024 +- 0.025 ps-1 were obtained, where in each case the first
error is statistical and the second systematic.Comment: 17 pages, 4 figures, submitted to Phys. Lett.
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