165 research outputs found
Spin diffusion in doped semiconductors
The behavior of spin diffusion in doped semiconductors is shown to be
qualitatively different than in undoped (intrinsic) ones. Whereas a spin packet
in an intrinsic semiconductor must be a multiple-band disturbance, involving
inhomogeneous distributions of both electrons and holes, in a doped
semiconductor a single-band disturbance is possible. For n-doped nonmagnetic
semiconductors the enhancement of diffusion due to a degenerate electron sea in
the conduction band is much larger for these single-band spin packets than for
charge packets, and can exceed an order of magnitude at low temperatures even
for equilibrium dopings as small as 10^16 cm^-3. In n-doped ferromagnetic and
semimagnetic semiconductors the motion of spin packets polarized antiparallel
to the equilibrium carrier spin polarization is predicted to be an order of
magnitude faster than for parallel polarized spin packets. These results are
reversed for p-doped semiconductors.Comment: 8 pages, 4 figure
Magnetic moments of Cu and Cu nuclei measured by in-source laser spectroscopy
We have obtained information on the atomic hyperfine splitting and, hence, on magnetic moments in neutron rich Cu isotopes by scanning the frequency of the narrow-band laser of the first excitation step in the resonance ionization laser ion source. The deduced magnetic moments are Cu, I = 1) = +2.48(2)(7) ; Cu, I=6) = +1.24(4)(6) and Cu, I=1) = +1.86(4)(6) ; Cu, I=6) = +1.50(7)(8). The results of the scans analysis point out on existence of a new isomer in Cu. It's deduced magnetic moment is (-)3.50(7)(11) that is in a good agreement with I=3 assignment. The method of in-source atomic spectroscopy, as well as the analysis of the obtained data, is described. The results are discussed in terms of single-particle configurations coupled to the Ni core
Scaling analysis of electron transport through metal-semiconducting carbon nanotube interfaces: Evolution from the molecular limit to the bulk limit
We present a scaling analysis of electronic and transport properties of
metal-semiconducting carbon nanotube interfaces as a function of the nanotube
length within the coherent transport regime, which takes fully into account
atomic-scale electronic structure and three-dimensional electrostatics of the
metal-nanotube interface using a real-space Green's function based
self-consistent tight-binding theory. As the first example, we examine devices
formed by attaching finite-size single-wall carbon nanotubes (SWNT) to both
high- and low- work function metallic electrodes through the dangling bonds at
the end. We analyze the nature of Schottky barrier formation at the
metal-nanotube interface by examining the electrostatics, the band lineup and
the conductance of the metal-SWNT molecule-metal junction as a function of the
SWNT molecule length and metal-SWNT coupling strength. We show that the
confined cylindrical geometry and the atomistic nature of electronic processes
across the metal-SWNT interface leads to a different physical picture of band
alignment from that of the planar metal-semiconductor interface. We analyze the
temperature and length dependence of the conductance of the SWNT junctions,
which shows a transition from tunneling- to thermal activation-dominated
transport with increasing nanotube length. The temperature dependence of the
conductance is much weaker than that of the planar metal-semiconductor
interface due to the finite number of conduction channels within the SWNT
junctions. We find that the current-voltage characteristics of the metal-SWNT
molecule-metal junctions are sensitive to models of the potential response to
the applied source/drain bias voltages.Comment: Minor revision to appear in Phys. Rev. B. Color figures available in
the online PRB version or upon request to: [email protected]
TRIGA-SPEC: A setup for mass spectrometry and laser spectroscopy at the research reactor TRIGA Mainz
The research reactor TRIGA Mainz is an ideal facility to provide neutron-rich
nuclides with production rates sufficiently large for mass spectrometric and
laser spectroscopic studies. Within the TRIGA-SPEC project, a Penning trap as
well as a beam line for collinear laser spectroscopy are being installed.
Several new developments will ensure high sensitivity of the trap setup
enabling mass measurements even on a single ion. Besides neutron-rich fission
products produced in the reactor, also heavy nuclides such as 235-U or 252-Cf
can be investigated for the first time with an off-line ion source. The data
provided by the mass measurements will be of interest for astrophysical
calculations on the rapid neutron-capture process as well as for tests of mass
models in the heavy-mass region. The laser spectroscopic measurements will
yield model-independent information on nuclear ground-state properties such as
nuclear moments and charge radii of neutron-rich nuclei of refractory elements
far from stability. This publication describes the experimental setup as well
as its present status.Comment: 20 pages, 17 figure
Remote System for Detection of Low-Levels of Methane Based on Photonic Crystal Fibres and Wavelength Modulation Spectroscopy
In this work we described an optical fibre sensing system for detecting low levels of methane. The properties of hollow-core photonic crystal fibres are explored to have a sensing head with favourable characteristics for gas sensing, particularly in what concerns intrinsic readout sensitivity and gas diffusion time in the sensing structure. The sensor interrogation was performed applying the Wavelength Modulation Spectroscopy technique, and a portable measurement unit was developed with performance suitable for remote detection of low levels of methane. This portable system has the capacity to simultaneously interrogate four remote photonic crystal fibre sensing heads
On-line yields obtained with the ISOLDE RILIS
The ISOLDE resonance ionization laser ion source (RILIS) allows to ionize efficiently and selectively many metallic elements. In recent yield surveys and on-line experiments with the ISOLDE RILIS we observed Mg , Al , Cd , Tb , Yb , Tl , Pb and Bi. The obtained yields are presented together with measured release parameters which allow to extrapolate the release efficiency towards more exotic (short-lived) nuclides of the same elements
Residential exposure to motor vehicle emissions and the risk of wheezing among 7-8 year-old schoolchildren: a city-wide cross-sectional study in Nicosia, Cyprus
<p>Abstract</p> <p>Background</p> <p>Several studies have reported associations between respiratory outcomes in children and a range of self-reported, administrative or geographical indicators of traffic pollution. First-time investigation into the frequency of asthmatic symptoms among 7-8 year-old Cypriot children in 1999-2000 showed increased prevalence in the capital Nicosia compared to other areas. Geographical differences on an island the size of Cyprus may reflect environmental and/or lifestyle factors. This study investigates the relationship between self-reported symptoms and residential exposure to motor vehicle emissions among Nicosia schoolchildren.</p> <p>Methods</p> <p>The addresses of children in the metropolitan area of Nicosia who participated in the original survey (N = 1,735) were geo-coded and the level of exposure of each child was assessed using distance- and emission-based indicators (i.e. estimated levels of particulate matter and nitrogen oxides emissions due to motor vehicles on main roads around the residence). Odds ratios of wheezing and asthma diagnosis in relation to levels of exposure were estimated in logistic regression models adjusting for person-based factors, co-morbidity and intra-school clustering.</p> <p>Results</p> <p>We found an increased risk of wheezing at distances less than 50 m from a main road and/or only among those experiencing the highest levels of exposure. The strongest effect estimates were observed when exposure was defined in terms of the cumulative burden at all roads around the residence. Adjusted odds ratios for current wheezing were 2.33 (95% CI 1.27, 4.30) amongst the quartile of participants exposed to the highest levels of PM at all roads 50 m of their residence and 2.14 (95% CI 1.05, 4.35) for NOx, with no effect at intermediate levels of exposure. While the direction of effect was apparent at longer distances, differences were generally not statistically significant.</p> <p>Conclusions</p> <p>Children experiencing the highest burden of emissions in Nicosia seem to be at a higher risk of reporting asthmatic symptoms. Due to the small number of children residing at close proximity to main roads and lack of evidence of risk at intermediate levels of exposure or longer distances, the observed pattern alone does not explain the generally higher prevalence observed in urban Nicosia compared to other areas.</p
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