1,187 research outputs found
A comparative DFT study of electronic properties of 2H-, 4H- and 6H-SiC(0001) and SiC(000-1) clean surfaces: Significance of the surface Stark effect
Electric field, uniform within the slab, emerging due to Fermi level pinning
at its both sides is analyzed using DFT simulations of the SiC surface slabs of
different thickness. It is shown that for thicker slab the field is nonuniform
and this fact is related to the surface state charge. Using the electron
density and potential profiles it is proved that for high precision simulations
it is necessary to take into account enough number of the Si-C layers. We show
that using 12 diatomic layers leads to satisfactory results. It is also
demonstrated that the change of the opposite side slab termination, both by
different type of atoms or by their location, can be used to adjust electric
field within the slab, creating a tool for simulation of surface properties,
depending on the doping in the bulk of semiconductor. Using these simulations
it was found that, depending on the electric field, the energy of the surface
states changes in a different way than energy of the bulk states. This
criterion can be used to distinguish Shockley and Tamm surface states. The
electronic properties, i.e. energy and type of surface states of the three
clean surfaces: 2H-, 4H-, 6H-SiC(0001), and SiC() are analyzed and
compared using field dependent DFT simulations.Comment: 18 pages, 10 figures, 4 table
Highly-ordered graphene for two dimensional electronics
With expanding interest in graphene-based electronics, it is crucial that
high quality graphene films be grown. Sublimation of Si from the 4H-SiC(0001)
Si-terminated) surface in ultrahigh vacuum is a demonstrated method to produce
epitaxial graphene sheets on a semiconductor. In this paper we show that
graphene grown from the SiC (C-terminated) surface are of higher
quality than those previously grown on SiC(0001). Graphene grown on the C-face
can have structural domain sizes more than three times larger than those grown
on the Si-face while at the same time reducing SiC substrate disorder from
sublimation by an order of magnitude.Comment: Submitted to Appl. Phys. Let
Photoexcited transients in disordered semiconductors: Quantum coherence at very short to intermediate times
We study theoretically electron transients in semiconductor alloys excited by
light pulses shorter than 100 femtoseconds and tuned above the absorption edge
during and shortly after the pulse, when disorder scattering is dominant.
We use non-equilibrium Green functions employing the field-dependent
self-consistent Born approximation. The propagators and the particle
correlation function are obtained by a direct numerical solution of the Dyson
equations in differential form. For the purely elastic scattering in our model
system the solution procedures for the retarded propagator and for the
correlation function can be decoupled.The propagator is used as an input in
calculating the correlation function. Numerical results combined with a
cumulant expansion permit to separate in a consistent fashion the dark and the
induced parts of the self-energy. The dark behavior reduces to propagation of
strongly damped quasi-particles; the field induced self-energy leads to an
additional time non-local coherence. The particle correlation function is
formed by a coherent transient and an incoherent back-scattered component. The
particle number is conserved only if the field induced coherence is fully
incorporated. The transient polarization and the energy balance are also
obtained and interpreted.Comment: Accepted for publication in Phys. Rev. B; 37 pages,17 figure
The structural properties of the multi-layer graphene/4H-SiC(000-1) system as determined by Surface X-ray Diffraction
We present a structural analysis of the multi-layer graphene-4HSiC(000-1})
system using Surface X-Ray Reflectivity. We show for the first time that
graphene films grown on the C-terminated (000-1}) surface have a
graphene-substrate bond length that is very short (0.162nm). The measured
distance rules out a weak Van der Waals interaction to the substrate and
instead indicates a strong bond between the first graphene layer and the bulk
as predicted by ab-initio calculations. The measurements also indicate that
multi-layer graphene grows in a near turbostratic mode on this surface. This
result may explain the lack of a broken graphene symmetry inferred from
conduction measurements on this system [C. Berger et al., Science 312, 1191
(2006)].Comment: 9 pages with 6 figure
Intrauterine exposure to mild analgesics during pregnancy and the occurrence of cryptorchidism and hypospadia in the offspring: The Generation R Study
This article is available open access through the publisher’s website. Copyright @ 2012 The Authors.BACKGROUND - Recently, over-the-counter mild analgesic use during pregnancy has been suggested to influence the risk of reproductive disorders in the offspring. We examined the influence of maternal exposure to mild analgesics during pregnancy on the occurrence of cryptorchidism and hypospadia in their offspring.
METHODS - Associations between maternal exposure to mild analgesics during pregnancy and cryptorchidism or hypospadia in the offspring were studied in 3184 women participating in a large population-based prospective birth cohort study from early pregnancy onwards in the Netherlands (2002–2006), the Generation R Study. Cryptorchidism and hypospadia were identified during routine screening assessments performed in child health care centres by trained physicians. The use of mild analgesics was assessed in three prenatal questionnaires in pregnancy, resulting in four periods of use, namely, periconception period, first 14 weeks of gestation, 14–22 weeks of gestation and 20–32 weeks of gestation. Logistic regression analyses were used to study the associations between maternal exposure to mild analgesics and cryptorchidism and hypospadia.
RESULTS - The cumulative prevalence over 30 months of follow up was 2.1% for cryptorchidism and 0.7% for hypospadia. Use of mild analgesics in the second period of pregnancy (14–22 weeks) increased the risk of congenital cryptorchidism [adjusted odds ratio (OR) 2.12; 95% confidence interval (CI) 1.17–3.83], primarily due to the use of acetaminophen (paracetamol) (adjusted OR 1.89; 95% CI 1.01–3.51). Among mothers of cryptorchid sons, 33.8% reported (23 of 68) the use of mild analgesics during pregnancy, compared with 31.8% (7 of 22) of mothers with a boy with hypospadia and 29.9% (926 of 3094) of mothers with healthy boys.
CONCLUSIONS - Our results suggest that intrauterine exposure to mild analgesics, primarily paracetamol, during the period in pregnancy when male sexual differentiation takes place, increases the risk of cryptorchidism.Erasmus University Rotterdam, School of Law and Faculty of Social Sciences, the Municipal Health Service Rotterdam area, Rotterdam, the Rotterdam Homecare Foundation, Rotterdam and the Stichting Trombosedienst & Artsenlaboratorium
Rijnmond (STAR), Rotterdam
Mechanisms of Manganese-Assisted Nonradiative Recombination in Cd(Mn)Se/Zn(Mn)Se Quantum Dots
Mechanisms of nonradiative recombination of electron-hole complexes in
Cd(Mn)Se/Zn(Mn)Se quantum dots accompanied by interconfigurational excitations
of Mn ions are analyzed within the framework of single electron model of
deep {\it 3d}-levels in semiconductors. In addition to the mechanisms caused by
Coulomb and exchange interactions, which are related because of the Pauli
principle, another mechanism due to {\it sp-d} mixing is considered. It is
shown that the Coulomb mechanism reduces to long-range dipole-dipole energy
transfer from photoexcited quantum dots to Mn ions. The recombination
due to the Coulomb mechanism is allowed for any states of Mn ions and
{\it e-h} complexes. In contrast, short-range exchange and
recombinations are subject to spin selection rules, which are the result of
strong {\it lh-hh} splitting of hole states in quantum dots. Estimates show
that efficiency of the {\it sp-d} mechanism can considerably exceed that of the
Coulomb mechanism. The phonon-assisted recombination and processes involving
upper excited states of Mn ions are studied. The increase in PL
intensity of an ensemble of quantum dots in a magnetic field perpendicular to
the sample growth plane observed earlier is analyzed as a possible
manifestation of the spin-dependent recombination.Comment: 14 pages, 2 figure
Low energy measurement of the 7Be(p,gamma)8B cross section
We have measured the cross section of the 7Be(p,gamma)8B reaction for E_cm =
185.8 keV, 134.7 keV and 111.7 keV using a radioactive 7Be target (132 mCi).
Single and coincidence spectra of beta^+ and alpha particles from 8B and 8Be^*
decay, respectively, were measured using a large acceptance spectrometer. The
zero energy S factor inferred from these data is 18.5 +/- 2.4 eV b and a
weighted mean value of 18.8 +/- 1.7 eV b (theoretical uncertainty included) is
deduced when combining this value with our previous results at higher energies.Comment: Accepted for publication in Phys. Rev. Let
First-Principles Calculations of Hyperfine Interactions in La_2CuO_4
We present the results of first-principles cluster calculations of the
electronic structure of La_2CuO_4. Several clusters containing up to nine
copper atoms embedded in a background potential were investigated.
Spin-polarized calculations were performed both at the Hartree-Fock level and
with density functional methods with generalized gradient corrections to the
local density approximation. The distinct results for the electronic structure
obtained with these two methods are discussed. The dependence of the
electric-field gradients at the Cu and the O sites on the cluster size is
studied and the results are compared to experiments. The magnetic hyperfine
coupling parameters are carefully examined. Special attention is given to a
quantitative determination of on-site and transferred hyperfine fields. We
provide a detailed analysis that compares the hyperfine fields obtained for
various cluster sizes with results from additional calculations of spin states
with different multiplicities. From this we conclude that hyperfine couplings
are mainly transferred from nearest neighbor Cu^{2+} ions and that
contributions from further distant neighbors are marginal. The mechanisms
giving rise to transfer of spin density are worked out. Assuming conventional
values for the spin-orbit coupling, the total calculated hyperfine interaction
parameters are compared to informations from experiments.Comment: 23 pages, 9 figure
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