5,862 research outputs found
Possible Effects of the Existence of the 4th Generation Neutrino
The 4th generation of fermions predicted by the phenomenology of heterotic
string models can possess new strictly conserved charge, which leads, in
particular, to the hypothesis of the existence of the 4th generation massive
stable neutrino. The compatibility of this hypothesis with the results of
underground experiment DAMA searching for weakly interactive particles of dark
matter and with the EGRET measurements of galactic gamma--background at
energies above 1 GeV fixes the possible mass of the 4th neutrino at the value
about 50 GeV. The possibility to test the hypothesis in accelerator experiments
is considered. Positron signal from the annihilation of relic massive neutrinos
in the galactic halo is calculated and is shown to be accessible for planned
cosmic ray experiments.Comment: 10 pages, 4 PostScript figure, Latex2
Size-dependent bandgap and particle size distribution of colloidal semiconductor nanocrystals
A new analytical expression for the size-dependent bandgap of colloidal
semiconductor nanocrystals is proposed within the framework of the finite-depth
square-well effective mass approximation in order to provide a quantitative
description of the quantum confinement effect. This allows one to convert
optical spectroscopic data (photoluminescence spectrum and absorbance edge)
into accurate estimates for the particle size distributions of colloidal
systems even if the traditional effective mass model is expected to fail, which
occurs typically for very small particles belonging to the so-called strong
confinement limit. By applying the reported theoretical methodologies to CdTe
nanocrystals synthesized through wet chemical routes, size distributions are
inferred and compared directly to those obtained from atomic force microscopy
and transmission electron microscopy. This analysis can be used as a
complementary tool for the characterization of nanocrystal samples of many
other systems such as the II-VI and III-V semiconductor materials.Comment: 9 pages, 5 figure
Biotic interactions, energy pathways and trigger factors of ecosystem dynamics in shallow saline lakes
Environmental variables, food-web dynamics, and energy pathways were investigated in ecosystems of six shallow lakes of the Crimea with salinity ranging from 24 to 340‰. There were considerable inter- and intra-annual #uctuations in the abiotic characteristics of the lakes. All the lakes had a very high concentration of total phosphorus in the water (up to 5.6 gP m-3) due to a great influence of the watershed on the lakes. A high level of primary production (up to 14.9 gC m-2 d-1) was found in the most of the lakes. The lowest primary production was found in the most saline lake with a dense population of the filtrator, Artemia urmiana. Grazing benthic energy pathways were dominant at salinities between 24 and 58‰. Greater levels of salinity led to a gradual reduction of benthic and increase of planktonic energy pathways. A fast shift from turbid to the clear-water regime was observed in one of the lakes, caused by annual variations in weather conditions
Scaling analysis of the magnetic-field-tuned quantum transition in superconducting amorphous In-O films
We have studied the magnetic-field-tuned superconductor-insulator quantum
transition (SIT) in amorphous In-O films with different oxygen content and,
hence, different electron density. While for states of the film near the
zero-field SIT the two-dimensional scaling behaviour is confirmed, for deeper
states in the superconducting phase the SIT scenario changes: in addition to
the scaling function that describes the conductivity of fluctuation-induced
Cooper pairs, there emerges a temperature-dependent contribution to the film
resistance. This contribution can originate from the conductivity of normal
electrons.Comment: 4 pages, 5 figures; accepted for publication in JETP Lett. 71 (4),
200
Self-consistency over the charge-density in dynamical mean-field theory: a linear muffin-tin implementation and some physical implications
We present a simple implementation of the dynamical mean-field theory
approach to the electronic structure of strongly correlated materials. This
implementation achieves full self-consistency over the charge density, taking
into account correlation-induced changes to the total charge density and
effective Kohn-Sham Hamiltonian. A linear muffin-tin orbital basis-set is used,
and the charge density is computed from moments of the many body
momentum-distribution matrix. The calculation of the total energy is also
considered, with a proper treatment of high-frequency tails of the Green's
function and self-energy. The method is illustrated on two materials with
well-localized 4f electrons, insulating cerium sesquioxide Ce2O3 and the
gamma-phase of metallic cerium, using the Hubbard-I approximation to the
dynamical mean-field self-energy. The momentum-integrated spectral function and
momentum-resolved dispersion of the Hubbard bands are calculated, as well as
the volume-dependence of the total energy. We show that full self-consistency
over the charge density, taking into account its modification by strong
correlations, can be important for the computation of both thermodynamical and
spectral properties, particularly in the case of the oxide material.Comment: 20 pages, 6 figures (submitted in The Physical Review B
Observation of the parallel-magnetic-field-induced superconductor-insulator transition in thin amorphous InO films
We study the response of a thin superconducting amorphous InO film with
variable oxygen content to a parallel magnetic field. A field-induced
superconductor-insulator transition (SIT) is observed that is very similar to
the one in normal magnetic fields. As the boson-vortex duality, which is the
key-stone of the theory of the field-induced SIT, is obviously absent in the
parallel configuration, we have to draw conclusion about the theory
insufficiency.Comment: 3 pages, 4 figure
Width of the Zero-Field Superconducting Resistive Transition in the Vicinity of the Localization Threshold
Resistive superconducting zero-field transition in amorphous In-O films in
states from the vicinity of the insulator-superconductor transition is analyzed
in terms of two characteristic temperatures: the upper one, , where the
finite amplitude of the order parameter is established and the lower one,
, where the phase ordering takes place. It follows from the
magnetoresistance measurements that the resistance in between, ,
cannot be ascribed to dissipation by thermally dissociated vortex pairs. So, it
is not Kosterlitz-Thouless-Berezinskii transition that happens at .Comment: 4 pages, 3 figure
The Distribution of Constituent Charm Quarks in the Hadron
Using a statistical approach in the framework of non-covariant perturbation
theory the distributions for light and charmed quarks in the hadron have been
derived, taking into account the mass of the charmed quark. The parameters of
the model have been extracted from the comparison with NA3 data on
hadroproduction of J/psi particles. A reanalysis of the EMC data on charm
production in muon-nucleon scattering has been performed. It has been found in
comparison with the conventional source of charmed quarks from photon-gluon
fusion, that the EMC data indicate the presence of an additional contribution
from deep-inelastic scattering on charmed quarks at large x. The resulting
admixture of the Fock states, containing charmed quarks in the decomposition of
the proton wave function is of the order of 1%. The approach presented for the
excitation of the Fock states with charmed quarks can also be applied to states
with beauty quarks as well as to the hadronic component of the virtual photon
(resolved photon component).Comment: 23 pages, 4 PostScript figures, Latex2e. In revised version in
comparison with the original one all (?) mistypings have been corrected, one
more thank has been added and the comparison of the pion and the proton J/psi
production is described in more detai
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