633 research outputs found
Kondo effect in quantum dots coupled to ferromagnetic leads with noncollinear magnetizations
Non-equilibrium Green's function technique has been used to calculate
spin-dependent electronic transport through a quantum dot in the Kondo regime.
The dot is described by the Anderson Hamiltonian and is coupled either
symmetrically or asymmetrically to ferromagnetic leads, whose magnetic moments
are noncollinear. It is shown that the splitting of the zero bias Kondo anomaly
in differential conductance decreases monotonically with increasing angle
between magnetizations, and for antiparallel configuration it vanishes in the
symmetrical case while remains finite in the asymmetrical one.Comment: 4 pages, 3 figure
Genus inequalities and four-dimensional surgery
AbstractWe obtain a new genus inequality for a topologically locally flat surface in a 4-dimensional manifold. This inequality provides new information about the fundamental group of the complement of such a surface and in many cases gives the minimum genus among surfaces within the same homology class. The general problem of finding an embedded surface of a small genus allowed by the inequality remains undecided and is directly related to the surgery conjecture of 4-dimensional topology
Critical exponents and phase transition in gold nuclei fragmentation at energies 10.6 and 4.0 GeV/nucleon
An attempt to extract critical exponents gamma, beta and tau from data on
gold nuclei fragmentation due to interactions with nuclear emulsion at energies
4.0 A GeV and 10.6 A GeV is presented. Based on analysis of Campi's 2nd charge
moments, two subsets of data at each energy are selected from the inclusive
data, corresponding to 'liquid' and 'gas' phases. The extracted values of
critical exponents from the selected data sets are in agreement with
predictions of 'liquid-gas' model of phase transition.Comment: 21 pages, 15 figure
Spin-dependent resonant tunneling in ZnSe/ZnMnSe heterostructures
Using the transfer matrix method and the effective-mass approximation, the
effect of resonant states on spin transport is studied in
ZnSe/ZnMnSe/ZnSe/ZnMnSe/ZnSe structures under the influence of both electric
and magnetic fields. The numerical results show that the ZnMnSe layers, which
act as spin filters, polarize the electric currents. Variation of thickness of
the central ZnSe layer shifts the resonant levels and exhibits an oscillatory
behavior in spin current densities. It is also shown that the spin polarization
of the tunneling current in geometrical asymmetry of the heterostructure where
two ZnMnSe layers have different Mn concentrations, depends strongly on the
thickness and the applied bias.Comment: 13 pages, 6 figure
Spin-polarized transport through a single-level quantum dot in the Kondo regime
Nonequilibrium electronic transport through a quantum dot coupled to
ferromagnetic leads (electrodes) is studied theoretically by the nonequilibrium
Green function technique. The system is described by the Anderson model with
arbitrary correlation parameter . Exchange interaction between the dot and
ferromagnetic electrodes is taken into account {\it via} an effective molecular
field. The following situations are analyzed numerically: (i) the dot is
symmetrically coupled to two ferromagnetic leads, (ii) one of the two
ferromagnetic leads is half-metallic with almost total spin polarization of
electron states at the Fermi level, and (iii) one of the two electrodes is
nonmagnetic whereas the other one is ferromagnetic. Generally, the Kondo peak
in the density of states (DOS) becomes spin-split when the total exchange field
acting on the dot is nonzero. The spin-splitting of the Kondo peak in DOS leads
to splitting and suppression of the corresponding zero bias anomaly in the
differential conductance.Comment: 9 pages, 7 figure
On a possible photon origin of the most-energetic AGASA events
In this work the ultra high energy cosmic ray events recorded by the AGASA
experiment are analysed. With detailed simulations of the extensive air showers
initiated by photons, the probabilities are determined of the photonic origin
of the 6 AGASA events for which the muon densities were measured and the
reconstructed energies exceeded 10^20 eV. On this basis a new, preliminary
upper limit on the photon fraction in cosmic rays above 10^20 eV is derived and
compared to the predictions of exemplary top-down cosmic-ray origin models.Comment: 3 pages, 1 figure, 2 tables; presented at XIII ISVHECRI, Pylos,
Greec
Strongly damped nuclear collisions: zero or first sound ?
The relaxation of the collective quadrupole motion in the initial stage of a
central heavy ion collision at beam energies AMeV is studied
within a microscopic kinetic transport model. The damping rate is shown to be a
non-monotonic function of E_{lab} for a given pair of colliding nuclei. This
fact is interpreted as a manifestation of the zero-to-first sound transition in
a finite nuclear system.Comment: 15 pages, 4 figure
Mixed infections with distinct cytomegalovirus glycoprotein B genotypes in Polish pregnant women, fetuses, and newborns
The purpose of this investigation was to describe a distribution of cytomegalovirus (CMV) single and multiple genotypes among infected pregnant women, their fetuses, and newborns coming from Central Poland, as well as congenital cytomegaly outcome. The study involved 278 CMV-seropositive pregnant women, of whom 192 were tested for viral DNAemia. Human cytomegalovirus (HCMV) genotyping was performed for 18 of 34 pregnant women carrying the viral DNA and for 12 of their 15 offspring with confirmed HCMV infections. Anti-HCMV antibodies levels were assessed by chemiluminescence immunoassay (CLIA) and enzyme-linked fluorescence assay (ELFA) tests. Viral DNA loads and genotypes were determined by real-time polymerase chain reaction (PCR) assays for the UL55 gene. In the pregnant women, we identified HCMV gB1, gB2, gB3, and gB4 genotypes. Single gB2, gB3, or gB4 genotypes were observed in 14 (77.8 %) women, while multiple gB1âgB2 or gB2âgB3 genotypes were observed in four (22.2 %). Maternal HCMV genotypes determined the genotypes identified in their fetuses and newborns (pââ€â0.050). Half of them were infected with single HCMV gB1, gB2, or gB3 genotypes and the other half with multiple gB1âgB2 or gB2âgB3 genotypes. Single and multiple genotypes were observed in both asymptomatic and symptomatic congenital cytomegaly, although no gB3 genotype was identified among asymptomatic cases. In Central Poland, infections with single and multiple HCMV strains occur in pregnant women, as well as in their fetuses and neonates, with both asymptomatic and symptomatic infections. HCMV infections identified in mothers seem to be associated with the viral genotypes in their children
Simulation of Ultra-High Energy Photon Propagation in the Geomagnetic Field
The identification of primary photons or specifying stringent limits on the
photon flux is of major importance for understanding the origin of ultra-high
energy (UHE) cosmic rays. We present a new Monte Carlo program allowing
detailed studies of conversion and cascading of UHE photons in the geomagnetic
field. The program named PRESHOWER can be used both as an independent tool or
together with a shower simulation code. With the stand-alone version of the
code it is possible to investigate various properties of the particle cascade
induced by UHE photons interacting in the Earth's magnetic field before
entering the Earth's atmosphere. Combining this program with an extensive air
shower simulation code such as CORSIKA offers the possibility of investigating
signatures of photon-initiated showers. In particular, features can be studied
that help to discern such showers from the ones induced by hadrons. As an
illustration, calculations for the conditions of the southern part of the
Pierre Auger Observatory are presented.Comment: 41 pages, 9 figures, added references in introduction, corrected
energy in row 1 of Table 3, extended caption of Table
Characteristics of geomagnetic cascading of ultra-high energy photons at the southern and northern sites of the Pierre Auger Observatory
Cosmic-ray photons above 10^19 eV can convert in the geomagnetic field and
initiate a preshower, i.e. a particle cascade before entering the atmosphere.
We compare the preshower characteristics at the southern and northern sites of
the Pierre Auger Observatory. In addition to a shift of the preshower patterns
on the sky due to the different pointing of the local magnetic field vectors,
the fact that the northern Auger site is closer to the geomagnetic pole results
in a different energy dependence of the preshower effect: photon conversion can
start at smaller energies, but large conversion probabilitites (>90%) are
reached for the whole sky at higher energies compared to the southern Auger
site. We show how the complementary preshower features at the two sites can be
used to search for ultra-high energy photons among cosmic rays. In particular,
the different preshower characteristics at the northern Auger site may provide
an elegant and unambiguous confirmation if a photon signal is detected at the
southern site.Comment: 25 pages, 14 figures, minor changes, conclusions unchanged, Appendix
A replaced, accepted by Astroparticle Physic
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