1,683 research outputs found
Sharp lines in the absorption edge of EuTe and PbEuTe in high magnetic fields
The optical absorption spectra in the region of the \fd transition energies
of epitaxial layers of of EuTe and \PbEuTe, grown by molecular beam epitaxy,
were studied using circularly polarized light, in the Faraday configuration.
Under \sigmam polarization a sharp symmetric absorption line (full width at
half-maximum 0.041 eV) emerges at the low energy side of the band-edge
absorption, for magnetic fields intensities greater than 6 T. The absorption
line shows a huge red shift (35 meV/T) with increasing magnetic fields. The
peak position of the absorption line as a function of magnetic field is
dominated by the {\em d-f} exchange interaction of the excited electron and the
\Euion spins in the lattice. The {\em d-f} exchange interaction energy was
estimated to be eV. In \PbEuTe the same absorption line
is detected, but it is broader, due to alloy disorder, indicating that the
excitation is localized within a finite radius. From a comparison of the
absorption spectra in EuTe and \PbEuTe the characteristic radius of the
excitation is estimated to be \AA.Comment: Journal of Physics: Condensed Matter (2004, at press
Evolution of population with sexual and asexual reproduction in changing environment
Using a lattice model based on Monte Carlo simulations, we study the role of
the reproduction pattern on the fate of an evolving population. Each individual
is under the selection pressure from the environment and random mutations. The
habitat ("climate") is changing periodically. Evolutions of populations
following two reproduction patterns are compared, asexual and sexual. We show,
via Monte Carlo simulations, that sexual reproduction by keeping more
diversified populations gives them better chances to adapt themselves to the
changing environment. However, in order to obtain a greater chance to mate, the
birth rate should be high. In the case of low birth rate and high mutation
probability there is a preference for the asexual reproduction.Comment: 11 pages including figs., for Int. J. Mod. Phys. C 15, issue 2 (2004
Risk analysis of marine activities in the Belgian part of the North Sea (RAMA): final report
RAMA is a 2-year project (04/2004 - 04/2006) executed by two Belgian partners, Ecolas NV (Environmental Consultancy Agency) and the Maritime Institute (University of Ghent), and financed by the SPSD II research program, specific actions, of the Belgian Science Policy (BELPSO). RAMA aims to assess the environmental risks of spills by commercial shipping activities on the Belgian Part of the North Sea. Shipping patterns, transports of dangerous goods, probability of risks and the potential impact of spill incidents (oil & hazardous and noxious substances) will be assessed. The risk analysis within this project studies both the chances of a spill accident happening and the environmental impacts in case of an accident. The valorisation of the RAMA project will result in a thorough analysis of the current status of the shipping at the North Sea in relation to the issue of safety. The scope of the project will however go beyond the mere result of a fundamental risk analysis of the commercial shipping at the North Sea. It is also aiming at the formulation of recommendations to improve the safety level for the environment and at an optimization of response in the framework of the Belgian "North Sea Disaster Plan"
Linear and nonlinear optical characteristics of the Falicov-Kimball model
We calculate the linear and nonlinear optical properties of the
Falicov-Kimball model for a mixed-valent system within the self-consistent
mean-field approximation. Second-harmonic generation can only occur if the
mixed-valent state has a built-in coherence between the itinerant d-electrons
and the localized f-holes. By contrast, second-harmonic generation cannot occur
for solutions of the model with f-site occupation as a good quantum number. As
an experimental test of coherence in mixed-valent compounds we propose a
measurement of the dynamic second-order susceptibility.Comment: 4 pages, 2 PostScript figures, to appear in Physical Review Letter
Polarised infrared emission from X-ray binary jets
Near-infrared (NIR) and optical polarimetric observations of a selection of
X-ray binaries are presented. The targets were observed using the Very Large
Telescope and the United Kingdom Infrared Telescope. We detect a significant
level (3 sigma) of linear polarisation in four sources. The polarisation is
found to be intrinsic (at the > 3 sigma level) in two sources; GRO J1655-40 (~
4-7% in H and Ks-bands during an outburst) and Sco X-1 (~ 0.1-0.9% in H and K),
which is stronger at lower frequencies. This is likely to be the signature of
optically thin synchrotron emission from the collimated jets in these systems,
whose presence indicates a partially-ordered magnetic field is present at the
inner regions of the jets. In Sco X-1 the intrinsic polarisation is variable
(and sometimes absent) in the H and K-bands. In the J-band (i.e. at higher
frequencies) the polarisation is not significantly variable and is consistent
with an interstellar origin. The optical light from GX 339-4 is also polarised,
but at a level and position angle consistent with scattering by interstellar
dust. The other polarised source is SS 433, which has a low level (0.5-0.8%) of
J-band polarisation, likely due to local scattering. The NIR counterparts of
GRO J0422+32, XTE J1118+480, 4U 0614+09 and Aql X-1 (which were all in or near
quiescence) have a linear polarisation level of < 16% (3 sigma upper limit,
some are < 6%). We discuss how such observations may be used to constrain the
ordering of the magnetic field close to the base of the jet in such systems.Comment: Accepted to be published in MNRAS; 13 pages, 6 figure
First Spitzer Space Telescope Observations of Magnetic Cataclysmic Variables: Evidence for Excess Emission at 3--8 microns
We present the first observations of magnetic cataclysmic variables with the
Spitzer Space Telescope. We used the Infrared Array Camera to obtain photometry
of the polars EF Eri, GG Leo, V347 Pav, and RX J0154.0-5947 at 3.6, 4.5, 5.8,
and 8.0 m. In all of our targets, we detect excess mid-infrared emission
over that expected from the component stars alone. We explore the origin of
this IR excess by examining bremsstrahlung, cyclotron emission, circumbinary
dust, and L/T brown dwarf secondary stars. Bremsstrahlung and cyclotron
emission appear unlikely to be significant contributors to the observed fluxes.
At present, the most likely candidate for the excess emission is dust that is
probably located in a circumbinary disk with an inner temperature near 800 K.
However, a simple dust disk plus any reasonable low mass or brown dwarf-like
secondary star is unable to fully explain the observed flux densities in the
3--8 m region.Comment: Accepted to ApJ Letter
Automated detection of tsunamigenic signatures and reporting using multi-scale offshore in situ measurements for early tsunami warning and critical decision-support
Valence and magnetic ordering in intermediate valence compounds : TmSe versus SmB6
The intermediate valent systems TmSe and SmB6 have been investigated up to 16
and 18 GPa by ac microcalorimetry with a pressure (p) tuning realized in situ
at low temperature. For TmSe, the transition from an antiferromagnetic
insulator for p<3 GPa to an antiferromagnetic metal at higher pressure has been
confirmed. A drastic change in the p variation of the Neel temperature (Tn) is
observed at 3 GPa. In the metallic phase (p>3 GPa), Tn is found to increase
linearly with p. A similar linear p increase of Tn is observed for the
quasitrivalent compound TmS which is at ambiant pressure equivalent to TmSe at
p=7 GPa. In the case of SmB6 long range magnetism has been detected above p=8
GPa, i.e. at a pressure slightly higher than the pressure of the insulator to
metal transition. However a homogeneous magnetic phase occurs only above 10
GPa. The magnetic and electronic properties are related to the renormalization
of the 4f wavefunction either to the divalent or the trivalent configurations.
As observed in SmS, long range magnetism in SmB6 occurs already far below the
pressure where a trivalent Sm3+ state will be reached. It seems possible, to
describe roughly the physical properties of the intermediate valence
equilibrium by assuming formulas for the Kondo lattice temperature depending on
the valence configuration. Comparison is also made with the appearance of long
range magnetism in cerium and ytterbium heavy fermion compounds.Comment: 22 pages including figure
Optical study of the electronic phase transition of strongly correlated YbInCu_4
Infrared, visible and near-UV reflectivity measurements are used to obtain
conductivity as a function of temperature and frequency in YbInCu_4, which
exhibits an isostructural phase-transition into a mixed-valent phase below
T_v=42 K. In addition to a gradual loss of spectral weight with decreasing
temperature extending up to 1.5 eV, a sharp resonance appears at 0.25 eV in the
mixed-valent phase. This feature can be described in terms of excitations into
the Kondo (Abrikosov-Suhl) resonance, and, like the sudden reduction of
resistivity, provides a direct reflection of the onset of coherence in this
strongly correlated electron system.Comment: 4 pages, 3 figures (to appear in Phys. Rev. B
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