911 research outputs found
Mission ornithologique sur l'île Matthew, 16-19 avril 2008
A census of the birds of Matthew Island, south-east of the New Hebrides chain, was taken during a trip of frigateship Vendémiaire of the French Navy, from 16 to 19 April 2008. Fourteen bird species were observed on the islets, including 8 seabirds breeding or potentially breeding (black-winged petrel Pterodroma nigripennis, wedge-tailed shearwater Ardenna pacifica, masked booby Sula dactylatra, brown booby S. leucogaster, red-tailed tropicbird Phaethon rubricauda, brown noddy Anous stolidus, grey noddy Anous albivitta and sooty tern Onychoprion fuscatus), three other seabirds (roosting red-footed booby S. sula, lesser frigatebird Fregata ariel and Pacific frigatebird F. minor), three shorebirds (turnstone Arenaria interpres, Pacific golden plover Pluvialis fulva and tattler Tringa sp.) and the sacred kingfisher Todirhamphus sanctus. No hen or cock (Gallus gallus) was sighted, unlike during previous visits to the island. Series of traps were set overnight for rats but no rodent was captured. Wedge-tailed shearwaters were abundant at sea and in the South-West lagoon of New Caledonia; flocks of migrating short-tailed shearwaters Ardenna tenuirostris were sighted in the Loyalty basin.Un recensement des oiseaux de l’île Matthew, au sud-est de la chaîne des Nouvelles-Hébrides, a été fait à l’occasion d’une mission de souveraineté de la frégate Vendémiaire de la Marine nationale, du 16 au 19 avril 2008. Quatorze espèces d’oiseaux furent observées sur l’île, dont huit oiseaux marins nicheurs ou potentiellement nicheurs (pétrel à ailes noires Pterodroma nigripennis, puffin fouquet Ardenna pacifica, fou masqué Sula dactylatra, fou brun S. leucogaster, phaéton à brins rouges Phaethon rubricauda, noddi brun Anous stolidus, noddi gris Anous albivitta et sterne fuligineuse Onychoprion fuscatus), trois oiseaux marins non nicheurs (fou à pieds rouges S. sula, frégate ariel Fregata ariel et frégate du Pacifique F. minor), trois limicoles (tournepierre à collier Arenaria interpres, pluvier fauve Pluvialis fulva et chevalier Tringa sp.) et le martin-chasseur sacré Todirhamphus sanctus. Aucun coq ni poule (Gallus gallus) n’a été aperçu, à la différence des missions précédentes. Une série de pièges à rats, posés pour la nuit, n’ont capturé aucun rongeur. Le puffin fouquet était abondant en mer et dans le lagon sud-ouest de la Grande Terre ; des rassemblements de puffins à bec grêle Ardenna tenuirostris en migration ont été observés dans le bassin des Loyauté
Probing spin dynamics and quantum relaxation in LiY0.998Ho0.002F4 via 19F NMR
We report measurements of 19F nuclear spin-lattice relaxation 1/T1 as a
function of temperature and external magnetic field in LiY0.998Ho0.002F4 single
crystal, a single-ion magnet exhibiting interesting quantum effects. The 19F
1/T1 is found to depend on the coupling with the diluted rare-earth (RE)
moments. Depending on the temperature range, a fast spin diffusion regime or a
diffusion limited regime is encountered. In both cases we find it possible to
use the 19F nucleus as a probe of the rare-earth spin dynamics. The results for
1/T1 show a behavior similar to that observed in molecular nanomagnets, a
result which we attribute to the discreteness of the energy levels in both
cases. At intermediate temperatures the lifetime broadening of the crystal
field split RE magnetic levels follows a T3 power law. At low temperature the
field dependence of 1/T1 shows peaks in correspondence to the critical magnetic
fields for energy level crossings (LC). The results can be explained by
inelastic scattering between the fluorine nuclear spins and the RE magnetic
levels. A key result of this study is that the broadening of the levels at LC
is found to be become extremely small at low temperatures, about 1.7 mT, a
value which is comparable to the weak dipolar fields at the RE lattice
positions. Thus, unlike the molecular magnets, decoherence effects are strongly
suppressed, and it may be possible to measure directly the level repulsions at
avoided level crossings.Comment: 21 pages, 5 figure
NMR and Relaxation in Superconductor
NMR and nuclear spin-lattice relaxation rate (NSLR) are reported at
7.2 Tesla and 1.4 Tesla in powder samples of the intermetallic compound
with superconducting transition temperature in zero field = 39.2 K. From
the first order quadrupole perturbed NMR specrum a quadrupole coupling
frequency of 835 5 kHz is obtained. The Knight shift is very small and it
decreases to zero in the superconducting phase. The NSLR follows a linear law
with = 165 10 (sec K) . The results in the normal phase indicate a
negligible -character of the wave function of the conduction electrons at
the Fermi level. Below the NSLR is strongly field dependent indicating
the presence of an important contribution related to the density and the
thermal motion of flux lines. No coherence peak is observed at the lower field
investigated (1.4 T)
Spin dynamics in hole-doped two-dimensional S=1/2 Heisenberg antiferromagnets: ^{63}Cu NQR relaxation in La_{2-x}Sr_xCuO_4 for
The effects on the correlated Cu^{2+} S = 1/2 spin dynamics in the
paramagnetic phase of La_{2-x}Sr_xCuO_4 (for ) due to the
injection of holes are studied by means of ^{63}Cu NQR spin-lattice relaxation
time T_1 measurements. The results are discussed in the framework of the
connection between T_1 and the in-plane magnetic correlation length
. It is found that at high temperatures the system remains in
the renormalized classical regime, with a spin stiffness constant
reduced by small doping to an extent larger than the one due to Zn doping. For
the effect of doping on appears to level off. The
values for derived from T_1 for K are much larger
than the ones estimated from the temperature behavior of sublattice
magnetization in the ordered phase (). It is argued that these
features are consistent with the hypothesis of formation of stripes of
microsegregated holes.Comment: 10 pages, 3 figure
Dynamics of Magnetic Defects in Heavy Fermion LiV2O4 from Stretched Exponential 7Li NMR Relaxation
7Li NMR measurements on LiV2O4 from 0.5 to 4.2 K are reported. A small
concentration of magnetic defects within the structure drastically changes the
7Li nuclear magnetization relaxation versus time from a pure exponential as in
pure LiV2O4 to a stretched exponential, indicating glassy behavior of the
magnetic defects. The stretched exponential function is described as arising
from a distribution of 7Li nuclear spin-lattice relaxation rates and we present
a model for the distribution in terms of the dynamics of the magnetic defects.
Our results explain the origin of recent puzzling 7Li NMR literature data on
LiV2O4 and our model is likely applicable to other glassy systems.Comment: Four typeset pages including four figure
19F nuclear spin relaxation and spin diffusion effects in the single ion magnet LiYF4:Ho3+
Temperature and magnetic field dependences of the 19F nuclear spin-lattice
relaxation in a single crystal of LiYF4 doped with holmium are described by an
approach based on a detailed consideration of the magnetic dipole-dipole
interactions between nuclei and impurity paramagnetic ions and nuclear spin
diffusion processes. The observed non-exponential long time recovery of the
nuclear magnetization after saturation at intermediate temperatures is in
agreement with predictions of the spin-diffusion theory in a case of the
diffusion limited relaxation. At avoided level crossings in the spectrum of
electron-nuclear states of the Ho3+ ion, rates of nuclear spin-lattice
relaxation increase due to quasi-resonant energy exchange between nuclei and
paramagnetic ions, in contrast to the predominant role played by electronic
cross-relaxation processes in the low-frequency ac-susceptibility.Comment: 27 pages total, 5 figures, accepted for publication, Eur. Phys. J.
High-Resolution Topography along Surface Rupture of the 16 October 1999 Hector Mine, California, Earthquake (M_w 7.1) from Airborne Laser Swath Mapping
In order to document surface rupture associated with the Hector Mine earthquake, in particular, the area of maximum slip and the deformed surface of Lavic Lake playa, we acquired high-resolution data using relatively new topographic-mapping methods. We performed a raster-laser scan of the main surface breaks along the entire rupture zone, as well as along an unruptured portion of the Bullion fault. The image of the ground surface produced by this method is highly detailed, comparable to that obtained when geologists make particularly detailed site maps for geomorphic or paleoseismic studies. In this case, however, for the first time after a surface-rupturing earthquake, the detailed mapping is along the entire fault zone rather than being confined to selected sites. These data are geodetically referenced, using the Global Positioning System, thus enabling more accurate mapping of the rupture traces. In addition, digital photographs taken along the same flight lines can be overlaid onto the precise topographic data, improving terrain visualization. We demonstrate the potential of these techniques for measuring fault-slip vectors
High-quality epitaxial iron nitride films grown by gas-assisted molecular-beam epitaxy
Thin films of γ’-Fe4N were grown on polished (001) MgO substrates by molecular-beam epitaxy of iron in the presence of a gas flow from a rf atomic source. By means of x-ray diffraction, Mössbauer Spectroscopy, Rutherford backscattering/channeling, and scanning probe microscopy, it is shown that, with this method, single-phase, high-quality epitaxial thin films can be grown with a very smooth surface (root-mean-square roughness ∼0.4 nm). Magnetic measurements reveal square hysteresis loops, moderate coercivities (45 Oe for a 33 nm thick film) and complete in-plane orientation of the magnetization. These properties make the films interesting candidates for device applications
Phase Separation of the Two-Dimensional t-J model
The boundary of phase separation of the two-dimensional t-J model is
investigated by the power-Lanczos method and Maxwell construction. The method
is similar to a variational approach and it determines the lower bound of the
phase separation boundary with in the limit . In
the physical interesting regime of high T_c superconductors where
there is no phase separation.Comment: LaTex 5 pages, 4 figure
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