105 research outputs found
Correlation between the Extraordinary Hall Effect and Resistivity
We study the contribution of different types of scattering sources to the
extraordinary Hall effect. Scattering by magnetic nano-particles embedded in
normal-metal matrix, insulating impurities in magnetic matrix, surface
scattering and temperature dependent scattering are experimentally tested. Our
new data, as well as previously published results on a variety of materials,
are fairly interpreted by a simple modification of the skew scattering model
Deep optical observations of the interaction of the SS 433 microquasar jet with the W 50 radio continuum shell
Four mosaics of deep, continuum-subtracted, CCD images have been obtained
over the extensive galactic radio continuum shell, W 50, which surrounds the
remarkable stellar system SS 433. Two of these mosaics in the Halpha+[N II] and
[O III] 5007 A emission lines respectively cover a field of ~2.3 x 2.5 degr^2
which contains all of W 50 but at a low angular resolution of 5 arcsec. The
third and fourth mosaics cover the eastern (in [O III] 5007 A) and western (in
Halpha+[N II]) filamentary nebulosity respectively but at an angular resolution
of 1 arcsec. These observations are supplemented by new low dispersion spectra
and longslit, spatially resolved echelle spectra. The [O III] 5007 A images
show for the first time the distribution of this emission in both the eastern
and western filaments while new Halpha+[N II] emission features are also found
in both of these regions. Approaching flows of faintly emitting material from
the bright eastern filaments of up 100 km/s in radial velocity are detected.
The present observations also suggest that the heliocentric systemic radial
velocity of the whole system is 56+-2 km/s. Furthermore, very deep imagery and
high resolution spectroscopy of a small part of the northern radio ridge of W
50 has revealed for the first time the very faint optical nebulosity associated
with this edge. It is suggested that patchy foreground dust along the ~5 kpc
sightline is inhibiting the detection of all of the optical nebulosity
associated with W 50. The interaction of the microquasar jets of SS 433 with
the W 50 shell is discussed.Comment: 19 pages, 13 figures, 2 tables. Accepted for pubication in MNRA
Diversité des familles de radiolaires au cours du temps
L’examen de la biodiversité des radiolaires, au niveau de la famille au cours du Phanérozoïque révèle quelques
tendances générales connues chez d’autres groupes d’organismes, surtout dans le plancton, alors que d’autres tendances
leur sont particulières. La crise permo-triasique, l’une des plus importantes dans l’évolution des organismes
marins, est marquée chez les radiolaires par l’extinction de deux familles (Albaillellaria et Latentifistularia) vers la fin
du Permien, mais surtout par une énorme diversification des spumellaires et nassellaires au Trias inférieur et moyen. La
diversité des radiolaires s’accroît du Cambrien au Jurassique, reste relativement stable au Crétacé et décroît légèremen
Theory of the anomalous Hall effect from the Kubo formula and the Dirac equation
A model to treat the anomalous Hall effect is developed. Based on the Kubo
formalism and on the Dirac equation, this model allows the simultaneous
calculation of the skew-scattering and side-jump contributions to the anomalous
Hall conductivity. The continuity and the consistency with the
weak-relativistic limit described by the Pauli Hamiltonian is shown. For both
approaches, Dirac and Pauli, the Feynman diagrams, which lead to the
skew-scattering and the side-jump contributions, are underlined. In order to
illustrate this method, we apply it to a particular case: a ferromagnetic bulk
compound in the limit of weak-scattering and free-electrons approximation.
Explicit expressions for the anomalous Hall conductivity for both
skew-scattering and side-jump mechanisms are obtained. Within this model, the
recently predicted ''spin Hall effect'' appears naturally
Supersonic water masers in 30 Doradus
We report on extremely high velocity molecular gas, up to -80 km/s relative
to the ambient medium, in the giant star-formation complex 30 Doradus in the
Large Magellanic Cloud (LMC), as observed in new 22 GHz H2O maser emission
spectra obtained with the Mopra radio telescope. The masers may trace the
velocities of protostars, and the observed morphology and kinematics indicate
that current star formation occurs near the interfaces of colliding
stellar-wind blown bubbles. The large space velocities of the protostars and
associated gas could result in efficient mixing of the LMC. A similar mechanism
in the Milky Way could seed the galactic halo with relatively young stars and
gas.Comment: 11 pages plus 1 PS and 1 EPS figure, uses AASTeX preprint style;
accepted for publication in Astrophysical Journal Letter
Textural variations in Neogene pelagic carbonate ooze at DSDP Site 593, southern Tasman Sea, and their paleoceanographic implications
Changes in Neogene sediment texture in pelagic carbonate-rich oozes on the Challenger Plateau, southern Tasman Sea, are used to infer changes in depositional paleocurrent velocities. The most obvious record of textural change is in the mud:sand ratio. Increases in the sand content are inferred to indicate a general up-core trend towards increasing winnowing of sediments resulting from increasing flow velocity of Southern Component Intermediate Water (SCIW), the forerunner of Antarctic Intermediate Water. In particular, the intervals c. 19-14.5 Ma, c. 9.5-8 Ma, and after 5 Ma are suggested to be times of increased SCIW velocity and strong sediment winnowing. Within the mud fraction, the fine silt to coarse clay sizes from 15.6 to 2 µm make the greatest contribution to the sediments and are composed of nannofossil plates. During extreme winnowing events it is the fine silt to very coarse clay material (13-3 µm) within this range that is preferentially removed, suggesting the 10 µm cohesive silt boundary reported for siliciclastic sediments does not apply to calcitic skeletal grains. The winnowed sediment comprises coccolithophore placoliths and spheres, represented by a mode at 4-7 µm.
Further support for seafloor winnowing is gained from the presence in Hole 593 of a condensed sedimentary section from c. 18 to 14 Ma where the sand content increases to c. 20% of the bulk sample. Associated with the condensed section is a 6 m thick orange unit representing sediments subjected to particularly oxygen-rich, late early to early middle Miocene SCIW. Together these are inferred to indicate increased SCIW velocity resulting in winnowed sediment associated with faster arrival of oxygen-rich surface water subducted to form SCIW. Glacial development of Antarctica has been recorded from many deep-sea sites, with extreme glacials providing the mechanism to increase watermass flow. Miocene glacial zones Mi1b-Mi6 are identified in an associated oxygen isotope record from Hole 593, and correspond with times of particularly invigorated paleocirculation, bottom winnowing, and sediment textural changes
A new candidate supernova remnant G 70.5+1.9
A compact complex of line emission filaments in the galactic plane has the
appearance of those expected of an evolved supernova remnant though non-thermal
radio and X-ray emission have not yet been detected. This optical emission line
region has now been observed with deep imagery and both low and high-dispersion
spectroscopy. Diagnostic diagrams of the line intensities from the present
spectra and the new kinematical observations both point to a supernova origin.
However, several features of the nebular complex still require an explanation
within this interpretation.Comment: 13 pages, 6 figures, accepted for publication in A&
The unprecedented optical outburst of the quasar 3C 454.3. The WEBT campaign of 2004-2005
The radio quasar 3C 454.3 underwent an exceptional optical outburst lasting
more than 1 year and culminating in spring 2005. The maximum brightness
detected was R = 12.0, which represents the most luminous quasar state thus far
observed (M_B ~ -31.4). In order to follow the emission behaviour of the source
in detail, a large multiwavelength campaign was organized by the Whole Earth
Blazar Telescope (WEBT). Continuous optical, near-IR and radio monitoring was
performed in several bands. ToO pointings by the Chandra and INTEGRAL
satellites provided additional information at high energies in May 2005. The
historical radio and optical light curves show different behaviours. Until
about 2001.0 only moderate variability was present in the optical regime, while
prominent and long-lasting radio outbursts were visible at the various radio
frequencies, with higher-frequency variations preceding the lower-frequency
ones. After that date, the optical activity increased and the radio flux is
less variable. This suggests that the optical and radio emissions come from two
separate and misaligned jet regions, with the inner optical one acquiring a
smaller viewing angle during the 2004-2005 outburst. Moreover, the colour-index
behaviour (generally redder-when-brighter) during the outburst suggests the
presence of a luminous accretion disc. A huge mm outburst followed the optical
one, peaking in June-July 2005. The high-frequency (37-43 GHz) radio flux
started to increase in early 2005 and reached a maximum at the end of our
observing period (end of September 2005). VLBA observations at 43 GHz during
the summer confirm theComment: 7 pages, 4 figures, to be published in A&
Anomalous Hall effect in Fe/Cu bilayers
The scaling of anomalous Hall resistivity on the longitudinal resistivity has
been intensively studied in the different magnetic systems, including
multilayers and granular films, to examine which mechanism, skew scattering or
side-jump, dominates. The basis of the scaling law is that both the
resistivities are due to the electron scattering at the imperfections in the
materials. By studying of anomalous Hall effect (AHE) in the simple Fe/Cu
bilayers, we demonstrate that the measured anomalous Hall effect should not
follow the scaling laws derived from skew scattering or side-jump mechanism due
to the short-circuit and shunting effects of the non-magnetic layers.Comment: 12 pages, 4 figures;
http://www.springerlink.com/content/1718722u75j24587
Faster maturation of selective attention in musically trained children and adolescents : Converging behavioral and event-related potential evidence
Previous work suggests that musical training in childhood is associated with enhanced executive functions. However, it is unknown whether this advantage extends to selective attention-another central aspect of executive control. We recorded a well-established event-related potential (ERP) marker of distraction, the P3a, during an audio-visual task to investigate the maturation of selective attention in musically trained children and adolescents aged 10-17 years and a control group of untrained peers. The task required categorization of visual stimuli, while a sequence of standard sounds and distracting novel sounds were presented in the background. The music group outperformed the control group in the categorization task and the younger children in the music group showed a smaller P3a to the distracting novel sounds than their peers in the control group. Also, a negative response elicited by the novel sounds in the N1/MMN time range (similar to 150-200 ms) was smaller in the music group. These results indicate that the music group was less easily distracted by the task-irrelevant sound stimulation and gated the neural processing of the novel sounds more efficiently than the control group. Furthermore, we replicated our previous finding that, relative to the control group, the musically trained children and adolescents performed faster in standardized tests for inhibition and set shifting. These results provide novel converging behavioral and electrophysiological evidence from a cross-modal paradigm for accelerated maturation of selective attention in musically trained children and adolescents and corroborate the association between musical training and enhanced inhibition and set shifting.Peer reviewe
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