7,659 research outputs found
Shear wave structure of a transect of the Los Angeles basin from multimode surface waves and H/V spectral ratio analysis
We use broad-band stations of the âLos Angeles Syncline Seismic Interferometry Experimentâ (LASSIE) to perform a joint inversion of the Horizontal to Vertical spectral ratios (H/V) and multimode dispersion curves (phase and group velocity) for both Rayleigh and Love waves at each station of a dense line of sensors. The H/V of the autocorrelated signal at a seismic station is proportional to the ratio of the imaginary parts of the Greenâs function. The presence of low-frequency peaks (âŒ0.2 Hz) in H/V allows us to constrain the structure of the basin with high confidence to a depth of 6 km. The velocity models we obtain are broadly consistent with the SCEC CVM-H community model and agree well with known geological features. Because our approach differs substantially from previous modelling of crustal velocities in southern California, this research validates both the utility of the diffuse field H/V measurements for deep structural characterization and the predictive value of the CVM-H community velocity model in the Los Angeles region. We also analyse a lower frequency peak (âŒ0.03 Hz) in H/V and suggest it could be the signature of the Moho. Finally, we show that the independent comparison of the H and V components with their corresponding theoretical counterparts gives information about the degree of diffusivity of the ambient seismic field
Structural domain and spin ordering induced glassy magnetic phase in single layered manganite PrSrMnO
The single layered manganite PrSrMnO undergoes
structural transition from high temperature tetragonal phase to low temperature
orthorhombic phase below room temperature. The orthorhombic phase was reported
to have two structural variants with slightly different lattice parameters and
Mn-3 levels show orbital ordering within both the variants, albeit having
mutually perpendicular ordering axis. In addition to orbital ordering, the
orthorhombic variants also order antiferromagnetically with different N\'eel
temperatures. Our magnetic investigation on the polycrystalline sample of
PrSrMnO shows large thermal hysteresis indicating the
first order nature of the tetragonal to orthorhombic transition. We observe
magnetic memory, large relaxation, frequency dependent ac susceptbility and
aging effects at low temperature, which indicate spin glass like magnetic
ground state in the sample. The glassy magnetic state presumably arises from
the interfacial frustration of orthorhombic domains with orbital and spin
orderings playing crucial role toward the competing magnetic interactions.Comment: 6 pages, 4 figures, Accepted in Europhysics Letter
Metastable states of a flux line lattice studied by transport and Small Angle Neutron Scattering
Flux Lines Lattice (FLL) states have been studied using transport
measurements and Small Angle Neutron Scattering in low T materials. In
Pb-In, the bulk dislocations in the FLL do not influence the transport
properties. In Fe doped NbSe, transport properties can differ after a
Field Cooling (FC) or a Zero Field Cooling (ZFC) procedure, as previously
reported. The ZFC FLL is found ordered with narrow Bragg Peaks and is linked to
a linear V(I) curve and to a superficial critical current. The FC FLL pattern
exhibits two Bragg peaks and the corresponding V(I) curve shows a S-shape. This
can be explained by the coexistence of two ordered FLL slightly tilted from the
applied field direction by different superficial currents. These currents are
wiped out when the transport current is increased.Comment: accepted for publication in Phys. Rev.
The Planetary Nebula System and Dynamics in the Outer Halo of NGC 5128
The halos of elliptical galaxies are faint and difficult to explore, but they
contain vital clues to both structure and formation. We present the results of
an imaging and spectroscopic survey for planetary nebulae (PNe) in the nearby
elliptical NGC 5128. We extend the work of Hui et al.(1995) well into the halo
of the galaxy--out to distances of 100 and 50 kpc along the major and minor
axes. We now know of 1141 PNe in NGC 5128, 780 of which are confirmed. Of these
780 PNe, 349 are new from this survey, and 148 are at radii beyond 20 kpc. PNe
exist at distances up to 80 kpc (~15 r_e), showing that the stellar halo
extends to the limit of our data. This study represents by far the largest
kinematic study of an elliptical galaxy to date, both in the number of velocity
tracers and in radial extent. We confirm the large rotation of the PNe along
the major axis, and show that it extends in a disk-like feature into the halo.
The rotation curve of the stars flattens at ~100 km/s with V/sigma between 1
and 1.5, and with the velocity dispersion of the PNe falling gradually at
larger radii. The two-dimensional velocity field exhibits a zero-velocity
contour with a pronounced twist, showing that the galaxy potential is likely
triaxial in shape, tending toward prolate. The total dynamical mass of the
galaxy within 80 kpc is ~5 x 10^{11} M_sun, with M/L_B ~ 13. This mass-to-light
ratio is much lower than what is typically expected for elliptical galaxies.Comment: 21 pages, 13 figures (figures 3-8 best viewed in color), accepted for
publication in the Astrophysical Journa
A Photometric Study of the Young Stellar Population Throughout the lambda Orionis Star-Forming Region
We present VRI photometry of 320,917 stars with 11 < R < 18 throughout the
lambda Orionis star-forming region. We statistically remove the field stars and
identify a representative PMS population throughout the interior of the
molecular ring. The spatial distribution of this population shows a
concentration of PMS stars around lambda Ori and in front of the B35 dark
cloud. Few PMS stars are found outside these pockets of high stellar density,
suggesting that star formation was concentrated in an elongated cloud extending
from B35 through lambda Ori to the B30 cloud. We find a lower limit for the
global stellar mass of about 500 Mo. We find that the global ratio of low- to
high-mass stars is similar to that predicted by the field initial mass
function, but this ratio varies strongly as a function of position in the
star-forming region. Locally, the star-formation process does not produce a
universal initial mass function. We construct a history of the star-forming
complex. This history incorporates a recent supernova to explain the
distribution of stars and gas today.Comment: 42 pages, 11 figures; to appear in the Astronomical Journa
Donner corps aux interactions (l'interaction enfin concrétisée)
Depuis plusieurs années, nous utilisons IODA comme méthode de description et de réalisation de simulations multi-agents. Cette méthode a pour originalité de concrétiser les interactions de maniÚre à ce qu'elles soient génériques et réutilisables dans différents contextes. Cet article a pour objectif d'identifier les problÚmes durs dans ce type de simulation et de montrer comment IODA apporte une aide à leur résolution. Since several years, we use in our team the IODA methodology to describe and realize multi-agent simulations. This method is original and not similar to the others because it makes Interactions becoming concrete and then able to become sufficiently general to be reused in many contexts. The aim of this paper is to identify some hard problems in this kind of simulation and to show how IODA can be helful to solve them
The nanoscale phase separation in hole-doped manganites
A macroscopic phase separation, in which ferromagnetic clusters are observed
in an insulating matrix, is sometimes observed, and believed to be essential to
the colossal magnetoresistive (CMR) properties of manganese oxides. The
application of a magnetic field may indeed trigger large magnetoresistance
effects due to the percolation between clusters allowing the movement of the
charge carriers. However, this macroscopic phase separation is mainly related
to extrinsic defects or impurities, which hinder the long-ranged charge-orbital
order of the system. We show in the present article that rather than the
macroscopic phase separation, an homogeneous short-ranged charge-orbital order
accompanied by a spin glass state occurs, as an intrinsic result of the
uniformity of the random potential perturbation induced by the solid solution
of the cations on the -sites of the structure of these materials. Hence the
phase separation does occur, but in a more subtle and interesting nanoscopic
form, here referred as ``homogeneous''. Remarkably, this ``nanoscale phase
separation'' alone is able to bring forth the colossal magnetoresistance in the
perovskite manganites, and is potentially relevant to a wide variety of other
magnetic and/or electrical properties of manganites, as well as many other
transition metal oxides, in bulk or thin film form as we exemplify throughout
the article.Comment: jpsj2 TeX style (J. Phys. Soc. Jpn); 18 pages, 7 figure
Symmetry Breaking of Relativistic Multiconfiguration Methods in the Nonrelativistic Limit
The multiconfiguration Dirac-Fock method allows to calculate the state of
relativistic electrons in atoms or molecules. This method has been known for a
long time to provide certain wrong predictions in the nonrelativistic limit. We
study in full mathematical details the nonlinear model obtained in the
nonrelativistic limit for Be-like atoms. We show that the method with sp+pd
configurations in the J=1 sector leads to a symmetry breaking phenomenon in the
sense that the ground state is never an eigenvector of L^2 or S^2. We thereby
complement and clarify some previous studies.Comment: Final version, to appear in Nonlinearity. Nonlinearity (2010) in
pres
Strategies for the prevention of autoimmune Type 1 diabetes
European experts on autoimmune Type 1 diabetes met for 2 days in October 2010 in Cambridge, to review the state-of-the-art and to discuss strategies for prevention of Type 1 diabetes (http://www-gene.cimr.cam.ac.uk/todd/sub_pages/T1D_prevention_Cambridge_workshop_20_21Oct2010.pdf). Meeting sessions examined the epidemiology of Type 1 diabetes; possible underlying causes of the continuing and rapid increase in Type 1 diabetes incidence at younger ages; and lessons learned from previous prevention trials. Consensus recommendations from the meeting were
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