631 research outputs found
Magnetic properties of the double perovskites LaPbMSbO6 (M = Mn, Co and Ni)
New double perovskites LaPbMSbO6, where M2+ = Mn2+, Co2+, and Ni2+, were
synthesized as polycrystals by an aqueous synthetic route at temperatures below
1000 oC. All samples are monoclinic, space group P21/n, as obtained from
Rietveld analysis of X-ray powder diffraction patterns. The distribution of M2+
and Sb5+ among the two octahedral sites have 3% of disorder for M2+ = Ni2+,
whereas for M2+ = Mn2+ and Co2+ less disorder is found. The three samples have
an antiferromagnetic transition, due to the antiferromagnetic coupling between
M2+ through super-superexchange paths M2+ - O2- - Sb5+ - O2- - M2+. Transition
temperatures are low: 8, 10 and 17 K for Mn2+, Co2+, and Ni2+ respectively, as
a consequence of the relatively long distances between the magnetic ions M2+.
Besides, for LaPbMnSbO6 a small transition at 45 K was found, with
ferrimagnetic characteristics, possibly as a consequence of a small disorder
between Mn2+ and Sb5+. This disorder would give additional and shorter
interaction paths: superexchange Mn2+ - O2- - Mn2+.Comment: 4 pages, 4 figures included. Manuscript submitted to IEEE
Transactions on Magnetics, proceedings of the LAW3M 2013 conferenc
Testing common classical LTE and NLTE model atmosphere and line-formation codes for quantitative spectroscopy of early-type stars
It is generally accepted that the atmospheres of cool/lukewarm stars of
spectral types A and later are described well by LTE model atmospheres, while
the O-type stars require a detailed treatment of NLTE effects. Here model
atmosphere structures, spectral energy distributions and synthetic spectra
computed with ATLAS9/SYNTHE and TLUSTY/SYNSPEC, and results from a hybrid
method combining LTE atmospheres and NLTE line-formation with DETAIL/SURFACE
are compared. Their ability to reproduce observations for effective
temperatures between 15000 and 35000 K are verified. Strengths and weaknesses
of the different approaches are identified. Recommendations are made as to how
to improve the models in order to derive unbiased stellar parameters and
chemical abundances in future applications, with special emphasis on Gaia
science.Comment: 12 pages, 8 figures; accepted for publication in Journal of Physics:
Conference Series, GREAT-ESF Workshop: Stellar Atmospheres in the Gaia Er
Tailoring the ground state of the ferrimagnet La2Ni(Ni1/3Sb2/3)O6
We report on the magnetic and structural properties of La2Ni(Ni1/3Sb2/3)O6 in
polycrystal, single crystal and thin film samples. We found that this material
is a ferrimagnet (Tc ~ 100 K) which possesses a very distinctive and uncommon
feature in its virgin curve of the hysteresis loops. We observe that bellow 20
K it lies outside the hysteresis cycle, and this feature was found to be an
indication of a microscopically irreversible process possibly involving the
interplay of competing antiferromagnetic interactions that hinder the initial
movement of domain walls. This initial magnetic state is overcome by applying a
temperature dependent characteristic field. Above this field, an isothermal
magnetic demagnetization of the samples yield a ground state different from the
initial thermally demagnetized one.Comment: 21 pages, 8 figures, submitted to JMM
Disorder-induced double resonant Raman process in graphene
An analytical study is presented of the double resonant Raman scattering
process in graphene, responsible for the D and D features in the
Raman spectra. This work yields analytical expressions for the D and
D integrated Raman intensities that explicitly show the dependencies
on laser energy, defect concentration, and electronic lifetime. Good agreement
is obtained between the analytical results and experimental measurements on
samples with increasing defect concentrations and at various laser excitation
energies. The use of Raman spectroscopy to identify the nature of defects is
discussed. Comparison between the models for the edge-induced and the
disorder-induced D band intensity suggests that edges or grain boundaries can
be distinguished from disorder by the different dependence of their Raman
intensity on laser excitation energy. Similarly, the type of disorder can
potentially be identified not only by the intensity ratio
, but also by its laser energy
dependence. Also discussed is a quantitative analysis of quantum interference
effects of the graphene wavefunctions, which determine the most important
phonon wavevectors and scattering processes responsible for the D and
D bands.Comment: 10 pages, 4 figure
HVS7: a chemically peculiar hyper-velocity star
Context: Hyper-velocity stars are suggested to originate from the dynamical
interaction of binary stars with the supermassive black hole in the Galactic
centre (GC), which accelerates one component of the binary to beyond the
Galactic escape velocity. Aims: The evolutionary status and GC origin of the
HVS SDSS J113312.12+010824.9 (HVS7) is constrained from a detailed study of its
stellar parameters and chemical composition. Methods: High-resolution spectra
of HVS7 obtained with UVES on the ESO VLT were analysed using state-of-the-art
NLTE/LTE modelling techniques that can account for a chemically-peculiar
composition via opacity sampling. Results: Instead of the expected slight
enrichments of alpha-elements and near-solar Fe, huge chemical peculiarities of
all elements are apparent. The He abundance is very low (<1/100 solar), C, N
and O are below the detection limit, i.e they are underabundant (<1/100, <1/3
and <1/10 solar). Heavier elements, however, are overabundant: the iron group
by a factor of ~10, P, Co and Cl by factors ~40, 80 and 440 and rare-earth
elements and Hg even by ~10000. An additional finding, relevant also for other
chemically peculiar stars are the large NLTE effects on abundances of TiII and
FeII (~0.6-0.7dex). The derived abundance pattern of HVS7 is characteristic for
the class of chemical peculiar magnetic B stars on the main sequence. The
chemical composition and high vsini=55+-2km/s render a low mass nature of HVS7
as a blue horizontal branch star unlikely. Conclusions: Such a surface
abundance pattern is caused by atomic diffusion in a possibly magnetically
stabilised, non-convective atmosphere. Hence all chemical information on the
star's place of birth and its evolution has been washed out. High precision
astrometry is the only means to validate a GC origin for HVS7.Comment: 9 pages, 3 figure
Dynamical mass of the O-type supergiant in Zeta Orionis A
A close companion of Zeta Orionis A was found in 2000 with the Navy Precision
Optical Interferometer (NPOI), and shown to be a physical companion. Because
the primary is a supergiant of type O, for which dynamical mass measurements
are very rare, the companion was observed with NPOI over the full 7-year orbit.
Our aim was to determine the dynamical mass of a supergiant that, due to the
physical separation of more than 10 AU between the components, cannot have
undergone mass exchange with the companion. The interferometric observations
allow measuring the relative positions of the binary components and their
relative brightness. The data collected over the full orbital period allows all
seven orbital elements to be determined. In addition to the interferometric
observations, we analyzed archival spectra obtained at the Calar Alto, Haute
Provence, Cerro Armazones, and La Silla observatories, as well as new spectra
obtained at the VLT on Cerro Paranal. In the high-resolution spectra we
identified a few lines that can be associated exclusively to one or the other
component for the measurement of the radial velocities of both. The combination
of astrometry and spectroscopy then yields the stellar masses and the distance
to the binary star. The resulting masses for components Aa of 14.0 solar masses
and Ab of 7.4 solar masses are low compared to theoretical expectations, with a
distance of 294 pc which is smaller than a photometric distance estimate of 387
pc based on the spectral type B0III of the B component. If the latter (because
it is also consistent with the distance to the Orion OB1 association) is
adopted, the mass of the secondary component Ab of 14 solar masses would agree
with classifying a star of type B0.5IV. It is fainter than the primary by about
2.2 magnitudes in the visual. The primary mass is then determined to be 33
solar masses
Anomalous Proximity Effect in Underdoped YBaCuO Josephson Junctions
Josephson junctions were photogenerated in underdoped thin films of the
YBaCuO family using a near-field scanning optical microscope.
The observation of the Josephson effect for separations as large as 100 nm
between two wires indicates the existence of an anomalously large proximity
effect and show that the underdoped insulating material in the gap of the
junction is readily perturbed into the superconducting state. The critical
current of the junctions was found to be consistent with the conventional
Josephson relationship. This result constrains the applicability of SO(5)
theory to explain the phase diagram of high critical temperature
superconductors.Comment: 11 pages, 4 figure
Pristine CNO abundances from Magellanic Cloud B stars II. Fast rotators in the LMC cluster NGC 2004
We present spectroscopic abundance analyses of three main-sequence B stars in
the young Large Magellanic Cloud cluster NGC 2004. All three targets have
projected rotational velocities around 130 km/s. Techniques are presented that
allow the derivation of stellar parameters and chemical abundances in spite of
these high v sin i values. Together with previous analyses of stars in this
cluster, we find no evidence among the main-sequence stars for effects due to
rotational mixing up to v sin i around 130 km/s. Unless the equatorial
rotational velocities are significantly larger than the v sin i values, this
finding is probably in line with theoretical expectations. NGC 2004/B30, a star
of uncertain evolutionary status located in the Blue Hertzsprung Gap, clearly
shows signs of mixing in its atmosphere. To verify the effects due to
rotational mixing will therefore require homogeneous analysis of statistically
significant samples of low-metallicity main-sequence B stars over a wide range
of rotational velocities.Comment: 12 pages, 5 figures, 2 tables; accepted for publication in ApJ (vol.
633, p. 899
Magnetic field geometry and chemical abundance distribution of the He-strong star CPD -57 3509
The magnetic field of CPD -57 3509 was recently detected in the framework of
the BOB (B fields in OB stars) collaboration. We acquired low-resolution
spectropolarimetric observations of CPD -57 3509 with FORS2 and high-resolution
UVES observations randomly distributed over a few months to search for
periodicity, to study the magnetic field geometry, and to determine the surface
distribution of silicon and helium. We also obtained supplementary photometric
observations at a timeline similar to the spectroscopic and spectropolarimetric
observations. A period of 6.36d was detected in the measurements of the mean
longitudinal magnetic field. A sinusoidal fit to our measurements allowed us to
constrain the magnetic field geometry and estimate the dipole strength in the
range of 3.9-4.5kG. Our application of the Doppler imaging technique revealed
the presence of He I spots located around the magnetic poles, with a strong
concentration at the positive pole and a weaker one around the negative pole.
In contrast, high concentration Si III spots are located close to the magnetic
equator. Further, our analysis of the spectral variability of CPD -57 3509 on
short time scales indicates distinct changes in shape and position of line
profiles possibly caused by the presence of beta Cep-like pulsations. A small
periodic variability in line with the changes of the magnetic field strength is
clearly seen in the photometric data.Comment: 11 pages, 5 tables, 7 figures, accepted for publication in MNRA
An asteroseismic study of the O9V star HD 46202 from CoRoT space-based photometry
The O9V star HD 46202, which is a member of the young open cluster NGC 2244,
was observed by the CoRoT satellite in October/November 2008 during a short run
of 34 days. From the very high-precision light curve, we clearly detect beta
Cep-like pulsation frequencies with amplitudes of ~0.1 mmag and below. A
comparison with stellar models was performed using a chi^2 as a measure for the
goodness-of-fit between the observed and theoretically computed frequencies.
The physical parameters of our best-fitting models are compatible with the ones
deduced spectroscopically. A core overshooting parameter alpha_ov = 0.10 +-
0.05 pressure scale height is required. None of the observed frequencies are
theoretically excited with the input physics used in our study. More
theoretical work is thus needed to overcome this shortcoming in how we
understand the excitation mechanism of pulsation modes in such a massive star.
A similar excitation problem has also been encountered for certain pulsation
modes in beta Cep stars recently modelled asteroseismically.Comment: Accepted for publication in Astronomy and Astrophysics on 17/12/2010,
9 pages, 7 figures, 4 table
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