2,951 research outputs found
Distribution of parallel vortices studied by spin-polarized neutron reflectivity and magnetization
We present the studies of non-uniformly distributed vortices in Nb/Al
multilayers at applied field near parallel to film surface by using
spin-polarized neutron reflectivity (SPNR) and DC magnetization measurements.
We have observed peaks above the lower critical field, Hc1, in the M-H curves
from the multilayers.
Previous works with a model calculation of minimizing Gibbs free energy have
suggested that the peaks could be ascribed to vortex line transitions for
spatial commensuration in a thin film superconductor. In order to directly
determine the distribution of vortices, we performed SPNR measurements on the
multilayer and found that the distribution and density of vortices are
different at ascending and descending fields. At ascending 2000 Oe which is
just below the first peak in the M-H curve, SPNR shows that vortices are mostly
localized near a middle line of the film meanwhile the vortices are distributed
in broader region at the descending 2000 Oe. That is related to the observation
of more vortices trapped at the descending field. As the applied field is
sightly tilted (< 3.5degree), we observe another peak at a smaller field. The
peak position is consistent with the parallel lower critical field (Hc1||). We
discuss that the vortices run along the applied field below Hc1|| and rotate
parallel to the surface at Hc1||.Comment: 17 pages, 9 figure
The extraordinary Hall effect in coherent epitaxial tau (Mn,Ni)Al thin films on GaAs
Ultrathin coherent epitaxial films of ferromagnetic tau(Mn,Ni)0.60Al0.40 have been grown by molecular beam epitaxy on GaAs substrates. X-ray scattering and cross-sectional transmission electron microscopy measurements confirm that the c axis of the tetragonal tau unit cell is aligned normal to the (001) GaAs substrate. Measurements of the extraordinary Hall effect (EHE) show that the films are perpendicularly magnetized, exhibiting EHE resistivities saturating in the range of 3.3-7.1 muOMEGA-cm at room temperature. These values of EHE resistivity correspond to signals as large as +7 and -7 mV for the two magnetic states of the film with a measurement current of 1 mA. Switching between the two magnetic states is found to occur at distinct field values that depend on the previously applied maximum field. These observations suggest that the films are magnetically uniform. As such, tau(Mn,Ni)Al films may be an excellent medium for high-density storage of binary information
Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars
(abridged) AIMS. We investigate the dynamics and stability of post-shock
plasma streaming along nonuniform stellar magnetic fields at the impact region
of accretion columns. We study how the magnetic field configuration and
strength determine the structure, geometry, and location of the shock-heated
plasma. METHODS. We model the impact of an accretion stream onto the
chromosphere of a CTTS by 2D axisymmetric magnetohydrodynamic simulations. Our
model takes into account the gravity, the radiative cooling, and the
magnetic-field-oriented thermal conduction. RESULTS. The structure, stability,
and location of the shocked plasma strongly depend on the configuration and
strength of the magnetic field. For weak magnetic fields, a large component of
B may develop perpendicular to the stream at the base of the accretion column,
limiting the sinking of the shocked plasma into the chromosphere. An envelope
of dense and cold chromospheric material may also develop around the shocked
column. For strong magnetic fields, the field configuration determines the
position of the shock and its stand-off height. If the field is strongly
tapered close to the chromosphere, an oblique shock may form well above the
stellar surface. In general, a nonuniform magnetic field makes the distribution
of emission measure vs. temperature of the shocked plasma lower than in the
case of uniform magnetic field. CONCLUSIONS. The initial strength and
configuration of the magnetic field in the impact region of the stream are
expected to influence the chromospheric absorption and, therefore, the
observability of the shock-heated plasma in the X-ray band. The field strength
and configuration influence also the energy balance of the shocked plasma, its
emission measure at T > 1 MK being lower than expected for a uniform field. The
above effects contribute in underestimating the mass accretion rates derived in
the X-ray band.Comment: 11 pages, 11 Figures; accepted for publication on A&A. Version with
full resolution images can be found at
http://www.astropa.unipa.it/~orlando/PREPRINTS/sorlando_accretion_shocks.pd
Liquid Xenon Detectors for Positron Emission Tomography
PET is a functional imaging technique based on detection of annihilation
photons following beta decay producing positrons. In this paper, we present the
concept of a new PET system for preclinical applications consisting of a ring
of twelve time projection chambers filled with liquid xenon viewed by avalanche
photodiodes. Simultaneous measurement of ionization charge and scintillation
light leads to a significant improvement to spatial resolution, image quality,
and sensitivity. Simulated performance shows that an energy resolution of <10%
(FWHM) and a sensitivity of 15% are achievable. First tests with a prototype
TPC indicate position resolution <1 mm (FWHM).Comment: Paper presented at the International Nuclear Physics Conference,
Vancouver, Canada, 201
Brains of verbal memory specialists show anatomical differences in language, memory and visual systems
Abstract We studied a group of verbal memory specialists to determine whether intensive oral text memory is associated with structural features of hippocampal and lateral-temporal regions implicated in language processing. Professional Vedic Sanskrit Pandits in India train from childhood for around 10 years in an ancient, formalized tradition of oral Sanskrit text memorization and recitation, mastering the exact pronunciation and invariant content of multiple 40,000–100,000 word oral texts. We conducted structural analysis of gray matter density, cortical thickness, local gyrification, and white matter structure, relative to matched controls. We found massive gray matter density and cortical thickness increases in Pandit brains in language, memory and visual systems, including i ) bilateral lateral temporal cortices and ii ) the anterior cingulate cortex and the hippocampus, regions associated with long and short-term memory. Differences in hippocampal morphometry matched those previously documented for expert spatial navigators and individuals with good verbal working memory. The findings provide unique insight into the brain organization implementing formalized oral knowledge systems
H spectroscopy of the high-inclination black hole transient Swift J1357.2-0933 during quiescence
Swift J1357.2-0933 is a transient low-mass X-ray binary hosting a
stellar-mass black hole. The source exhibits optical dips and very broad
emission lines during both outburst and quiescence, which are thought to be the
result of a high orbital inclination. We present phase-resolved spectroscopy
obtained with the 10.4m Gran Telescopio Canarias (GTC). The spectra focus on
the spectral region during X-ray quiescence. The emission line is
exceptionally broad (full width at half maximum, FWHM > 4000 \AA), in agreement
with previous studies focused on . A two-Gaussian fit to the
prominent double-peaked profile reveals a periodic variability in the centroid
position of the line. We also produced a diagnostic diagram aimed at
constraining additional orbital parameters. Together, they allow us to
independently confirm the orbital period of the system using a new dataset
obtained five years after the previous outburst. However, our estimates for
both the systemic velocity and the radial velocity semi-amplitude of the black
hole reveal larger values than those found in previous studies. We argue that
this could be explained by the precession of the disc and the presence of a
hotspot. We found evidence of a narrow inner core in the double-peaked H
emission profile. We studied its evolution across the orbit, finding that it is
likely to result from the occultation of inner material by the outer rim bulge,
further supporting the high orbital inclination hypothesis.Comment: 6 pages, 4 figure
Evidence for Distinct Components of the Galactic Stellar Halo from 838 RR Lyrae Stars Discovered in the LONEOS-I Survey
We present 838 ab-type RR Lyrae stars from the Lowell Observatory Near Earth
Objects Survey Phase I (LONEOS-I). These objects cover 1430 deg^2 and span
distances ranging from 3-30 kpc from the Galactic Center. Object selection is
based on phased, photometric data with 28-50 epochs. We use this large sample
to explore the bulk properties of the stellar halo, including the spatial
distribution. The period-amplitude distribution of this sample shows that the
majority of these RR Lyrae stars resemble Oosterhoff type I, but there is a
significant fraction (26 %) which have longer periods and appear to be
Oosterhoff type II. We find that the radial distributions of these two
populations have significantly different profiles (rho_{OoI} ~ R^(-2.26 +-
0.07) and rho_{OoII} ~ R^(-2.88 +- 0.11). This suggests that the stellar halo
was formed by at least two distinct accretion processes and supports dual-halo
models.Comment: 18 pages, 28 figures, apjemulated, minor corrections and
clarifications. Accepted to ApJ on Jan 21, 200
High-resolution Compton spectroscopy using X-ray microcalorimeters
X-ray Compton spectroscopy is one of the few direct probes of the electron
momentum distribution of bulk materials in ambient and operando environments.
We report high-resolution inelastic X-ray scattering experiments with high
momentum and energy transfer performed at a storage-ring-based high-energy
X-ray light source facility using an X-ray microcalorimeter detector. Compton
profiles were measured for lithium and cobalt oxide powders relevant to
lithium-ion battery research. Spectroscopic analysis of the measured Compton
profiles shows high-sensitivity to the low-Z elements and oxidation states. The
lineshape analysis of the measured Compton profiles in comparison with computed
Hartree-Fock profiles is limited by the resolution of the energy-resolving
semiconductor detector. We have characterized an X-ray transition-edge sensor
microcalorimeter detector for high-resolution Compton scattering experiments
using a bending magnet source at the Advanced Photon Source (APS) with a double
crystal monochromator providing monochromatic photon energies near 27.5 keV.
The momentum resolution below 0.16 atomic units was measured yielding an
improvement of more than a factor of 7 over a state-of-the-art silicon drift
detector for the same scattering geometry. Furthermore, the lineshapes of
narrow valence and broad core electron profiles of sealed lithium metal were
clearly resolved using an X-ray microcalorimeter detector compared to smeared
and broadened lineshapes observed when using a silicon drift detector.
High-resolution Compton scattering using the energy-resolving detector shown
here presents new opportunities for spatial imaging of electron momentum
distributions for a wide class of materials with applications ranging from
electrochemistry to condensed matter physics.Comment: The following article has been submitted to Applied Physics Letter
Evidence for past interaction with an asymmetric circumstellar shell in the young SNR Cassiopeia A
Context. Observations of the supernova remnant (SNR) Cassiopeia A (Cas A) show significant asymmetries in the reverse shock that cannot be explained by models describing a remnant expanding through a spherically symmetric wind of the progenitor star.Aims. We investigate whether a past interaction of Cas A with a massive asymmetric shell of the circumstellar medium can account for the observed asymmetries of the reverse shock.Methods. We performed three-dimensional (3D) (magneto)-hydrodynamic simulations that describe the remnant evolution from the SN explosion to its interaction with a massive circumstellar shell. The initial conditions (soon after the shock breakout at the stellar surface) are provided by a 3D neutrino-driven SN model whose morphology closely resembles Cas A and the SNR simulations cover approximate to 2000 yr of evolution. We explored the parameter space of the shell, searching for a set of parameters able to produce an inward-moving reverse shock in the western hemisphere of the remnant at the age of approximate to 350 yr, analogous to that observed in Cas A.Results. The interaction of the remnant with the shell can produce asymmetries resembling those observed in the reverse shock if the shell was asymmetric with the densest portion in the (blueshifted) nearside to the northwest (NW). According to our favorite model, the shell was thin (thickness sigma approximate to 0.02 pc) with a radius r(sh) approximate to 1.5 pc from the center of the explosion. The reverse shock shows the following asymmetries at the age of Cas A: (i) it moves inward in the observer frame in the NW region, while it moves outward in most other regions; (ii) the geometric center of the reverse shock is offset to the NW by approximate to 0.1 pc from the geometric center of the forward shock; and (iii) the reverse shock in the NW region has enhanced nonthermal emission because, there, the ejecta enter the reverse shock with a higher relative velocity (between 4000 and 7000 km s(-1)) than in other regions (below 2000 km s(-1)).Conclusions. The large-scale asymmetries observed in the reverse shock of Cas A can be interpreted as signatures of the interaction of the remnant with an asymmetric dense circumstellar shell that occurred between approximate to 180 and approximate to 240 yr after the SN event. We suggest that the shell was, most likely, the result of a massive eruption from the progenitor star that occurred between 10(4) and 10(5) yr prior to core-collapse. We estimate a total mass of the shell of the order of 2 M-circle dot
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