9,174 research outputs found
Autoignition test cell Patent
Test chamber for determining decomposition and autoignition of materials used in spacecraft under controlled environmental condition
Model-free reconstruction of magnetic correlations in frustrated magnets
Frustrated magnetic systems exhibit extraordinary physical properties but
quantification of their magnetic correlations poses a serious challenge to
experiment and theory. Current insight into frustrated magnetic correlations
relies on modelling techniques such as reverse Monte Carlo methods, which
require knowledge about the exact ordered atomic structure. Here we present a
method for direct reconstruction of magnetic correlations in frustrated magnets
by three-dimensional difference pair distribution function analysis of neutron
total scattering data. The methodology is applied to the disordered frustrated
magnet bixbyite, (Mn1-xFex)2O3, which reveals nearest-neighbor
antiferromagnetic correlations for the metal sites up to a range of
approximately 15 {\AA}. Importantly, this technique allows for magnetic
correlations to be determined directly from the experimental data without any
assumption about the atomic structure
Unusual Phase Transitions and Magnetoelastic Coupling in TlFe1.6Se2 Single Crystals
Structural, magnetic, electrical transport, and heat capacity data are
reported for single crystals of TlFe1.6Se2. This compound crystallizes in a
tetragonal structure similar to the ThCr2Si2 structure, but with vacancies in
the Fe layer. The vacancies can be ordered or disordered depending on
temperature and thermal history. If the vacancies are ordered, the basal plane
lattice constant increases from a to \sqrt{5}a. Antiferromagnetic order with
the Fe spins along the c-axis occurs below T_N ~ 430K as shown by single
crystal neutron diffraction and the magnetic structure is reported. In
addition, for the vacancy ordered crystal, two other phase transitions are
found at T_1 ~ 140K, and T_2 ~ 100K. The phase transitions at T_1 and T_2 are
evident in heat capacity, magnetic susceptibility, resistivity data, a and c
lattice parameters, and in the unusual temperature dependence of the magnetic
order parameter determined from neutron scattering. The phase transitions at
T_1 and T_2 result in significant changes in the magnetic moment per iron, with
1.72(6)\mu_B observed at 300K, 2.07(9)\mu_B at 140\,K, 1.90(9)\,\mu_B at
115\,K, and 1.31(8)\mu_B for 5\,K if the same "block checkerboard" magnetic
structure is used at all temperatures. The phase transitions appear to be
driven by small changes in the c lattice constant, large magnetoelastic
coupling, and the localization of carriers with decreasing temperature.Comment: Accepted for publication in Physical Review
Explaining the entropy excess in clusters and groups of galaxies without additional heating
The X-ray luminosity and temperature of clusters and groups of galaxies do
not scale in a self-similar manner. This has often been interpreted as a sign
that the intracluster medium has been substantially heated by non-gravitational
sources. In this paper, we propose a simple model which, instead, uses the
properties of galaxy formation to explain the observations. Drawing on
available observations, we show that there is evidence that the efficiency of
galaxy formation was higher in groups than in clusters. If confirmed, this
would deplete the low-entropy gas in groups, increase their central entropy and
decrease their X-ray luminosity. A simple, empirical, hydrostatic model appears
to match both the luminosity-temperature relation of clusters and properties of
their internal structure as well.Comment: 5 pages, 4 figures, accepted in ApJL; added one reference, otherwise
unchange
Vortices in Thin, Compressible, Unmagnetized Disks
We consider the formation and evolution of vortices in a hydrodynamic
shearing-sheet model. The evolution is done numerically using a version of the
ZEUS code. Consistent with earlier results, an injected vorticity field evolves
into a set of long-lived vortices, each of which has a radial extent comparable
to the local scale height. But we also find that the resulting velocity field
has a positive shear stress, . This effect appears
only at high resolution. The transport, which decays with time as t^-1/2,
arises primarily because the vortices drive compressive motions. This result
suggests a possible mechanism for angular momentum transport in low-ionization
disks, with two important caveats: a mechanism must be found to inject
vorticity into the disk, and the vortices must not decay rapidly due to
three-dimensional instabilities.Comment: 8 pages, 10 figures (high resolution figures available in ApJ
electronic edition
Multidisciplinary research from ERTS-1 data
There are no author-identified significant results in this report
Use of ERTS-1 data: Summary report of work on ten tasks
The author has identified the following significant results. Depth mapping's for a portion of Lake Michigan and at the Little Bahama Bank test site have been verified by use of navigation charts and on-site visits. A thirteen category recognition map of Yellowstone Park has been prepared. Model calculation of atmospheric effects for various altitudes have been prepared. Radar, SLAR, and ERTS-1 data for flooded areas of Monroe County, Michigan are being studied. Water bodies can be reliably recognized and mapped using maximum likelihood processing of ERTS-1 digital data. Wetland mapping has been accomplished by slicing of single band and/or ratio processing of two bands for a single observation date. Both analog and digital processing have been used to map the Lake Ontario basin using ERTS-1 data. Operating characteristic curves were developed for the proportion estimation algorithm to determine its performance in the measurement of surface water area. The signal in band MSS-5 was related to sediment content of waters by modelling approach and by relating surface measurements of water to processed ERTS data. Radiance anomalies in ERTS-1 data could be associated with the presence of oil on water in San Francisco Bay, but the anomalies were of the same order as those caused by variations in sediment concentration and tidal flushing
Spin reorientation in TlFe1.6Se2 with complete vacancy ordering
The relationship between vacancy ordering and magnetism in TlFe1.6Se2 has
been investigated via single crystal neutron diffraction, nuclear forward
scattering, and transmission electron microscopy. The examination of chemically
and structurally homogenous crystals allows the true ground state to be
revealed, which is characterized by Fe moments lying in the ab-plane below
100K. This is in sharp contrast to crystals containing regions of order and
disorder, where a competition between c-axis and ab-plane orientations of the
moments is observed. The properties of partially-disordered TlFe1.6Se2 are
therefore not associated with solely the ordered or disordered regions. This
contrasts the viewpoint that phase separation results in independent physical
properties in intercalated iron selenides, suggesting a coupling between
ordered and disordered regions may play an important role in the
superconducting analogues.Comment: Minor changes; updated references and funding acknowledgemen
- …