8,025 research outputs found
Coupled valence and spin state transition in (Pr0.7Sm0.3)0.7Ca0.3CoO3
The coupled valence and spin state transition (VSST) taking place in
(Pr0.7Sm0.3)0.7Ca0.3CoO3 was investigated by soft x-ray absorption spectroscopy
(XAS) experiments carried out at the Pr-M4,5, Co-L2,3, and O-1s edges. This
VSST is found to be composed of a sharp Pr/Co valence and Co spin state
transition centered at T*=89.3 K, followed by a smoother Co spin-state
evolution at higher temperatures. At T < T*, we found that the praseodymium
displays a mixed valence Pr3+/Pr4+ with about 0.13 Pr4+/f.u., while all the
Co3+ is in the low-spin (LS) state. At T around T*, the sharp valence
transition converts all the Pr4+ to Pr3+ with a corresponding Co3+ to Co4+
compensation. This is accompanied by an equally sharp spin state transition of
the Co3+ from the low to an incoherent mixture of low and high spin (HS)
states. An involvement of the intermediate spin (IS) state can be discarded for
the Co3+. While above T* and at high temperatures the system shares rather
similar properties as Sr-doped LaCoO3, at low temperatures it behaves much more
like EuCoO3 with its highly stable LS configuration for the Co3+. Apparently,
the mechanism responsible for the formation of Pr4+ at low temperatures also
helps to stabilize the Co3+ in the LS configuration despite the presence of
Co4+ ions. We also found out that that the Co4+ is in an IS state over the
entire temperature range investigated in this study (10-290 K). The presence of
Co3+ HS and Co4+ IS at elevated temperatures facilitates the conductivity of
the material.Comment: 19 pages, 7 figures, Accepted in PR
First-order magnetic and structural phase transitions in FeSeTe
We use bulk magnetic susceptibility, electronic specific heat, and neutron
scattering to study structural and magnetic phase transitions in FeSe%
Te. FeTe exhibits a first order phase transition near 67
K with a tetragonal to monoclinic structural transition and simultaneously
develops a collinear antiferromagnetic (AF) order responsible for the entropy
change across the transition. Systematic studies of FeSeTe system
reveal that the AF structure and lattice distortion in these materials are
different from those of FeAs-based pnictides. These results call into question
the conclusions of present density functional calculations, where
FeSeTe and FeAs-based pnictides are expected to have similar Fermi
surfaces and therefore the same spin-density-wave AF order.Comment: 5 pages, 3 figure
Supernova neutrinos in the light of FCNC
We study the effect of including flavor changing neutral currents (FCNC) in
the analysis of the neutrino signal of a supernova burst. When we include the
effect of the FCNC which are beyond the standard model (SM) in the study of the
MSW resonant conversion, we obtain dramatic changes in the \Delta
m^2-sin^2(2\theta) probability contours for neutrino detection.Comment: 8 pages in ReVTeX,3 figures. Revised manuscript submitted to Phys.
Rev.
A chamber experiment for the feasibility study of an artificial plasma reflector for OTH radar applications
The feasibility of using two intersecting beams for plasma generation in the upper atmosphere as an over-the-horizon radar reflector was investigated. A cube was filled with dry air to a pressure corresponding to the simulated altitude, and two components of a split microwave beam were fed into the cube at right angles. Plasma layers were generated where the two beams intersected. Three critical issues were addressed: (1) reflectivity of the generated plasma layers; (2) propagation of high power microwave pulses; and (3) lifetime of the plasma
Probing for cosmological parameters with LAMOST measurement
In this paper we study the sensitivity of the Large Sky Area Multi-Object
Fiber Spectroscopic Telescope (LAMOST) project to the determination of
cosmological parameters, employing the Monte Carlo Markov Chains (MCMC) method.
For comparison, we first analyze the constraints on cosmological parameters
from current observational data, including WMAP, SDSS and SN Ia. We then
simulate the 3D matter power spectrum data expected from LAMOST, together with
the simulated CMB data for PLANCK and the SN Ia from 5-year Supernovae Legacy
Survey (SNLS). With the simulated data, we investigate the future improvement
on cosmological parameter constraints, emphasizing the role of LAMOST. Our
results show the potential of LAMOST in probing for the cosmological
parameters, especially in constraining the equation-of-state (EoS) of the dark
energy and the neutrino mass.Comment: 7 pages and 3 figures. Replaced with version accepted for publication
in JCA
Thin deformable mirrors for a reconfigurable space telescope
As part of a small satellite technology demonstration that will utilize autonomous assembly, reconfiguration, and docking technology to form the primary mirror for the mission’s telescope payload, the mirror segments are required to modify and control their shape, in order to allow for imaging in different configurations. This paper focuses on the development of 10 cm diameter active lightweight mirrors. The current mirror design, control scheme, and fabrication methods are described, as well as experimental results on initial samples. The data demonstrates that the mirrors are capable of at least 100 microns of displacement during operation, and that fabrication on polished molds can result in high quality reflective surfaces
The hidden sterile neutrino and the (2+2) sum rule
We discuss oscillations of atmospheric and solar neutrinos into sterile
neutrinos in the 2+2 scheme. A zeroth order sum rule requires equal
probabilities for oscillation into nu_s and nu_tau in the solar+atmospheric
data sample. Data does not favor this claim. Here we use scatter plots to
assess corrections of the zeroth order sum rule when (i) the 4 x 4 neutrino
mixing matrix assumes its full range of allowed values, and (ii) matter effects
are included. We also introduce a related "product rule". We find that the sum
rule is significantly relaxed, due to both the inclusion of the small mixing
angles (which provide a short-baseline contribution) and to matter effects. The
product rule is also dramatically altered. The observed relaxation of the sum
rule weakens the case against the 2+2 model and the sterile neutrino. To
invalidate the 2+2 model, a global fit to data with the small mixing angles
included seems to be required.Comment: 43 pages, 11 figures (same as v2, accidental replacement
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