728 research outputs found
A method for interactive satellite failure diagnosis: Towards a connectionist solution
Various kinds of processes which allow one to make a diagnosis are analyzed. The analyses then focuses on one of these processes used for satellite failure diagnosis. This process consists of sending the satellite instructions about system status alterations: to mask the effects of one possible component failure or to look for additional abnormal measures. A formal model of this process is given. This model is an extension of a previously defined connectionist model which allows computation of ratios between the likelihoods of observed manifestations according to various diagnostic hypotheses. The expected mean value of these likelihood measures for each possible status of the satellite can be computed in a similar way. Therefore, it is possible to select the most appropriate status according to three different purposes: to confirm an hypothesis, to eliminate an hypothesis, or to choose between two hypotheses. Finally, a first connectionist schema of computation of these expected mean values is given
Two spatially separated phases in semiconducting RbFeS
We report neutron scattering and transport measurements on semiconducting
RbFeS, a compound isostructural and isoelectronic to the
well-studied FeSe K, Rb, Cs, Tl/K) superconducting
systems. Both resistivity and DC susceptibility measurements reveal a magnetic
phase transition at K. Neutron diffraction studies show that the 275 K
transition originates from a phase with rhombic iron vacancy order which
exhibits an in-plane stripe antiferromagnetic ordering below 275 K. In
addition, interdigitated mesoscopically with the rhombic phase is an ubiquitous
phase with iron vacancy order. This phase has a
magnetic transition at K and an iron vacancy order-disorder
transition at K. These two different structural phases are closely
similar to those observed in the isomorphous Se materials. Based on the close
similarities of the in-plane antiferromagnetic structures, moments sizes, and
ordering temperatures in semiconducting RbFeS and
KFeSe, we argue that the in-plane antiferromagnetic order
arises from strong coupling between local moments. Superconductivity,
previously observed in the FeSeS system, is absent
in RbFeS, which has a semiconducting ground state. The
implied relationship between stripe/block antiferromagnetism and
superconductivity in these materials as well as a strategy for further
investigation is discussed in this paper.Comment: 7 pages, 5 figure
Universal magnetic and structural behaviors in the iron arsenides
Commonalities among the order parameters of the ubiquitous antiferromagnetism
present in the parent compounds of the iron arsenide high temperature
superconductors are explored. Additionally, comparison is made between the well
established two-dimensional Heisenberg-Ising magnet, KNiF and iron
arsenide systems residing at a critical point whose structural and magnetic
phase transitions coincide. In particular, analysis is presented regarding two
distinct classes of phase transition behavior reflected in the development of
antiferromagnetic and structural order in the three main classes of iron
arsenide superconductors. Two distinct universality classes are mirrored in
their magnetic phase transitions which empirically are determined by the
proximity of the coupled structural and magnetic phase transitions in these
materials.Comment: 6 pages, 4 figure
Modification of Charge Trapping at Particle/Particle Interfaces by Electrochemical Hydrogen Doping of Nanocrystalline TiO2
Particle/particle interfaces play a crucial role in the functionality and performance of nanocrystalline materials such as mesoporous metal oxide electrodes. Defects at these interfaces are known to impede charge separation via slow-down of transport and increase of charge recombination, but can be passivated via electrochemical doping (i.e., incorporation of electron/proton pairs), leading to transient but large enhancement of photoelectrode performance. Although this process is technologically very relevant, it is still poorly understood. Here we report on the electrochemical characterization and the theoretical modeling of electron traps in nanocrystalline rutile TiO2 films. Significant changes in the electrochemical response of porous films consisting of a random network of TiO2 particles are observed upon the electrochemical accumulation of electron/proton pairs. The reversible shift of a capacitive peak in the voltammetric profile of the electrode is assigned to an energetic modification of trap states at particle/particle interfaces. This hypothesis is supported by first-principles theoretical calculations on a TiO2 grain boundary, providing a simple model for particle/particle interfaces. In particular, it is shown how protons readily segregate to the grain boundary (being up to 0.6 eV more stable than in the TiO2 bulk), modifying its structure and electron-trapping properties. The presence of hydrogen at the grain boundary increases the average depth of traps while at the same time reducing their number compared to the undoped situation. This provides an explanation for the transient enhancement of the photoelectrocatalytic activity toward methanol photooxidation which is observed following electrochemical hydrogen doping of rutile TiO2 nanoparticle electrodes
Antiferromagnetic Critical Fluctuations in BaFeAs
Magnetic correlations near the magneto-structural phase transition in the
bilayer iron pnictide parent compound, BaFeAs, are measured. In close
proximity to the antiferromagnetic phase transition in BaFeAs, a
crossover to three dimensional critical behavior is anticipated and has been
preliminarily observed. Here we report complementary measurements of
two-dimensional magnetic fluctuations over a broad temperature range about
T. The potential role of two-dimensional critical fluctuations in the
magnetic phase behavior of BaFeAs and their evolution near the
anticipated crossover to three dimensional critical behavior and long-range
order are discussed.Comment: 6 pages, 4 figures; Accepted for publication in Physical Review
Magnetic order tuned by Cu substitution in Fe1.1-zCuzTe
We study the effects of Cu substitution in Fe1.1Te, the non-superconducting
parent compound of the iron-based superconductor, Fe1+yTe1-xSex, utilizing
neutron scattering techniques. It is found that the structural and magnetic
transitions, which occur at \sim 60 K without Cu, are monotonically depressed
with increasing Cu content. By 10% Cu for Fe, the structural transition is
hardly detectable, and the system becomes a spin glass below 22 K, with a
slightly incommensurate ordering wave vector of (0.5-d, 0, 0.5) with d being
the incommensurability of 0.02, and correlation length of 12 angstrom along the
a axis and 9 angstrom along the c axis. With 4% Cu, both transition
temperatures are at 41 K, though short-range incommensurate order at (0.42, 0,
0.5) is present at 60 K. With further cooling, the incommensurability decreases
linearly with temperature down to 37 K, below which there is a first order
transition to a long-range almost-commensurate antiferromagnetic structure. A
spin anisotropy gap of 4.5 meV is also observed in this compound. Our results
show that the weakly magnetic Cu has large effects on the magnetic
correlations; it is suggested that this is caused by the frustration of the
exchange interactions between the coupled Fe spins.Comment: 7 pages, 7 figures, version as appeared on PR
Heat capacity study of BaFeAs: effects of annealing
Heat-capacity, X-ray diffraction, and resistivity measurements on a
high-quality BaFeAs sample show an evolution of the
magneto-structural transition with successive annealing periods. After a 30-day
anneal the resistivity in the (ab) plane decreases by more than an order of
magnitude, to 12 cm, with a residual resistance ratio 36; the
heat-capacity anomaly at the transition sharpens, to an overall width of less
than K, and shifts from 135.4 to 140.2 K. The heat-capacity anomaly in both the
as-grown sample and after the 30-day anneal shows a hysteresis of 0.15 K,
and is unchanged in a magnetic field H = 14 T. The X-ray and
heat-capacity data combined suggest that there is a first order jump in the
structural order parameter. The entropy of the transition is reported
Experimental elucidation of the origin of the `double spin resonances' in Ba(FeCo)As
We report a combined study of the spin resonances and superconducting gaps
for underdoped ( K), optimally doped ( K), and overdoped
( K) Ba(FeCo)As single crystals with inelastic
neutron scattering and angle resolved photoemission spectroscopy. We find a
quasi two dimensional spin resonance whose energy scales with the
superconducting gap in all three compounds. In addition, anisotropic low energy
spin excitation enhancements in the superconducting state have been deduced and
characterized for the under and optimally doped compounds. Our data suggest
that the quasi two dimensional spin resonance is a spin exciton that
corresponds to the spin singlet-triplet excitations of the itinerant electrons.
However, the intensity enhancements of the anisotropic spin excitations are
dominated by the out-of-plane spin excitations of the ordered moments due to
the suppression of damping in the superconducting state. Hence we offer a new
interpretation of the double energy scales differing from previous
interpretations based on anisotropic superconducting energy gaps, and
systematically explain the doping-dependent trend across the phase diagram.Comment: 8 pages, 7 figures, 1 table. Accepted for publication on Physical
Review
Crystal growth and elasticity
The purpose of this paper is to review some elasticity effects in epitaxial
growth. We start by a description of the main ingredients needed to describe
elasticity effects (elastic interactions, surface stress, bulk and surface
elasticity, thermodynamics of stressed solids). Then we describe how bulk and
surface elasticity affect growth mode and surface morphology by means of
stress-driven instability. At last stress-strain evolution during crystal
growth is reported.Comment: 12 page
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