4,789 research outputs found
Superconductivity in higher titanium oxides
Recent renewal of the highest transition temperature in a conventional
superconductor of the sulfer hydride attracts much attention to exploring
simple compounds with the lighter elements, situated in unconventional
conditions. We report the discovery of superconductivity in simple oxides of
Ti4O7 and g-Ti3O5 in a thin-film form having deliberately tuned epitaxial
structures and off-stoichiometry. These higher titanium oxides join in a class
of simple-oxide superconductors, and g-Ti3O5 now holds the highest
superconducting transition temperature of 7.1 kelvin among them. The mechanism
behind the superconductivity is discussed on the basis of electrical
measurements and theoretical predictions. We conclude that superconductivity
arises from unstabilized bipolaronic insulating states.Comment: 25 pages, 4 figures in main text, 14 pages, 11 figures in
supplemental informatio
A Mean-field Approach for an Intercarrier Interference Canceller for OFDM
The similarity of the mathematical description of random-field spin systems
to orthogonal frequency-division multiplexing (OFDM) scheme for wireless
communication is exploited in an intercarrier-interference (ICI) canceller used
in the demodulation of OFDM. The translational symmetry in the Fourier domain
generically concentrates the major contribution of ICI from each subcarrier in
the subcarrier's neighborhood. This observation in conjunction with mean field
approach leads to a development of an ICI canceller whose necessary cost of
computation scales linearly with respect to the number of subcarriers. It is
also shown that the dynamics of the mean-field canceller are well captured by a
discrete map of a single macroscopic variable, without taking the spatial and
time correlations of estimated variables into account.Comment: 7pages, 3figure
A comparison of measured and calculated thermal stresses in a hybrid metal matrix composite spar cap element
A hybrid spar of titanium with an integrally brazed composite, consisting of an aluminum matrix reinforced with boron-carbide-coated fibers, was heated in an oven and the resulting thermal stresses were measured. Uniform heating of the spar in an oven resulted in thermal stresses arising from the effects of dissimilar materials and anisotropy of the metal matrix composite. Thermal stresses were calculated from a finite element structural model using anisotropic material properties deduced from constituent properties and rules of mixtures. Comparisons of calculated thermal stresses with measured thermal stresses on the spar are presented. It was shown that failure to account for anisotropy in the metal matrix composite elements would result in large errors in correlating measured and calculated thermal stresses. It was concluded that very strong material characterization efforts are required to predict accurate thermal stresses in anisotropic composite structures
The co-pyrolysis of flame retarded high impact polystyrene and polyolefins
The co-pyrolysis of brominated high impact polystyrene (Br-HIPS) with polyolefins using a fixed bed reactor has been investigated, in particular, the effect that different types brominated aryl compounds and antimony trioxide have on the pyrolysis products. The pyrolysis products were analysed using FT-IR, GC-FID, GC-MS, and GC-ECD. Liquid chromatography was used to separate the oils/waxes so that a more detailed analysis of the aliphatic, aromatic, and polar fractions could be carried out. It was found that interaction occurs between Br-HIPS and polyolefins during co-pyrolysis and that the presence of antimony trioxide influences the pyrolysis mass balance. Analysis of the Br-HIPS + polyolefin co-pyrolysis products showed that the presence of polyolefins led to an increase in the concentration of alkyl and vinyl mono-substituted benzene rings in the pyrolysis oil/wax resulting from Br-HIPS pyrolysis. The presence of Br-HIPS also had an impact on the oil/wax products of polyolefin pyrolysis, particularly on the polyethylene oil/wax composition which converted from being a mixture of 1-alkenes and n-alkanes to mostly n-alkanes. Antimony trioxide had very little impact on the polyolefin wax/oil composition but it did suppress the formation of styrene and alpha-methyl styrene and increase the formation of ethylbenzene and cumene during the pyrolysis of the Br-HIPS
To what extent does the self-consistent mean-field exist?
A non-convergent difficulty near level-repulsive region is discussed within
the self-consistent mean-field theory. It is shown by numerical and analytic
studies that the mean-field is not realized in the many-fermion system when
quantum fluctuations coming from two-body residual interaction and quadrupole
deformation are larger than an energy difference between two avoided crossing
orbits. An analytic condition indicating a limitation of the mean-field concept
is derived for the first time
Electromagnon and phonon excitations in multiferroic TbMnO3
We have performed Raman measurements on TbMnO3 single crystal under magnetic
field along the three crystallographic directions. The flip of the spin spiral
plane creates an electromagnon excitation. In addition to the electromagnons
induced by the Heisenberg coupling, we have detected the electromagnon created
by the Dzyaloshinskii-Moriya interaction along the c axis. We have identified
all the vibrational modes of TbMnO3. Their temperature dependences show that
only one phonon observed along the polarization axis is sensitive to the
ferroelectric transition. This mode is tied to the Tb3+ ion displacements that
contribute to the ferroelectric polarization
Magneto-electric excitations in multiferroic TbMnO3 by Raman scattering
Low energy excitations in the multiferroic material TbMnO3 have been
investigated by Raman spectroscopy. Our observations reveal the existence of
two peaks at 30 cm-1 and 60 cm-1. They are observed in the cycloidal phase
below the Curie temperature but not in the sinusoidal phase, suggesting their
magnetoelectric origin. While the peak energies coincide with the frequencies
of electromagnons measured previously by transmission spectroscopy, they show
surprisingly different selection rules, with the 30 cm-1 excitation enhanced by
the electric field of light along the spontaneous polarization. The origins of
the modes are discussed under Raman and infrared selection rules
considerations
Statistical Mechanics of Dictionary Learning
Finding a basis matrix (dictionary) by which objective signals are
represented sparsely is of major relevance in various scientific and
technological fields. We consider a problem to learn a dictionary from a set of
training signals. We employ techniques of statistical mechanics of disordered
systems to evaluate the size of the training set necessary to typically succeed
in the dictionary learning. The results indicate that the necessary size is
much smaller than previously estimated, which theoretically supports and/or
encourages the use of dictionary learning in practical situations.Comment: 6 pages, 4 figure
Beyond homozygosity mapping: family-control analysis based on Hamming distance for prioritizing variants in exome sequencing
A major challenge in current exome sequencing in autosomal recessive (AR) families is the lack of an effective method to prioritize single-nucleotide variants (SNVs). AR families are generally too small for linkage analysis, and length of homozygous regions is unreliable for identification of causative variants. Various common filtering steps usually result in a list of candidate variants that cannot be narrowed down further or ranked. To prioritize shortlisted SNVs we consider each homozygous candidate variant together with a set of SNVs flanking it. We compare the resulting array of genotypes between an affected family member and a number of control individuals and argue that, in a family, differences between family member and controls should be larger for a pathogenic variant and SNVs flanking it than for a random variant. We assess differences between arrays in two individuals by the Hamming distance and develop a suitable test statistic, which is expected to be large for a causative variant and flanking SNVs. We prioritize candidate variants based on this statistic and applied our approach to six patients with known pathogenic variants and found these to be in the top 2 to 10 percentiles of ranks
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