3,231 research outputs found
Inferring Species Trees from Incongruent Multi-Copy Gene Trees Using the Robinson-Foulds Distance
We present a new method for inferring species trees from multi-copy gene
trees. Our method is based on a generalization of the Robinson-Foulds (RF)
distance to multi-labeled trees (mul-trees), i.e., gene trees in which multiple
leaves can have the same label. Unlike most previous phylogenetic methods using
gene trees, this method does not assume that gene tree incongruence is caused
by a single, specific biological process, such as gene duplication and loss,
deep coalescence, or lateral gene transfer. We prove that it is NP-hard to
compute the RF distance between two mul-trees, but it is easy to calculate the
generalized RF distance between a mul-tree and a singly-labeled tree. Motivated
by this observation, we formulate the RF supertree problem for mul-trees
(MulRF), which takes a collection of mul-trees and constructs a species tree
that minimizes the total RF distance from the input mul-trees. We present a
fast heuristic algorithm for the MulRF supertree problem. Simulation
experiments demonstrate that the MulRF method produces more accurate species
trees than gene tree parsimony methods when incongruence is caused by gene tree
error, duplications and losses, and/or lateral gene transfer. Furthermore, the
MulRF heuristic runs quickly on data sets containing hundreds of trees with up
to a hundred taxa.Comment: 16 pages, 11 figure
Conservatism implications of shock test tailoring for multiple design environments
A method for analyzing shock conservation in test specifications that have been tailored to qualify a structure for multiple design environments is discussed. Shock test conservation is qualified for shock response spectra, shock intensity spectra and ranked peak acceleration data in terms of an Index of Conservation (IOC) and an Overtest Factor (OTF). The multi-environment conservation analysis addresses the issue of both absolute and average conservation. The method is demonstrated in a case where four laboratory tests have been specified to qualify a component which must survive seven different field environments. Final judgment of the tailored test specification is shown to require an understanding of the predominant failure modes of the test item
Quantum Phase Transition in the Itinerant Antiferromagnet (V0.9Ti0.1)2O3
Quantum-critical behavior of the itinerant electron antiferromagnet
(V0.9Ti0.1)2O3 has been studied by single-crystal neutron scattering. By
directly observing antiferromagnetic spin fluctuations in the paramagnetic
phase, we have shown that the characteristic energy depends on temperature as
c_1 + c_2 T^{3/2}, where c_1 and c_2 are constants. This T^{3/2} dependence
demonstrates that the present strongly correlated d-electron antiferromagnet
clearly shows the criticality of the spin-density-wave quantum phase transition
in three space dimensions.Comment: 4 pages, 4 figure
Magnetic Interaction in the Geometrically Frustrated Triangular Lattice Antiferromagnet
The spin wave excitations of the geometrically frustrated triangular lattice
antiferromagnet (TLA) have been measured using high resolution
inelastic neutron scattering. Antiferromagnetic interactions up to third
nearest neighbors in the ab plane (J_1, J_2, J_3, with
and ), as well as out-of-plane coupling (J_z, with
) are required to describe the spin wave dispersion
relations, indicating a three dimensional character of the magnetic
interactions. Two energy dips in the spin wave dispersion occur at the
incommensurate wavevectors associated with multiferroic phase, and can be
interpreted as dynamic precursors to the magnetoelectric behavior in this
system.Comment: 4 pages, 4 figures, published in Phys. Rev. Let
Magnons in Ferromagnetic Metallic Manganites
Ferromagnetic (FM) manganites, a group of likely half-metallic oxides, are of
special interest not only because they are a testing ground of the classical
doubleexchange interaction mechanism for the colossal magnetoresistance, but
also because they exhibit an extraordinary arena of emergent phenomena. These
emergent phenomena are related to the complexity associated with strong
interplay between charge, spin, orbital, and lattice. In this review, we focus
on the use of inelastic neutron scattering to study the spin dynamics, mainly
the magnon excitations in this class of FM metallic materials. In particular,
we discussed the unusual magnon softening and damping near the Brillouin zone
boundary in relatively narrow band compounds with strong Jahn-Teller lattice
distortion and charge/orbital correlations. The anomalous behaviors of magnons
in these compounds indicate the likelihood of cooperative excitations involving
spin, lattice, as well as orbital degrees of freedom.Comment: published in J. Phys.: Cond. Matt. 20 figure
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