1,537 research outputs found
A congruent phylogenomic signal places eukaryotes within the Archaea
Determining the relationships among the major groups of cellular life is important for understanding the evolution of biological diversity, but is difficult given the enormous time spans involved. In the textbook ‘three domains’ tree based on informational genes, eukaryotes and Archaea share a common ancestor to the exclusion of Bacteria. However, some phylogenetic analyses of the same data have placed eukaryotes within the Archaea, as the nearest relatives of different archaeal lineages. We compared the support for these competing hypotheses using sophisticated phylogenetic methods and an improved sampling of archaeal biodiversity. We also employed both new and existing tests of phylogenetic congruence to explore the level of uncertainty and conflict in the data. Our analyses suggested that much of the observed incongruence is weakly supported or associated with poorly fitting evolutionary models. All of our phylogenetic analyses, whether on small subunit and large subunit ribosomal RNA or concatenated protein-coding genes, recovered a monophyletic group containing eukaryotes and the TACK archaeal superphylum comprising the Thaumarchaeota, Aigarchaeota, Crenarchaeota and Korarchaeota. Hence, while our results provide no support for the iconic three-domain tree of life, they are consistent with an extended eocyte hypothesis whereby vital components of the eukaryotic nuclear lineage originated from within the archaeal radiation
Observations of the 8 December 1987 occultation of AG+40 deg 0783 by 324 Bamberga
The occultation of AG+40 deg 0783 by 324 Bamberga on 8 December 1987 was observed at 13 sites in the United States, Japan, and China. At four sites the event was observed photoelectrically; the other observations were visual. A least-squares fit of a circular limb profile to the data gives a diameter of 227.6 + or - 1.9 km. However, this solution is inconsistent with a negative visual observation near the northern edge of the ground track. The inconsistency cannot be removed by assuming an elliptical profile. The data suggest that Bamberga, despite its low-amplitude lightcurve, may depart significantly from a spherical or ellipsoidal shape. The asteroid also appears to be at least 10 percent smaller than indicated by infrared radiometry
Propagation of charged particle waves in a uniform magnetic field
This paper considers the probability density and current distributions
generated by a point-like, isotropic source of monoenergetic charges embedded
into a uniform magnetic field environment. Electron sources of this kind have
been realized in recent photodetachment microscopy experiments. Unlike the
total photocurrent cross section, which is largely understood, the spatial
profiles of charge and current emitted by the source display an unexpected
hierarchy of complex patterns, even though the distributions, apart from
scaling, depend only on a single physical parameter. We examine the electron
dynamics both by solving the quantum problem, i. e., finding the energy Green
function, and from a semiclassical perspective based on the simple cyclotron
orbits followed by the electron. Simulations suggest that the semiclassical
method, which involves here interference between an infinite set of paths,
faithfully reproduces the features observed in the quantum solution, even in
extreme circumstances, and lends itself to an interpretation of some (though
not all) of the rich structure exhibited in this simple problem.Comment: 39 pages, 16 figure
Electric-field-controlled directional growth of ferroelectric domains in multiferroic BiFeO3 films
We describe the directional growth of ferroelectric domains in a multiferroic
BiFeO3 thin film, which was grown epitaxially on a vicinal (001) SrTiO3
substrate. A detailed structural analysis of the film shows that a strain
gradient, which can create a symmetry breaking in a ferroelectric double well
potential, causes ferroelectric domains to grow with preferred directionality
under the influence of an electric field. Our results suggest the possibility
of controlling the direction of domain growth with an electric field by
imposing constraints on ferroelectric films, such as a strain gradient.Comment: 16 pages, 3 figure
Optical Generation of Vortices in trapped Bose-Einstein Condensates
We demonstrate numerically the efficient generation of vortices in
Bose-Einstein condensates (BEC) by using a ``phase imprinting'' method. The
method consist of passing a far off resonant laser pulse through an absorption
plate with azimuthally dependent absorption coefficient, imaging the laser beam
onto a BEC, and thus creating the corresponding non-dissipative Stark shift
potential and condensate phase shift. In our calculations we take into account
experimental imperfections. We also propose an interference method to detect
vortices by coherently pushing part of the condensate using optically induced
Bragg scattering.Comment: 8 pages, 4 figure
Application of elastostatic Green function tensor technique to electrostriction in cubic, hexagonal and orthorhombic crystals
The elastostatic Green function tensor approach, which was recently used to
treat electrostriction in numerical simulation of domain structure formation in
cubic ferroelectrics, is reviewed and extended to the crystals of hexagonal and
orthorhombic symmetry. The tensorial kernels appearing in the expressions for
effective nonlocal interaction of electrostrictive origin are derived
explicitly and their physical meaning is illustrated on simple examples. It is
argued that the bilinear coupling between the polarization gradients and
elastic strain should be systematically included in the Ginzburg-Landau free
energy expansion of electrostrictive materials.Comment: 4 page
Effects of macroscopic polarization in III-V nitride multi-quantum-wells
Huge built-in electric fields have been predicted to exist in wurtzite III-V
nitrides thin films and multilayers. Such fields originate from heterointerface
discontinuities of the macroscopic bulk polarization of the nitrides. Here we
discuss the background theory, the role of spontaneous polarization in this
context, and the practical implications of built-in polarization fields in
nitride nanostructures. To support our arguments, we present detailed
self-consistent tight-binding simulations of typical nitride QW structures in
which polarization effects are dominant.Comment: 11 pages, 9 figures, uses revtex/epsf. submitted to PR
Going global or going nowhere? Chinese media in a time of flux
This article focuses on how Chinese media industries have sought to expand into new territories, identifying several mechanisms by which Chinese cultural and media products are traded and consumed: namely finished content, co-productions, formats and online platforms. The article also considers the value of overseas location shooting for domestic programmes and acquisition of international media assets. The key idea that is discussed in regards to China's outward-bound trade mission is know-how. In respect to media production, producers tend to opt for risk avoidance rather than creative exploration. Consequently, Chinese producers, investors and practitioners are deficient in knowledge of international markets and audience perceptions because the government has chosen to intervene, select and guide Chinese products. The article suggests this is gradually changing
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