575 research outputs found
The mechanism of hole carrier generation and the nature of pseudogap- and 60K-phases in YBCO
In the framework of the model assuming the formation of NUC on the pairs of
Cu ions in CuO plane the mechanism of hole carrier generation is
considered and the interpretation of pseudogap and 60 K-phases in
. is offered. The calculated dependences of hole
concentration in on doping and temperature
are found to be in a perfect quantitative agreement with experimental data. As
follows from the model the pseudogap has superconducting nature and arises at
temperature in small clusters uniting a number of
NUC's due to large fluctuations of NUC occupation. Here and
are the superconducting transition temperatures of infinite and finite
clusters of NUC's, correspondingly. The calculated and
dependences are in accordance with experiment. The area between
and corresponds to the area of fluctuations
where small clusters fluctuate between superconducting and normal states owing
to fluctuations of NUC occupation. The results may serve as important arguments
in favor of the proposed model of HTSC.Comment: 12 pages, 7 figure
Disorder and transport in cuprates: weak localization and magnetic contributions
We report resistivity measurements in underdoped YBaCuOand overdoped TlBaCuO single crystals in which the
concentration of defects in the CuO planes is controlled by electron
irradiation. Low upturns of the resistivity are observed in both cases for
large defect content. In the Tl compound the decrease of conductivity scales as
expected from weak localization theory. On the contrary in
YBaCuO the much larger low T contribution to the
resistivity is proportional to the defect content and might then be associated
to a Kondo like spin flip scattering term. This would be consistent with the
results on the magnetic properties induced by spinless defects.Comment: latex rullier1.tex, 5 files, 4 pages [SPEC-S01/003], submitted to
Phys. Rev. Let
Decoupled CuO_2 and RuO_2 layers in superconducting and magnetically ordered RuSr_2GdCu_2O_8
Comprehensive measurements of dc and ac susceptibility, dc resistance,
magnetoresistance, Hall resistivity, and microwave absorption and dispersion in
fields up to 8 T have been carried out on RuSr_2GdCu_2O_8 with the aim to
establish the properties of RuO_2 and CuO_2 planes. At ~130 K, where the
magnetic order develops in the RuO_2 planes, one observes a change in the slope
of dc resistance, change in the sign of magnetoresistance, and the appearance
of an extraordinary Hall effect. These features indicate that the RuO_2 planes
are conducting. A detailed analysis of the ac susceptibility and microwave data
on both, ceramic and powder samples show that the penetration depth remains
frequency dependent and larger than the London penetration depth even at low
temperatures. We conclude that the conductivity in the RuO_2 planes remains
normal even when superconducting order is developed in the CuO_2 planes below
\~45 K. Thus, experimental evidence is provided in support of theoretical
models which base the coexistence of superconductivity and magnetic order on
decoupled CuO_2 and RuO_2 planes.Comment: 11 pages, 11 figures, submitted to PR
The highly rearranged mitochondrial genomes of the crabs Maja crispata and Maja squinado (Majidae) and gene order evolution in Brachyura
Abstract
We sequenced the mitochondrial genomes of the spider crabs Maja crispata and Maja squinado (Majidae, Brachyura). Both genomes contain the whole set of 37 genes characteristic of Bilaterian genomes, encoded on both \u3b1- and \u3b2-strands. Both species exhibit the same gene order, which is unique among known animal genomes. In particular, all the genes located on the \u3b2-strand form a single block. This gene order was analysed together with the other nine gene orders known for the Brachyura. Our study confirms that the most widespread gene order (BraGO) represents the plesiomorphic condition for Brachyura and was established at the onset of this clade. All other gene orders are the result of transformational pathways originating from BraGO. The different gene orders exhibit variable levels of genes rearrangements, which involve only tRNAs or all types of genes. Local homoplastic arrangements were identified, while complete gene orders remain unique and represent signatures that can have a diagnostic value. Brachyura appear to be a hot-spot of gene order diversity within the phylum Arthropoda. Our analysis, allowed to track, for the first time, the fully evolutionary pathways producing the Brachyuran gene orders. This goal was achieved by coupling sophisticated bioinformatic tools with phylogenetic analysis
Self-trapping of excitons, violation of condon approximation, and efficient fluorescence in conjugated cycloparaphenylenes
Cycloparaphenylenes, the simplest structural unit of armchair carbon nanotubes, have unique optoelectronic properties counterintuitive in the class of conjugated organic materials. Our time-dependent density functional theory study and excited state dynamics simulations of cycloparaphenylene chromophores provide a simple and conceptually appealing physical picture explaining experimentally observed trends in optical properties in this family of molecules. Fully delocalized degenerate second and third excitonic states define linear absorption spectra. Self-trapping of the lowest excitonic state due to electron-phonon coupling leads to the formation of spatially localized excitation in large cycloparaphenylenes within 100 fs. This invalidates the commonly used Condon approximation and breaks optical selection rules, making these materials superior fluorophores. This process does not occur in the small molecules, which remain inefficient emitters. A complex interplay of symmetry, Ο-conjugation, conformational distortion and bending strain controls all photophysics of cycloparaphenylenes.Fil: Adamska, Lyudmyla. Los Alamos National Laboratory. Los Alamos; Estados UnidosFil: Nayyar, Iffat. Los Alamos National Laboratory. Los Alamos; Estados UnidosFil: Chen, Hang. Boston University; Estados UnidosFil: Swan, Anna K.. Boston University; Estados UnidosFil: Oldani, Andres Nicolas. Universidad Nacional de Quilmes; ArgentinaFil: FernΓ‘ndez Alberti, SebastiΓ‘n. Consejo Nacional de Investigaciones CientΓficas y TΓ©cnicas; Argentina. Universidad Nacional de Quilmes; ArgentinaFil: Golder, Matthew R.. University of Oregon; Estados UnidosFil: Jasti, Ramesh. University of Oregon; Estados UnidosFil: Doorn, Stephen K.. Los Alamos National Laboratory. Los Alamos; Estados UnidosFil: Tretiak, Sergei. Los Alamos National Laboratory. Los Alamos; Estados Unido
A systematic review of the evidence for single stage and two stage revision of infected knee replacement
BACKGROUND:
Periprosthetic infection about the knee is a devastating complication that may affect between 1% and 5% of knee replacement. With over 79 000 knee replacements being implanted each year in the UK, periprosthetic infection (PJI) is set to become an important burden of disease and cost to the healthcare economy. One of the important controversies in treatment of PJI is whether a single stage revision operation is superior to a two-stage procedure. This study sought to systematically evaluate the published evidence to determine which technique had lowest reinfection rates.
METHODS:
A systematic review of the literature was undertaken using the MEDLINE and EMBASE databases with the aim to identify existing studies that present the outcomes of each surgical technique. Reinfection rate was the primary outcome measure. Studies of specific subsets of patients such as resistant organisms were excluded.
RESULTS:
63 studies were identified that met the inclusion criteria. The majority of which (58) were reports of two-stage revision. Reinfection rated varied between 0% and 41% in two-stage studies, and 0% and 11% in single stage studies. No clinical trials were identified and the majority of studies were observational studies.
CONCLUSIONS:
Evidence for both one-stage and two-stage revision is largely of low quality. The evidence basis for two-stage revision is significantly larger, and further work into direct comparison between the two techniques should be undertaken as a priority
Multigene Phylogeny of Choanozoa and the Origin of Animals
Animals are evolutionarily related to fungi and to the predominantly unicellular protozoan phylum Choanozoa, together known as opisthokonts. To establish the sequence of events when animals evolved from unicellular ancestors, and understand those key evolutionary transitions, we need to establish which choanozoans are most closely related to animals and also the evolutionary position of each choanozoan group within the opisthokont phylogenetic tree. Here we focus on Ministeria vibrans, a minute bacteria-eating cell with slender radiating tentacles. Single-gene trees suggested that it is either the closest unicellular relative of animals or else sister to choanoflagellates, traditionally considered likely animal ancestors. Sequencing thousands of Ministeria protein genes now reveals about 14 with domains of key significance for animal cell biology, including several previously unknown from deeply diverging Choanozoa, e.g. domains involved in hedgehog, Notch and tyrosine kinase signaling or cell adhesion (cadherin). Phylogenetic trees using 78 proteins show that Ministeria is not sister to animals or choanoflagellates (themselves sisters to animals), but to Capsaspora, another protozoan with thread-like (filose) tentacles. The Ministeria/Capsaspora clade (new class Filasterea) is sister to animals and choanoflagellates, these three groups forming a novel clade (filozoa) whose ancestor presumably evolved filose tentacles well before they aggregated as a periciliary collar in the choanoflagellate/sponge common ancestor. Our trees show ichthyosporean choanozoans as sisters to filozoa; a fusion between ubiquitin and ribosomal small subunit S30 protein genes unifies all holozoa (filozoa plus Ichthyosporea), being absent in earlier branching eukaryotes. Thus, several successive evolutionary innovations occurred among their unicellular closest relatives prior to the origin of the multicellular body-plan of animals
Anomalous Optoelectronic Properties of Chiral Carbon Nanorings...and One Ring to Rule Them All
Carbon nanorings are hoop-shaped, {\pi}-conjugated macrocycles which form the
fundamental annular segments of single-walled carbon nanotubes (SWNTs). In a
very recent report, the structures of chiral carbon nanorings (which may serve
as chemical templates for synthesizing chiral nanotubes) were experimentally
synthesized and characterized for the first time. Here, in our communication,
we show that the excited-state properties of these unique chiral nanorings
exhibit anomalous and extremely interesting optoelectronic properties, with
excitation energies growing larger as a function of size (in contradiction with
typical quantum confinement effects). While the first electronic excitation in
armchair nanorings is forbidden with a weak oscillator strength, we find that
the same excitation in chiral nanorings is allowed due to a strong geometric
symmetry breaking. Most importantly, among all the possible nanorings
synthesized in this fashion, we show that only one ring, corresponding to a
SWNT with chiral indices (n+3,n+1), is extremely special with large
photoinduced transitions that are most readily observable in spectroscopic
experiments.Comment: Accepted by the Journal of Physical Chemistry Letter
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