Polarisation properties of exciton-polaritons in semiconductor microcavities

Interactions of exciton-polaritons in semiconductor microcavities and the resulting polarisation dynamics are investigated theoretically. Within the coboson framework of polariton-polariton scattering, it is shown that the matrix element of direct Coulomb scattering is proportional to the transferred momentum, q, cubed in the limit of small q. In the same limit, the magnitude of superexchange/exchange interactions can be considered constant. These results are applied to the elastic circle geometry, where a system of equations describing the steady-state pseudospin components is derived. It is shown, that for this geometry, polariton-polariton scattering can account for the generation of circularly/linearly polarised final states from linearly/circularly polarised initial states, depolarisation and the generation of spin currents. In the low density regime polaritons are good bosons and the dynamics of polariton Bose-Einstein condensation (BEC) are investigated. A stochastic model is derived, resulting in a Langevin type equation describing the time dynamics of the condensate spinor order parameter. The build up in condensate polarisation degree is shown to evidence macroscopic ground state population, while the stochastic choice of polarisation vector evidences the symmetry breaking nature of the phase transition. The decrease of polarisation degree above threshold is demonstrated to be a consequence of polariton-polariton interactions, a result which is complemented by recent experimental work. The stochastic model is extended to include Josephson coupling of spatially separate condensates. The coupling results in polarisation and phase correlations between the condensates, explaining the polarisation locking and spatial coherence seen experimentally. Finally, the effect of polarisation pinning by local effective fields is examined

Tetracycline Selective Pressure and Homologous Recombination Shape the Evolution of Chlamydia suis: A Recently Identified Zoonotic Pathogen

Species closely related to the human pathogen Chlamydia trachomatis (Ct) have recently been found to cause zoonotic infections, posing a public health threat especially in the case of tetracycline resistant Chlamydia suis (Cs) strains. These strains acquired a tet(C)-containing cassette via horizontal gene transfer (HGT). Genomes of 11 Cs strains from various tissues were sequenced to reconstruct evolutionary pathway(s) for tet(C) HGT. Cs had the highest recombination rate of Chlamydia species studied to date. Admixture occurred among Cs strains and with Chlamydia muridarum but not with Ct. Although in vitro tet(C) cassette exchange with Ct has been documented, in vivo evidence may require examining human samples from Ct and Cs co-infected sites. Molecular-clock dating indicated that ancestral clades of resistant Cs strains predated the 1947 discovery of tetracycline, which was subsequently used in animal feed. The cassette likely spread throughout Cs strains by homologous recombination after acquisition from an external source, and our analysis suggests Betaproteobacteria as the origin. Selective pressure from tetracycline may be responsible for recent bottlenecks in Cs populations. Since tetracycline is an important antibiotic for treating Ct, zoonotic infections at mutual sites of infection indicate the possibility for cassette transfer and major public health repercussions

Interplay of recombination and selection in the genomes of Chlamydia trachomatis

<p>Abstract</p> <p>Background</p> <p><it>Chlamydia trachomatis </it>is an obligate intracellular bacterial parasite, which causes several severe and debilitating diseases in humans. This study uses comparative genomic analyses of 12 complete published <it>C. trachomatis </it>genomes to assess the contribution of recombination and selection in this pathogen and to understand the major evolutionary forces acting on the genome of this bacterium.</p> <p>Results</p> <p>The conserved core genes of <it>C. trachomatis </it>are a large proportion of the pan-genome: we identified 836 core genes in <it>C. trachomatis </it>out of a range of 874-927 total genes in each genome. The ratio of recombination events compared to mutation (ρ/θ) was 0.07 based on ancestral reconstructions using the ClonalFrame tool, but recombination had a significant effect on genetic diversification (r/m = 0.71). The distance-dependent decay of linkage disequilibrium also indicated that <it>C. trachomatis </it>populations behaved intermediately between sexual and clonal extremes. Fifty-five genes were identified as having a history of recombination and 92 were under positive selection based on statistical tests. Twenty-three genes showed evidence of being under both positive selection and recombination, which included genes with a known role in virulence and pathogencity (e.g., <it>ompA, pmps, tarp</it>). Analysis of inter-clade recombination flux indicated non-uniform currents of recombination between clades, which suggests the possibility of spatial population structure in <it>C. trachomatis </it>infections.</p> <p>Conclusions</p> <p><it>C. trachomatis </it>is the archetype of a bacterial species where recombination is relatively frequent yet gene gains by horizontal gene transfer (HGT) and losses (by deletion) are rare. Gene conversion occurs at sites across the whole <it>C. trachomatis </it>genome but may be more often fixed in genes that are under diversifying selection. Furthermore, genome sequencing will reveal patterns of serotype specific gene exchange and selection that will generate important research questions for understanding <it>C. trachomatis </it>pathogenesis.</p> <p>Reviewers</p> <p>This article was reviewed by Dr. Jeremy Selengut, Dr. Lee S. Katz (nominated by Dr. I. King Jordan) and Dr. Arcady Mushegian.</p

Comparison of the density-matrix renormalization group method applied to fractional quantum Hall systems in different geometries

We report a systematic study of the fractional quantum Hall effect (FQHE) using the density-matrix renormalization group (DMRG) method on two different geometries: the sphere and the cylinder. We provide convergence benchmarks based on model Hamiltonians known to possess exact zero-energy ground states, as well as an analysis of the number of sweeps and basis elements that need to be kept in order to achieve the desired accuracy.The ground state energies of the Coulomb Hamiltonian at $\nu=1/3$ and $\nu=5/2$ filling are extracted and compared with the results obtained by previous DMRG implementations in the literature. A remarkably rapid convergence in the cylinder geometry is noted and suggests that this boundary condition is particularly suited for the application of the DMRG method to the FQHE.Comment: 5 pages, 7 figure

Predicting Phenotype and Emerging Strains among Chlamydia trachomatis Infections

Single nucleotide polymorphisms can be used for epidemiologic and evolutionary studies worldwide

High orientation of long chain branched poly (lactic acid) with enhanced blood compatibility and bionic structure

YesHighly-oriented poly (lactic acid) (PLA) with bionic micro-grooves was fabricated through solid hot drawing technology for further improving the mechanical properties and blood biocompatibility of PLA. In order to enhance the melt strength and thus obtain high orientation degree, long chain branched PLA (LCB-PLA) was prepared at first through a two-step ring-opening reaction during processing. Linear viscoelasticity combined with branch-on-branch (BOB) model was used to predict probable compositions and chain topologies of the products, and it was found that the molecular weight of PLA increased and topological structures with star like chain with three arms and tree-like chain with two generations formed during reactive processing, and consequently draw ratio as high as1200% can be achieved during the subsequent hot stretching. With the increase of draw ratio, the tensile strength and orientation degree of PLA increased dramatically. Long chain branching and orientation could significantly enhance the blood compatibility of PLA by prolonging clotting time and decreasing platelet activation. Micro-grooves can be observed on the surface of the oriented PLA which were similar to the intimal layer of blood vessel, and such bionic structure resulted from the formation of the oriented shish kebab-like crystals along the draw direction

Inhibition of prolyl oligopeptidase with a synthetic unnatural dipeptide

A new inhibitor, containing a linked proline-piperidine structure, for the enzyme prolyl oligopeptidase (POP) has been synthesised and demonstrated to bind covalently with the enzyme at the active site. This provides evidence that covalent inhibitors of POP do not have to be limited to structures containing five-membered N-containing heterocyclic rings

Hypervirulent Chlamydia trachomatis Clinical Strain Is a Recombinant between Lymphogranuloma Venereum (L2) and D Lineages

Chlamydia trachomatis is an obligate intracellular bacterium that causes a diversity of severe and debilitating diseases worldwide. Sporadic and ongoing outbreaks of lymphogranuloma venereum (LGV) strains among men who have sex with men (MSM) support the need for research on virulence factors associated with these organisms. Previous analyses have been limited to single genes or genomes of laboratory-adapted reference strain L2/434 and outbreak strain L2b/UCH-1/proctitis. We characterized an unusual LGV strain, termed L2c, isolated from an MSM with severe hemorrhagic proctitis. L2c developed nonfusing, grape-like inclusions and a cytotoxic phenotype in culture, unlike the LGV strains described to date. Deep genome sequencing revealed that L2c was a recombinant of L2 and D strains with conserved clustered regions of genetic exchange, including a 78-kb region and a partial, yet functional, toxin gene that was lost with prolonged culture. Indels (insertions/deletions) were discovered in an ftsK gene promoter and in the tarp and hctB genes, which encode key proteins involved in replication, inclusion formation, and histone H1-like protein activity, respectively. Analyses suggest that these indels affect gene and/or protein function, supporting the in vitro and disease phenotypes. While recombination has been known to occur for C. trachomatis based on gene sequence analyses, we provide the first whole-genome evidence for recombination between a virulent, invasive LGV strain and a noninvasive common urogenital strain. Given the lack of a genetic system for producing stable C. trachomatis mutants, identifying naturally occurring recombinants can clarify gene function and provide opportunities for discovering avenues for genomic manipulation

A redistribution of water due to pileus cloud formation near the tropopause

International audienceThin stratiform clouds called pileus can form in the earth's atmosphere when humid air is lifted above rising convection. In the lower troposphere pileus lifetimes are short, so they have been considered little more than an attractive curiosity. This paper describes pileus cloud forming near the tropopause at low-latitudes, and discusses how they may be associated with a redistribution of water vapor and ice at cold temperatures