1,832 research outputs found
Strong Enhancement of Superconducting Correlation in a Two-Component Fermion Gas
We study high-density electron-hole (e-h) systems with the electron density
slightly larger than the hole density. We find a new superconducting phase, in
which the excess electrons form Cooper pairs moving in an e-h BCS phase. The
coexistence of the e-h and e-e orders is possible because e and h have opposite
charges, whereas analogous phases are impossible in the case of two fermion
species that have the same charge or are neutral. Most strikingly, the e-h
order enhances the superconducting e-h order parameter by more than one order
of magnitude as compared with that given by the BCS formula, for the same value
of the effective e-e attractive potential \lambda^{ee}. This new phase should
be observable in an e-h system created by photoexcitation in doped
semiconductors at low temperatures.Comment: 5 pages including 5 PostScript figure
Critical temperature and giant isotope effect in presence of paramagnons
We reconsider the long-standing problem of the effect of spin fluctuations on
the critical temperature and isotope effect in a phonon-mediated
superconductor. Although the general physics of the interplay between phonons
and paramagnons had been rather well understood, the existing approximate
formulas fail to describe the correct behavior of for general phonon
and paramagnon spectra. Using a controllable approximation, we derive an
analytical formula for which agrees well with exact numerical solutions
of the Eliashberg equations for a broad range of parameters. Based on both
numerical and analytical results, we predict a strong enhancement of the
isotope effect when the frequencies of spin fluctuation and phonons are of the
same order. This effect may have important consequences for near-magnetic
superconductors such as MgCNiComment: 5 pages, 2 figure
The Discovery of Argon in Comet C/1995 O1 (Hale-Bopp)
On 30.14 March 1997 we observed the EUV spectrum of the bright comet C/1995
O1 (Hale-Bopp) at the time of its perihelion, using our EUVS sounding rocket
telescope/spectrometer. The spectra reveal the presence H Ly beta, O+, and,
most notably, Argon. Modelling of the retrieved Ar production rates indicates
that comet Hale-Bopp is enriched in Ar relative to cosmogonic expectations.
This in turn indicates that Hale-Bopp's deep interior has never been exposed to
the 35-40 K temperatures necessary to deplete the comet's primordial argon
supply.Comment: 9 pages, 2 figures. ApJ, 545, in press (2000
Vortex Imaging in the pi-Band of Magnesium Diboride
We report scanning tunneling spectroscopy imaging of the vortex lattice in
single crystalline MgB2. By tunneling parallel to the c-axis, a single
superconducting gap (Delta = 2.2 meV) associated with the pi-band is observed.
The vortices in the pi-band have a large core size compared to estimates based
on Hc2, and show an absence of localized states in the core. Furthermore,
superconductivity between the vortices is rapidly suppressed by an applied
field. These results suggest that superconductivity in the pi-band is, at least
partially, induced by the intrinsically superconducting sigma-band.Comment: 4 pages, 3 figure
Chemical dynamics of triacetylene formation and implications to the synthesis of polyynes in Titan's atmosphere
For the last four decades, the role of polyynes such as diacetylene (HCCCCH) and triacetylene (HCCCCCCH) in the chemical evolution of the atmosphere of Saturn's moon Titan has been a subject of vigorous research. These polyacetylenes are thought to serve as an UV radiation shield in planetary environments; thus, acting as prebiotic ozone, and are considered as important constituents of the visible haze layers on Titan. However, the underlying chemical processes that initiate the formation and control the growth of polyynes have been the least understood to date. Here, we present a combined experimental, theoretical, and modeling study on the synthesis of the polyyne triacetylene (HCCCCCCH) via the bimolecular gas phase reaction of the ethynyl radical (CCH) with diacetylene (HCCCCH). This elementary reaction is rapid, has no entrance barrier, and yields the triacetylene molecule via indirect scattering dynamics through complex formation in a single collision event. Photochemical models of Titan's atmosphere imply that triacetylene may serve as a building block to synthesize even more complex polyynes such as tetraacetylene (HCCCCCCCCH)
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Politics, Portraiture and Power: Reassessing the Public Image of William Ewart Gladstone
Clonal expansion within pneumococcal serotype 6C after use of seven-valent vaccine
Streptococcus pneumoniae causes invasive infections, primarily at the extremes of life. A seven-valent conjugate vaccine (PCV7) is used to protect against invasive pneumococcal disease in children. Within three years of PCV7 introduction, we observed a fourfold increase in serotype 6C carriage, predominantly due to a single clone. We determined the whole-genome sequences of nineteen S. pneumoniae serotype 6C isolates, from both carriage (n = 15) and disease (n = 4) states, to investigate the emergence of serotype 6C in our population, focusing on a single multi-locus sequence type (MLST) clonal complex 395 (CC395). A phylogenetic network was constructed to identify different lineages, followed by analysis of variability in gene sets and sequences. Serotype 6C isolates from this single geographical site fell into four broad phylogenetically distinct lineages. Variation was seen in the 6C capsular locus and in sequences of genes encoding surface proteins. The largest clonal complex was characterised by the presence of lantibiotic synthesis locus. In our population, the 6C capsular locus has been introduced into multiple lineages by independent capsular switching events. However, rapid clonal expansion has occurred within a single MLST clonal complex. Worryingly, plasticity exists within current and potential vaccine-associated loci, a consideration for future vaccine use, target selection and design
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