3,835 research outputs found
Observation of Fermi-surface-dependent nodeless superconducting gaps in Ba0.6K0.4Fe2As2
The recent discovery of superconductivity in iron-arsenic compounds below a
transition temperature (Tc) as high as 55K ended the monopoly of copper oxides
(cuprates) in the family of high-Tc superconductors. A critical issue in
understanding this new superconductor, as in the case of cuprates, is the
nature, in particular the symmetry and orbital dependence, of the
superconducting gap. There are conflicting experimental results, mostly from
indirect measurements of the low energy excitation gap, ranging from one gap to
two gaps, from line nodes to nodeless gap function in momentum space. Here we
report a direct observation of the superconducting gap, including its momentum,
temperature, and Fermi surface (FS) dependence in Ba0.6K0.4Fe2As2 (Tc = 37 K)
using angle-resolved photoelectron spectroscopy. We find two superconducting
gaps with different values: a large gap (~ 12 meV) on the two small hole-like
and electron-like FS sheets, and a small gap (~ 6 meV) on the large hole-like
FS. Both gaps, closing simultaneously at the bulk Tc, are nodeless and nearly
isotropic around their respective FS sheets. The isotropic pairing interactions
are strongly orbital dependent, as the ratio 2Delta/kBTc switches from weak to
strong coupling on different bands. The same and surprisingly large
superconducting gap due to strong pairing on the two small FS, which are
connected by the (pi, 0) spin-density-wave vector in the parent compound,
strongly suggests that the pairing mechanism originates from the inter-band
interactions between these two nested FS sheets.Comment: 4 figure
Compact Nuclei in Galaxies at Moderate Redshift:II. Their Nature and Implications for the AGN Luminosity Function
This study explores the space density and properties of active galaxies to
z=0.8. We have investigated the frequency and nature of unresolved nuclei in
galaxies at moderate redshift as indicators of nuclear activity such as Active
Galactic Nuclei (AGN) or starbursts. Candidates are selected by fitting imaged
galaxies with multi-component models using maximum likelihood estimate
techniques to determine the best model fit. We select those galaxies requiring
an unresolved point-source component in the galaxy nucleus, in addition to a
disk and/or bulge component, to adequately model the galaxy light. We have
searched 70 WFPC2 images primarily from the Medium Deep Survey for galaxies
containing compact nuclei. In our survey of 1033 galaxies, the fraction
containing an unresolved nuclear component greater than 5% of the total galaxy
light is 9+/-1% corrected for incompleteness. In this second of two papers in
this series, we discuss the nature of the compact nuclei and their hosts.
We present the upper limit luminosity function (LF) for low-luminosity AGN
(LLAGN) in two redshift bins to z=0.8. Mild number density evolution is
detected for nuclei at -18 -16
and this flatness, combined with the increase in number density, is
inconsistent with pure luminosity evolution. Based on the amount of density
evolution observed for these objects, we find that almost all present-day
spiral galaxies could have hosted a LLAGN at some point in their lives. We also
comment on the likely contribution of these compact nuclei to the soft X-ray
background.Comment: 50 pages, 14 figures, to appear in ApJ, April 199
Assembly and architecture of the EBV B cell entry triggering complex.
Epstein-Barr Virus (EBV) is an enveloped double-stranded DNA virus of the gammaherpesvirinae sub-family that predominantly infects humans through epithelial cells and B cells. Three EBV glycoproteins, gH, gL and gp42, form a complex that targets EBV infection of B cells. Human leukocyte antigen (HLA) class II molecules expressed on B cells serve as the receptor for gp42, triggering membrane fusion and virus entry. The mechanistic role of gHgL in herpesvirus entry has been largely unresolved, but it is thought to regulate the activation of the virally-encoded gB protein, which acts as the primary fusogen. Here we study the assembly and function of the reconstituted B cell entry complex comprised of gHgL, gp42 and HLA class II. The structure from negative-stain electron microscopy provides a detailed snapshot of an intermediate state in EBV entry and highlights the potential for the triggering complex to bring the two membrane bilayers into proximity. Furthermore, gHgL interacts with a previously identified, functionally important hydrophobic pocket on gp42, defining the overall architecture of the complex and playing a critical role in membrane fusion activation. We propose a macroscopic model of the initiating events in EBV B cell fusion centered on the formation of the triggering complex in the context of both viral and host membranes. This model suggests how the triggering complex may bridge the two membrane bilayers, orienting critical regions of the N- and C- terminal ends of gHgL to promote the activation of gB and efficient membrane fusion
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