488 research outputs found
Backflow in a Fermi Liquid
We calculate the backflow current around a fixed impurity in a Fermi liquid.
The leading contribution at long distances is radial and proportional to 1/r^2.
It is caused by the current induced density modulation first discussed by
Landauer. The familiar 1/r^3 dipolar backflow obtained in linear response by
Pines and Nozieres is only the next to leading term, whose strength is
calculated here to all orders in the scattering. In the charged case the
condition of perfect screening gives rise to a novel sum rule for the phase
shifts. Similar to the behavior in a classical viscous liquid, the friction
force is due only to the leading contribution in the backflow while the dipolar
term does not contribute.Comment: 4 pages, 1 postscript figure, uses ReVTeX and epsfig macro, submitted
to Physical Review Letter
The role of the spin in quasiparticle interference
Quasiparticle interference patterns measured by scanning tunneling microscopy
(STM) can be used to study the local electronic structure of metal surfaces and
high temperature superconductors. Here, we show that even in non-magnetic
systems the spin of the quasiparticles can have a profound effect on the
interference patterns. On Bi(110), where the surface state bands are not
spin-degenerate, the patterns are not related to the dispersion of the
electronic states in a simple way. In fact, the features which are expected for
the spin-independent situation are absent and the observed interference
patterns can only be interpreted by taking spin-conserving scattering events
into account.Comment: 4 pages, 2 figure
Radiocarbon Date List X: Baffin Bay, Baffin Island, Iceland, Labrador Sea, and the Northern North Atlantic
Date List X contains an annotated listing of 213 radiocarbon dates determined on samples from marine and terrestrial environments. The marine samples were collected from the East Greenland, Iceland, Spitzbergen, and Norwegian margins, Baffin Bay, and Labrador Sea. The terrestrial samples were collected from Vestfirdir, Iceland and Baffin Island. The samples were submitted by INSTAAR and researchers affiliated with INSTAAR\u27s Micropaleontology Laboratory under the direction of Dr.’s John T. Andrews and Anne E. Jennings. All of the dates from marine sediment cores were determined from either shells or foraminifera (both benthic and planktic). All dates were obtained by the Accelerator Mass Spectrometry (AMS) method. Regions of concentrated marine research include: Baffin Bay, Baffin Island, Labrador Sea, East Greenland fjords, shelf and slope, Denmark Strait, the southwestern and northwestern Iceland shelves, and Vestfirdir, Iceland. The non-marine radiocarbon dates are from peat, wood, plant microfossils, and mollusc. The radiocarbon dates have been used to address a variety of research objectives such as: 1. determining the timing of northern hemisphere high latitude environmental changes including glacier advance and retreat, and 2. assessing the accuracy of a fluctuating reservoir correction. Thus, most of the dates constrain the timing, rate, and interaction of late Quaternary paleoenvironmental fluctuations in sea level, glacier extent, sediment input, and changes in ocean circulation patterns. Where significant, stratigraphic and sample contexts are presented for each core to document the basis for interpretations
Effective and efficient committee work: A Systematic overview of multidisciplinary literatures
An era of financial constraints calls for effective and efficient committee work when making collective decisions. A systematic search identified research literatures in business administration, health research and service development, and social psychology addressing decision making about highly technical issues by mixed groups of people. Existing empirical and theoretical syntheses were drawn together to identify learning about the structure, processes and environment of committees and the characteristics of effective chairing. Committee performance depends upon the individuals involved, their attributes and relationships; and the time available for a committee to explore their knowledge to make choices or solve problems. In general, groups with six to twelve members tend to perform better than those in either smaller or larger groups, especially when relying on virtual communication. Diverse groups take account of a range of opinions and enhance credibility and widespread acceptance and implementation of decisions but may be more difficult to convene and manage appropriately. However, where chairs manage conflict constructively, more varied membership leads to better performance and more reliable judgements. These small-scale interactions reflect the larger scale institutional relationships, hierarchies and cultures which act as a backdrop to committee activities. These findings suggest that effective committee performance is enhanced by: appointing members from all key stakeholder groups who between them bring the appropriate range in educational and functional background, while keeping the group size close to 6-12; appointing committee chairs for their facilitation skills and generalist background rather than specialist knowledge; allowing sufficient time to allow all relevant knowledge to be shared and evaluated through discussion, especially when judgements need to be made by committees with members who vary in status; applying formal consensus development processes; and, particularly when working virtually, considering the challenges of developing trust and cohesion, and integrating divergent perspectives
Adsorption and dissociation of molecular oxygen on the (0001) surface of double hexagonal close packed americium
In our continuing attempts to understand theoretically various surface
properties such as corrosion and potential catalytic activity of actinide
surfaces in the presence of environmental gases, we report here the first ab
initio study of molecular adsorption on the double hexagonal packed (dhcp)
americium (0001) surface. Dissociative adsorption is found to be energetically
more favorable compared to molecular adsorption. The most stable configuration
corresponds to a horizontal approach molecular dissociation with the oxygen
atoms occupying neighboring h3 sites, with chemisorption energies at the NSOC
and SOC theoretical levels being 9.395 eV and 9.886 eV, respectively. The
corresponding distances of the oxygen molecule from the surface and
oxygen-oxygen distance were found to be 0.953 Ang. and 3.731 Ang.,
respectively. Overall our calculations indicate that chemisorption energies in
cases with SOC are slightly more stable than the cases with NSOC in the
0.089-0.493 eV range. The work functions and net magnetic moments respectively
increased and decreased in all cases compared with the corresponding quantities
of the bare dhcp Am (0001) surface. The adsorbate-substrate interactions have
been analyzed in detail using the partial charges inside the muffin-tin
spheres, difference charge density distributions, and the local density of
states. The effects, if any, of chemisorption on the Am 5f electron
localization-delocalization characteristics in the vicinity of the Fermi level
are also discussed.Comment: 6 tables, 10 figure
Implication of the overlap representation for modelling generalized parton distributions
Based on a field theoretically inspired model of light-cone wave functions,
we derive valence-like generalized parton distributions and their double
distributions from the wave function overlap in the parton number conserved
s-channel. The parton number changing contributions in the t-channel are
restored from duality. In our construction constraints of positivity and
polynomiality are simultaneously satisfied and it also implies a model
dependent relation between generalized parton distributions and transverse
momentum dependent parton distribution functions. The model predicts that the
t-behavior of resulting hadronic amplitudes depends on the Bjorken variable
x_Bj. We also propose an improved ansatz for double distributions that embeds
this property.Comment: 15 pages, 8 eps figure
Cosmogenic Ages Indicate No MIS 2 Refugia in the Alexander Archipelago, Alaska
The late-Pleistocene history of the coastal Cordilleran Ice Sheet remains relatively unstudied compared to chronologies of the Laurentide Ice Sheet. Yet accurate reconstructions of Cordilleran Ice Sheet extent and the timing of ice retreat along the Pacific Coast are essential for paleoclimate modeling, assessing meltwater contribution to the North Pacific, and determining the availability of ice-free land along the coastal Cordilleran Ice Sheet margin for human migration from Beringia into the rest of the Americas. To improve the chronology of Cordilleran Ice Sheet history in the Alexander Archipelago, Alaska, we applied 10Be and 36Cl dating to boulders and glacially sculpted bedrock in areas previously hypothesized to have remained ice-free throughout the local Last Glacial Maximum (LLGM; 20–17 ka). Results indicate that these sites, and more generally the coastal northern Alexander Archipelago, became ice-free by 15.1 ± 0.9 ka (n = 12 boulders; 1 SD). We also provide further age constraints on deglaciation along the southern Alexander Archipelago and combine our new ages with data from two previous studies. We determine that ice retreatedfrom the outer coast of the southern Alexander Archipelago at 16.3 ± 0.8 ka (n = 14 boulders; 1 SD). These results collectively indicate that areas above modern sea level that were previously mapped as glacial refugia were covered by ice during the LLGM until between ∼ 16.3 and 15.1 ka. As no evidence was found for ice-free land during the LLGM, our results suggest that previous ice-sheet reconstructions underestimate the regional maximum Cordilleran Ice Sheet extent, and that all ice likely terminated on the continental shelf. Future work should investigate whether presently submerged areas of the continental shelf were ice-free
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