692 research outputs found
Electron-electron interactions and two-dimensional - two-dimensional tunneling
We derive and evaluate expressions for the dc tunneling conductance between
interacting two-dimensional electron systems at non-zero temperature. The
possibility of using the dependence of the tunneling conductance on voltage and
temperature to determine the temperature-dependent electron-electron scattering
rate at the Fermi energy is discussed. The finite electronic lifetime produced
by electron-electron interactions is calculated as a function of temperature
for quasiparticles near the Fermi circle. Vertex corrections to the random
phase approximation substantially increase the electronic scattering rate. Our
results are in an excellent quantitative agreement with experiment.Comment: Revtex style, 21 pages and 8 postscript figures in a separate file;
Phys. Rev. B (in press
Coulomb scattering lifetime of a two-dimensional electron gas
Motivated by a recent tunneling experiment in a double quantum-well system,
which reports an anomalously enhanced electronic scattering rate in a clean
two-dimensional electron gas, we calculate the inelastic quasiparticle lifetime
due to electron-electron interaction in a single loop dynamically screened
Coulomb interaction within the random-phase-approximation. We obtain excellent
quantitative agreement with the inelastic scattering rates in the tunneling
experiment without any adjustable parameter, finding that the reported large
( a factor of six) disagreement between theory and experiment arises from
quantitative errors in the existing theoretical work and from the off-shell
energy dependence of the electron self-energy.Comment: 11 pages, RevTex, figures included. Also available at
http://www-cmg.physics.umd.edu/~lzheng
Interaction corrections at intermediate temperatures: dephasing time
We calculate the temperature dependence of the weak localization correction
in a two dimensional system at arbitrary relation between temperature, and
the elastic mean free time. We describe the crossover in the dephasing time
between the high temperature, ,
and the low temperature behaviors. The prefactors in
these dependences are not universal, but are determined by the Fermi liquid
constant characterising the spin exchange interaction.Comment: 4 pages, to appear in PRB, minor errors corrected, added reference
Large-space shell-model calculations for light nuclei
An effective two-body interaction is constructed from a new Reid-like
potential for a large no-core space consisting of six major shells and is used
to generate the shell-model properties for light nuclei from =2 to 6. (For
practical reasons, the model space is partially truncated for =6.) Binding
energies and other physical observables are calculated and compare favorably
with experiment.Comment: prepared using LaTex, 21 manuscript pages, no figure
-symmetric Gauge Mediation With Fayet-Iliopoulos Term
We have studied -symmetrc gauge mediation models with Fayet-Iliopoulos
terms. We give a concrete example of hidden sector with an gauge
theory and a Fayet-Iliopoulos term, which can induce distinctive soft terms in
the visible sector, and help solving fine tuning problems in models of
-symmetric gauge mediation.Comment: 11 pages, 1 figur
Molecular Phylogenies indicate a Paleo-Tibetan Origin of Himalayan Lazy Toads (Scutiger)
The Himalaya presents an outstanding geologically active orogen and biodiversity hotspot. However, our understanding of the historical biogeography of its fauna is far from comprehensive. Many taxa are commonly assumed to have originated from China-Indochina and dispersed westward along the Himalayan chain. Alternatively, the "Tibetan-origin hypothesis" suggests primary diversification of lineages in Paleo-Tibet, and secondary diversification along the slopes of the later uplifted Greater Himalaya. We test these hypotheses in high-mountain megophryid anurans (Scutiger). Extensive sampling from High Asia, and analyses of mitochondrial (2839\u2009bp) and nuclear DNA (2208\u2009bp), using Bayesian and Maximum likelihood phylogenetics, suggest that the Himalayan species form a distinct clade, possibly older than those from the eastern Himalaya-Tibet orogen. While immigration from China-Indochina cannot be excluded, our data may indicate that Himalayan Scutiger originated to the north of the Himalaya by colonization from Paleo-Tibet and then date back to the Oligocene. High intraspecific diversity of Scutiger implies limited migration across mountains and drainages along the Himalaya. While our study strengthens support for a "Tibetan-origin hypothesis", current sampling (10/22 species; 1 revalidated\ufeff: S. occidentalis) remains insufficient to draw final conclusions on Scutiger but urges comparative phylogeographers to test alternative, geologically supported hypotheses for a true future understanding of Himalayan biogeography
Inelastic lifetimes of confined two-component electron systems in semiconductor quantum wire and quantum well structures
We calculate Coulomb scattering lifetimes of electrons in two-subband quantum
wires and in double-layer quantum wells by obtaining the quasiparticle
self-energy within the framework of the random-phase approximation for the
dynamical dielectric function. We show that, in contrast to a single-subband
quantum wire, the scattering rate in a two-subband quantum wire contains
contributions from both particle-hole excitations and plasmon excitations. For
double-layer quantum well structures, we examine individual contributions to
the scattering rate from quasiparticle as well as acoustic and optical plasmon
excitations at different electron densities and layer separations. We find that
the acoustic plasmon contribution in the two-component electron system does not
introduce any qualitatively new correction to the low energy inelastic
lifetime, and, in particular, does not produce the linear energy dependence of
carrier scattering rate as observed in the normal state of high-
superconductors.Comment: 16 pages, RevTeX, 7 figures. Also available at
http://www-cmg.physics.umd.edu/~lzheng
Global epidemiology of avian influenza A H5N1 virus infection in humans, 1997–2015: a systematic review of individual case data
Avian influenza A H5N1 viruses have caused many, typically severe, human infections since the first human case was reported in 1997. However, no comprehensive epidemiological analysis of global human cases of H5N1 from 1997 to 2015 exists. Moreover, few studies have examined in detail the changing epidemiology of human H5N1 cases in Egypt, especially given the outbreaks since November, 2014, which have the highest number of cases ever reported worldwide in a similar period. Data on individual patients were collated from different sources using a systematic approach to describe the global epidemiology of 907 human H5N1 cases between May, 1997, and April, 2015. The number of affected countries rose between 2003 and 2008, with expansion from east and southeast Asia, then to west Asia and Africa. Most cases (67·2%) occurred from December to March, and the overall case-fatality risk was 483 (53·5%) of 903 cases which varied across geographical regions. Although the incidence in Egypt has increased dramatically since November, 2014, compared with the cases beforehand, there were no significant differences in the fatality risk, history of exposure to poultry, history of patient contact, and time from onset to hospital admission in the recent cases
A study of charged kappa in
Based on events collected by BESII, the decay
is studied. In the invariant mass
spectrum recoiling against the charged , the charged
particle is found as a low mass enhancement. If a Breit-Wigner function of
constant width is used to parameterize the kappa, its pole locates at MeV/. Also in this channel,
the decay is observed for the first time.
Its branching ratio is .Comment: 14 pages, 4 figure
Measurements of the observed cross sections for exclusive light hadrons containing at , 3.650 and 3.6648 GeV
By analyzing the data sets of 17.3, 6.5 and 1.0 pb taken,
respectively, at , 3.650 and 3.6648 GeV with the BES-II
detector at the BEPC collider, we measure the observed cross sections for
, , ,
and at the three energy
points. Based on these cross sections we set the upper limits on the observed
cross sections and the branching fractions for decay into these
final states at 90% C.L..Comment: 7 pages, 2 figure
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