355 research outputs found
Development of an improved oxygen electrode for use in alkaline H2-O2 fuel cells Quarterly report, Apr. 1 - Jun. 30, 1967
Preparation of institial compounds of transition metals for hydrogen oxygen fuel cell cathode
Ultrafast Optical-Pump Terahertz-Probe Spectroscopy of the Carrier Relaxation and Recombination Dynamics in Epitaxial Graphene
The ultrafast relaxation and recombination dynamics of photogenerated
electrons and holes in epitaxial graphene are studied using optical-pump
Terahertz-probe spectroscopy. The conductivity in graphene at Terahertz
frequencies depends on the carrier concentration as well as the carrier
distribution in energy. Time-resolved studies of the conductivity can therefore
be used to probe the dynamics associated with carrier intraband relaxation and
interband recombination. We report the electron-hole recombination times in
epitaxial graphene for the first time. Our results show that carrier cooling
occurs on sub-picosecond time scales and that interband recombination times are
carrier density dependent.Comment: 4 pages, 5 figure
Electronic Structure of Te and As Covered Si(211)
Electronic and atomic structures of the clean, and As and Te covered Si(211)
surface are studied using pseudopotential density functional method. The clean
surface is found to have (2 X 1) and rebonded (1 X 1) reconstructions as stable
surface structures, but no \pi-bonded chain reconstruction. Binding energies of
As and Te adatoms at a number of symmetry sites on the ideal and (2 X 1)
reconstructed surfaces have been calculated because of their importance in the
epitaxial growth of CdTe and other materials on the Si(211) surface. The
special symmetry sites on these surfaces having the highest binding energies
for isolated As and Te adatoms are identified. But more significantly, several
sites are found to be nearly degenerate in binding energy values. This has
important consequences for epitaxial growth processes. Optimal structures
calculated for 0.5 ML of As and Te coverage reveal that the As adatoms dimerize
on the surface while the Te adatoms do not. However, both As and Te covered
surfaces are found to be metallic in nature.Comment: 17 pages, 9 figures, accepted for publication in Phys. Rev.
An isotopic effect in phi photoproduction at a few GeV
A distinct isotopic effect in phi photoproduction at 2-5 GeV region is
identified by examining the production amplitudes due to Pomeron-exchange and
meson-exchange mechanisms. This effect is mainly caused by the pi-eta
interference constrained by SU(3) symmetry and the isotopic structure of the
gamma NN coupling in the direct phi-radiation amplitude. It can be tested
experimentally by measuring differences in the polarization observables between
the gamma-p and gamma-n reactions.Comment: 11 pages, 6 figure
Functional approach to the electromagnetic response function: the Longitudinal Channel
In this paper we address the (charge) longitudinal electromagnetic response
for a homogeneous system of nucleons interacting via meson exchanges in the
functional framework. This approach warrants consistency if the calculation is
carried on order-by-order in the mesonic loop expansion with RPA-dressed
mesonic propagators. At the 1-loop order and considering pion, rho and omega
exchanges we obtain a quenching of the response, in line with the experimental
results.Comment: RevTeX, 18 figures available upon request - to be published in
Physical Review
The size of the proton - closing in on the radius puzzle
We analyze the recent electron-proton scattering data from Mainz using a
dispersive framework that respects the constraints from analyticity and
unitarity on the nucleon structure. We also perform a continued fraction
analysis of these data. We find a small electric proton charge radius, r_E^p =
0.84_{-0.01}^{+0.01} fm, consistent with the recent determination from muonic
hydrogen measurements and earlier dispersive analyses. We also extract the
proton magnetic radius, r_M^p = 0.86_{-0.03}^{+0.02} fm, consistent with
earlier determinations based on dispersion relations.Comment: 4 pages, 2 figures, fit improved, small modifications, section on
continued fractions modified, conclusions on the proton charge radius
unchanged, version accepted for publication in European Physical Journal
Effects of impurity scattering on electron-phonon resonances in semiconductor superlattice high-field transport
A non-equilibrium Green's function method is applied to model high-field
quantum transport and electron-phonon resonances in semiconductor
superlattices. The field-dependent density of states for elastic (impurity)
scattering is found non-perturbatively in an approach which can be applied to
both high and low electric fields. I-V curves, and specifically electron-phonon
resonances, are calculated by treating the inelastic (LO phonon) scattering
perturbatively. Calculations show how strong impurity scattering suppresses the
electron-phonon resonance peaks in I-V curves, and their detailed sensitivity
to the size, strength and concentration of impurities.Comment: 7 figures, 1 tabl
Te covered Si(001): a variable surface reconstruction
At a given temperature, clean and adatom covered silicon surfaces usually
exhibit well-defined reconstruction patterns. Our finite temperature ab-initio
molecular dynamics calculations show that the tellurium covered Si(001) surface
is an exception. Soft longitudinal modes of surface phonons due to the strongly
anharmonic potential of the bridged tellurium atoms prevent the reconstruction
structure from attaining any permanent, two dimensional periodic geometry. This
explains why experiments attempting to find a definite model for the
reconstruction have reached conflicting conclusions.Comment: 4 pages, 3 gif figure
Photo- and Electroproduction of Eta Mesons
Eta photo- and electroproduction off the nucleon is investigated in an
effective lagrangian approach that contains Born terms and both vector meson
and nucleon resonance contributions. In particular, we review and develop the
formalism for coincidence experiments with polarization degrees of freedom. The
different response functions appearing in single and double polarization
experiments have been studied. We will present calculations for structure
functions and kinematical conditions that are most sensitive to details of the
lagrangian, in particular with regard to contributions of nucleon resonances
beyond the dominant (1535) resonance.Comment: 24 pages RevTeX/LaTeX2.09, NFSS1, 13 figures (in separate file
(tar,gzip and uue)), accepted for publication in Z. Phys.
Quasi-Elastic Scattering in the Inclusive (He, t) Reaction
The triton energy spectra of the charge-exchange C(He,t) reaction
at 2 GeV beam energy are analyzed in the quasi-elastic nucleon knock-out
region. Considering that this region is mainly populated by the charge-exchange
of a proton in He with a neutron in the target nucleus and the final proton
going in the continuum, the cross-sections are written in the distorted-wave
impulse approximation. The t-matrix for the elementary exchange process is
constructed in the DWBA, using one pion- plus rho-exchange potential for the
spin-isospin nucleon- nucleon potential. This t-matrix reproduces the
experimental data on the elementary pn np process. The calculated
cross-sections for the C(He,t) reaction at to triton
emission angle are compared with the corresponding experimental data, and are
found in reasonable overall accord.Comment: 19 pages, latex, 11 postscript figures available at
[email protected], submitted to Phy.Rev.
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