4,239 research outputs found
Research and education in management of large- scale technical programs Semiannual progress report
Research and education in management of large scale technical programs - education and integration of interdisciplinary tea
Technology and management of large-scale programs Semiannual progress report
Technology and management of large scale program
Measurement of 0.25-3.2 GeV antiprotons in the cosmic radiation
The balloon-borne Isotope Matter-Antimatter Experiment (IMAX) was flown from Lynn Lake, Manitoba, Canada on 16–17 July 1992. Using velocity and magnetic rigidity to determine mass, we have directly measured the abundances of cosmic ray antiprotons and protons in the energy range from 0.25 to 3.2 GeV. Both the absolute flux of antiprotons and the antiproton/proton ratio are consistent with recent theoretical work in which antiprotons are produced as secondary products of cosmic ray interactions with the interstellar medium. This consistency implies a lower limit to the antiproton lifetime of ∼10 to the 7th yr
Data systems concepts for space systems, phase 1
Deviations from the traditional spacecraft data systems were studied. A data system architecture was developed from the top down
Manifestation of spin-charge separation in the dynamic dielectric response of one--dimensional Sr2CuO3
We have determined the dynamical dielectric response of a one-dimensional,
correlated insulator by carrying out electron energy-loss spectroscopy on
Sr2CuO3 single crystals. The observed momentum and energy dependence of the
low-energy features, which correspond to collective transitions across the gap,
are well described by an extended one-band Hubbard model with moderate nearest
neighbor Coulomb interaction strength. An exciton-like peak appears with
increasing momentum transfer. These observations provide experimental evidence
for spin-charge separation in the relevant excitations of this compound, as
theoretically expected for the one-dimensional Hubbard model.Comment: RevTex, 4 pages+2 figures, to appear in PRL (July 13
Gold-induced nanowires on the Ge(100) surface yield a 2D, and not a 1D electronic structure
Atomic nanowires on semiconductor surfaces induced by the adsorption of
metallic atoms have attracted a lot of attention as possible hosts of the
elusive, Tomonaga-Luttinger liquid. The Au/Ge(100) system in particular is the
subject of controversy as to whether the Au-induced nanowires do indeed host
exotic, 1D metallic states. We report on a thorough study of the electronic
properties of high quality nanowires formed at the Au/Ge(100) surface. High
resolution ARPES data show the low-lying Au-induced electronic states to
possess a dispersion relation that depends on two orthogonal directions in
k-space. Comparison of the E(k,k) surface measured using ARPES to
tight-binding calculations yields hopping parameters in the two different
directions that differ by a factor of two. We find that the larger of the two
hopping parameters corresponds, in fact, to the direction perpendicular to the
nanowires (t). This, the topology of the = contour in
k, and the fact that / proves that the
Au-induced electron pockets possess a 2D, closed Fermi surface, this firmly
places the Au/Ge(100) nanowire system outside being a potential hosts of a
Tomonaga-Luttinger liquid. We combine these ARPES data with STS measurements of
the spatially-resolved electronic structure and find that the spatially
straight conduction channels observed up to energies of order one electron volt
below the Fermi level do not originate from the Au-induced states seen in the
ARPES data. The former are more likely to be associated with bulk Ge states
that are localized to the subsurface region. Despite our proof of the 2D nature
of the Au-induced nanowire and sub-surface Ge-related states, an anomalous
suppression of the density of states at the Fermi level is observed in both the
STS and ARPES data, this phenomenon is discussed in the light of the effects of
disorder.Comment: 17 pages, 8 figure
Collective excitations in electron-hole bilayers
We report a combined analytic and Molecular Dynamics analysis of the
collective mode spectrum of an electron-hole (bipolar) bilayer in the strong
coupling quasi-classical limit. A robust, isotropic energy gap is identified in
the out-of-phase spectra, generated by the combined effect of correlations and
of the excitation of the bound dipoles; the in-phase spectra exhibit a
correlation governed acoustic dispersion for the longitudinal and transverse
modes. Strong nonlinear generation of higher harmonics of the fundamental
dipole oscillation frequency and the transfer of harmonics between different
modes is observed. The mode dispersions in the liquid state are compared with
the phonon spectrum in the crystalline solid phase, reinforcing a coherent
physical picture.Comment: 4 pages, 5 figure
Chemical kinetic and photochemical data for use in stratospheric modelling
An evaluated set of rate constants and photochemical cross sections were compiled for use in modelling stratospheric processes. The data are primarily relevant to the ozone layer, and its possible perturbation by anthropogenic activities. The evaluation is current to, approximately, January, 1979
Chemical kinetics and photochemical data for use in stratospheric modeling. Evaluation number 6
Evaluated sets of rate constants and photochemical cross sections are presented. The primary application of the data is in the modeling of stratospheric processes, with particular emphasis on the ozone layer and its possible perturbation by anthropogenic and natural phenomena
Research and education in management of large-scale technical programs
A research effort is reported which was conducted by NASA in conjunction with Drexel University, and which was aimed at an improved understanding of large scale systems technology and management
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