4,056 research outputs found
Classification and reduction of pilot error
Human error is a primary or contributing factor in about two-thirds of commercial aviation accidents worldwide. With the ultimate goal of reducing pilot error accidents, this contract effort is aimed at understanding the factors underlying error events and reducing the probability of certain types of errors by modifying underlying factors such as flight deck design and procedures. A review of the literature relevant to error classification was conducted. Classification includes categorizing types of errors, the information processing mechanisms and factors underlying them, and identifying factor-mechanism-error relationships. The classification scheme developed by Jens Rasmussen was adopted because it provided a comprehensive yet basic error classification shell or structure that could easily accommodate addition of details on domain-specific factors. For these purposes, factors specific to the aviation environment were incorporated. Hypotheses concerning the relationship of a small number of underlying factors, information processing mechanisms, and error types types identified in the classification scheme were formulated. ASRS data were reviewed and a simulation experiment was performed to evaluate and quantify the hypotheses
Strain Modulated Electronic Properties of Ge Nanowires - A First Principles Study
We used density-functional theory based first principles simulations to study
the effects of uniaxial strain and quantum confinement on the electronic
properties of germanium nanowires along the [110] direction, such as the energy
gap and the effective masses of the electron and hole. The diameters of the
nanowires being studied are up to 50 {\AA}. As shown in our calculations, the
Ge [110] nanowires possess a direct band gap, in contrast to the nature of an
indirect band gap in bulk. We discovered that the band gap and the effective
masses of charge carries can be modulated by applying uniaxial strain to the
nanowires. These strain modulations are size-dependent. For a smaller wire (~
12 {\AA}), the band gap is almost a linear function of strain; compressive
strain increases the gap while tensile strain reduces the gap. For a larger
wire (20 {\AA} - 50 {\AA}), the variation of the band gap with respect to
strain shows nearly parabolic behavior: compressive strain beyond -1% also
reduces the gap. In addition, our studies showed that strain affects effective
masses of the electron and hole very differently. The effective mass of the
hole increases with a tensile strain while the effective mass of the electron
increases with a compressive strain. Our results suggested both strain and size
can be used to tune the band structures of nanowires, which may help in design
of future nano-electronic devices. We also discussed our results by applying
the tight-binding model.Comment: 1 table, 8 figure
Collective excitation spectrum of a disordered Hubbard model
We study the collective excitation spectrum of a d=3 site-disordered
Anderson-Hubbard model at half-filling, via a random-phase approximation (RPA)
about broken-symmetry, inhomogeneous unrestricted Hartree-Fock (UHF) ground
states. We focus in particular on the density and character of low-frequency
collective excitations in the transverse spin channel. In the absence of
disorder, these are found to be spin-wave-like for all but very weak
interaction strengths, extending down to zero frequency and separated from a
Stoner-like band, to which there is a gap. With disorder present, a prominent
spin-wave-like band is found to persist over a wide region of the
disorder-interaction phase plane in which the mean-field ground state is a
disordered antiferromagnet, despite the closure of the UHF single-particle gap.
Site resolution of the RPA excitations leads to a microscopic rationalization
of the evolution of the spectrum with disorder and interaction strength, and
enables the observed localization properties to be interpreted in terms of the
fraction of strong local moments and their site-differential distribution.Comment: 25 pages (revtex), 9 postscript figure
Functional renormalization group approach to zero-dimensional interacting systems
We apply the functional renormalization group method to the calculation of
dynamical properties of zero-dimensional interacting quantum systems. As case
studies we discuss the anharmonic oscillator and the single impurity Anderson
model. We truncate the hierarchy of flow equations such that the results are at
least correct up to second order perturbation theory in the coupling. For the
anharmonic oscillator energies and spectra obtained within two different
functional renormalization group schemes are compared to numerically exact
results, perturbation theory, and the mean field approximation. Even at large
coupling the results obtained using the functional renormalization group agree
quite well with the numerical exact solution. The better of the two schemes is
used to calculate spectra of the single impurity Anderson model, which then are
compared to the results of perturbation theory and the numerical
renormalization group. For small to intermediate couplings the functional
renormalization group gives results which are close to the ones obtained using
the very accurate numerical renormalization group method. In particulare the
low-energy scale (Kondo temperature) extracted from the functional
renormalization group results shows the expected behavior.Comment: 22 pages, 8 figures include
Symmetric Anderson impurity model with a narrow band
The single channel Anderson impurity model is a standard model for the
description of magnetic impurities in metallic systems. Usually, the bandwidth
represents the largest energy scale of the problem. In this paper, we analyze
the limit of a narrow band, which is relevant for the Mott-Hubbard transition
in infinite dimensions. For the symmetric model we discuss two different
effects: i) The impurity contribution to the density of states at the Fermi
surface always turns out to be negative in such systems. This leads to a new
crossover in the thermodynamic quantities that we investigate using the
numerical renormalization group. ii) Using the Lanczos method, we calculate the
impurity spectral function and demonstrate the breakdown of the skeleton
expansion on an intermediate energy scale. Luttinger's theorem, as an example
of the local Fermi liquid property of the model, is shown to still be valid.Comment: 4 pages RevTeX, 2 eps figures included, final versio
A Local Moment Approach to magnetic impurities in gapless Fermi systems
A local moment approach is developed for the single-particle excitations of a
symmetric Anderson impurity model (AIM), with a soft-gap hybridization
vanishing at the Fermi level with a power law r > 0. Local moments are
introduced explicitly from the outset, and a two-self-energy description is
employed in which the single-particle excitations are coupled dynamically to
low-energy transverse spin fluctuations. The resultant theory is applicable on
all energy scales, and captures both the spin-fluctuation regime of strong
coupling (large-U), as well as the weak coupling regime. While the primary
emphasis is on single particle dynamics, the quantum phase transition between
strong coupling (SC) and (LM) phases can also be addressed directly; for the
spin-fluctuation regime in particular a number of asymptotically exact results
are thereby obtained. Results for both single-particle spectra and SC/LM phase
boundaries are found to agree well with recent numerical renormalization group
(NRG) studies. A number of further testable predictions are made; in
particular, for r < 1/2, spectra characteristic of the SC state are predicted
to exhibit an r-dependent universal scaling form as the SC/LM phase boundary is
approached and the Kondo scale vanishes. Results for the `normal' r = 0 AIM are
moreover recovered smoothly from the limit r -> 0, where the resultant
description of single-particle dynamics includes recovery of Doniach-Sunjic
tails in the Kondo resonance, as well as characteristic low-energy Fermi liquid
behaviour.Comment: 52 pages, 19 figures, submitted to Journal of Physics: Condensed
Matte
Tidal flat deposits of the Lower Proterozoic Campbell Group along the southwestern margin of the Kaapvaal Craton, Northern Cape Province, South Africa
Lower Proterozoic stromatolites and associated clastic carbonate deposits of the Campbell Group, from the southern margin (Prieska area) of the Kaapvaal Craton, northern Cape Province, are described. Contrary to previous interpretations (Beukes, 1978; 1980a) shallow subtidal to supratidal facies are recognised and discussed in regional context. An alternative model for the facies development of the Campbell Group is proposed
Spectral function of the Kondo model in high magnetic fields
Using a recently developed perturbative renormalization group (RG) scheme, we
calculate analytically the spectral function of a Kondo impurity for either
large frequencies w or large magnetic field B and arbitrary frequencies. For
large w >> max[B,T_K] the spectral function decays as 1/ln^2[ w/T_K ] with
prefactors which depend on the magnetization. The spin-resolved spectral
function displays a pronounced peak at w=B with a characteristic asymmetry. In
a detailed comparison with results from numerical renormalization group (NRG)
and bare perturbation theory in next-to-leading logarithmic order, we show that
our perturbative RG scheme is controlled by the small parameter 1/ln[
max(w,B)/T_K]. Furthermore, we assess the ability of the NRG to resolve
structures at finite frequencies.Comment: 8 pages, version published in PRB, minor change
Kondo effect in real quantum dots
Exchange interaction within a quantum dot strongly affects the transport
through it in the Kondo regime. In a striking difference with the results of
the conventional model, where this interaction is neglected, here the
temperature and magnetic field dependence of the conductance may become
non-monotonic: its initial increase follows by a drop when temperature and
magnetic field are lowered
- …