15,114 research outputs found
Study of state-of-the-art static inverter design Final report, 6 Jan. - 6 Jun. 1966
Multiple purpose inverter design based on phase demodulated inverter circuit selected from state-of-the-art assessment of ten inverter circuit
New Consequences of Induced Transparency in a Double-Lambda scheme: Destructive Interference In Four-wave Mixing
We investigate a four-state system interacting with long and short laser
pulses in a weak probe beam approximation. We show that when all lasers are
tuned to the exact unperturbed resonances, part of the four-wave mixing (FWM)
field is strongly absorbed. The part which is not absorbed has the exact
intensity required to destructively interfere with the excitation pathway
involved in producing the FWM state. We show that with this three-photon
destructive interference, the conversion efficiency can still be as high as
25%. Contrary to common belief,our calculation shows that this process, where
an ideal one-photon electromagnetically induced transparency is established, is
not most suitable for high efficiency conversion. With appropriate
phase-matching and propagation distance, and when the three-photon destructive
interference does not occur, we show that the photon flux conversion efficiency
is independent of probe intensity and can be close to 100%. In addition, we
show clearly that the conversion efficiency is not determined by the maximum
atomic coherence between two lower excited states, as commonly believed. It is
the combination of phase-matching and constructive interference involving the
two terms arising in producing the mixing wave that is the key element for the
optimized FWM generation. Indeed, in this scheme no appreciable excited state
is produced, so that the atomic coherence between states |0> and |2> is always
very small.Comment: Submitted to Phys. Rev. A, 7 pages, 4 figure
A second eigenvalue bound for the Dirichlet Schroedinger operator
Let be the th eigenvalue of the Schr\"odinger
operator with Dirichlet boundary conditions on a bounded domain and with the positive potential . Following the spirit of the
Payne-P\'olya-Weinberger conjecture and under some convexity assumptions on the
spherically rearranged potential , we prove that . Here denotes the ball, centered at the
origin, that satisfies the condition .
Further we prove under the same convexity assumptions on a spherically
symmetric potential , that decreases
when the radius of the ball increases.
We conclude with several results about the first two eigenvalues of the
Laplace operator with respect to a measure of Gaussian or inverted Gaussian
density
Ensemble density-functional theory for ab-initio molecular dynamics of metals and finite-temperature insulators
A new method is presented for performing first-principles molecular-dynamics
simulations of systems with variable occupancies. We adopt a matrix
representation for the one-particle statistical operator Gamma, to introduce a
``projected'' free energy functional G that depends on the Kohn-Sham orbitals
only and that is invariant under their unitary transformations. The Liouville
equation [ Gamma , H ] = 0 is always satisfied, guaranteeing a very efficient
and stable variational minimization algorithm that can be extended to
non-conventional entropic formulations or fictitious thermal distributions.Comment: 5 pages, two-column style with 2 postscript figures embedded. Uses
REVTEX and epsf macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#nm_meta
Shell-model calculations for the three-nucleon system
We use Faddeev's decomposition to solve the shell-model problem for three
nucleons. The dependence on harmonic-oscillator excitations allowed in the
model space, up to in the present calculations, and on the
harmonic-oscillator frequency is studied. Effective interactions derived from
Nijmegen II and Reid93 potentials are used in the calculations. The binding
energies obtained are close to those calculated by other methods. The structure
of the Faddeev equations is discussed and a simple formula for matrix elements
of the permutation operators in a harmonic-oscillator basis is given. The Pauli
principle is properly treated in the calculations.Comment: 11 pages. REVTEX. 6 PostScript figure
Feeding Interactions Between Juvenile and Adult Flightless Cormorants.
We report observations on chick feedings by adult Flightless Cormorants Phalacrocorax harrisi, indicating that, contrary to the literature, the sequence of interaction is similar to that of other Pelecaniformes
Comparing verbal media for alarm handling: Speech versus textual displays
The rise of computers in command and control domains has meant that control operations can be performed via desk-based visual display terminals. This trend has also produced the potential to display information to operators in a variety of formats. Of particular interest has been the use of text-based displays for alarm presentation. There are possible limitations to the use of text for alarm presentation, not least of which is the need for a dedicated alarms display screen (or, at least, a display page). Given the capability of computers to synthesize speech, it is possible that speech-based alarms could generate the same information as text-based displays without the need for dedicated screen space. In this paper an experimental comparison of speech-based and text-based displays for presentation of alarms is reported. The findings show that speech leads to longer response times than text displays, but that it has minimal effect on the efficacy of fault handling. The results are discussed within the alarm initiated activities framework and implications for alarm system design are outlined
Cholesterol and coronary heart disease: screening and treatment
Coronary heart disease (CHD) is a major cause of morbidity and mortality in the United Kingdom, accounting for just under one quarter of all deaths in 1995: 27% among men and 21% among women.1 Although many CHD deaths occur among elderly people, CHD accounts for 31% of male and 13% of female deaths within the 45–64 age group
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How alliteration enhances conceptual–attentional interactions in reading
In linguistics, the relationship between phonological word form and meaning is mostly considered arbitrary. Why, then, do literary authors traditionally craft sound relationships between words? We set out to characterise how dynamic interactions between word form and meaning may account for this literary practice. Here, we show that alliteration influences both meaning integration and attentional engagement during reading. We presented participants with adjective-noun phrases, having manipulated semantic relatedness (congruent, incongruent) and form repetition (alliterating, non-alliterating) orthogonally, as in "dazzling-diamond"; "sparkling-diamond"; "dangerous-diamond"; and "creepy-diamond". Using simultaneous recording of event-related brain potentials and pupil dilation (PD), we establish that, whilst semantic incongruency increased N400 amplitude as expected, it reduced PD, an index of attentional engagement. Second, alliteration affected semantic evaluation of word pairs, since it reduced N400 amplitude even in the case of unrelated items (e.g., "dangerous-diamond"). Third, alliteration specifically boosted attentional engagement for related words (e.g., "dazzling-diamond"), as shown by a sustained negative correlation between N400 amplitudes and PD change after the window of lexical integration. Thus, alliteration strategically arouses attention during reading and when comprehension is challenged, phonological information helps readers link concepts beyond the level of literal semantics. Overall, our findings provide a tentative mechanism for the empowering effect of sound repetition in literary constructs
X-ray Spectral Formation in a Converging Fluid Flow: Spherical Accretion into Black Holes
We study Compton upscattering of low-frequency photons in a converging flow
of thermal plasma. The photons escape diffusively and electron scattering is
the dominant source of opacity. We solve numerically and approximately
analytically the equation of radiative transfer in the case of spherical,
steady state accretion into black holes. Unlike previous work on this subject,
we consider the inner boundary at a finite radius and this has a significant
effect on the emergent spectrum. It is shown that the bulk motion of the
converging flow is more efficient in upscattering photons than thermal
Comptonization, provided that the electron temperature in the flow is of order
a few keV or less. In this case, the spectrum observed at infinity consists of
a soft component coming from those input photons which escaped after a few
scatterings without any significant energy change and of a power law which
extends to high energies and is made of those photons which underwent
significant upscattering. The luminosity of the power law is relatively small
compared to that of the soft component. The more reflective the inner boundary
is, the flatter the power-law spectrum becomes. The spectral energy power-law
index for black-hole accretion is always higher than 1 and it is approximately
1.5 for high accretion rates. This result tempts us to say that bulk motion
Comptonization might be the mechanism behind the power-law spectra seen in
black-hole X-ray sources.Comment: 37 pages, LaTex, AAS Macros, 8 ps figures, to appear in Ap
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