5,673 research outputs found
Closed-form expressions for correlated density matrices: application to dispersive interactions and example of (He)2
Empirically correlated density matrices of N-electron systems are
investigated. Exact closed-form expressions are derived for the one- and
two-electron reduced density matrices from a general pairwise correlated wave
function. Approximate expressions are proposed which reflect dispersive
interactions between closed-shell centro-symmetric subsystems. Said expressions
clearly illustrate the consequences of second-order correlation effects on the
reduced density matrices. Application is made to a simple example: the (He)2
system. Reduced density matrices are explicitly calculated, correct to second
order in correlation, and compared with approximations of independent electrons
and independent electron pairs. The models proposed allow for variational
calculations of interaction energies and equilibrium distance as well as a
clear interpretation of dispersive effects on electron distributions. Both
exchange and second order correlation effects are shown to play a critical role
on the quality of the results.Comment: 22 page
The refined transfer, bundle structures and algebraic K-theory
We give new homotopy theoretic criteria for deciding when a fibration with
homotopy finite fibers admits a reduction to a fiber bundle with compact
topological manifold fibers. The criteria lead to a new and unexpected result
about homeomorphism groups of manifolds. A tool used in the proof is a
surjective splitting of the assembly map for Waldhausen's functor A(X).
We also give concrete examples of fibrations having a reduction to a fiber
bundle with compact topological manifold fibers but which fail to admit a
compact fiber smoothing. The examples are detected by algebraic K-theory
invariants.
We consider a refinement of the Becker-Gottlieb transfer. We show that a
version of the axioms described by Becker and Schultz uniquely determines the
refined transfer for the class of fibrations admitting a reduction to a fiber
bundle with compact topological manifold fibers.
In an appendix, we sketch a theory of characteristic classes for fibrations.
The classes are primary obstructions to finding a compact fiber smoothing.Comment: This version contains mostly minor revision
Relativistic and Radiative Corrections to the Mollow Spectrum
The incoherent, inelastic part of the resonance fluorescence spectrum of a
laser-driven atom is known as the Mollow spectrum [B. R. Mollow, Phys. Rev.
188, 1969 (1969)]. Starting from this level of description, we discuss
theoretical foundations of high-precision spectroscopy using the resonance
fluorescence light of strongly laser-driven atoms. Specifically, we evaluate
the leading relativistic and radiative corrections to the Mollow spectrum, up
to the relative orders of (Z alpha)^2 and alpha(Z alpha)^2, respectively, and
Bloch-Siegert shifts as well as stimulated radiative corrections involving
off-resonant virtual states. Complete results are provided for the hydrogen
1S-2P_{1/2} and 1S-2P_{3/2} transitions; these include all relevant correction
terms up to the specified order of approximation and could directly be compared
to experimental data. As an application, the outcome of such experiments would
allow for a sensitive test of the validity of the dressed-state basis as the
natural description of the combined atom-laser system.Comment: 20 pages, 1 figure; RevTe
Strong-field approximation for intense-laser atom processes: the choice of gauge
The strong-field approximation can be and has been applied in both length
gauge and velocity gauge with quantitatively conflicting answers. For
ionization of negative ions with a ground state of odd parity, the predictions
of the two gauges differ qualitatively: in the envelope of the angular-resolved
energy spectrum, dips in one gauge correspond to humps in the other. We show
that the length-gauge SFA matches the exact numerical solution of the
time-dependent Schr\"odinger equation.Comment: 5 pages, 3 figures, revtex
Correlated electron emission in laser-induced nonsequence double ionization of Helium
In this paper, we have investigated the correlated electron emission of the
nonsequence double ionization (NSDI) in an intense linearly polarized field.
The theoretical model we employed is the semiclassical rescattering model, the
model atom we used is the helium. We find a significant correlation between
magnitude and direction of the momentum of two emission electrons, and give a
good explanation for this striking phenomenon by observing the classical
collisional trajectories. We argue that this correlation phenomenon is
universal in NSDI process, as revealed by the recent experiment on the argon.Comment: 4 pages, 3 figures, accepted for publication in Phys. Rev.
Electron impact double ionization of helium from classical trajectory calculations
With a recently proposed quasiclassical ansatz [Geyer and Rost, J. Phys. B 35
(2002) 1479] it is possible to perform classical trajectory ionization
calculations on many electron targets. The autoionization of the target is
prevented by a M\o{}ller type backward--forward propagation scheme and allows
to consider all interactions between all particles without additional
stabilization. The application of the quasiclassical ansatz for helium targets
is explained and total and partially differential cross sections for electron
impact double ionization are calculated. In the high energy regime the
classical description fails to describe the dominant TS1 process, which leads
to big deviations, whereas for low energies the total cross section is
reproduced well. Differential cross sections calculated at 250 eV await their
experimental confirmation.Comment: LaTeX, 22 pages, 10 figures, submitted to J. Phys.
Three channel model of meson-meson scattering and scalar meson spectroscopy
New solutions on the scalar -- isoscalar phase shifts are analysed
together with previous results using a separable potential model of
three coupled channels (, and an effective
system). Model parameters are fitted to two sets of solutions obtained in a
recent analysis of the CERN-Cracow-Munich measurements of the reaction on a polarized target. A relatively
narrow (90 -- 180 MeV) scalar resonance is found, in contrast
to a much broader ( MeV) state emerging from the analysis
of previous unpolarized target data.Comment: 10 Latex pages + 6 postscript figure
Radio spectra and polarisation properties of a bright sample of Radio-Loud Broad Absorption Line Quasars
The origin of broad-absorption-line quasars (BAL QSOs) is still an open
issue. Accounting for ~20% of the QSO population, these objects present broad
absorption lines in their optical spectra generated from outflows with
velocities up to 0.2c. In this work we present the results of a multi-frequency
study of a well-defined radio-loud BAL QSO sample, and a comparison sample of
radio-loud non-BAL QSOs, both selected from the Sloan Digital Sky Survey
(SDSS).
We aim to test which of the currently-popular models for the BAL phenomenon -
`orientation' or 'evolutionary' - best accounts for the radio properties of BAL
quasars. Observations from 1.4 to 43 GHz have been obtained with the VLA and
Effelsberg telescopes, and data from 74 to 408 MHz have been compiled from the
literature.
The fractions of candidate GHz-peaked sources are similar in the two samples
(36\pm12% vs 23\pm8%), suggesting that BAL QSOs are not generally younger than
non-BAL QSOs. BAL and non-BAL QSOs show a large range of spectral indices,
consistent with a broad range of orientations. There is weak evidence (91%
confidence) that the spectral indices of the BAL QSOs are steeper than those of
non-BAL QSOs, mildly favouring edge-on orientations. At a higher level of
significance (\geq97%), the spectra of BAL QSOs are not flatter than those of
non-BAL QSOs, which suggests that a polar orientation is not preferred.Comment: Accepted by A&
The prenatal ultrasonographic detection of myelomeningocele in patients referred to Children's Hospital Medical Center: a cross sectional study
BACKGROUND: To find out about the prenatal diagnosis rate of myelomeningocele (MMC) by ultrasound scan in patients referred to the Children's Hospital Medical Center in Tehran, Iran from July 2004 to July 2005. METHODS: We included 140 children born with MMC and who were referred for management, surgery and treatment of complications associated with it. The ultrasound reports were examined. Data on sex, age, location of MMC, time of prenatal ultrasound and the trimester in which the diagnosis was made along with the results of the diagnosis (MMC, hydrocephalus, or both), were collected. RESULTS: Among the studied patients, 136 (97.1%) cases had prenatal ultrasound, amongst those, 58 (42.6%) sonographic evaluations were diagnostic for hydrocephalus and/or MMC. The prenatal ultrasound was positive for MMC in 16 (11.8%), hydrocephalus in 25 (18.4%) and both MMC and hydrocephalus in 17 (12.5%) cases. Among all cases with prenatal diagnosis of MMC, 3.4% were detected in the first, 31% in the second and 65.5% in the third trimester. Thoracic/thoracolumbar lesions were found prenatally in 40% of cases, which is significantly higher than the detection rate of other locations including cervical/cervicothoracic and lumbar/lumbosacral/sacral regions diagnosed only in 0% and 21% of cases respectively. CONCLUSION: There is a large difference between the detection rate of our population (24.3%) compared to others (68%). Pregnant women should have an ultrasound at 20â22 week for detection of congenital anomalies including MMC
Exploration and exploitation in the presence of network externalities
This paper examines the conditions under which exploration of a new, incompatible technologyis
conducive to firm growth in the presence of network externalities. In particular,
this studyis motivated bythe divergent evolutions of the PC and the workstation markets in
response to a new technology: reduced instruction set computing (RISC). In the PC market,
Intel has developed new microprocessors bymaintaining compatibilitywith the established
architecture, whereas it was radicallyr eplaced byRISC in the workstation market. History
indicates that unlike the PC market, the workstation market consisted of a large number
of power users, who are less sensitive to compatibilitythan ordinaryusers. Our numerical
analysis indicates that the exploration of a new, incompatible technologyis more likelyto
increase the chance of firm growth when there are a substantial number of power users or
when a new technologyis introduced before an established technologytakes off.
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