46,766 research outputs found
Positron scattering and annihilation on noble gas atoms
Positron scattering and annihilation on noble gas atoms below the positronium
formation threshold is studied ab initio using many-body theory methods. The
many-body theory provides a near-complete understanding of the
positron-noble-gas-atom system at these energies and yields accurate numerical
results. It accounts for positron-atom and electron-positron correlations,
e.g., polarization of the atom by the incident positron and the
non-perturbative process of virtual positronium formation. These correlations
have a large effect on the scattering dynamics and result in a strong
enhancement of the annihilation rates compared to the independent-particle
mean-field description. Computed elastic scattering cross sections are found to
be in good agreement with recent experimental results and Kohn variational and
convergent close-coupling calculations. The calculated values of the
annihilation rate parameter (effective number of electrons
participating in annihilation) rise steeply along the sequence of noble gas
atoms due to the increasing strength of the correlation effects, and agree well
with experimental data.Comment: 24 pages, 17 figure
What is novel in quantum transport for mesoscopics?
The understanding of mesoscopic transport has now attained an ultimate
simplicity. Indeed, orthodox quantum kinetics would seem to say little about
mesoscopics that has not been revealed - nearly effortlessly - by more popular
means. Such is far from the case, however. The fact that kinetic theory remains
very much in charge is best appreciated through the physics of a quantum point
contact. While discretization of its conductance is viewed as the exclusive
result of coherent, single-electron-wave transmission, this does not begin to
address the paramount feature of all metallic conduction: dissipation. A
perfect quantum point contact still has finite resistance, so its ballistic
carriers must dissipate the energy gained from the applied field. How do they
manage that? The key is in standard many-body quantum theory, and its
conservation principles.Comment: 10 pp, 3 figs. Invited talk at 50th Golden Jubilee DAE Symposium,
BARC, Mumbai, 200
Ballistic transport is dissipative: the why and how
In the ballistic limit, the Landauer conductance steps of a mesoscopic
quantum wire have been explained by coherent and dissipationless transmission
of individual electrons across a one-dimensional barrier. This leaves untouched
the central issue of conduction: a quantum wire, albeit ballistic, has finite
resistance and so must dissipate energy. Exactly HOW does the quantum wire shed
its excess electrical energy? We show that the answer is provided, uniquely, by
many-body quantum kinetics. Not only does this inevitably lead to universal
quantization of the conductance, in spite of dissipation; it fully resolves a
baffling experimental result in quantum-point-contact noise. The underlying
physics rests crucially upon the action of the conservation laws in these open
metallic systems.Comment: Invited Viewpoint articl
On certain other sets of integers
We show that if A is a subset of {1,...,N} containing no non-trivial
three-term arithmetic progressions then |A|=O(N/ log^{3/4-o(1)} N).Comment: 29 pp. Corrected typos. Added definitions for some non-standard
notation and remarks on lower bound
High-field noise in metallic diffusive conductors
We analyze high-field current fluctuations in degenerate conductors by
mapping the electronic Fermi-liquid correlations at equilibrium to their
semiclassical non-equilibrium form. Our resulting Boltzmann description is
applicable to diffusive mesoscopic wires. We derive a non-equilibrium
connection between thermal fluctuations of the current and resistive
dissipation. In the weak-field limit this is the canonical fluctuation-
dissipation theorem. Away from equilibrium, the connection enables explicit
calculation of the excess ``hot-electron'' contribution to the thermal
spectrum. We show that excess thermal noise is strongly inhibited by Pauli
exclusion. This behaviour is generic to the semiclassical metallic regime.Comment: 13 pp, one fig. Companion paper to cond-mat/9911251. Final version,
to appear in J. Phys.: Cond. Ma
Assessment of the dimensionality of the Wijma delivery expectancy/experience questionnaire using factor analysis and Rasch analysis
Background: Fear of childbirth has negative consequences for a woman's physical and emotional wellbeing. The most commonly used measurement tool for childbirth fear is the Wijma Delivery Expectancy Questionnaire (WDEQ-A). Although originally conceptualized as unidimensional, subsequent investigations have suggested it is multidimensional. This study aimed to undertake a detailed psychometric assessment of the WDEQ-A; exploring the dimensionality and identifying possible subscales that may have clinical and research utility.
Methods: WDEQ-A was administered to a sample of 1410 Australian women in mid-pregnancy. The dimensionality of WDEQ-A was explored using exploratory (EFA) and confirmatory factor analysis (CFA), and Rasch analysis.
Results: EFA identified a four factor solution. CFA failed to support the unidimensional structure of the original WDEQ-A, but confirmed the four factor solution identified by EFA. Rasch analysis was used to refine the four subscales (Negative emotions: five items; Lack of positive emotions: five items; Social isolation: four items; Moment of birth: three items). Each WDEQ-A Revised subscale showed good fit to the Rasch model and adequate internal consistency reliability. The correlation between Negative emotions and Lack of positive emotions was strong, however Moment of birth and Social isolation showed much lower intercorrelations, suggesting they should not be added to create a total score.
Conclusion: This study supports the findings of other investigations that suggest the WDEQ-A is multidimensional and should not be used in its original form. The WDEQ-A Revised may provide researchers with a more refined, psychometrically sound tool to explore the differential impact of aspects of childbirth fear.Full Tex
Coulomb screening in mesoscopic noise: a kinetic approach
Coulomb screening, together with degeneracy, is characteristic of the
metallic electron gas. While there is little trace of its effects in transport
and noise in the bulk, at mesoscopic scales the electronic fluctuations start
to show appreciable Coulomb correlations. Within a strictly standard Boltzmann
and Fermi-liquid framework, we analyze these phenomena and their relation to
the mesoscopic fluctuation-dissipation theorem, which we prove. We identify two
distinct screening mechanisms for mesoscopic fluctuations. One is the
self-consistent response of the contact potential in a non-uniform system. The
other couples to scattering, and is an exclusively non-equilibrium process.
Contact-potential effects renormalize all thermal fluctuations, at all scales.
Collisional effects are relatively short-ranged and modify non-equilibrium
noise. We discuss ways to detect these differences experimentally.Comment: Source: REVTEX. 16 pp.; 7 Postscript figs. Accepted for publication
in J. Phys.: Cond. Ma
Far-infrared rotational emission by carbon monoxide
Accurate theoretical collisional excitation rates are used to determine the emissivities of CO rotational lines 10 to the 4th power/cu cm n(H2), 100 K T 2000 K, and J 50. An approximate analytic expression for the emissitivities which is valid over most of this region is obtained. Population inversions in the lower rotational levels occur for densities n(H2) approximately 10 (to the 3rd to 5th power)/cu cm and temperatures T approximately 50 K. Interstellar shocks observed edge on are a potential source of millimeter wave CO maser emission. The CO rotational cooling function suggested by Hollenbach and McKee (1979) is verified, and accurate numerical values given. Application of these results to other linear molecules should be straightforward
Radio Spectral Index and Expansion of 3C58
We present new observations of the plerionic supernova remnant 3C58 with the
VLA at 74 and 327 MHz. In addition, we re-reduced earlier observations at 1.4
and 4.9 GHz taken in 1973 and 1984. Comparing these various images, we find
that: 1. the remnant has a flat and relatively uniform spectral index
distribution, 2. any expansion of the remnant with time is significantly less
than that expected for uniform, undecelerated expansion since the generally
accepted explosion date in 1181 A.D., and 3. there is no evidence for a
non-thermal synchrotron emission shell generated by a supernova shock wave,
with any such emission having a surface brightness of <1 x 10^(-21) W / (m^2 Hz
sr) at 327 MHz.Comment: 18 pages, 7 Figures, Latex, Accepted for publication in the
Astrophysical Journa
Effect of positron-atom interactions on the annihilation gamma spectra of molecules
Calculations of gamma spectra for positron annihilation on a selection of
molecules, including methane and its fluoro-substitutes, ethane, propane,
butane and benzene are presented. The annihilation gamma spectra characterise
the momentum distribution of the electron-positron pair at the instant of
annihilation. The contribution to the gamma spectra from individual molecular
orbitals is obtained from electron momentum densities calculated using modern
computational quantum chemistry density functional theory tools. The
calculation, in its simplest form, effectively treats the low-energy
(thermalised, room-temperature) positron as a plane wave and gives annihilation
gamma spectra that are about 40% broader than experiment, although the main
chemical trends are reproduced. We show that this effective "narrowing" of the
experimental spectra is due to the action of the molecular potential on the
positron, chiefly, due to the positron repulsion from the nuclei. It leads to a
suppression of the contribution of small positron-nuclear separations where the
electron momentum is large. To investigate the effect of the nuclear repulsion,
as well as that of short-range electron-positron and positron-molecule
correlations, a linear combination of atomic orbital description of the
molecular orbitals is employed. It facilitates the incorporation of correction
factors which can be calculated from atomic many-body theory and account for
the repulsion and correlations. Their inclusion in the calculation gives gamma
spectrum linewidths that are in much better agreement with experiment.
Furthermore, it is shown that the effective distortion of the electron momentum
density, when it is observed through positron annihilation gamma spectra, can
be approximated by a relatively simple scaling factor.Comment: 26 pages, 12 figure
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