57,612 research outputs found
Mesoscopic Noise Theory: Microscopics, or Phenomenology?
We argue, physically and formally, that existing diffusive models of noise
yield inaccurate microscopic descriptions of nonequilibrium current
fluctuations. The theoretical shortfall becomes pronounced in quantum-confined
metallic systems, such as the two-dimensional electron gas. In such systems we
propose a simple experimental test of mesoscopic validity for diffusive
theory's central claim: the smooth crossover between Johnson-Nyquist and shot
noise.Comment: Invited paper, UPoN'99 Conference, Adelaide. 13 pp, no figs. Minor
revisions to text and reference
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Three-dimensional cometary dust coma modelling in the collisionless regime: strengths and weaknesses
Inverse coma and tail modelling of comets based on the method developed by Finson & Probstein is commonly used to analyse cometary coma images. Models of this type often contain a large number of assumptions that may not be constrained unless wide temporal or spectral coverage is available and the comets are bright and at relatively small geocentric distance. They are used to predict physical parameters, such as the mass distribution of the dust, but rarely give assessments of the accuracy of the estimate. A three-dimensional cometary dust coma model in the collisionless regime has been developed to allow the effectiveness of such models to constrain dust coma properties to be tested. The model is capable of simulating the coma morphology for the following input parameters: the comet nucleus shape, size, rotation, emission function (including active fraction and jets), grain velocity distribution (and dispersion), size distribution, dust production rate, grain material and light scattering from the cometary dust.
Characterization of the model demonstrates that the mass distribution cannot be well constrained as is often assumed; the cumulative mass distribution index ? can only be constrained to within ±0.15. The model is highly sensitive to the input grain terminal velocity distribution so model input can be tested with a large degree of confidence. Complex secondary parameters such as jets, rotation and grain composition all have an effect on the structure of the coma in similar ways, so unique solutions for these parameters cannot be derived from a single optical image alone. Multiple images at a variety of geometries close in time can help constrain these effects.
The model has been applied to photometric observations of comets 126P/IRAS and 46P/Wirtanen to constrain a number of physical properties including the dust production rate and mass distribution index. The derived dust production rate (Qdust) for 46P/Wirtanen was 3+7/1.5 kg s1 at a pre-perihelion heliocentric distance of 1.8 au, and for P/IRAS was 50+100/20 kg s1 at a pre-perihelion heliocentric distance of 1.7 au; both comets exhibited a mass distribution index ? = 0.8 ± 0.15
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
Physics at COSY
The COSY accelerator in J\'ulich is presented together with its internal and
external detectors. The physics programme performed recently is discussed with
emphasis on strangeness physics.Comment: Invited talk given at the "10th International Symposium on
Meson-Nucleon Physics and the Structure of the Nucleon (MENU04)," IHEP,
Beijing, China, 30/Aug.-4/Sept./0
Cosmic microwave background constraints on coupled dark matter
We study CMB constraints on a model with a cosmological constant and a
fraction of dark matter non-minimally coupled to a massless scalar field. In
this scenario, there is an extra gravity-like fifth force which can affect the
evolution of the Universe enough to have a discernible effect on measurements
of cosmological parameters. Using Planck and WMAP polarisation data, we find
that up to half of the dark matter can be coupled. The coupling can also be
several times larger than in models with a single species of cold dark matter
coupled to a quintessence scalar field, as the scalar field does not play the
role of dark energy and is therefore less constrained by the data.Comment: 5 pages, 4 figure
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
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