534 research outputs found

    Electron microscopic demonstration of centrifugal nerve fibers in the human optic nerve

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    Electron microscopic views of centrifugal nerve fibers in the optic nerve stump of a 56-year-old man are presented. These nerve fibers had survived for 16 days after removal of the corresponding eyeball and exhibited terminal swellings pointing in a distal direction and indicating axoplasmic flow towards the removed eye. The centrifugal nerves in this adult lack any evidence of attempted regeneration that has earlier been observed under similar conditions in the optic nerve stump of a child. Zentrifugale (antidrome, efferente) Nervenfasern sind hier zum ersten Mal mit dem Elektronenmikroskop im menschlichen Sehnerven dargestellt worden. Diese Nervenfasern wurden in dem Sehnervenstumpf eines 56jährigen Mannes 16 Tage nach der Entfernung des dazugehörigen Auges gefunden. Endschwellungen dieser Nervenfasern waren distal ausgerichtet und deuteten damit einen Axoplasmafluß in Richtung des entfernten Auges an. Während deutliche Regenerationsversuche an den distalen Enden unterbrochener zentrifugaler Nervenfasern im Sehnervenstumpf eines Kindes früher beobachtet worden sind, fanden sich im Sehnerven dieses Erwachsenen keinerlei Zeichen von Regeneration der zentrifugalen Fasern.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47369/1/417_2004_Article_BF00414787.pd

    Extended M1 sum rule for excited symmetric and mixed-symmetry states in nuclei

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    A generalized M1 sum rule for orbital magnetic dipole strength from excited symmetric states to mixed-symmetry states is considered within the proton-neutron interacting boson model of even-even nuclei. Analytic expressions for the dominant terms in the B(M1) transition rates from the first and second 2+2^+ states are derived in the U(5) and SO(6) dynamic symmetry limits of the model, and the applicability of a sum rule approach is examined at and in-between these limits. Lastly, the sum rule is applied to the new data on mixed-symmetry states of 94Mo and a quadrupole d-boson ratio nd(01+)/nd(22+)0.6nd(0^+_1)/nd(2^+_2) \approx 0.6 is obtained in a largely parameter-independent wayComment: 19 pages, 3 figures, Revte

    Simulation of dimensionality effects in thermal transport

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    The discovery of nanostructures and the development of growth and fabrication techniques of one- and two-dimensional materials provide the possibility to probe experimentally heat transport in low-dimensional systems. Nevertheless measuring the thermal conductivity of these systems is extremely challenging and subject to large uncertainties, thus hindering the chance for a direct comparison between experiments and statistical physics models. Atomistic simulations of realistic nanostructures provide the ideal bridge between abstract models and experiments. After briefly introducing the state of the art of heat transport measurement in nanostructures, and numerical techniques to simulate realistic systems at atomistic level, we review the contribution of lattice dynamics and molecular dynamics simulation to understanding nanoscale thermal transport in systems with reduced dimensionality. We focus on the effect of dimensionality in determining the phononic properties of carbon and semiconducting nanostructures, specifically considering the cases of carbon nanotubes, graphene and of silicon nanowires and ultra-thin membranes, underlying analogies and differences with abstract lattice models.Comment: 30 pages, 21 figures. Review paper, to appear in the Springer Lecture Notes in Physics volume "Thermal transport in low dimensions: from statistical physics to nanoscale heat transfer" (S. Lepri ed.

    Observation of exclusive DVCS in polarized electron beam asymmetry measurements

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    We report the first results of the beam spin asymmetry measured in the reaction e + p -> e + p + gamma at a beam energy of 4.25 GeV. A large asymmetry with a sin(phi) modulation is observed, as predicted for the interference term of Deeply Virtual Compton Scattering and the Bethe-Heitler process. The amplitude of this modulation is alpha = 0.202 +/- 0.028. In leading-order and leading-twist pQCD, the alpha is directly proportional to the imaginary part of the DVCS amplitude.Comment: 6 pages, 5 figure

    A Kinematically Complete Measurement of the Proton Structure Function F2 in the Resonance Region and Evaluation of Its Moments

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    We measured the inclusive electron-proton cross section in the nucleon resonance region (W < 2.5 GeV) at momentum transfers Q**2 below 4.5 (GeV/c)**2 with the CLAS detector. The large acceptance of CLAS allowed for the first time the measurement of the cross section in a large, contiguous two-dimensional range of Q**2 and x, making it possible to perform an integration of the data at fixed Q**2 over the whole significant x-interval. From these data we extracted the structure function F2 and, by including other world data, we studied the Q**2 evolution of its moments, Mn(Q**2), in order to estimate higher twist contributions. The small statistical and systematic uncertainties of the CLAS data allow a precise extraction of the higher twists and demand significant improvements in theoretical predictions for a meaningful comparison with new experimental results.Comment: revtex4 18 pp., 12 figure
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