25,644 research outputs found
Are the Perseus-Pisces chain and the Pavo-Indus wall connected?
A significant empty region was found between the southern Pavo- Indus (PI)
wall and the northern Perseus-Pisces (PP) chain. This survey tests the reality
of this void which may simply reflect previous poor sampling of the galaxies in
this region. Redshifts for a magnitude selected sample of 379 galaxies were
obtained covering the four UKST/SERC survey fields with Bt <= 17.0. All
redshifts were obtained with the FLAIR multi-object spectroscopy system on the
1.2 m U.K. Schmidt Telescope at Siding Spring, Australia. Two highly
significant density enhancements were found in the galaxy distribution at 133
Mpc and 200 Mpc (Ho=75 km/s/Mpc). We claim that no connexion exists between PP
and PI. However, a southern extension of PP was detected and makes the total
length of this chain of more than 150 Mpc.Comment: 14 pages, postscript including tables and figures
Fundamental Limits of Low-Density Spreading NOMA with Fading
Spectral efficiency of low-density spreading non-orthogonal multiple access
channels in the presence of fading is derived for linear detection with
independent decoding as well as optimum decoding. The large system limit, where
both the number of users and number of signal dimensions grow with fixed ratio,
called load, is considered. In the case of optimum decoding, it is found that
low-density spreading underperforms dense spreading for all loads. Conversely,
linear detection is characterized by different behaviors in the underloaded vs.
overloaded regimes. In particular, it is shown that spectral efficiency changes
smoothly as load increases. However, in the overloaded regime, the spectral
efficiency of low- density spreading is higher than that of dense spreading
Hard sphere-like dynamics in a non hard sphere liquid
The collective dynamics of liquid Gallium close to the melting point has been
studied using Inelastic X-ray Scattering to probe lengthscales smaller than the
size of the first coordination shell. %(momentum transfers, , 15
nm). Although the structural properties of this partially covalent
liquid strongly deviate from a simple hard-sphere model, the dynamics, as
reflected in the quasi-elastic scattering, are beautifully described within the
framework of the extended heat mode approximation of Enskog's kinetic theory,
analytically derived for a hard spheres system. The present work demonstrates
the applicability of Enskog's theory to non hard- sphere and non simple
liquids.Comment: 5 pages, 2 figures, accepted in Phys. Rev. Let
Medium modifications of the nucleon-nucleon elastic cross section in neutron-rich intermediate energy HICs
Several observables of unbound nucleons which are to some extent sensitive to
the medium modifications of nucleon-nucleon elastic cross sections in
neutron-rich intermediate energy heavy ion collisions are investigated. The
splitting effect of neutron and proton effective masses on cross sections is
discussed. It is found that the transverse flow as a function of rapidity, the
as a function of momentum, and the ratio of halfwidths of the
transverse to that of longitudinal rapidity distribution are very
sensitive to the medium modifications of the cross sections. The transverse
momentum distribution of correlation functions of two-nucleons does not yield
information on the in-medium cross section.Comment: 14 pages, 5 figure
Axial vector form factor of nucleons in a light-cone diquark model
The nucleon axial vector form factor is investigated in a light-cone quark
spectator diquark model, in which Melosh rotations are applied to both the
quark and vector diquark. It is found that this model gives a very good
description of available experimental data and the results have very little
dependence on the parameters of the model. The relation between the nucleon
axial constant and the anomalous magnetic moment of nucleons is also discussed.Comment: 8 pages, Revtex4, 1 figure, version to be published in Phys. Rev.
Field emission from single multi-wall carbon nanotubes
Electron field emission characteristics of individual multiwalled carbon
nanotubes have been investigated by a piezoelectric nanomanipulation system
operating inside a scanning electron microscopy chamber. The experimental setup
ensures a high control capability on the geometric parameters of the field
emission system (CNT length, diameter and anode-cathode distance). For several
multiwalled carbon nanotubes, reproducible and quite stable emission current
behaviour has been obtained with a dependence on the applied voltage well
described by a series resistance modified Fowler-Nordheim model. A turn-on
field of about 30 V/um and a field enhancement factor of around 100 at a
cathode-anode distance of the order of 1 um have been evaluated. Finally, the
effect of selective electron beam irradiation on the nanotube field emission
capabilities has been extensively investigated.Comment: 16 pages, 5 figure
Electronic Excitations and Insulator-Metal Transition in Poly(3-hexylthiophene) Organic Field-Effect Transistors
We carry out a comprehensive theoretical and experimental study of charge
injection in Poly(3-hexylthiophene) (P3HT) to determine the most likely
scenario for metal-insulator transition in this system. We calculate the
optical absorption frequencies corresponding to a polaron and a bipolaron
lattice in P3HT. We also analyze the electronic excitations for three possible
scenarios under which a first-- or a second--order metal--insulator transition
can occur in doped P3HT. These theoretical scenarios are compared with data
from infrared absorption spectroscopy on P3HT thin film field-effect
transistors (FET). Our measurements and theoretical predictions suggest that
charge-induced localized states in P3HT FETs are bipolarons and that the
highest doping level achieved in our experiments approaches that required for a
first-order metal--insulator transition.Comment: 9 pages, 4 figures. Phys. Rev. B, in pres
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