55,366 research outputs found
Analytic invariant charge and the lattice static quark-antiquark potential
A recently developed model for the QCD analytic invariant charge is compared
with quenched lattice simulation data on the static quark-antiquark potential.
By employing this strong running coupling one is able to obtain the confining
quark-antiquark potential in the framework of the one-gluon exchange model. To
achieve this objective a technique for evaluating the integrals of a required
form is developed. Special attention is paid here to removing the divergences
encountered the calculations. All this enables one to examine the asymptotic
behavior of the potential at both small and large distances with high accuracy.
An explicit expression for the quark-antiquark potential, which interpolates
between these asymptotics, and satisfies the concavity condition, is proposed.
The derived potential coincides with the perturbative results at small
distances, and it is in a good agreement with the lattice data in the
nonperturbative physically-relevant region. An estimation of the parameter
is obtained for the case of pure gluodynamics. It is found to
be consistent with all the previous estimations of in the
framework of approach in hand.Comment: LaTeX2e, 10 pages with 3 EPS figure
Anomalous Josephson effect in semiconducting nanowires as a signature of the topologically nontrivial phase
We study Josephson junctions made of semiconducting nanowires with Rashba
spin-orbit coupling, where superconducting correlations are induced by the
proximity effect. In the presence of a suitably directed magnetic field, the
system displays the anomalous Josephson effect: a nonzero supercurrent in the
absence of a phase bias between two superconductors. We show that this
anomalous current can be increased significantly by tuning the nanowire into
the helical regime. In particular, in a short junction, a large anomalous
current is a signature for topologically nontrivial superconductivity in the
nanowire.Comment: 10 pages, 9 figures; published versio
Measurement of macroscopic plasma parameters with a radio experiment: Interpretation of the quasi-thermal noise spectrum observed in the solar wind
The ISEE-3 SBH radio receiver has provided the first systematic observations of the quasi-thermal (plasma waves) noise in the solar wind plasma. The theoretical interpretation of that noise involves the particle distribution function so that electric noise measurements with long antennas provide a fast and independent method of measuring plasma parameters: densities and temperatures of a two component (core and halo) electron distribution function have been obtained in that way. The polarization of that noise is frequency dependent and sensitive to the drift velocity of the electron population. Below the plasma frequency, there is evidence of a weak noise spectrum with spectral index -1 which is not yet accounted for by the theory. The theoretical treatment of the noise associated with the low energy (thermal) proton population shows that the moving electrical antenna radiates in the surrounding plasma by Carenkov emission which becomes predominant at the low frequencies, below about 0.1 F sub P
Evaluation of a hybrid hydrostatic bearing for cryogenic turbopump application
A hybrid hydrostatic bearing was designed to operate in liquid hydrogen at speeds to 80,000 rpm and radial loads to 440 n (100 lbf). The bearing assembly consisted of a pair of 20-mm angular-contact ball bearings encased in a journal, which was in turn supported by a fluid film of liquid hydrogen. The size and operating conditions of the bearing were selected to be compatible with the operating requirements of an advanced technology turbopump. Several test parameters were varied to characterize the bearing's steady-state operation. The rotation of the tester shaft was varied between 0 and 80,000 rpm. Bearing inlet fluid pressure was varied between 2.07 and 4.48 MPa (300 and 650 psia), while the fluid sump pressure was independently varied between 0.34 and 2.07 MPa (50 and 300 psia). The maximum radial load applied to the bearing was 440 N (110 lbf). Measured hybrid-hydrostatic-bearing stiffness was 1.5 times greater than predicted, while the fluid flow rate through the bearing was 35 to 65 percent less than predicted. Under two-phase fluid conditions, the stiffness was even greater and the flow rate was less. The optimal pressure ratio for the bearing should be between 0.2 and 0.55 depending on the balance desired between bearing efficiency and stiffness. Startup and shutdown cyclic tests were conducted to demonstrate the ability of the hybrid-hydrostatic-bearing assembly to survive at least a 300-firing-duty cycle. For a typical cycle, the shaft was accelerated to 50,000 rpm in 1.8 sec. The bearing operated for 337 start-stop cycles without failure
Acceleration of weakly collisional solar-type winds
One of the basic properties of the solar wind, that is the high speed of the
fast wind, is still not satisfactorily explained. This is mainly due to the
theoretical difficulty of treating weakly collisional plasmas. The fluid
approach implies that the medium is collision dominated and that the particle
velocity distributions are close to Maxwellians. However the electron velocity
distributions observed in the solar wind depart significantly from Maxwellians.
Recent kinetic collisionless models (called exospheric) using velocity
distributions with a suprathermal tail have been able to reproduce the high
speeds of the fast solar wind. In this letter we present new developments of
these models by generalizing them over a large range of corona conditions. We
also present new results obtained by numerical simulations that include
collisions. Both approaches calculate the heat flux self-consistently without
any assumption on the energy transport. We show that both approaches - the
exospheric and the collisional one - yield a similar variation of the wind
speed with the basic parameters of the problem; both produce a fast wind speed
if the coronal electron distribution has a suprathermal tail. This suggests
that exospheric models contain the necessary ingredients for the powering of a
transonic stellar wind, including the fast solar one.Comment: Accepted for publication in The Astrophysical Journal Letters
(accepted: 13 May 2005
Two-temperature coronal flow above a thin disk
We extended the disk corona model (Meyer & Meyer-Hofmeister 1994; Meyer, Liu,
& Meyer-Hofmeister 2000a) to the inner region of galactic nuclei by including
different temperatures in ions and electrons as well as Compton cooling. We
found that the mass evaporation rate and hence the fraction of accretion energy
released in the corona depend strongly on the rate of incoming mass flow from
outer edge of the disk, a larger rate leading to more Compton cooling, less
efficient evaporation and a weaker corona. We also found a strong dependence on
the viscosity, higher viscosity leading to an enhanced mass flow in the corona
and therefore more evaporation of gas from the disk below. If we take accretion
rates in units of the Eddington rate our results become independent on the mass
of the central black hole. The model predicts weaker contributions to the hard
X-rays for objects with higher accretion rate like narrow-line Seyfert 1
galaxies (NLS1s), in agreement with observations. For luminous active galactic
nuclei (AGN) strong Compton cooling in the innermost corona is so efficient
that a large amount of additional heating is required to maintain the corona
above the thin disk.Comment: 17 pages, 6 figures. ApJ accepte
Nanodust detection near 1 AU from spectral analysis of Cassini/RPWS radio data
Nanodust grains of a few nanometer in size are produced near the Sun by
collisional break-up of larger grains and picked-up by the magnetized solar
wind. They have so far been detected at 1 AU by only the two STEREO spacecraft.
Here we analyze the spectra measured by the radio and plasma wave instrument
onboard Cassini during the cruise phase close to Earth orbit; they exhibit
bursty signatures similar to those observed by the same instrument in
association to nanodust stream impacts on Cassini near Jupiter. The observed
wave level and spectral shape reveal impacts of nanoparticles at about 300
km/s, with an average flux compatible with that observed by the radio and
plasma wave instrument onboard STEREO and with the interplanetary flux models
Dust detection by the wave instrument on STEREO: nanoparticles picked up by the solar wind?
The STEREO/WAVES instrument has detected a very large number of intense
voltage pulses. We suggest that these events are produced by impact ionisation
of nanoparticles striking the spacecraft at a velocity of the order of
magnitude of the solar wind speed. Nanoparticles, which are half-way between
micron-sized dust and atomic ions, have such a large charge-to-mass ratio that
the electric field induced by the solar wind magnetic field accelerates them
very efficiently. Since the voltage produced by dust impacts increases very
fast with speed, such nanoparticles produce signals as high as do much larger
grains of smaller speeds. The flux of 10-nm radius grains inferred in this way
is compatible with the interplanetary dust flux model. The present results may
represent the first detection of fast nanoparticles in interplanetary space
near Earth orbit.Comment: In press in Solar Physics, 13 pages, 5 figure
Phase Diagram for Ultracold Bosons in Optical Lattices and Superlattices
We present an analytic description of the finite-temperature phase diagram of
the Bose-Hubbard model, successfully describing the physics of cold bosonic
atoms trapped in optical lattices and superlattices. Based on a standard
statistical mechanics approach, we provide the exact expression for the
boundary between the superfluid and the normal fluid by solving the
self-consistency equations involved in the mean-field approximation to the
Bose-Hubbard model. The zero-temperature limit of such result supplies an
analytic expression for the Mott lobes of superlattices, characterized by a
critical fractional filling.Comment: 8 pages, 6 figures, submitted to Phys. Rev.
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