36,770 research outputs found
A Note on Asymptotic Freedom at High Temperatures
This short note considers, within the external field approach outlined in
hep-ph/0202026, the role of the lowest lying gluon Landau mode in QCD in the
high temperature limit. Its influence on a temperature- and field-dependent
running coupling constant is examined. The thermal imaginary part of the mode
is temperature-independent in our approach and exactly cancels the well-known
zero temperature imaginary part, thus rendering the Savvidy vacuum stable.
Combining the real part of the mode with the contributions from the higher
lying Landau modes and the vacuum contribution, a field-independent coupling
alpha_s(T) is obtained. It can be interpreted as the ordinary zero temperature
running coupling constant with average thermal momenta \approx 2pi T for
gluons and \approx pi T for quarks.Comment: 4 pages; minor changes, version to appear in Phys. Rev.
Massive Infrared-Quiet Dense Cores: Unveiling the Initial Conditions of High-Mass Star Formation
As Pr. Th. Henning said at the conference, cold precursors of high-mass stars
are now "hot topics". We here propose some observational criteria to identify
massive infrared-quiet dense cores which can host the high-mass analogs of
Class 0 protostars and pre-stellar condensations. We also show how far-infrared
to millimeter imaging surveys of entire complexes forming OB stars are starting
to unveil the initial conditions of high-mass star formation
Hall effect in laser ablated Co_2(Mn,Fe)Si thin films
Pulsed laser deposition was employed to grow thin films of the Heusler
compounds Co_2MnSi and Co_2FeSi. Epitaxial growth was realized both directly on
MgO (100) and on a Cr or Fe buffer layer. Structural analysis by x-ray and
electron diffraction shows for both materials the ordered L2_1 structure. Bulk
magnetization was determined with a SQUID magnetometer. The values agree with
the Slater-Pauling rule for half-metallic Heusler compounds. On the films grown
directly on the substrate measurements of the Hall effect have been performed.
The normal Hall effect is nearly temperature independent and points towards a
compensated Fermi surface. The anomalous contribution is found to be dominated
by skew scattering. A remarkable sign change of both normal and anomalous Hall
coefficients is observed on changing the valence electron count from 29 (Mn) to
30 (Fe).Comment: 9 pages, 6 figures submitted to J Phys
A consistent interpretation of the low temperature magneto-transport in graphite using the Slonczewski--Weiss--McClure 3D band structure calculations
Magnetotransport of natural graphite and highly oriented pyrolytic graphite
(HOPG) has been measured at mK temperatures. Quantum oscillations for both
electron and hole carriers are observed with orbital angular momentum quantum
number up to . A remarkable agreement is obtained when comparing
the data and the predictions of the Slonczewski--Weiss--McClure tight binding
model for massive fermions. No evidence for Dirac fermions is observed in the
transport data which is dominated by the crossing of the Landau bands at the
Fermi level, corresponding to , which occurs away from the point
where Dirac fermions are expected.Comment: 3 figure
Feasibility study of a 110 watt per kilogram lightweight solar array system
An investigation of the feasibility of a solar array panel subsystem which will produce 10,000 watts of electrical output at 1 A.U. with an overall beginning-of-life power-to-weight ratio of at least 110 watt/kg is reported. A description of the current baseline configuration which meets these requirements is presented. A parametric analysis of the single boom, two blanket planar solar array system was performed to arrive at the optimum system aspect ratio. A novel concept for the stiffening of a lightweight solar array by canting the solar cell blankets at a small angle to take advantage of the inherent in-plane stiffness to increase the symmetric out-of-plane frequency is introduced along with a preliminary analysis of the stiffening effect. A comparison of welded and soldered solar cell interconnections leads to the conclusion that welding is required on this ultralightweight solar array. The use of a boron/aluminum composite material in a BI-STEM type deployable boom is investigated as a possible advancement in the state-of-the-art
Self consistent kinetic simulations of SPT and HEMP thrusters including the near-field plume region
The Particle-in-Cell (PIC) method was used to study two different ion
thruster concepts - Stationary Plasma Thrusters (SPT) and High Efficiency
Multistage Plasma Thrusters (HEMP-T), in particular the plasma properties in
the discharge chamber due to the different magnetic field configurations.
Special attention was paid to the simulation of plasma particle fluxes on the
thrusters channel surfaces. In both cases, PIC proved itself as a powerful
tool, delivering important insight into the basic physics of the different
thruster concepts. The simulations demonstrated that the new HEMP thruster
concept allows for a high thermal efficiency due to both minimal energy
dissipation and high acceleration efficiency. In the HEMP thruster the plasma
contact to the wall is limited only to very small areas of the magnetic field
cusps, which results in much smaller ion energy flux to the thruster channel
surface as compared to SPT. The erosion yields for dielectric discharge channel
walls of SPT and HEMP thrusters were calculated with the binary collision code
SDTrimSP. For SPT, an erosion rate on the level of 1 mm of sputtered material
per hour was observed. For HEMP, thruster simulations have shown that there is
no erosion inside the dielectric discharge channel.Comment: 14 pages, 11 figures This work was presented at 21st International
Conference on Numerical Simulation of Plasmas (ICNSP'09
Computing in Additive Networks with Bounded-Information Codes
This paper studies the theory of the additive wireless network model, in
which the received signal is abstracted as an addition of the transmitted
signals. Our central observation is that the crucial challenge for computing in
this model is not high contention, as assumed previously, but rather
guaranteeing a bounded amount of \emph{information} in each neighborhood per
round, a property that we show is achievable using a new random coding
technique.
Technically, we provide efficient algorithms for fundamental distributed
tasks in additive networks, such as solving various symmetry breaking problems,
approximating network parameters, and solving an \emph{asymmetry revealing}
problem such as computing a maximal input.
The key method used is a novel random coding technique that allows a node to
successfully decode the received information, as long as it does not contain
too many distinct values. We then design our algorithms to produce a limited
amount of information in each neighborhood in order to leverage our enriched
toolbox for computing in additive networks
Exploiting the Temporal Logic Hierarchy and the Non-Confluence Property for Efficient LTL Synthesis
The classic approaches to synthesize a reactive system from a linear temporal
logic (LTL) specification first translate the given LTL formula to an
equivalent omega-automaton and then compute a winning strategy for the
corresponding omega-regular game. To this end, the obtained omega-automata have
to be (pseudo)-determinized where typically a variant of Safra's
determinization procedure is used. In this paper, we show that this
determinization step can be significantly improved for tool implementations by
replacing Safra's determinization by simpler determinization procedures. In
particular, we exploit (1) the temporal logic hierarchy that corresponds to the
well-known automata hierarchy consisting of safety, liveness, Buechi, and
co-Buechi automata as well as their boolean closures, (2) the non-confluence
property of omega-automata that result from certain translations of LTL
formulas, and (3) symbolic implementations of determinization procedures for
the Rabin-Scott and the Miyano-Hayashi breakpoint construction. In particular,
we present convincing experimental results that demonstrate the practical
applicability of our new synthesis procedure
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