2,743 research outputs found
Stability of negative ionization fronts: regularization by electric screening?
We recently have proposed that a reduced interfacial model for streamer
propagation is able to explain spontaneous branching. Such models require
regularization. In the present paper we investigate how transversal Fourier
modes of a planar ionization front are regularized by the electric screening
length. For a fixed value of the electric field ahead of the front we calculate
the dispersion relation numerically. These results guide the derivation of
analytical asymptotes for arbitrary fields: for small wave-vector k, the growth
rate s(k) grows linearly with k, for large k, it saturates at some positive
plateau value. We give a physical interpretation of these results.Comment: 11 pages, 2 figure
Detecting Photon-Photon Interactions in a Superconducting Circuit
A local interaction between photons can be engineered by coupling a nonlinear
system to a transmission line. The required high impedance transmission line
can be conveniently formed from a chain of Josephson junctions. The
nonlinearity is generated by side-coupling this chain to a Cooper pair box. We
propose to probe the resulting photon-photon interactions via their effect on
the current-voltage characteristic of a voltage-biased Josephson junction
connected to the transmission line. Considering the Cooper pair box to be in
the weakly anharmonic regime, we find that the dc current through the probe
junction yields features around the voltages , where
is the plasma frequency of the superconducting circuit. The features
at are a direct signature of the photon-photon interaction in the
system.Comment: 10 pages, 7 figure
Whitening of the Quark-Gluon Plasma
Parton-parton collisions do not neutralize local color charges in the
quark-gluon plasma as they only redistribute the charges among momentum modes.
We discuss color diffusion and color conductivity as the processes responsible
for the neutralization of the plasma. For this purpose, we first compute the
conductivity and diffusion coefficients in the plasma that is significantly
colorful. Then, the time evolution of the color density due to the conductivity
and diffusion is studied. The conductivity is shown to be much more efficient
than the diffusion in neutralizing the plasma at the scale longer than the
screening length. Estimates of the characteristic time scales, which are based
on close to global equilibrium computations, suggest that first the plasma
becomes white and then the momentum degrees of freedom thermalize.Comment: 9 pages, revised, to appear in Phys. Rev.
Interpreters for the Defense: Due Process for the Non-English-Speaking Defendant
The authors of this Comment contend that the communications problems of non-English-speaking indigent defendants can best be solved by the appointment of court-compensated interpreters. Although they evaluate recent legislative proposals directed at these problems, the authors stress the arguments derived from considerations of equal protection and due process which support a possible constitutional right to interpreters
Thermalization vs. Isotropization & Azimuthal Fluctuations
Hydrodynamic description requires a local thermodynamic equilibrium of the
system under study but an approximate hydrodynamic behaviour is already
manifested when a momentum distribution of liquid components is not of
equilibrium form but merely isotropic. While the process of equilibration is
relatively slow, the parton system becomes isotropic rather fast due to the
plasma instabilities. Azimuthal fluctuations observed in relativistic heavy-ion
collisions are argued to distinguish between a fully equilibrated and only
isotropic parton system produced in the collision early stage.Comment: 12 pages, presented at `Correlations and Fluctuations in Relativistic
Nuclear Collisions', MIT, April 05, minor correction
Are Coronae of Magnetically Active Stars Heated by Flares? III. Analytical Distribution of Superimposed Flares
(abridged) We study the hypothesis that observed X-ray/extreme ultraviolet
emission from coronae of magnetically active stars is entirely (or to a large
part) due to the superposition of flares, using an analytic approach to
determine the amplitude distribution of flares in light curves. The
flare-heating hypothesis is motivated by time series that show continuous
variability suggesting the presence of a large number of superimposed flares
with similar rise and decay time scales. We rigorously relate the amplitude
distribution of stellar flares to the observed histograms of binned counts and
photon waiting times, under the assumption that the flares occur at random and
have similar shapes. Applying these results to EUVE/DS observations of the
flaring star AD Leo, we find that the flare amplitude distribution can be
represented by a truncated power law with a power law index of 2.3 +/- 0.1. Our
analytical results agree with existing Monte Carlo results of Kashyap et al.
(2002) and Guedel et al. (2003). The method is applicable to a wide range of
further stochastically bursting astrophysical sources such as cataclysmic
variables, Gamma Ray Burst substructures, X-ray binaries, and spatially
resolved observations of solar flares.Comment: accepted for publication in Ap
SR and Neutron Diffraction Investigations on Reentrant Ferromagnetic Superconductor Eu(Fe{0.86}Ir{0.14})2As2
Results of muon spin relaxation (SR) and neutron powder diffraction
measurements on a reentrant superconductor Eu(FeIr)As
are presented. Eu(FeIr)As exhibits superconductivity
at ~K competing with long range ordered Eu
moments below K. A reentrant behavior (manifested by nonzero
resistivity in the temperature range 10--17.5 K) results from an exquisite
competition between the superconductivity and magnetic order. The zero field
SR data confirm the long range magnetic ordering below K. The transition temperature is found to increase with increasing
magnetic field in longitudinal field SR which along with the neutron
diffraction results, suggests the transition to be ferromagnetic. The neutron
diffraction data reveal a clear presence of magnetic Bragg peaks below which could be indexed with propagation vector k = (0, 0, 0), confirming a
long range magnetic ordering in agreement with SR data. Our analysis of
the magnetic structure reveals an ordered magnetic moment of (at 1.8 K) on the Eu atoms and they form a ferromagnetic structure with
moments aligned along the -axis. No change in the magnetic structure is
observed in the reentrant or superconducting phases and the magnetic structure
remains same for 1.8 K . No clear evidence of
structural transition or Fe moment ordering was found.Comment: 9 pages, 7 figures, to appear in Phys. Rev.
Fluctuations from dissipation in a hot non-Abelian plasma
We consider a transport equation of the Boltzmann-Langevin type for
non-Abelian plasmas close to equilibrium to derive the spectral functions of
the underlying microscopic fluctuations from the entropy. The correlator of the
stochastic source is obtained from the dissipative processes in the plasma.
This approach, based on classical transport theory, exploits the well-known
link between a linearized collision integral, the entropy and the spectral
functions. Applied to the ultra-soft modes of a hot non-Abelian (classical or
quantum) plasma, the resulting spectral functions agree with earlier findings
obtained from the microscopic theory. As a by-product, it follows that
B\"odeker's effective theory is consistent with the fluctuation-dissipation
theorem.Comment: 9 pages, revtex, no figures, identical to published versio
Spontaneous Branching of Anode-Directed Streamers between Planar Electrodes
Non-ionized media subject to strong fields can become locally ionized by
penetration of finger-shaped streamers. We study negative streamers between
planar electrodes in a simple deterministic continuum approximation. We observe
that for sufficiently large fields, the streamer tip can split. This happens
close to Firsov's limit of `ideal conductivity'. Qualitatively the tip
splitting is due to a Laplacian instability quite like in viscous fingering.
For future quantitative analytical progress, our stability analysis of planar
fronts identifies the screening length as a regularization mechanism.Comment: 4 pages, 6 figures, submitted to PRL on Nov. 16, 2001, revised
version of March 10, 200
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