202,522 research outputs found
Optical conductivity of one-dimensional narrow-gap semiconductors
The optical conductivities of two one-dimensional narrow-gap semiconductors,
anticrossing quantum Hall edge states and carbon nanotubes, are studied using
bosonization method. A lowest order renormalization group analysis indicates
that the bare band gap can be treated perturbatively at high
frequency/temperature. At very low energy scale the optical conductivity is
dominated by the excitonic contribution, while at temperature higher than a
crossover temperature the excitonic features are eliminated by thermal
fluctuations. In case of carbon nanotubes the crossover temperature scale is
estimated to be 300 K.Comment: RevTeX4 file, 6 pages, no figur
Shuttle system ascent aerodynamic and plume heating
The shuttle program provided a challenge to the aerothermodynamicist due to the complexity of the flow field around the vehicle during ascent, since the configuration causes multiple shock interactions between the elements. Wind tunnel tests provided data for the prediction of the ascent design heating environment which involves both plume and aerodynamic heating phenomena. The approach for the heating methodology based on ground test firings and the use of the wind tunnel data to formulate the math models is discussed
Particle-in-cell and weak turbulence simulations of plasma emission
The plasma emission process, which is the mechanism for solar type II and
type III radio bursts phenomena, is studied by means of particle-in-cell and
weak turbulence simulation methods. By plasma emission, it is meant as a loose
description of a series of processes, starting from the solar flare associated
electron beam exciting Langmuir and ion-acoustic turbulence, and subsequent
partial conversion of beam energy into the radiation energy by nonlinear
processes. Particle-in-cell (PIC) simulation is rigorous but the method is
computationally intense, and it is difficult to diagnose the results. Numerical
solution of equations of weak turbulence (WT) theory, termed WT simulation, on
the other hand, is efficient and naturally lends itself to diagnostics since
various terms in the equation can be turned on or off. Nevertheless, WT theory
is based upon a number of assumptions. It is, therefore, desirable to compare
the two methods, which is carried out for the first time in the present paper
with numerical solutions of the complete set of equations of the WT theory and
with two-dimensional electromagnetic PIC simulation. Upon making quantitative
comparisons it is found that WT theory is largely valid, although some
discrepancies are also found. The present study also indicates that it requires
large computational resources in order to accurately simulate the radiation
emission processes, especially for low electron beam speeds. Findings from the
present paper thus imply that both methods may be useful for the study of solar
radio emissions as they are complementary.Comment: 21 pages, 9 figure
Scaling of Reaction Zones in the A+B->0 Diffusion-Limited Reaction
We study reaction zones in three different versions of the A+B->0 system. For
a steady state formed by opposing currents of A and B particles we derive
scaling behavior via renormalization group analysis. By use of a previously
developed analogy, these results are extended to the time-dependent case of an
initially segregated system. We also consider an initially mixed system, which
forms reaction zones for dimension d<4. In this case an extension of the
steady-state analogy gives scaling results characterized by new exponents.Comment: 4 pages, REVTeX 3.0 with epsf, 2 uuencoded postscript figures
appended, OUTP-94-33
Multiple Thresholds in a Model System of Noisy Ion Channels
Voltage-activated ion channels vary randomly between open and closed states,
influenced by the membrane potential and other factors. Signal transduction is
enhanced by noise in a simple ion channel model. The enhancement occurs in a
finite range of signals; the range can be extended using populations of
channels. The range increases more rapidly in multiple-threshold channel
populations than in single-threshold populations. The diversity of ion channels
may thus be present as a strategy to reduce the metabolic costs of handling a
broad class of electrochemical signals.Comment: REVTeX 4, 5 pages, 4 figures; added paragrap
Relativistic r-modes in Slowly Rotating Neutron Stars: Numerical Analysis in the Cowling Approximation
We investigate the properties of relativistic -modes of slowly rotating
neutron stars by using a relativistic version of the Cowling approximation. In
our formalism, we take into account the influence of the Coriolis like force on
the stellar oscillations, but ignore the effects of the centrifugal like force.
For three neutron star models, we calculated the fundamental -modes with
and 3. We found that the oscillation frequency of the
fundamental -mode is in a good approximation given by , where is defined in the corotating frame at the
spatial infinity, and is the angular frequency of rotation of the
star. The proportional coefficient is only weakly dependent on
, but it strongly depends on the relativistic parameter ,
where and are the mass and the radius of the star. All the fundamental
-modes with computed in this study are discrete modes with distinct
regular eigenfunctions, and they all fall in the continuous part of the
frequency spectrum associated with Kojima's equation (Kojima 1998). These
relativistic -modes are obtained by including the effects of rotation higher
than the first order of so that the buoyant force plays a role, the
situation of which is quite similar to that for the Newtonian -modes.Comment: 22 pages, 8 figures, accepted for publication in Ap
Muon Detection of TeV Gamma Rays from Gamma Ray Bursts
Because of the limited size of the satellite-borne instruments, it has not
been possible to observe the flux of gamma ray bursts (GRB) beyond GeV energy.
We here show that it is possible to detect the GRB radiation of TeV energy and
above, by detecting the muon secondaries produced when the gamma rays shower in
the Earth's atmosphere. Observation is made possible by the recent
commissioning of underground detectors (AMANDA, the Lake Baikal detector and
MILAGRO) which combine a low muon threshold of a few hundred GeV or less, with
a large effective area of 10^3 m^2 or more. Observations will not only provide
new insights in the origin and characteristics of GRB, they also provide
quantitative information on the diffuse infrared background.Comment: Revtex, 12 pages, 3 postscript figures, uses epsfig.st
Extended BRST invariance in topological Yang Mills theory revisited
Extended BRST invariance (BRST plus anti-BRST invariances) provides in
principle a natural way of introducing the complete gauge fixing structure
associated to a gauge field theory in the minimum representation of the
algebra. However, as it happens in topological Yang Mills theory, not all gauge
fixings can be obtained from a symmetrical extended BRST algebra, where
antighosts belong to the same representation of the Lorentz group of the
corresponding ghosts. We show here that, at non interacting level, a simple
field redefinition makes it possible to start with an extended BRST algebra
with symmetric ghost antighost spectrum and arrive at the gauge fixing action
of topological Yang Mills theory.Comment: Interaction terms heve been included in all the calculations. Two
references added. Version to be published in Phys. Rev. D. 7 pages, Latex, no
figure
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