2,190 research outputs found
Multicomponent theory of buoyancy instabilities in magnetized plasmas: The case of magnetic field parallel to gravity
We investigate electromagnetic buoyancy instabilities of the electron-ion
plasma with the heat flux based on not the magnetohydrodynamic (MHD) equations,
but using the multicomponent plasma approach when the momentum equations are
solved for each species. We consider a geometry in which the background
magnetic field, gravity, and stratification are directed along one axis. The
nonzero background electron thermal flux is taken into account. Collisions
between electrons and ions are included in the momentum equations. No
simplifications usual for the one-fluid MHD-approach in studying these
instabilities are used. We derive a simple dispersion relation, which shows
that the thermal flux perturbation generally stabilizes an instability for the
geometry under consideration. This result contradicts to conclusion obtained in
the MHD-approach. We show that the reason of this contradiction is the
simplified assumptions used in the MHD analysis of buoyancy instabilities and
the role of the longitudinal electric field perturbation which is not captured
by the ideal MHD equations. Our dispersion relation also shows that the medium
with the electron thermal flux can be unstable, if the temperature gradients of
ions and electrons have the opposite signs. The results obtained can be applied
to the weakly collisional magnetized plasma objects in laboratory and
astrophysics.Comment: Accepted for publication in Astrophysics & Space Scienc
Photonic production of B_c-mesons
The cross section of the
process is calculated. It is shown that near threshold the pseudoscalar state
production is much suppressed with respect to the vector one. At large energies
their ratio becaims . The process of heavy
quark recombination dominates in the production of states. The
fragmentation process comes to play at high values only, while
its contribution will remain nondominant.Comment: Preprint IHEP 94-8
Reorientation of magnetic anisotropy in epitaxial cobalt ferrite thin films
Spin reorientation has been observed in CoFe2O4 thin single crystalline films epitaxially grown on (100) MgO substrate upon varying the film thickness. The critical thickness for such a spin-reorientation transition was estimated to be 300 nm. The reorientation is driven by a structural transition in the film from a tetragonal to cubic symmetry. At low thickness, the in-plane tensile stress induces a tetragonal distortion of the lattice that generates a perpendicular anisotropy, large enough to overcome the shape anisotropy and to stabilize the magnetization easy axis out of plane. However, in thicker films, the lattice relaxation toward the cubic structure of the bulk allows the shape anisotropy to force the magnetization to be in plane aligned
A Cointegration Analysis of Agricultural, Energy and Bio-Fuel Spot and Futures Prices
This paper features an analysis of the cointegration relationships among agricultural commodity, ethanol and Cushing crude oil spot and futures prices. The use of grains for the creation of bio-fuels has sparked fears that these demands are inflating food prices. We analyse approximately 10 years of daily spot and futures prices for corn, wheat, sugar ethanol and oil prices from Datastream for the period 19 July 2006 to 2 July 2015. The analysis, featuring Engle-Granger pairwise cointegration and Markov-switching VECM and Impulse Response Analysis, confirms that these markets have significant linkages which vary according to whether they are in low or high volatility regimes
Semileptonic B_c-meson decays in sum rules of QCD and NRQCD
The semileptonic B_c-meson decays into the heavy quarkonia J/\psi (\eta_c)
and a pair of leptons are investigated in the framework of three-point sum
rules of QCD and NRQCD. Calculations of analytical expressions for the spectral
densities of QCD and NRQCD correlators with account for the Coulomb-like
\alpha_s/v terms are presented. The generalized relations due to the spin
symmetry of NRQCD for the form factors of B_c\to J/\psi (\eta_c)l\nu_l
transitions with l denoting one of the leptons e, \mu or \tau, are derived at
the recoil momentum close to zero. This allows one to express all NRQCD form
factors through a single universal quantity, an analogue of Isgur-Wise function
at the maximal invariant mass of lepton pair. The gluon condensate corrections
to three-point functions are calculated both in full QCD in the Borel transform
scheme and in NRQCD in the moment scheme. This enlarges the parameteric
stability region of sum rule method, that makes the results of the approach to
be more reliable. Numerical estimates of widths for the transitions of B_c\to
J/\psi (\eta_c)l\nu_l are presented.Comment: 37 pages, Latex file, 10 figures, 15 epsf-insertions, revised version
to appear in Nucl.Phys.
A Geometrical Characterization of the Twin Paradox and its Variants
The aim of this paper is to provide a logic-based conceptual analysis of the
twin paradox (TwP) theorem within a first-order logic framework. A geometrical
characterization of TwP and its variants is given. It is shown that TwP is not
logically equivalent to the assumption of the slowing down of moving clocks,
and the lack of TwP is not logically equivalent to the Newtonian assumption of
absolute time. The logical connection between TwP and a symmetry axiom of
special relativity is also studied.Comment: 22 pages, 3 figure
Hexagonal dielectric resonators and microcrystal lasers
We study long-lived resonances (lowest-loss modes) in hexagonally shaped
dielectric resonators in order to gain insight into the physics of a class of
microcrystal lasers. Numerical results on resonance positions and lifetimes,
near-field intensity patterns, far-field emission patterns, and effects of
rounding of corners are presented. Most features are explained by a
semiclassical approximation based on pseudointegrable ray dynamics and boundary
waves. The semiclassical model is also relevant for other microlasers of
polygonal geometry.Comment: 12 pages, 17 figures (3 with reduced quality
Possibility of extracting the weak phase from decays
We explore the possibility of extracting the weak phase from pure
tree decays in a model
independent way. The CP violating weak phase can be determined
cleanly, without any hadronic uncertainties, as these decay modes are free from
the penguin pollutions. Furthermore, neither tagging nor time dependent studies
are required to extract the angle with these modes.Comment: 12 pages, LaTex, Minor changes in the text, version to appear in
Phys. Rev.
An O(N) symmetric extension of the Sine-Gordon Equation
We discuss an O(N) exension of the Sine-Gordon (S-G)equation which allows us
to perform an expansion around the leading order in large-N result using
Path-Integral methods. In leading order we show our methods agree with the
results of a variational calculation at large-N. We discuss the striking
differences for a non-polynomial interaction between the form for the effective
potential in the Gaussian approximation that one obtains at large-N when
compared to the N=1 case. This is in contrast to the case when the classical
potential is a polynomial in the field and no such drastic differences occur.
We find for our large-N extension of the Sine-Gordon model that the unbroken
ground state is unstable as one increases the coupling constant (as it is for
the original S-G equation) and we determine the stability criteria.Comment: 21 pages, Latex (Revtex4) v3:minor grammatical changes and addition
Adjustment of the electric current in pulsar magnetospheres and origin of subpulse modulation
The subpulse modulation of pulsar radio emission goes to prove that the
plasma flow in the open field line tube breaks into isolated narrow streams. I
propose a model which attributes formation of streams to the process of the
electric current adjustment in the magnetosphere. A mismatch between the
magnetospheric current distribution and the current injected by the polar cap
accelerator gives rise to reverse plasma flows in the magnetosphere. The
reverse flow shields the electric field in the polar gap and thus shuts up the
plasma production process. I assume that a circulating system of streams is
formed such that the upward streams are produced in narrow gaps separated by
downward streams. The electric drift is small in this model because the
potential drop in narrow gaps is small. The gaps have to drift because by the
time a downward stream reaches the star surface and shields the electric field,
the corresponding gap has to shift. The transverse size of the streams is
determined by the condition that the potential drop in the gaps is sufficient
for the pair production. This yields the radius of the stream roughly 10% of
the polar cap radius, which makes it possible to fit in the observed
morphological features such as the "carousel" with 10-20 subbeams and the
system of the core - two nested cone beams.Comment: 8 pages, 1 figur
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