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 $\gamma \gamma \rightarrow B_c(B_c^*) \bar b c$ 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 $\sigma(B_c^*)/sigma(B_c)\sim 4$. The process of heavy quark recombination dominates in the production of $B_c(B_c^*)$ states. The fragmentation process $b\to B_c$ comes to play at high $p_t$ 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 γ\gamma from Λb→ΛD0\Lambda_b \to \Lambda D^0 decays

We explore the possibility of extracting the weak phase $\gamma$ from pure tree decays $\Lambda_b \to \Lambda (D^0, \bar{D^0}, D^0_{CP})$ in a model independent way. The CP violating weak phase $\gamma$ 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 $\gamma$ 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