37,436 research outputs found
Are Magnetic Wind-Driving Disks Inherently Unstable?
There have been claims in the literature that accretion disks in which a
centrifugally driven wind is the dominant mode of angular momentum transport
are inherently unstable. This issue is considered here by applying an
equilibrium-curve analysis to the wind-driving, ambipolar diffusion-dominated,
magnetic disk model of Wardle & Konigl (1993). The equilibrium solution curves
for this class of models typically exhibit two distinct branches. It is argued
that only one of these branches represents unstable equilibria and that a real
disk/wind system likely corresponds to a stable solution.Comment: 5 pages, 2 figures, to be published in ApJ, vol. 617 (2004 Dec 20).
Uses emulateapj.cl
Fundamental Oscillation Periods of the Interlayer Exchange Coupling beyond the RKKY Approximation
A general method for obtaining the oscillation periods of the interlayer
exchange coupling is presented. It is shown that it is possible for the
coupling to oscillate with additional periods beyond the ones predicted by the
RKKY theory. The relation between the oscillation periods and the spacer Fermi
surface is clarified, showing that non-RKKY periods do not bear a direct
correspondence with the Fermi surface. The interesting case of a FCC(110)
structure is investigated, unmistakably proving the existence and relevance of
non-RKKY oscillations. The general conditions for the occurrence of non-RKKY
oscillations are also presented.Comment: 34 pages, 10 figures ; to appear in J. Phys.: Condens. Mat
Explicit Actions for Electromagnetism with Two Gauge Fields with Only one Electric and one Magnetic Physical Fields
We extend the work of Mello et al. based in Cabbibo and Ferrari concerning
the description of electromagnetism with two gauge fields from a variational
principle, i.e. an action. We provide a systematic independent derivation of
the allowed actions which have only one magnetic and one electric physical
fields and are invariant under the discrete symmetries and . We conclude
that neither the Lagrangian, nor the Hamiltonian, are invariant under the
electromagnetic duality rotations. This agrees with the weak-strong coupling
mixing characteristic of the duality due to the Dirac quantization condition
providing a natural way to differentiate dual theories related by the duality
rotations (the energy is not invariant). Also the standard electromagnetic
duality rotations considered in this work violate both and by inducing
Hopf terms (theta terms) for each sector and a mixed Maxwell term. The
canonical structure of the theory is briefly addressed and the 'magnetic' gauge
sector is interpreted as a ghost sector.Comment: v2: 12 pages; References added, discussion concerning degrees of
freedom corrected; v3: is now used the standard normalization of 1/4 in the
actions; the possibility of theta being a pseudo-scalar implied a title
changing; eq (23) added; signs corrected in equations (39,45-47); references
adde
Magnetized Accretion-Ejection Structures: 2.5D MHD simulations of continuous Ideal Jet launching from resistive accretion disks
We present numerical magnetohydrodynamic (MHD) simulations of a magnetized
accretion disk launching trans-Alfvenic jets. These simulations, performed in a
2.5 dimensional time-dependent polytropic resistive MHD framework, model a
resistive accretion disk threaded by an initial vertical magnetic field. The
resistivity is only important inside the disk, and is prescribed as eta =
alpha_m V_AH exp(-2Z^2/H^2), where V_A stands for Alfven speed, H is the disk
scale height and the coefficient alpha_m is smaller than unity. By performing
the simulations over several tens of dynamical disk timescales, we show that
the launching of a collimated outflow occurs self-consistently and the ejection
of matter is continuous and quasi-stationary. These are the first ever
simulations of resistive accretion disks launching non-transient ideal MHD
jets. Roughly 15% of accreted mass is persistently ejected. This outflow is
safely characterized as a jet since the flow becomes super-fastmagnetosonic,
well-collimated and reaches a quasi-stationary state. We present a complete
illustration and explanation of the `accretion-ejection' mechanism that leads
to jet formation from a magnetized accretion disk. In particular, the magnetic
torque inside the disk brakes the matter azimuthally and allows for accretion,
while it is responsible for an effective magneto-centrifugal acceleration in
the jet. As such, the magnetic field channels the disk angular momentum and
powers the jet acceleration and collimation. The jet originates from the inner
disk region where equipartition between thermal and magnetic forces is
achieved. A hollow, super-fastmagnetosonic shell of dense material is the
natural outcome of the inwards advection of a primordial field.Comment: ApJ (in press), 32 pages, Higher quality version available at
http://www-laog.obs.ujf-grenoble.fr/~fcass
Antimicrobial Resistance: the use of antimicrobials in the Livestock Sector
The use of antimicrobials in livestock production provides a basis for improving animal health and productivity. This in turn contributes to food security, food safety, animal welfare, protection of livelihoods and animal resources. However, there is increasing concern about levels of antimicrobial resistance in bacteria isolated from human, animal, food and environmental samples and how this relates to use of antimicrobials in livestock production. The report examines antimicrobial usage in livestock and its impact on public health and the food economy. Policy issues and knowledge gaps to manage antimicrobial use and the risk of antimicrobial resistance are identified and discussed
The cosmological behavior of Bekenstein's modified theory of gravity
We study the background cosmology governed by the Tensor-Vector-Scalar theory
of gravity proposed by Bekenstein. We consider a broad family of potentials
that lead to modified gravity and calculate the evolution of the field
variables both numerically and analytically. We find a range of possible
behaviors, from scaling to the late time domination of either the additional
gravitational degrees of freedom or the background fluid.Comment: 10 pages, 8 figures, A few typos corrected in the text and figures.
Version published in PR
Strain-Modified RKKY Interaction in Carbon Nanotubes
For low-dimensional metallic structures, such as nanotubes, the exchange
coupling between localized magnetic dopants is predicted to decay slowly with
separation. The long-range character of this interaction plays a significant
role in determining the magnetic order of the system. It has previously been
shown that the interaction range depends on the conformation of the magnetic
dopants in both graphene and nanotubes. Here we examine the RKKY interaction in
carbon nanotubes in the presence of uniaxial strain for a range of different
impurity configurations. We show that strain is capable of amplifying or
attenuating the RKKY interaction, significantly increasing certain interaction
ranges, and acting as a switch: effectively turning on or off the interaction.
We argue that uniaxial strain can be employed to significantly manipulate
magnetic interactions in carbon nanotubes, allowing an interplay between
mechanical and magnetic properties in future spintronic devices. We also
examine the dimensional relationship between graphene and nanotubes with
regards to the decay rate of the RKKY interaction.Comment: 7 pages, 6 figures, submitte
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