1,610 research outputs found
Regional and Transregional Currents in the Shallows of Lake Chad
Der Beitrag analysiert die Spannung zwischen der Interpretation eines Konflikts und den regionalen oder transregionalen Reaktionen darauf. Den empirischen Fokus bildet das militärische Vorgehen der Multinational Joint Task Force (MNJTF) – Benin, Kamerun, Tschad, Niger und Nigeria – gegen Boko Haram. Dieses Vorgehen ist zwar in einem regionalen Kontext verankert und wird von der Lake Chad Basin Commission (LCBC) koordiniert, doch zugleich auch in einem transregionalen Kontext, im Rahmen der Kooperation zwischen zwei Regional Economic Communities, der Economic Community of West African States (ECOWAS) und der Economic Community of Central African States (ECCAS). Keine dieser beiden räumlichen Verankerungen sind einfach gegeben, vielmehr sind es bestimmte Akteure, die den regionalen oder transregionalen Charakter des Konflikts und der entsprechenden Reaktionen produzieren und reproduzieren.This article addresses the tension between the conception of a conflict and the ensuing response as regional or as transregional. The empirical focus is the military response within the Multinational Joint Task Force (MNJTF) – comprised of Benin, Cameroon, Chad, Niger and Nigeria – against Boko Haram. This response is situated in a regional context, as its coordinating forum is the Lake Chad Basin Commission (LCBC), but also in a transregional one, through the cooperation between two Regional Economic Communities (RECs), the Economic Community of West African States (ECOWAS) and the Economic Community of Central African States (ECCAS). This article cautions that neither of these two framings is a given. Rather, particular actors produce and reproduce the regional or respectively transregional character of a security concern as well as its response
Inertia and chiral edge modes of a skyrmion magnetic bubble
The dynamics of a vortex in a thin-film ferromagnet resembles the motion of a
charged massless particle in a uniform magnetic field. Similar dynamics is
expected for other magnetic textures with a nonzero skyrmion number. However,
recent numerical simulations revealed that skyrmion magnetic bubbles show
significant deviations from this model. We show that a skyrmion bubble
possesses inertia and derive its mass from the standard theory of a thin-film
ferromagnet. Besides center-of-mass motion, other low energy modes are waves on
the edge of the bubble traveling with different speeds in opposite directions.Comment: updated simulation detail
Optimal time-dependent polarized current pattern for fast domain wall propagation in nanowires: Exact solutions for biaxial and uniaxial anisotropies
One of the important issues in nanomagnetism is to lower the current needed
for a technologically useful domain wall (DW) propagation speed. Based on the
modified Landau-Lifshitz-Gilbert (LLG) equation with both Slonczewski
spin-transfer torque and the field-like torque, we derive the optimal spin
current pattern for fast DW propagation along nanowires. Under such conditions,
the DW velocity in biaxial wires can be enhanced as much as ten times compared
to the velocities achieved in experiments so far. Moreover, the fast variation
of spin polarization can help DW depinning. Possible experimental realizations
are discussed.Comment: 4 pages, 1 figur
The role of the N*(1535) resonance and the pi^- p --> KY amplitudes in the OZI forbidden pi N --> phi N reaction
We study the pi N --> phi N reaction close to the phi N threshold within the
chiral unitary approach, by combining the pi^- p --> K^+ Sigma^-, pi^- p -->
K^0 Sigma^0 and pi^- p --> K^0 Lambda amplitudes with the coupling of the phi
to the K components of the final states of these reactions via quantum loops.
We obtain a good agreement with experiment when the dominant pi^- p --> K^0
Lambda amplitude is constrained with its experimental cross section. We also
evaluate the coupling of the N*(1535) to phi N and find a moderate coupling as
a consequence of partial cancellation of the large KY components of the
N*(1535). We also show that the N*(1535) pole approximation is too small to
reproduce the measured cross section for the pi N --> phi N reaction.Comment: 10 pages, 6 figure
Thermal fluctuation field for current-induced domain wall motion
Current-induced domain wall motion in magnetic nanowires is affected by
thermal fluctuation. In order to account for this effect, the
Landau-Lifshitz-Gilbert equation includes a thermal fluctuation field and
literature often utilizes the fluctuation-dissipation theorem to characterize
statistical properties of the thermal fluctuation field. However, the theorem
is not applicable to the system under finite current since it is not in
equilibrium. To examine the effect of finite current on the thermal
fluctuation, we adopt the influence functional formalism developed by Feynman
and Vernon, which is known to be a useful tool to analyze effects of
dissipation and thermal fluctuation. For this purpose, we construct a quantum
mechanical effective Hamiltonian describing current-induced domain wall motion
by generalizing the Caldeira-Leggett description of quantum dissipation. We
find that even for the current-induced domain wall motion, the statistical
properties of the thermal noise is still described by the
fluctuation-dissipation theorem if the current density is sufficiently lower
than the intrinsic critical current density and thus the domain wall tilting
angle is sufficiently lower than pi/4. The relation between our result and a
recent result, which also addresses the thermal fluctuation, is discussed. We
also find interesting physical meanings of the Gilbert damping alpha and the
nonadiabaticy parameter beta; while alpha characterizes the coupling strength
between the magnetization dynamics (the domain wall motion in this paper) and
the thermal reservoir (or environment), beta characterizes the coupling
strength between the spin current and the thermal reservoir.Comment: 16 page, no figur
Origin of adiabatic and non-adiabatic spin transfer torques in current-driven magnetic domain wall motion
A consistent theory to describe the correlated dynamics of quantum mechanical
itinerant spins and semiclassical local magnetization is given. We consider the
itinerant spins as quantum mechanical operators, whereas local moments are
considered within classical Lagrangian formalism. By appropriately treating
fluctuation space spanned by basis functions, including a zero-mode wave
function, we construct coupled equations of motion for the collective
coordinate of the center-of-mass motion and the localized zero-mode coordinate
perpendicular to the domain wall plane. By solving them, we demonstrate that
the correlated dynamics is understood through a hierarchy of two time scales:
Boltzmann relaxation time when a non-adiabatic part of the spin-transfer torque
appears, and Gilbert damping time when adiabatic part comes up.Comment: 4 pages, 2 figure
Quantum projection noise limited interferometry with coherent atoms in a Ramsey type setup
Every measurement of the population in an uncorrelated ensemble of two-level
systems is limited by what is known as the quantum projection noise limit.
Here, we present quantum projection noise limited performance of a Ramsey type
interferometer using freely propagating coherent atoms. The experimental setup
is based on an electro-optic modulator in an inherently stable Sagnac
interferometer, optically coupling the two interfering atomic states via a
two-photon Raman transition. Going beyond the quantum projection noise limit
requires the use of reduced quantum uncertainty (squeezed) states. The
experiment described demonstrates atom interferometry at the fundamental noise
level and allows the observation of possible squeezing effects in an atom
laser, potentially leading to improved sensitivity in atom interferometers.Comment: 8 pages, 8 figures, published in Phys. Rev.
Scalar mesons moving in a finite volume and the role of partial wave mixing
Phase shifts and resonance parameters can be obtained from finite-volume
lattice spectra for interacting pairs of particles, moving with nonzero total
momentum. We present a simple derivation of the method that is subsequently
applied to obtain the pi pi and pi K phase shifts in the sectors with total
isospin I=0 and I=1/2, respectively. Considering different total momenta, one
obtains extra data points for a given volume that allow for a very efficient
extraction of the resonance parameters in the infinite-volume limit.
Corrections due to the mixing of partial waves are provided. We expect that our
results will help to optimize the strategies in lattice simulations, which aim
at an accurate determination of the scattering and resonance properties.Comment: 19 pages, 12 figure
Transport magnetic currents driven by moving kink crystal in chiral helimagnets
We show that the bulk transport magnetic current is generated by the moving
magnetic kink crystal (chiral soliton lattice) formed in the chiral helimagnet
under the static magnetic field applied perpendicular to the helical axis. The
current is caused by the non-equilibrium transport momentum with the kink mass
being determined by the spin fluctuations around the kink crystal state. An
emergence of the transport magnetic currents is then a consequence of the
dynamical off-diagonal long range order along the helical axis. We derive an
explicit formula for the inertial mass of the kink crystal and the current in
the weak field limit.Comment: 5 pages, 3 figures, to appear in Phys. Rev.
Baryon Resonance Analysis from SAID
We discuss the analysis of data from piN elastic scattering and single pion
photo- and electroproduction. The main focus is a study of low-lying
non-strange baryon resonances. Here we concentrate on some difficulties
associated with resonance identification, in particular the Roper and higher
P11 states.Comment: 4 pages, 6 figures; Nstar2009 Conf Proceedings; small revisio
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