10,220 research outputs found
Gamma Rays From The Galactic Center and the WMAP Haze
Recently, an analysis of data from the Fermi Gamma Ray Space Telescope has
revealed a flux of gamma rays concentrated around the inner ~0.5 degrees of the
Milky Way, with a spectrum that is sharply peaked at 2-4 GeV. If interpreted as
the products of annihilating dark matter, this signal implies that the dark
matter consists of particles with a mass between 7.3 and 9.2 GeV annihilating
primarily to charged leptons. This mass range is very similar to that required
to accommodate the signals reported by CoGeNT and DAMA/LIBRA. In addition to
gamma rays, the dark matter is predicted to produce energetic electrons and
positrons in the Inner Galaxy, which emit synchrotron photons as a result of
their interaction with the galactic magnetic field. In this letter, we
calculate the flux and spectrum of this synchrotron emission assuming that the
gamma rays from the Galactic Center originate from dark matter, and compare the
results to measurements from the WMAP satellite. We find that a sizable flux of
hard synchrotron emission is predicted in this scenario, and that this can
easily account for the observed intensity, spectrum, and morphology of the
"WMAP Haze".Comment: 5 pages, 4 figure
Free energy density for mean field perturbation of states of a one-dimensional spin chain
Motivated by recent developments on large deviations in states of the spin
chain, we reconsider the work of Petz, Raggio and Verbeure in 1989 on the
variational expression of free energy density in the presence of a mean field
type perturbation. We extend their results from the product state case to the
Gibbs state case in the setting of translation-invariant interactions of finite
range. In the special case of a locally faithful quantum Markov state, we
clarify the relation between two different kinds of free energy densities (or
pressure functions).Comment: 29 pages, Section 5 added, to appear in Rev. Math. Phy
Quantum Markov fields on graphs
We introduce generalized quantum Markov states and generalized d-Markov
chains which extend the notion quantum Markov chains on spin systems to that on
-algebras defined by general graphs. As examples of generalized d-Markov
chains, we construct the entangled Markov fields on tree graphs. The concrete
examples of generalized d-Markov chains on Cayley trees are also investigated.Comment: 23 pages, 1 figure. accepted to "Infinite Dimensional Anal. Quantum
Probability & Related Topics
Theory of Magnetic Anisotropy in III_{1-x}Mn_{x}V Ferromagnets
We present a theory of magnetic anisotropy in diluted magnetic semiconductors with carrier-induced
ferromagnetism. The theory is based on four and six band envelope functions
models for the valence band holes and a mean-field treatment of their exchange
interactions with ions. We find that easy-axis reorientations
can occur as a function of temperature, carrier density , and strain. The
magnetic anisotropy in strain-free samples is predicted to have a
hole-density dependence at small , a dependence at large , and
remarkably large values at intermediate densities. An explicit expression,
valid at small , is given for the uniaxial contribution to the magnetic
anisotropy due to unrelaxed epitaxial growth lattice-matching strains. Results
of our numerical simulations are in agreement with magnetic anisotropy
measurements on samples with both compressive and tensile strains. We predict
that decreasing the hole density in current samples will lower the
ferromagnetic transition temperature, but will increase the magnetic anisotropy
energy and the coercivity.Comment: 15 pages, 15 figure
Spin injection through the depletion layer: a theory of spin-polarized p-n junctions and solar cells
A drift-diffusion model for spin-charge transport in spin-polarized {\it p-n}
junctions is developed and solved numerically for a realistic set of material
parameters based on GaAs. It is demonstrated that spin polarization can be
injected through the depletion layer by both minority and majority carriers,
making all-semiconductor devices such as spin-polarized solar cells and bipolar
transistors feasible. Spin-polarized {\it p-n} junctions allow for
spin-polarized current generation, spin amplification, voltage control of spin
polarization, and a significant extension of spin diffusion range.Comment: 4 pages, 3 figure
CoFeB Thickness Dependence of Thermal Stability Factor in CoFeB/MgO Perpendicular Magnetic Tunnel Junctions
Thermal stability factor (delta) of recording layer was studied in
perpendicular anisotropy CoFeB/MgO magnetic tunnel junctions (p-MTJs) with
various CoFeB recording layer thicknesses and junction sizes. In all series of
p-MTJs with different thicknesses, delta is virtually independent of the
junction sizes of 48-81 nm in diameter. The values of delta increase linearly
with increasing the recording layer thickness. The slope of the linear fit is
explained well by a model based on nucleation type magnetization reversal.Comment: 12 pages, 5 figure
Rate of Evaporation of Manganese, Copper, Tin, Chromium, and Sulphur from Molten Iron under Vacuum
The vacuum melting of Fe-Mn, Fe-Cu, Fe-Sn, Fe-Cr, Fe-S, and Fe-Si-S alloys was carried out at 1600℃ by using a high-frequency induction furnace. The evaporation of Mn, Cu, Sn, Cr, and S follows first-order kinetics and the specific evaporation constant K^s of each of these elements was obtained. The K^s of sulphur in Fe-Si-S alloys increases with the increase of silicon content. For each of these elements in the binary alloys, the rate constant of evaporation and of transport in melt and the ratio of surface concentration to bulk concentration C^s/C^m were obtained. Furthermore, the evaporation coefficient α of each of these elements was obtained both experimentally and theoretically according to the equation derived by Olette. It was confirmed that for each of these elements in the binary alloys the theoretical value of the product α. C^s/C^m is close to the experimental value of α
Activity ageing in growing networks
We present a model for growing information networks where the ageing of a
node depends on the time at which it entered the network and on the last time
it was cited. The model is shown to undergo a transition from a small-world to
large-world network. The degree distribution may exhibit very different shapes
depending on the model parameters, e.g. delta-peaked, exponential or power-law
tailed distributions.Comment: 9 pages, 2 figure
A theory of ferromagnetism in planar heterostructures of (Mn,III)-V semiconductors
A density functional theory of ferromagnetism in heterostructures of compound
semiconductors doped with magnetic impurities is presented. The variable
functions in the density functional theory are the charge and spin densities of
the itinerant carriers and the charge and localized spins of the impurities.
The theory is applied to study the Curie temperature of planar heterostructures
of III-V semiconductors doped with manganese atoms. The mean-field,
virtual-crystal and effective-mass approximations are adopted to calculate the
electronic structure, including the spin-orbit interaction, and the magnetic
susceptibilities, leading to the Curie temperature. By means of these results,
we attempt to understand the observed dependence of the Curie temperature of
planar -doped ferromagnetic structures on variation of their
properties. We predict a large increase of the Curie Temperature by additional
confinement of the holes in a -doped layer of Mn by a quantum well.Comment: 8 pages, 7 figure
Spin depolarization in the transport of holes across GaMnAs/GaAlAs/p-GaAs
We study the spin polarization of tunneling holes injected from ferromagnetic
GaMnAs into a p-doped semiconductor through a tunneling barrier. We obtain an
upper limit to the spin injection rate. We find that spin-orbit interaction
interaction in the barrier and in the drain limits severely spin injection.
Spin depolarization is stronger when the magnetization is parallel to the
current than when is perpendicular to it.Comment: Accepted in Phys. Rev. B. 4 pages, 4 figure
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