11,801 research outputs found
On the mechanism of irradiation enhanced exchange bias
By means of layer resolved ion irradiation the mechanisms involved in the
irradiation driven modifications of the exchange bias effect in NiFe/FeMn
bilayers have been investigated. It is shown that not only the locations of the
defects but also the magnetic coupling between both layers during the
irradiation process is of crucial importance. This requires an extension of
current models accounting for defects in exchange bias systems.Comment: 3 pages, 3 figures, revised version, added results from further
structural characterization by TEM, submitted to Europhysics Letter
Investigation of ion induced bending mechanism for nanostructures
Ion induced bending is a promising controlled technique for manipulating nanoscale structures. However, the underlying mechanism of the process is not well understood. In this letter, we report a detailed study of the bending mechanism of Si nanowires (NWs) under Ga+ irradiation. The microstructural changes in the NW due to ion beam irradiation are studied and molecular dynamics simulations are used to explore the ion–NW interaction processes. The simulation results are compared with the microstructural studies of the NW. The investigations inform a generic understanding of the bending process in crystalline materials, which we suggest to be feasible as a versatile manipulation and integration technique in nanotechnology
Very high energy emission from the hard spectrum sources HESS J1641-463, HESS J1741-302 and HESS J1826-130
A recent study of the diffuse -ray emission in the Central Molecular
Zone using very high energy (VHE, E 0.1 TeV) H.E.S.S. data suggests that
the Galactic Center (GC) is the most plausible supplier of Galactic
ultra-relativistic cosmic-rays (CRs) up to the knee at about 10 eV
(PeV). However, the GC might not be the only source capable to accelerate CRs
up to PeV energies in the Galaxy. Here we present H.E.S.S. data analysis
results and interpretation of three H.E.S.S. sources, with spectra extending
beyond 10 TeV and relatively hard spectral indices compared with the average
spectral index of H.E.S.S. sources, namely HESS J1641-463, HESS J1741-302 and
HESS J1826-130. Although the nature of these VHE -ray sources is still
open, their spectra suggest that the astrophysical objects producing such
emission must be capable of accelerating the parental particle population up to
energies of at least several hundreds of TeV. Assuming a hadronic scenario,
dense gas regions can provide rich target material for accelerated particles to
produce VHE -ray emission via proton-proton interactions followed by a
subsequent decay. Thus, detailed investigations of the interstellar
medium along the line of sight to all of these sources have been performed by
using data from available atomic and molecular hydrogen surveys. The results
point out the existence of dense interstellar gas structures coincident with
the best fit positions of these sources. One can find possible hadronic models
with CRs being accelerated close to the PeV energies to explain the
-ray emission from all of these sources, which opens up the possibility
that a population of PeV CR accelerators might be active in the Galaxy.Comment: 8 pages, 2 figures, in Proceedings of 35th ICRC, Busan (Korea) 201
HESS J1826130: A Very Hard -Ray Spectrum Source in the Galactic Plane
HESS J1826130 is an unidentified hard spectrum source discovered by
H.E.S.S. along the Galactic plane, the spectral index being = 1.6 with
an exponential cut-off at about 12 TeV. While the source does not have a clear
counterpart at longer wavelengths, the very hard spectrum emission at TeV
energies implies that electrons or protons accelerated up to several hundreds
of TeV are responsible for the emission. In the hadronic case, the VHE emission
can be produced by runaway cosmic-rays colliding with the dense molecular
clouds spatially coincident with the H.E.S.S. source.Comment: 6 pages, 3 figures, Proceedings of the 6th International Symposium on
High Energy Gamma-Ray Astronomy (Gamma2016), Heidelberg, German
On the Construction of Common Size, Value and Momentum Factors in International Stock Markets: A Guide with Applications
Demand is growing for a better understanding of how assets are priced in countries outside of the U.S. While financial data are available for many firms world-wide, it is important to have a reliable and replicable method of constructing high-quality systematic risk factors from these data. This paper first documents that appropriately screened data from Thomson Reuters Datastream and Thomson Reuters Worldscope can be used to replicate closely not only U.S. market returns and the corresponding momentum risk factor (as existing work has suggested), but also the widely-used U.S. size and value risk factors. We then build novel pan-European and country-specific momentum, size, and value risk factors. By comparing our pan-European market returns and risk factors with their counterparts in the U.S., we find that they are astonishingly highly correlated. The factors we compute are made available to other researchers.Risk factors; value, size, momentum, international equity markets, asset pricing anomalies
The reaction 13C(alpha,n)16O: a background for the observation of geo-neutrinos
The absolute cross section of the C(,n)O reaction has
been measured at E = 0.8 to 8.0 MeV with an overall accuracy of 4%.
The precision is needed to subtract reliably a background in the observation of
geo-neutrinos, e.g. in the KamLAND detector.Comment: LaTex file, 13 pages including 3 ps figures. Any request to
[email protected]. Phys. Rev . C, to appea
Convex Hull of Planar H-Polyhedra
Suppose are planar (convex) H-polyhedra, that is, $A_i \in
\mathbb{R}^{n_i \times 2}$ and $\vec{c}_i \in \mathbb{R}^{n_i}$. Let $P_i =
\{\vec{x} \in \mathbb{R}^2 \mid A_i\vec{x} \leq \vec{c}_i \}$ and $n = n_1 +
n_2$. We present an $O(n \log n)$ algorithm for calculating an H-polyhedron
with the smallest such that
Functionalizing self-assembled GaN quantum dot superlattices by Eu-implantation
Self-assembled GaN quantum dots (QDs) stacked in superlattices (SL) with AlN spacer layers were implanted with Europium ions to fluences of 1013, 1014, and 1015 cm−2. The damage level introduced in the QDs by the implantation stays well below that of thick GaN epilayers. For the lowest fluence, the structural properties remain unchanged after implantation and annealing while for higher fluences the implantation damage causes an expansion of the SL in the [0001] direction which increases with implantation fluence and is only partly reversed after thermal annealing at 1000 °C. Nevertheless, in all cases, the SL quality remains very good after implantation and annealing with Eu ions incorporated preferentially into near-substitutional cation sites. Eu3+ optical activation is achieved after annealing in all samples. In the sample implanted with the lowest fluence, the Eu3+ emission arises mainly from Eu incorporated inside the QDs while for the higher fluences only the emission from Eu inside the AlN-buffer, capping, and spacer layers is observed.
© 2010 American Institute of PhysicsFCT-PTDC/CTM/100756/2008program PESSOA EGIDE/GRICESFCT-SFRH/BD/45774/2008FCT-SFRH/BD/44635/200
Strong Balmer lines in old stellar populations: No need for young ages in ellipticals?
Comparing models of Simple Stellar Populations (SSP) with observed line
strengths generally provides a tool to break the age-metallicity degeneracy in
elliptical galaxies. Due to the wide range of Balmer line strengths observed,
ellipticals have been interpreted to exhibit an appreciable scatter in age. In
this paper, we analyze Composite Stellar Population models with a simple mix of
an old metal-rich and an old metal-poor component. We show that these models
simultaneously produce strong Balmer lines and strong metallic lines without
invoking a young population. The key to this result is that our models are
based on SSPs that better match the steep increase of Hbeta in metal-poor
globular clusters than models in the literature. Hence, the scatter of Hbeta
observed in cluster and luminous field elliptical galaxies can be explained by
a spread in the metallicity of old stellar populations. We check our model with
respect to the so-called G-dwarf problem in ellipticals. For a galaxy subsample
covering a large range in UV-V colors we demonstrate that the addition of an
old metal-poor subcomponent does not invalidate other observational constraints
like colors and the flux in the mid-UV.Comment: Accepted for publication in ApJ Main Journal, 9 pages, 5 figure
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