5,226 research outputs found
Current driven switching of magnetic layers
The switching of magnetic layers is studied under the action of a spin
current in a ferromagnetic metal/non-magnetic metal/ferromagnetic metal spin
valve. We find that the main contribution to the switching comes from the
non-equilibrium exchange interaction between the ferromagnetic layers. This
interaction defines the magnetic configuration of the layers with minimum
energy and establishes the threshold for a critical switching current.
Depending on the direction of the critical current, the interaction changes
sign and a given magnetic configuration becomes unstable. To model the time
dependence of the switching process, we derive a set of coupled Landau-Lifshitz
equations for the ferromagnetic layers. Higher order terms in the
non-equilibrium exchange coupling allow the system to evolve to its
steady-state configuration.Comment: 8 pages, 2 figure. Submitted to Phys. Rev.
PFTijah: text search in an XML database system
This paper introduces the PFTijah system, a text search system that is integrated with an XML/XQuery database management system. We present examples of its use, we explain some of the system internals, and discuss plans for future work. PFTijah is part of the open source release of MonetDB/XQuery
Sound ranking algorithms for XML search
Ranking algorithms for XML should reflect the actual combined content and structure constraints of queries, while at the same time producing equal rankings for queries that are semantically equal. Ranking algorithms that produce different rankings for queries that are semantically equal are easily detected by tests on large databases: We call such algorithms not sound. We report the behavior of different approaches to ranking content-and-structure queries on pairs of queries for which we expect equal ranking results from the query semantics. We show that most of these approaches are not sound. Of the remaining approaches, only 3 adhere to the W3C XQuery Full-Text standard
Effect of geometrical size of the particles in a hot and dense hadron gas
Incorporation of the finite size of baryons into the equation of state (EOS)
of a hot and dense hadron gas (HG) in a thermodynamically consistent manner has
been a much studied problem. We first review its current status. Various models
have been proposed in order to account for the repulsive force generated by the
hard-core geometrical size of the baryons resulting in an excluded volume
effect in the EOS. We examine the criterion of the thermodynamical consistency
of these models and summarize their shortcomings. In order to remove the
shortcomings, we propose a new model which incorporates the excluded volume
effect in a thermodynamically consistent manner. We find that the new model
works even for the cases of extremely large temperatures and densities where
most of other approaches fail. Furthermore, the new expressions for
thermodynamical variables resemble in form with those obtained from
thermodynamically inconsistent models and thus a useful correction factor has
been suggested here which converts inconsistent expressions into
thermodynamically consistent ones. Finally we compare the predictions of new
model with those obtained from various old models.Comment: 19 pages, 9 figures, accepted for publication in Phys. Rev.
Meter-baseline tests of sterile neutrinos at Daya Bay
We explore the sensitivity of an experiment at the Daya Bay site, with a
point radioactive source and a few meter baseline, to neutrino oscillations
involving one or more eV mass sterile neutrinos. We find that within a year,
the entire 3+2 and 1+3+1 parameter space preferred by global fits can be
excluded at the 3\sigma level, and if an oscillation signal is found, the 3+1
and 3+2 scenarios can be distinguished from each other at more than the 3\sigma
level provided one of the sterile neutrinos is lighter than 0.5 eV.Comment: 4 pages, 5 figures, 1 table. Version to appear in PL
Center vortex model for the infrared sector of SU(4) Yang-Mills theory: String tensions and deconfinement transition
A random vortex world-surface model for the infrared sector of SU(4)
Yang-Mills theory is constructed, focusing on the confinement properties and
the behavior at the deconfinement phase transition. Although the corresponding
data from lattice Yang-Mills theory can be reproduced, the model requires a
more complex action and considerably more tuning than the SU(2) and SU(3) cases
studied previously. Its predictive capabilities are accordingly reduced. This
behavior has a definite physical origin, which is elucidated in detail in the
present work. As the number of colors is raised in Yang-Mills theory, the
corresponding infrared effective vortex description cannot indefinitely
continue to rely on dynamics determined purely by vortex world-surface
characteristics; additional color structures present on the vortices begin to
play a role. As evidenced by the modeling effort reported here, definite
signatures of this behavior appear in the case of four colors.Comment: 24 pages, 7 figures containing 8 ps file
A Comparative Analysis of Public and Private Political Risk Insurance Policies with Strategic Applications for Risk Mitigation
Generation of non-classical light is both of fundamental interest and a common condition for quantum information applications (QIA). One feasible type of single photon emitter for QIA is based on semiconductor quantum dots (QDs), due to their atomic-like energy structure and their possibility to be integrated with other semiconductor devices on the same chip. Sitecontrolled QDs with highly linear polarized emission are a prerequisite for certain QIA and a close to room temperature operation is demanded for widespread applications. III-nitride QD can have the deep connement potentials needed for high temperature operation, and the demonstration of single photon emission at room temperature was recently reported for a GaN QD [Nano Lett. 14, 982 (2014)]. Asymmetric III-nitride QD emits light with a high degree of linear polarization. To make site-controlled nitride-based QDs a promising approach is to deposit a thin layer of InGaN on top of hexagonal GaN micropyramids. QDs formed on the apex of the pyramids grown with this approach have been shown to exhibit single and sharp InGaN related emission lines with a high degree of linear polarization [Nano Lett. 11, 2415 (2011)]. A simple elongation of the pyramid base gives control of the polarization direction [Light: Sci. Appl. 3, e139 (2014)]. The work presented in this thesis deals with time correlation measurements, to measure, for the rst time, the single photon properties of these pyramidal QDs. A time correlated single photon spectroscopy (TCSPS) setup was assembled, tested and used to perform measurements on these pyramidal QDs. The TCSPS apparatus measures the time dierences between subsequent photons emitted from the sample. In the spectrally ltered light of one emission line in the emission spectra, e.g. exciton emission, of a QD two or more photons cannot be emitted simultaneously, i.e. the photons are sent out one by one. A histogram of the ensemble of measured time dierences (~106 events) will then for the ideal case have no events for τ = 0, and very few for close to zero. This histogram, when normalized, is under certain conditions equal to the second order coherence function g(2)(τ ). In reality, however, there are photons coming from other sources close to the QD, i.e. background emission, that reach the detector and reduce the dip in the correlation histogram for small τ. There is also an statistical uncertainty in the measured time dierences and nally the nite bin width used in the histogram that deteriorate the measured correlation function. To understand the in uence on g(2)(τ) from background emission, instrument response function and the bin width, on the measurement on excitonic emission, simulations and calculations were made. The crucial variables were, for our samples and setup, the level of the background emission and the instrument response function. A post growth process was developed to cover the lower parts of the pyramid sides as well as the area between the pyramids with a metal lm, to reduce the background emission. This reduces the background emission and largely improves the relative QD signal. As a result, signicant improved single photon characteristics were demonstrated. A measurement of the second order coherence function for the excitonic autocorrelation at a temperature of 12 K, gave for zero time delay ( = 0) a value of g(2)(0) = 0.24 and the residual value of the second order coherence function (0.24) could be in full explained by the three variables, background emission, instrument response function and bin width. The g(2)(0) value for correlation measurements at higher temperatures of 50 K and 80 K is also fully explained by the three variables, showing that the emission from the QD itself is ideal up to 80 K. This result underlines the great potential of these site controlled pyramidal dots as sources of fast polarized single photon emission, and provides the rst rigorous evidence of InGaN quantum dot formation on hexagonal GaN pyramids. We also show the rst proof of biexcitonic emission in this pyramidal QDs
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