3,493 research outputs found
Direct observation of the influence of the As-Fe-As angle on the Tc of superconducting SmFeAsOF
The electrical resistivity, crystalline structure and electronic properties
calculated from the experimentally measured atomic positions of the compound
SmFeAsOF have been studied up to pressures ~20GPa. The
correlation between the pressure dependence of the superconducting transition
temperature (Tc) and crystallographic parameters on the same sample shows
clearly that a regular FeAs tetrahedron maximizes Tc, through
optimization of carrier transfer to the FeAs planes as indicated by the
evolution of the electronic band structures.Comment: 15pages, 4 figure
Large lianas as hyperdynamic elements of the tropical forest canopy
Lianas (woody vines) are an important component of lowland tropical forests.
We report large liana and tree inventory and dynamics data from Amazonia over periods
of up to 24 years, making this the longest geographically extensive study of liana ecology
to date. We use these results to address basic questions about the ecology of large lianas
in mature forests and their interactions with trees. In one intensively studied site we find
that large lianas (â„10 cm diameter) represent ,5% of liana stems, but 80% of biomass of
well-lit upper canopy lianas. Across sites, large lianas and large trees are both most successful
in terms of structural importance in richer soil forests, but large liana success may
be controlled more by the availability of large tree supports rather than directly by soil
conditions. Long-term annual turnover rates of large lianas are 5â8%, three times those of
trees. Lianas are implicated in large tree mortality: liana-infested large trees are three times
more likely to die than liana-free large trees, and large lianas are involved in the death of
at least 30% of tree basal area. Thus large lianas are a much more dynamic component of
Amazon forests than are canopy trees, and they play a much more significant functional
role than their structural contribution suggests
Two approaches for effective modelling of rain-rate time-series for radiocommunication system simulations
The paper presents a model which allows to synthetically generate rain rate time-series for a fixed location. Rain rate time-series are very much correlated with signal attenuation in Ka band and above and, thus, enable to realistically simulate propagation effects on Earth-satellite links. The model presented are based on Markov chains
Current Induced Order Parameter Dynamics: Microscopic Theory Applied to Co/Cu/Co spin valves
Transport currents can alter alter order parameter dynamics and change steady
states in superconductors, in ferromagnets, and in hybrid systems. In this
article we present a scheme for fully microscopic evaluation of order parameter
dynamics that is intended for application to nanoscale systems. The approach
relies on time-dependent mean-field-theory, on an adiabatic approximation, and
on the use of non-equilibrium Greens function (NEGF) theory to calculate the
influence of a bias voltage across a system on its steady-state density matrix.
We apply this scheme to examine the spin-transfer torques which drive
magnetization dynamics in Co/Cu/Co spin-valve structures. Our microscopic
torques are peaked near Co/Cu interfaces, in agreement with most previous
pictures, but suprisingly act mainly on Co transition metal -orbitals rather
than on -orbitals as generally supposed.Comment: 9 pages, 5 figure
Ab-initio GMR and current-induced torques in Au/Cr multilayers
We report on an {\em ab-initio} study of giant magnetoresistance (GMR) and
current-induced-torques (CITs) in Cr/Au multilayers that is based on
non-equilibrium Green's functions and spin density functional theory. We find
substantial GMR due primarily to a spin-dependent resonance centered at the
Cr/Au interface and predict that the CITs are strong enough to switch the
antiferromagnetic order parameter at current-densities times smaller
than typical ferromagnetic metal circuit switching densities.Comment: 8 pages, 6 figure
Strong, Weak and Branching Bisimulation for Transition Systems and Markov Reward Chains: A Unifying Matrix Approach
We first study labeled transition systems with explicit successful
termination. We establish the notions of strong, weak, and branching
bisimulation in terms of boolean matrix theory, introducing thus a novel and
powerful algebraic apparatus. Next we consider Markov reward chains which are
standardly presented in real matrix theory. By interpreting the obtained matrix
conditions for bisimulations in this setting, we automatically obtain the
definitions of strong, weak, and branching bisimulation for Markov reward
chains. The obtained strong and weak bisimulations are shown to coincide with
some existing notions, while the obtained branching bisimulation is new, but
its usefulness is questionable
Limitations on the superposition principle: superselection rules in non-relativistic quantum mechanics
The superposition principle is a very basic ingredient of quantum theory.
What may come as a surprise to many students, and even to many practitioners of
the quantum craft, is tha superposition has limitations imposed by certain
requirements of the theory. The discussion of such limitations arising from the
so-called superselection rules is the main purpose of this paper. Some of their
principal consequences are also discussed. The univalence, mass and particle
number superselection rules of non-relativistic quantum mechanics are also
derived using rather simple methods.Comment: 22 pages, no figure
Using genetic algorithms to generate test sequences for complex timed systems
The generation of test data for state based specifications is a computationally expensive process. This problem is magnified if we consider that time con- straints have to be taken into account to govern the transitions of the studied system. The main goal of this paper is to introduce a complete methodology, sup- ported by tools, that addresses this issue by represent- ing the test data generation problem as an optimisa- tion problem. We use heuristics to generate test cases. In order to assess the suitability of our approach we consider two different case studies: a communication protocol and the scientific application BIPS3D. We give details concerning how the test case generation problem can be presented as a search problem and automated. Genetic algorithms (GAs) and random search are used to generate test data and evaluate the approach. GAs outperform random search and seem to scale well as the problem size increases. It is worth to mention that we use a very simple fitness function that can be eas- ily adapted to be used with other evolutionary search techniques
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