1,070 research outputs found
Manipulators with flexible links: A simple model and experiments
A simple dynamic model proposed for flexible links is briefly reviewed and experimental control results are presented for different flexible systems. A simple dynamic model is useful for rapid prototyping of manipulators and their control systems, for possible application to manipulator design decisions, and for real time computation as might be applied in model based or feedforward control. Such a model is proposed, with the further advantage that clear physical arguments and explanations can be associated with its simplifying features and with its resulting analytical properties. The model is mathematically equivalent to Rayleigh's method. Taking the example of planar bending, the approach originates in its choice of two amplitude variables, typically chosen as the link end rotations referenced to the chord (or the tangent) motion of the link. This particular choice is key in establishing the advantageous features of the model, and it was used to support the series of experiments reported
Universal relationship between crystallinity and irreversibility field of MgB2
The relationship between irreversibility field, Hirr, and crystallinity of
MgB2 bulks including carbon substituted samples was studied. The Hirr was found
to increase with an increase of FWHM of MgB2 (110) peak, which corresponds to
distortion of honeycomb boron sheet, and their universal correlation was
discovered even including carbon substituted samples. Excellent Jc
characteristics under high magnetic fields were observed in samples with large
FWHM of (110) due to the enhanced intraband scattering and strengthened grain
boundary flux pinning. The relationship between crystallinity and Hirr can
explain the large variation of Hirr for MgB2 bulks, tapes, single crystals and
thin films.Comment: 3 pages, 4 figures, to be published in Appl. Phys. Lett. (in press
Lyapunov exponent and natural invariant density determination of chaotic maps: An iterative maximum entropy ansatz
We apply the maximum entropy principle to construct the natural invariant
density and Lyapunov exponent of one-dimensional chaotic maps. Using a novel
function reconstruction technique that is based on the solution of Hausdorff
moment problem via maximizing Shannon entropy, we estimate the invariant
density and the Lyapunov exponent of nonlinear maps in one-dimension from a
knowledge of finite number of moments. The accuracy and the stability of the
algorithm are illustrated by comparing our results to a number of nonlinear
maps for which the exact analytical results are available. Furthermore, we also
consider a very complex example for which no exact analytical result for
invariant density is available. A comparison of our results to those available
in the literature is also discussed.Comment: 16 pages including 6 figure
Self-organization in systems of self-propelled particles
We investigate a discrete model consisting of self-propelled particles that
obey simple interaction rules. We show that this model can self-organize and
exhibit coherent localized solutions in one- and in two-dimensions.In
one-dimension, the self-organized solution is a localized flock of finite
extent in which the density abruptly drops to zero at the edges.In
two-dimensions, we focus on the vortex solution in which the particles rotate
around a common center and show that this solution can be obtained from random
initial conditions, even in the absence of a confining boundary. Furthermore,
we develop a continuum version of our discrete model and demonstrate that the
agreement between the discrete and the continuum model is excellent.Comment: 4 pages, 5 figure
Structural Features of Layered Iron Pnictide Oxides (Fe2As2)(Sr4M2O6)
Structural features of newly found perovskite-based iron pnictide oxide
system have been systematically studied. Compared to REFePnO system,
perovskite-based system tend to have lower Pn-Fe-Pn angle and higher pnictogen
height owing to low electronegativity of alkaline earth metal and small
repulsive force between pnictogen and oxygen atoms. As-Fe-As angles of
(Fe2As2)(Sr4Cr2O6), (Fe2As2)(Sr4V2O6) and (Fe2Pn2)(Sr4MgTiO6) are close to
ideal tetrahedron and those pnictogen heights of about 1.40 A are close to
NdFeAsO with optimized carrier concentration. These structural features of this
system may leads to realization of high Tc superconductivity.Comment: 3pages, 2figures, 1table, proceedings of M2S 200
Successive transition from superconducting to antiferromagnetic phase in (Ca_6(Al, Ti)_4O_y)Fe_2As_2 studied via ^{75}As and ^{27}Al NMR
An unusual successive phase transition from superconducting (SC) to
antiferromagnetic (AF) phases was discovered via ^{75}As and ^{27}Al nuclear
magnetic resonance (NMR) in (Fe_2As_2)(Ca_6(Al, Ti)_4O_y) with four (Al, Ti)O
layers intercalated between FeAs planes. Although the spatially-uniform AF
ordering is clearly visible from ^{27}Al spectra, the ordered moments are very
small and the low-frequency fluctuation is much suppressed, contrary to
existing pnictides with localized magnetic elements. Furthermore, the
temperature (T) dependence of the fluctuation at both nuclei is very similar
throughout the entire temperature range. These facts suggest that some
hybridization between Ti and Fe orbitals induces a uniform electronic state
within FeAs and (Al, Ti)O layers accompanied by the SC and AF transitions. The
iron-based pnictide with Ti-doped blocking layers is the first high-T_c
compound having metallic blocking layers
Role of magnetic dopants in the phase diagram of Sm1111 pnictides: The Mn case
The deliberate insertion of magnetic Mn dopants in the Fe sites of the
optimally-doped SmFeAsO0.88-F0.12 iron-based superconductor can modify in a
controlled way its electronic properties. The resulting phase diagram was
investigated across a wide range of manganese contents (x) by means of
muon-spin spectroscopy (muSR), both in zero- and in transverse fields,
respectively, to probe the magnetic and the superconducting order. The pure
superconducting phase (at x < 0.03) is replaced by a crossover region at
intermediate Mn values (0.03 =< x < 0.08), where superconductivity coexists
with static magnetic order. After completely suppressing superconductivity for
x = 0.08, a further increase in Mn content reinforces the natural tendency
towards antiferromagnetic correlations among the magnetic Mn ions. The sharp
drop of Tc and the induced magnetic order in the presence of magnetic
disorder/dopants, such as Mn, are both consistent with a recent theoretical
model of unconventional superconductors [M. Gastiasoro et al., ArXiv
1606.09495], which includes correlation-enhanced RKKY-couplings between the
impurity moments.Comment: 10 pages, 13 figures. Accepted on Physical Review
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