7,550 research outputs found
Neural networks for modelling and control of a non-linear dynamic system
The authors describe the use of neural nets to model and control a nonlinear second-order electromechanical model of a drive system with varying time constants and saturation effects. A model predictive control structure is used. This is compared with a proportional-integral (PI) controller with regard to performance and robustness against disturbances. Two feedforward network types, the multilayer perceptron and radial-basis-function nets, are used to model the system. The problems involved in the transfer of connectionist theory to practice are discussed
Monoclinic and triclinic phases in higher-order Devonshire theory
Devonshire theory provides a successful phenomenological description of many
cubic perovskite ferroelectrics such as BaTiO3 via a sixth-order expansion of
the free energy in the polar order parameter. However, the recent discovery of
a novel monoclinic ferroelectric phase in the PZT system by Noheda et al.
(Appl. Phys. Lett. 74, 2059 (1999)) poses a challenge to this theory. Here, we
confirm that the sixth-order Devonshire theory cannot support a monoclinic
phase, and consider extensions of the theory to higher orders. We show that an
eighth-order theory allows for three kinds of equilibrium phases in which the
polarization is confined not to a symmetry axis but to a symmetry plane. One of
these phases provides a natural description of the newly observed monoclinic
phase. Moreover, the theory makes testable predictions about the nature of the
phase boundaries between monoclinic, tetragonal, and rhombohedral phases. A
ferroelectric phase of the lowest (triclinic) symmetry type, in which the
polarization is not constrained by symmetry, does not emerge until the
Devonshire theory is carried to twelfth order. A topological analysis of the
critical points of the free-energy surface facilitates the discussion of the
phase transition sequences.Comment: 10 pages, with 5 postscript figures embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/dv_pzt/index.htm
Creating pseudo Kondo-resonances by field-induced diffusion of atomic hydrogen
In low temperature scanning tunneling microscopy (STM) experiments a cerium
adatom on Ag(100) possesses two discrete states with significantly different
apparent heights. These atomic switches also exhibit a Kondo-like feature in
spectroscopy experiments. By extensive theoretical simulations we find that
this behavior is due to diffusion of hydrogen from the surface onto the Ce
adatom in the presence of the STM tip field. The cerium adatom possesses
vibrational modes of very low energy (3-4meV) and very high efficiency (> 20%),
which are due to the large changes of Ce-states in the presence of hydrogen.
The atomic vibrations lead to a Kondo-like feature at very low bias voltages.
We predict that the same low-frequency/high-efficiency modes can also be
observed at lanthanum adatoms.Comment: five pages and four figure
Rb*He_n exciplexes in solid 4_He
We report the observation of emission spectra from Rb*He_n exciplexes in
solid 4He. Two different excitation channels were experimentally identified,
viz., exciplex formation via laser excitation to the atomic 5P3/2 and to the
5P1/2 levels. While the former channel was observed before in liquid helium, on
helium nanodroplets and in helium gas by different groups, the latter creation
mechanism occurs only in solid helium or in gaseous helium above 10 Kelvin. The
experimental results are compared to theoretical predictions based on the
extension of a model, used earlier by us for the description of Cs*He_n
exciplexes. We also report the first observation of fluorescence from atomic
rubidium in solid helium, and discuss striking differences between the
spectroscopic feature of Rb-He and Cs-He systems.Comment: 8 pages, 8 figure
Noncommutative Common Cause Principles in Algebraic Quantum Field Theory
States in algebraic quantum field theory "typically" establish correlation
between spacelike separated events. Reichenbach's Common Cause Principle,
generalized to the quantum field theoretical setting, offers an apt tool to
causally account for these superluminal correlations. In the paper we motivate
first why commutativity between the common cause and the correlating events
should be abandoned in the definition of the common cause. Then we show that
the Noncommutative Weak Common Cause Principle holds in algebraic quantum field
theory with locally finite degrees of freedom. Namely, for any pair of
projections A, B supported in spacelike separated regions V_A and V_B,
respectively, there is a local projection C not necessarily commuting with A
and B such that C is supported within the union of the backward light cones of
V_A and V_B and the set {C, non-C} screens off the correlation between A and B
Heritabilities of osteochondral lesions and genetic correlations with production and exterior traits in station-tested pigs
Osteochondrosis might reduce the performance of slaughter pigs, longevity of sows and animal welfare. The aim of the present work was to describe the prevalence in Swiss breeds and to analyse the genetic background of osteochondral lesions. Between January 2002 and December 2005, about 9500 station-tested pigs were examined for several exterior traits before slaughtering at the Swiss pig performance testing station using the Swiss linear description system with a scale from 1 to 7 per trait. The animals belonged to three breeds: Large White dam line, Swiss Landrace and Large White sire line. Additionally, a random sample of these pigs (n = 2622) was examined for osteochondral lesions at seven positions of the carcass after dissection. At first, the surface and shape of the femur, humerus, radius and ulna at the joints were evaluated by a trained person. Afterwards these bones were sawed and the state of the cartilage and the distal epiphyseal cartilage of the ulna was examined at the cutting surface. Osteochondral lesions were scored on a scale from 1 to 6. The prevalence of osteochondral lesions was low at head of humerus, condylus lateralis humeri, radius and ulna proximal and head of femur. Osteochondral lesions at condylus medialis humeri (CMH), distal epiphyseal cartilage of ulna (DEU) and condylus lateralis femoris (CMF) exhibited phenotypic and genetic variance. Their heritabilities ranged from 0.16 to 0.18 using linear mixed animal models. Therefore, it is possible to reduce the prevalence of osteochondral lesions by selection in principle. Exterior traits showed low heritabilities (0.10 to 0.26) but several favourable genetic correlations with osteochondral lesions at CMH, DEU and CMF with low to moderate magnitude. Genetic correlations between osteochondral lesions and production traits were lo
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