46,919 research outputs found
Euler equation of the optimal trajectory for the fastest magnetization reversal of nano-magnetic structures
Based on the modified Landau-Lifshitz-Gilbert equation for an arbitrary
Stoner particle under an external magnetic field and a spin-polarized electric
current, differential equations for the optimal reversal trajectory, along
which the magnetization reversal is the fastest one among all possible reversal
routes, are obtained. We show that this is a Euler-Lagrange problem with
constrains. The Euler equation of the optimal trajectory is useful in designing
a magnetic field pulse and/or a polarized electric current pulse in
magnetization reversal for two reasons. 1) It is straightforward to obtain the
solution of the Euler equation, at least numerically, for a given magnetic
nano-structure characterized by its magnetic anisotropy energy. 2) After
obtaining the optimal reversal trajectory for a given magnetic nano-structure,
finding a proper field/current pulse is an algebraic problem instead of the
original nonlinear differential equation
Mathematical control of complex systems 2013
Mathematical control of complex systems have already become an ideal research area for control engineers, mathematicians, computer scientists, and biologists to understand, manage, analyze, and interpret functional information/dynamical behaviours from real-world complex dynamical systems, such as communication systems, process control, environmental systems, intelligent manufacturing systems, transportation systems, and structural systems. This special issue aims to bring together the latest/innovative knowledge and advances in mathematics for handling complex systems. Topics include, but are not limited to the following: control systems theory (behavioural systems, networked control systems, delay systems, distributed systems, infinite-dimensional systems, and positive systems); networked control (channel capacity constraints, control over communication networks, distributed filtering and control, information theory and control, and sensor networks); and stochastic systems (nonlinear filtering, nonparametric methods, particle filtering, partial identification, stochastic control, stochastic realization, system identification)
Fuzzy-logic-based control, filtering, and fault detection for networked systems: A Survey
This paper is concerned with the overview of the recent progress in fuzzy-logic-based filtering, control, and fault detection problems. First, the network technologies are introduced, the networked control systems are categorized from the aspects of fieldbuses and industrial Ethernets, the necessity of utilizing the fuzzy logic is justified, and the network-induced phenomena are discussed. Then, the fuzzy logic control strategies are reviewed in great detail. Special attention is given to the thorough examination on the latest results for fuzzy PID control, fuzzy adaptive control, and fuzzy tracking control problems. Furthermore, recent advances
on the fuzzy-logic-based filtering and fault detection problems are reviewed. Finally, conclusions are given and some possible future research directions are pointed out, for example, topics on two-dimensional networked systems, wireless networked control systems, Quality-of-Service (QoS) of networked systems, and fuzzy access control in open networked systems.This work was supported in part by the National Natural Science Foundation of China under Grants 61329301,
61374039, 61473163, and 61374127, the Hujiang Foundation of China under Grants C14002 andD15009, the Engineering and Physical Sciences Research Council (EPSRC) of the UK, the Royal Society of the UK, and the Alexander von Humboldt Foundation of Germany
Ultrafast pump-probe spectroscopic signatures of superconducting and pseudogap phases in YBa2Cu3O7-{\delta} films
Femtosecond pump-probe spectroscopy is applied to identify transient optical
signatures of phase transitions in optimally doped YBa2Cu3O7-{\delta} films. To
elucidate the dynamics of superconducting and pseudogap phases, the slow
thermal component is removed from the time-domain traces of photo-induced
reflectivity in a high-flux regime with low frequency pulse rate. The rescaled
data exhibit distinct signatures of the phase separation with abrupt changes at
the onsets of TSC and TPG in excellent agreement with transport data. Compared
to the superconducting phase, the response of the pseudogap phase is
characterized by the strongly reduced reflectivity change accompanied by a
faster recovery time.Comment: 14 pages, 3 figure
A 3,500-year tree-ring record of annual precipitation on the northeastern Tibetan Plateau
An annually resolved and absolutely dated ring-width chronology spanning 4,500 y has been constructed using subfossil, archaeological, and living-tree juniper samples from the northeastern Tibetan Plateau. The chronology represents changing mean annual precipitation and is most reliable after 1500 B.C. Reconstructed precipitation for this period displays a trend toward more moist conditions: the last 10-, 25-, and 50-y periods all appear to be the wettest in at least three and a half millennia. Notable historical dry periods occurred in the 4th century BCE and in the second half of the 15th century CE. The driest individual year reconstructed (since 1500 B.C.) is 1048 B.C., whereas the wettest is 2010. Precipitation variability in this region appears not to be associated with inferred changes in Asian monsoon intensity during recent millennia. The chronology displays a statistical association with the multidecadal and longer-term variability of reconstructed mean Northern Hemisphere temperatures over the last two millennia. This suggests that any further large-scale warming might be associated with even greater moisture supply in this region
Effect of iron content and potassium substitution in AFeSe (A = K, Rb, Tl) superconductors: a Raman-scattering investigation
We have performed Raman-scattering measurements on high-quality single
crystals of the superconductors KFeSe ( = 32 K),
TlKFeSe ( = 29 K), and
TlRbFeSe ( = 31 K), as well as of the
insulating compound KFeSe. To interpret our results, we have made
first-principles calculations for the phonon modes in the ordered iron-vacancy
structure of KFeSe. The modes we observe can be assigned
very well from our symmetry analysis and calculations, allowing us to compare
Raman-active phonons in the AFeSe compounds. We find a clear frequency
difference in most phonon modes between the superconducting and
non-superconducting potassium crystals, indicating the fundamental influence of
iron content. By contrast, substitution of K by Tl or Rb in
AFeSe causes no substantial frequency shift for any modes
above 60 cm, demonstrating that the alkali-type metal has little effect
on the microstructure of the FeSe layer. Several additional modes appear below
60 cm in Tl- and Rb-substituted samples, which are vibrations of heavier
Tl and Rb ions. Finally, our calculations reveal the presence of "chiral"
phonon modes, whose origin lies in the chiral nature of the
KFeSe structure.Comment: 11 pages, 10 figures and 2 table
Synthesis, Structure and Magnetic Properties of New Layered Iron-Oxychalcogenide Na2Fe2OSe2
A new layered iron-oxychalcogenide Na2Fe2OSe2 has been synthesized and
structurally characterized by powder X-ray diffraction. The structure is formed
by alternate stacking of the newly discovered [Fe2OSe2] blocks and double
layers of Na. Conductivity study shows that Na2Fe2OSe2 is a semiconductor with
activation energy of 0.26 eV. Magnetic susceptibility and heat capacity
measurements reveal an antiferromagnetic phase transition occurs at TN=73 K. A
broad maximum of magnetic susceptibility and a slow decay of the specific heat
above TN, arises as a result of two-dimensional short-range spin correlation.Comment: 4 pages, 4 figure
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