42,675 research outputs found
Resonance tongues and patterns in periodically forced reaction-diffusion systems
Various resonant and near-resonant patterns form in a light-sensitive
Belousov-Zhabotinsky (BZ) reaction in response to a spatially-homogeneous
time-periodic perturbation with light. The regions (tongues) in the forcing
frequency and forcing amplitude parameter plane where resonant patterns form
are identified through analysis of the temporal response of the patterns.
Resonant and near-resonant responses are distinguished. The unforced BZ
reaction shows both spatially-uniform oscillations and rotating spiral waves,
while the forced system shows patterns such as standing-wave labyrinths and
rotating spiral waves. The patterns depend on the amplitude and frequency of
the perturbation, and also on whether the system responds to the forcing near
the uniform oscillation frequency or the spiral wave frequency. Numerical
simulations of a forced FitzHugh-Nagumo reaction-diffusion model show both
resonant and near-resonant patterns similar to the BZ chemical system
Magnetic properties of undoped Cu2O fine powders with magnetic impurities and/or cation vacancies
Fine powders of micron- and submicron-sized particles of undoped Cu2O
semiconductor, with three different sizes and morphologies have been
synthesized by different chemical processes. These samples include nanospheres
200 nm in diameter, octahedra of size 1 micron, and polyhedra of size 800 nm.
They exhibit a wide spectrum of magnetic properties. At low temperature, T = 5
K, the octahedron sample is diamagnetic. The nanosphere is paramagnetic. The
other two polyhedron samples synthesized in different runs by the same process
are found to show different magnetic properties. One of them exhibits weak
ferromagnetism with T_C = 455 K and saturation magnetization, M_S = 0.19 emu/g
at T = 5 K, while the other is paramagnetic. The total magnetic moment
estimated from the detected impurity concentration of Fe, Co, and Ni, is too
small to account for the observed magnetism by one to two orders of magnitude.
Calculations by the density functional theory (DFT) reveal that cation
vacancies in the Cu2O lattice are one of the possible causes of induced
magnetic moments. The results further predict that the defect-induced magnetic
moments favour a ferromagnetically coupled ground state if the local
concentration of cation vacancies, n_C, exceeds 12.5%. This offers a possible
scenario to explain the observed magnetic properties. The limitations of the
investigations in the present work, in particular in the theoretical
calculations, are discussed and possible areas for further study are suggested.Comment: 20 pages, 5 figures 2 tables, submitted to J Phys Condense Matte
An MHD Model For Magnetar Giant Flares
Giant flares on soft gamma-ray repeaters that are thought to take place on
magnetars release enormous energy in a short time interval. Their power can be
explained by catastrophic instabilities occurring in the magnetic field
configuration and the subsequent magnetic reconnection. By analogy with the
coronal mass ejection (CME) events on the Sun, we develop a theoretical model
via an analytic approach for magnetar giant flares. In this model, the rotation
and/or displacement of the crust causes the field to twist and deform, leading
to flux rope formation in the magnetosphere and energy accumulation in the
related configuration. When the energy and helicity stored in the configuration
reach a threshold, the system loses its equilibrium, the flux rope is ejected
outward in a catastrophic way, and magnetic reconnection helps the catastrophe
develop to a plausible eruption. By taking SGR 1806 - 20 as an example, we
calculate the free magnetic energy released in such an eruptive process and
find that it is more than ergs, which is enough to power a giant
flare. The released free magnetic energy is converted into radiative energy,
kinetic energy and gravitational energy of the flux rope. We calculated the
light curves of the eruptive processes for the giant flares of SGR 1806 - 20,
SGR 0526-66 and SGR 1900+14, and compared them with the observational data. The
calculated light curves are in good agreement with the observed light curves of
giant flares.Comment: Accepted to Ap
Time evolution, cyclic solutions and geometric phases for general spin in an arbitrarily varying magnetic field
A neutral particle with general spin and magnetic moment moving in an
arbitrarily varying magnetic field is studied. The time evolution operator for
the Schr\"odinger equation can be obtained if one can find a unit vector that
satisfies the equation obeyed by the mean of the spin operator. There exist at
least cyclic solutions in any time interval. Some particular time
interval may exist in which all solutions are cyclic. The nonadiabatic
geometric phase for cyclic solutions generally contains extra terms in addition
to the familiar one that is proportional to the solid angle subtended by the
closed trace of the spin vector.Comment: revtex4, 8 pages, no figur
Harnessing heterogeneous social networks for better recommendations: A grey relational analysis approach
Most of the extant studies in social recommender system are based on explicit social relationships, while the potential of implicit relationships in the heterogeneous social networks remains largely unexplored. This study proposes a new approach to designing a recommender system by employing grey relational analysis on the heterogeneous social networks. It starts with the establishment of heterogeneous social networks through the user-item bipartite graph, user social network graph and user-attribute bipartite graph; and then uses grey relational analysis to identify implicit social relationships, which are then incorporated into the matrix factorization model. Five experiments were conducted to test the performance of our approach against four state-of-the-art baseline methods. The results show that compared with the baseline methods, our approach can effectively alleviate the sparsity problem, because the heterogeneous social network provides richer information. In addition, the grey relational analysis method has the advantage of low requirements for data size and efficiently relieves the cold start problem. Furthermore, our approach saves processing time, thus increases recommendation efficiency. Overall, the proposed approach can effectively improve the accuracy of rating prediction in social recommendations and provide accurate and efficient recommendation service for users
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