1,720,979 research outputs found
Topology of the fittest transportation network
No full textThe presence or absence of loops in the emergent transportation networks, that are characterized by a minimum overall cost, is shown to depend on the convexity of the cost function for the local transportation of material. Our results are directly applicable to a variety of situations across disciplines
Detecting communities in large networks
No full textWe develop an algorithm to detect community structure in complex networks. The algorithm is based on spectral methods and takes into account weights and link orientation. Since the method detects efficiently clustered nodes in large networks even when these are not sharply partitioned, it turns to be specially suitable for the analysis of social and information networks. We test the algorithm on a large-scale data-set from a psychological experiment of word association. In this case, it proves to be successful both in clustering words, and in uncovering mental association patterns. © 2005 Elsevier B.V. All rights reserved
Signature of negative domain wall mass in soft magnetic materials
No full textMagnetic properties of ferromagnetic materials often originate from domain wall motion, involving different damping mechanisms, an effective mass and various interactions with the surrounding media. In metallic materials, eddy current damping overwhelms inertia and thus the effect of the mass is usually neglected. We have recently reported experimental evidence that in soft metallic ferromagnets eddy currents yield an observable negative contribution to the effective domain wall mass. The weight of this mass is of the order of 10-5 kg/m2, much larger than the positive Doring mass (∼10-9 kg/m2). This negative effective mass is responsible for the leftward asymmetry of Barkhausen noise pulse shapes. In particular, this asymmetry depends on the pulse duration and it is found to encode important information on the characteristic time of the underlying domain wall dynamics. Only on long timescales the pulse shapes are symmetric and show the universal features typical of the Barkhausen effect. This result clarifies the general significance of pulse shape asymmetry commonly observed in systems showing a similar crackling noise, and contributes to better understand the microscopic phenomena responsible of magnetic hysteresis
Community structure from spectral properties in complex networks
No full textWe analyze the spectral properties of complex networks focusing on their relation to the community structure, and develop an algorithm based on correlations among components of different eigenvectors. The algorithm applies to general weighted networks, and, in a suitably modified version, to the case of directed networks. Our method allows to correctly detect communities in sharply partitioned graphs, however it is useful to the analysis of more complex networks, without a well defined cluster structure, as social and information networks. As an example, we test the algorithm on a large scale data-set from a psychological experiment of free word association, where it proves to be successful both in clustering words, and in uncovering mental association patterns. © 2005 American Institute of Physics
Loss separation for dynamic hysteresis in ferromagnetic thin films
No full textWe develop a theory for dynamic hysteresis in ferromagnetic thin films, on the basis of the phenomenological principle of loss separation. We observe that, remarkably, the theory of loss separation, originally derived for bulk metallic materials, is applicable to disordered magnetic systems under fairly general conditions regardless of the particular damping mechanism. We confirm our theory both by numerical simulations of a driven random-field Ising model, and by reexamining several experimental data reported in the literature on dynamic hysteresis in thin films. All the experiments examined and the simulations find a natural interpretation in terms of loss separation. The power losses' dependence on the driving field rate predicted by our theory fits satisfactorily all the data in the entire frequency range, thus reconciling the apparent lack of universality observed in different materials
Communities Detection in Large Networks
No full textWe develop an algorithm to detect community structure in complex networks. The algorithm is based on spectral methods and takes into account weights and links orientations. Since the method detects efficiently clustered nodes in large networks even when these are not sharply partitioned, it turns to be specially suitable to the analysis of social and information networks. We test the algorithm on a large-scale data-set from a psychological experiment of word association. In this case, it proves to be successful both in clustering words, and in uncovering mental association patterns. © Springer-Verlag 2004
Eddy current damping of a moving domain wall: Beyond the quasistatic approximation
Full text linkWe investigate dynamic hysteresis in ferromagnetic thin films with zigzag domain walls. We introduce a discrete model describing the motion of a wall in a disordered ferromagnet with in-plane magnetization, driven by an external magnetic field, considering the effects of dipolar interactions and anisotropy. We analyze the effects of external field frequency and temperature on the coercive field by Monte Carlo simulations, and find a good agreement with the experimental data reported in literature for Fe/GaAs films. This implies that dynamic hysteresis in this case can be explained by a single propagating domain wall model without invoking domain nucleation
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Rayleigh loops in the random-field Ising model on the Bethe lattice
Full text linkWe analyze the demagnetization properties of the random-field Ising model on the Bethe lattice focusing on the behavior near the disorder induced phase transition. We derive an exact recursion relation for the magnetization and integrate it numerically. Our analysis shows that demagnetization is possible only in the continuous high disorder phase, where at low field the loops are described by the Rayleigh law. In the low disorder phase, the saturation loop displays a discontinuity that is reflected by a nonvanishing magnetization m∞ after a series of nested loops. In this case, at low fields the loops are not symmetric and the Rayleigh law does not hold
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