1,721,073 research outputs found
Hopping of elliptical pebbles on the beach and laboratory results
No full textAbstract On sandy ocean beaches, amongst many irregularly shaped pebbles, a surprisingly large number of pebbles that are nearly perfect ellipsoids may also be found. These elliptical stones are characterized by the density ρ of the mineral and by the length of their symmetry axes ( a , b , c ) with ( a > b > c ). The grinding process which forms the ab-plane of the ellipsoid relates to the rotation around the axis of the greatest moment of inertia ( c -axis) in the surf. In a recent paper by fairly simple physical arguments, it was shown that by this rotation around the c -axis the ellipticity ε always increases. This is in agreement with the empirical observation that on sandy beaches of the oceans no spherical or disc-like pebbles can be found. If the pebble velocity during the rotation becomes too large, then hopping of the pebble sets in. This is caused by the fact that the centre-of-mass acceleration during fast rotation exceeds the gravity acceleration g . Since greater forces occur for jumping than for rolling, the hopping can have a strong influence on the b / a -ratio of the elliptical pebble. In this work, it will be shown that the b -axis is worn away more than the a -axis by erosion with jumping. In a laboratory study with a model of an elliptical cylinder, the theoretical results were checked and partially confirmed
Experimental study of electron-phonon properties in ZrB 2
No full textHigh quality samples and absence of superconductivity down to 40 mK make of ZrB 2 the best normal state reference system for the superconducting isostructural MgB 2. Actually, the question of pairing has to be focused on the electron-phonon interaction in the normal state. After presenting the resistivity measurements of ZrB 2, we explain the details of the Bloch-Grüneisen and Einstein models used to deduce the first results. We then compare experimental de Haas-van Alphen effect data with theoretical Fermi surfaces to present additional results on electron quasi-particle renormalization. The estimations reveal an isotropic and negligible coupling constant of in average 0.145. The contribution of the coupling to the optical phonon modes is 0.082, in contrast to the known larger coupling of 0.283 [3] to the E 2g phonon mode in MgB 2. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 200671.18.+y Fermi surface: calculations and measurements; effective mass, g factor, 71.38.-k Polarons and electron-phonon interactions, 72.10.-d Theory of electronic transport; scattering mechanisms, 74.25.Fy Transport properties,
Electronic properties of boron-doped diamond on the border between the normal and the superconducting state
No full textWe report on measurements of resistivity and magnetoresistivity, Hall effect and upper critical field on normal and superconducting heavily boron-doped diamond layer produced by microwave plasma assisted chemical vapour deposition. The magnetoresistivity shows a characteristic change as a function of the boron concentration with strong evidence for a closed Fermi surface for superconducting samples. Although experimental and theoretical results favour a conventional, weak coupling electron-phonon interpretation of the superconducting mechanism, the observed dependence of the transition temperature T-C on the hole concentration cannot be consistently described by the conventional theory. (c) 2005 Elsevier B.V. All rights reserved
Magnetic phase diagram and reentrant superconductivity in DyNi2B2C single crystals
No full textIn order to investigate the interaction between superconductivity and magnetic ordering resistivity, magnetisation measurements were performed on DyNi2B2C single crystals for T = 0.05-15 K and for B = 0-5 T in the crystallographic ab-plane. Measurements of the magnetisation with a torque cantilever magnetometer give strong evidence for metamagnetic states built of different ordered ferromagnetic domains. For B parallel to [100]. an anomalous large hysteresis effect of the metamagnetic transitions and also of the superconducting transitions were observed in the low-temperature range (T < 2 K). In some extreme cases, the metamagnetic state can be frozen after a field cycle sweeping up to 2 T and down to zero at 50 mK. In this case, superconductivity is totally suppressed. The superconducting state can be restored in two ways: Either with increasing temperature, or by applying a small field with the opposite field direction. (C) 2001 Elsevier Science B.V. All rights reserved
Electronic properties of the semiconductor RuIn3
No full textTemperature-dependent measurements of the resistivity of RuIn3 single crystals show a semiconducting behaviour, in contrast to previously published results ( Roof et al 1986 Powder Diffr. 1 20; Pottgen 1995 J. Alloys Compounds 226 59). In the high- temperature range the semiconducting gap was measured to be 0.4 - 0.5 eV. We observe an anisotropy of the resistivity along [ 110] and [ 001] orientations of the tetragonal single crystals. At low temperatures two activation energies of impurities were estimated to 1 and 10 meV. The temperature dependence of the specific heat and the band structure calculations also show a semiconducting behaviour of RuIn3
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
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Magnetic ordering in TbNi2B2C and Tb0.4Y0.6Ni2B2C single crystals
No full textThe complex magnetic ordering of TbNi2B2C and Tb0.4Y0.6Ni2B2C single crystals has been investigated by AC-susceptibility and magnetization measurements. Metamagnetic transitions and an anisotropic behavior within (a, b)-plane were revealed for TbNi2B2C. Alloying with Y reduces the critical field of antiferromagnetic to paramagnetic transition, eliminates the metamagnetic transitions and induces superconductivity below 4.1 K. (C) 2000 Elsevier Science B.V. All rights reserved
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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