1,720,980 research outputs found
Conductance switching and nonequilibrium phase coexistence in superconductors with intermediate bias
No full textInternational audienceSuperconducting systems may display different types of nonequilibrium states depending on the specific constraints imposed for measurement. We probe current-voltage relations of three-dimensional superconducting films by allowing finite voltages to develop across their length. Our experiments reveal sharp features of negative differential conductance which highlight the validity of the principle of minimum entropy production at the critical current transition. We have observed dissipative states with resistances intermediate between those of superconducting and normal phases at zero applied magnetic field, indicating a phenomenon of phase coexistence under nonequilibrium conditions. The features of steady states reported here are not accessible in conventional transport experiments with current-biasing methods
Conductance switching in a voltage-biased superconductor
No full textSuperconductors are distinguished by the ability to conduct electricity without any resistance. Consequently, a finite electric field is incompatible with the zero resistance state. Here we devise an experiment to apply a voltage-bias across a superconducting film, in order to probe the electric-field-induced breakdown of superconductivity. It is shown that voltage-biasing leads to an extremely sharp feature of negative differential conductance (NDC) with a discontinuity in the current-voltage relation, such that the normal state appears at currents much lesser than the critical current. Alongside the conductance, the net power dissipated also undergoes a drastic reduction with increasing voltage across the superconductor-normal transition. We further observe that an electric field may induce a state with a resistance intermediate between that of superconducting and normal phases. The NDC phenomenon is useful for designing switching devices with low power consumption. We demonstrate how the device parameters can be controlled by the adjustment of an external resistor, offering a great degree of adaptability for different applications. The bistable current-voltage relation demonstrated here is promising for realizing complex non-linear networks having multiple steady state solutions, including states with symmetry-broken voltage distributions.</div
Fish Symbolism in Indus Valley Epigraphy and Protohistoric Accounts
Full text linkThe contribution of the Indus Valley civilization to the historic cultures of South Asia is a matter of debate due to a discontinuity in material culture, from the time of its decline to the reappearance of urbanization several centuries later. Progress in the epigraphy of the Indus Valley has been hindered by the absence of a bilingual inscription and the brevity of its texts. One of the most frequent signs encountered in its undeciphered writing system is the pictogram of ‘fish’. On a few seal inscriptions, this sign appears alone, suggesting that it represented a meaningful word or a name. It is noteworthy that Indian literature of later centuries recounts a protohistoric kingdom named Matsya in the vicinity of the Indus Valley sites, as matsya is the Sanskrit word for ‘fish’ and a divinity in the form of a fish is celebrated in the Indian version of the flood myth. An analysis of these narratives is presented in this paper, revealing the possibility of an association with the Indus Valley civilization of the more distant past. These observations indicate that fish symbolism may have occupied a place of prominence in Indus culture from political and religious perspectives. The Matsya territory mentioned in Vedic and epic literature is discussed in light of the chalcolithic cultures of Rajasthan, and it is suggested that this region witnessed successive waves of migration of different cultural groups due to its economic importance related to the exploitation of copper reserves
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
Quantum Phase Transitions of YxSi1-x
No full textLe désordre joue un rôle important dans le transport électronique à basse température et est toujours un problème actuel de la matière condensée : la combinaison des interactions coulombiennes et du phénomène de localisation peut modifier l'état fondamental d'un système électronique. La compréhension des effets du désordre à l'approche des transitions de phase quantiques métal-isolant (TMI) et métal-supraconducteur sur le système amorphe YₓSi₁₋ₓ est l'objectif de cette thèse.Nous avons étudié les propriétés de transport à très basse température (jusqu'à 15 mK) de films amorphes de YₓSi₁₋ₓ en fonction du désordre. Ce système a été étudié en modifiant la stœchiométrie (x) afin de construire un diagramme de phase électronique du YₓSi₁₋ₓ bulk (3D). La TMI se produit à x ~ 16 %, et le système est supraconducteur entre x ~ 21 % et x ~ 45 %. Nous avons également montré qu'un traitement thermique permet de modifier la conductivité et la température critique dans le cas supraconducteur, rendant le système moins conducteur. En parallèle, nous avons caractérisé la morphologie des films par microscopie électronique à transmission (TEM) et avons montré que l' YₓSi₁₋ₓ reste amorphe et homogène jusqu'à des températures de recuit de 500°C. Au-delà, une ségrégation des atomes d'yttrium a été observée, et le matériau commence à cristalliser sous forme d'oxydes d'yttrium et de silicium au-delà de 700°C environ.Dans l'état isolant (x=15%), nous avons mis en évidence des sauts hystérétiques de courant lorsque l’YₓSi₁₋ₓ est soumis à une tension importante. Ces sauts de courant évoluent avec le désordre du système et peuvent atteindre plusieurs ordres de grandeur à fort désordre. Nous avons caractérisé ces variations brutales de courant avec la température et le désordre, et montré qu'ils traduisent un fort découplage thermique entre les électrons et les phonons du film. Le modèle usuel de découplage électron-phonon, dit "d'électrons chauds", permet d'expliquer la tension caractéristique à laquelle ont lieu ces sauts, mais ne permet pas d'expliquer la disparition du saut de courant que nous observons à faible désordre.Disorder plays an important role in low-temperature electronic transport and is still an active problem in condensed matter physics: the combination of Coulomb interactions and localization can alter the ground state of an electronic system. Understanding the effects of disorder approaching the metal-insulator (MIT) and the metal-superconductor quantum phase transitions in the amorphous YₓSi₁₋ₓ system is the aim of this thesis. We have studied the transport properties of amorphous YₓSi₁₋ₓ films at very low temperatures (down to 15 mK) as a function of disorder. This system was studied by modifying the stoichiometry (x) in order to construct an electronic phase diagram of bulk YₓSi₁₋ₓ (3D). The MIT occurs at x ~ 16%, and the system is superconducting between x ~ 21% and x ~ 45%. We have also shown that thermal treatments can modify the conductivity and critical temperature in the superconducting case, making the system less conductive. In parallel, we characterized the morphology of the films through transmission electron microscopy (TEM) and showed that YₓSi₁₋ₓ remains amorphous and homogeneous up to annealing temperatures of 500°C. Beyond this, the segregation of yttrium atoms was observed, and the material begins to crystallize as yttrium-silicon oxides above about 700°C.In the insulating state (x=15%), we have demonstrated hysteretic current jumps when YₓSi₁₋ₓ is subjected to a high voltage. These current jumps evolve with the disorder of the system and can reach several orders of magnitude with high disorder. We have characterized these sudden variations in current with temperature and disorder, and shown that they reflect strong thermal decoupling between the electrons and the phonons in the film. The usual "hot electron" model of electron-phonon decoupling can explain the characteristic voltage at which these jumps occur, but cannot explain the disappearance of the current jump we observe at low disorder
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
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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