8,364 research outputs found

    Tearing of free-standing graphene

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    We examine the fracture mechanics of tearing graphene. We present a molecular dynamics simulation of the propagation of cracks in clamped, free-standing graphene as a function of the out-of-plane force. The geometry is motivated by experimental configurations that expose graphene sheets to out-of-plane forces, such as back-gate voltage. We establish the geometry and basic energetics of failure and obtain approximate analytical expressions for critical crack lengths and forces. We also propose a method to obtain graphene's toughness. We observe that the cracks' path and the edge structure produced are dependent on the initial crack length. This work may help avoid the tearing of graphene sheets and aid the production of samples with specific edge structures.CAPESNational Science Foundation DMR 1002428Physic

    Energy in an Expanding Universe in the Teleparallel Geometry

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    The main purpose of this paper is to explicitly verify the consistency of the energy-momentum and angular momentum tensor of the gravitational field established in the Hamiltonian structure of the Teleparallel Equivalent of General Relativity (TEGR). In order to reach these objectives, we obtained the total energy and angular momentum (matter plus gravitational field) of the closed universe of the Friedmann-Lemaitre-Robertson-Walker (FLRW). The result is compared with those obtained from the pseudotensors of Einstein and Landau-Lifshitz. We also applied the field equations (TEGR) in an expanding FLRW universe. Considering the stress energy-momentum tensor for a perfect fluid, we found a teleparallel equivalent of Friedmann equations of General Relativity (GR).Comment: 19 pages, no figures. Revised in view of Referee's comments. Version to appear in the Brazilian Journal of Physic

    Homotopy Type Theory in Lean

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    We discuss the homotopy type theory library in the Lean proof assistant. The library is especially geared toward synthetic homotopy theory. Of particular interest is the use of just a few primitive notions of higher inductive types, namely quotients and truncations, and the use of cubical methods.Comment: 17 pages, accepted for ITP 201

    Zipf Law for Brazilian Cities

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    This work studies the Zipf Law for cities in Brazil. Data from censuses of 1970, 1980, 1991 and 2000 were used to select a sample containing only cities with 30,000 inhabitants or more. The results show that the population distribution in Brazilian cities does follow a power law similar to the ones found in other countries. Estimates of the power law exponent were found to be 2.22 +/- 0.34 for the 1970 and 1980 censuses, and 2.26 +/- 0.11 for censuses of 1991 and 2000. More accurate results were obtained with the maximum likelihood estimator, showing an exponent equal to 2.41 for 1970 and 2.36 for the other three years.Comment: 12 pages, 6 figures, 3 tables, Elsevier LaTeX, accepted for publication in "Physica A". Correction of minor mistyping (eq. 8

    Policy approaches regarding technology transfer: Portugal and Switzerland compared

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    The environment in which technology transfer takes place plays a key role in defining the best approaches and, ultimately, their success. In the present paper we analyse the extent to which Technology Transfer Offices (TTOs) efficiency is influenced by framework conditions and, in particular, by the innovation policies and programmes. We hypothesise that countries with higher technology transfer efficiency levels would have innovation policies more supportive to technology transfer efforts. Results based on an in depth account and statistical analysis of over 60 innovation policies from Switzerland (widely associated to high levels of technology transference efficiency) and Portugal (a laggard country in this particular) corraborate our initial hypothesis. Switzerland policies overall include more references to knowledge and technology transfer, in the form of licenses, R&D collaboration and spin-offs, than Portuguese policies. One exception is the case of patents (intellectual property rights, in general) with stronger weight in Portuguese policies and, to some extent, the support to spin-off creation and venture capital. The findings highlighted significant differences in variables with impact in technology transfer, namely the priorities addressed, target groups and funding eligibility, aspects of the innovation process targeted and forms of funding. From the exercise it was possible to derive some policy implications. Specifically, we advance that if a country wishes to increase technology transfer efficiency then it should implement a mandate for R&D cooperation between different actors, give priority to fund cutting edge science and research performers, and attribute a higher emphasis on applied industrial research and prototype creation aspects of the innovation process.Technology transfer, innovation policies, technology transfer efficiency
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