3,057 research outputs found

    A one-dimensional Fermi accelerator model with moving wall described by a nonlinear van der Pol oscillator

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    A modification of the one-dimensional Fermi accelerator model is considered in this work. The dynamics of a classical particle of mass mm, confined to bounce elastically between two rigid walls where one is described by a non-linear van der Pol type oscillator while the other one is fixed, working as a re-injection mechanism of the particle for a next collision, is carefully made by the use of a two-dimensional non-linear mapping. Two cases are considered: (i) the situation where the particle has mass negligible as compared to the mass of the moving wall and does not affect the motion of it; (ii) the case where collisions of the particle does affect the movement of the moving wall. For case (i) the phase space is of mixed type leading us to observe a scaling of the average velocity as a function of the parameter (hi¸\c{hi}) controlling the non-linearity of the moving wall. For large hi¸\c{hi}, a diffusion on the velocity is observed leading us to conclude that Fermi acceleration is taking place. On the other hand for case (ii), the motion of the moving wall is affected by collisions with the particle. However due to the properties of the van der Pol oscillation, the moving wall relaxes again to a limit cycle. Such kind of motion absorbs part of the energy of the particle leading to a suppression of the unlimited energy gain as observed in case (i). The phase space shows a set of attractors of different periods whose basin of attraction has a complicate organization

    The Brazilian Interbank Network Structure and Systemic Risk

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    We explore the structure and dynamics of interbank exposures in Brazil using a unique data set of all mutual exposures of financial institutions in Brazil, as well as their capital reserves, at various periods in 2007 and 2008. We show that the network of exposures can be adequately modeled as a directed scale-free (weighted) graph with heavy-tailed degree and weight distributions. We also explore the relationship between connectivity of a financial institution and its capital buffer. Finally, we use the network structure to explore the extent of systemic risk generated in the system by the individual institutions.

    Lepton Flavor Violating Photoleptonic Effect

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    We study lepton flavor violating analogs of the photoelectric effect, with a final μ\mu or τ\tau instead of an electron: γe→μ\gamma e\to \mu and γe→τ\gamma e\to \tau. On the basis of the general parametrization of the matrix element of the electromagnetic current we estimate the upper limits for the cross sections and event rates of these processes, imposed by the current experimental bounds on μ→eγ\mu\to e \gamma and τ→eγ\tau\to e \gamma decays.Comment: 6 pages, 1 figure, RevTeX

    Antiferromagnetic spin chain behavior and a transition to 3D magnetic order in Cu(D,L-alanine)2: Roles of H-bonds

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    We study the spin chain behavior, a transition to 3D magnetic order and the magnitudes of the exchange interactions for the metal-amino acid complex Cu(D,L-alanine)2.H2O, a model compound to investigate exchange couplings supported by chemical paths characteristic of biomolecules. Thermal and magnetic data were obtained as a function of temperature (T) and magnetic field (B0). The magnetic contribution to the specific heat, measured between 0.48 and 30 K, displays above 1.8 K a 1D spin-chain behavior that can be fitted with an intrachain antiferromagnetic (AFM) exchange coupling constant 2J0 = (-2.12 ±\pm 0.08) cm−1^{-1}, between neighbor coppers at 4.49 {\AA} along chains connected by non-covalent and H-bonds. We also observe a narrow specific heat peak at 0.89 K indicating a phase transition to a 3D magnetically ordered phase. Magnetization curves at fixed T = 2, 4 and 7 K with B0 between 0 and 9 T, and at T between 2 and 300 K with several fixed values of B0 were globally fitted by an intrachain AFM exchange coupling constant 2J0 = (-2.27 ±\pm 0.02) cm−1^{-1} and g = 2.091 ±\pm 0.005. Interchain interactions J1 between coppers in neighbor chains connected through long chemical paths with total length of 9.51 {\AA} are estimated within the range 0.1 < |2J1| < 0.4 cm−1^{-1}, covering the predictions of various approximations. We analyze the magnitudes of 2J0 and 2J1 in terms of the structure of the corresponding chemical paths. The main contribution in supporting the intrachain interaction is assigned to H-bonds while the interchain interactions are supported by paths containing H-bonds and carboxylate bridges, with the role of the H-bonds being predominant. We compare the obtained intrachain coupling with studies of compounds showing similar behavior and discuss the validity of the approximations allowing to calculate the interchain interactions.Comment: 10 pages, 4 figure

    Information Systems For Shuttle Processing: An Enterprise Approach

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    The purpose of this paper is to describe a process being utilized by Lockheed Space Operations Company (LSOC) for planning, developing and supporting an integrated information system for the Shuttle Processing Contract (SPC). This process was developed by Electronic Data Systems (EDS) to address the complex modernization and integration issues facing General Motors (GM). LSOC has contracted EDS to participate in adapting the process to the environment at Kennedy Space Center (KSC) for shuttle processing

    Impact of Level of Use on Plant Vigor and Weight of \u3cem\u3eFestuca humilior\u3c/em\u3e and \u3cem\u3eCalamagrostis vicunarum\u3c/em\u3e

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    Tussock grasslands of Festuca humilior and Calamagrostis vicunarum cover a significant extension of the central Andes of Peru, constituting a plant association highly appreciated by cattle ranchers. The objective of this research was to estimate the cumulative impact of the level of use of the key grasses Festuca humilior and Calamagrostis vicunarum on plant height, vigor, and weight. This study was conducted in a humid grassland of regular condition located at 4186 masl in the Puna ecoregion. The variables evaluated were: plant height, basal and canopy diameter and plant weight. The experiment lasted two years in which individual plants from both species were cut every four months. The vigor resulted from the regression of the weight (g) and the volume (cm3) of the plant. Height (cm) was the length from the soil surface to the flag leaf. The experimental design was a randomized block design with a 2x3 factorial arrangement, two species and three height removal levels: zero, 40 and 80%. The height, vigor and weight of Festuca humilior and Calamagrostis vicunarum decreased in response to increasing use levels, but the changes were more evident in Festuca than in Calamagrostis. While the decrease in the evaluated variables was constant for Festuca humilior as the level of use increased, Calamagrostis also decreased, but to a lesser extent, remaining almost unchanged when biomass removal levels changed from moderate to heavy, suggesting that this species was more tolerant to grazing. The results helped explaining why in fields dominated by Festuca humilior and sub-dominated by Calamagrostis vicunarum subjected to intense grazing, dominance relationships change in favor of Calamagrostis, a species of lower forage value than Festuca humilior. It is recommended to manage the grazing system under a low-intensity and low frequency regime

    Scaling properties for a classical particle in a time-dependent potential well.

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    Some scaling properties for a classical particle interacting with a time-dependent square-well potential are studied. The corresponding dynamics is obtained by use of a two-dimensional nonlinear area-preserving map. We describe dynamics within the chaotic sea by use of a scaling function for the variance of the average energy, thereby demonstrating that the critical exponents are connected by an analytic relationship
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