1,164 research outputs found

    Elastic Wave Transmission at an Abrupt Junction in a Thin Plate, with Application to Heat Transport and Vibrations in Mesoscopic Systems

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    The transmission coefficient for vibrational waves crossing an abrupt junction between two thin elastic plates of different widths is calculated. These calculations are relevant to ballistic phonon thermal transport at low temperatures in mesoscopic systems and the Q for vibrations in mesoscopic oscillators. Complete results are calculated in a simple scalar model of the elastic waves, and results for long wavelength modes are calculated using the full elasticity theory calculation. We suggest that thin plate elasticty theory provide a useful and tractable approximation to the full three dimensional geometry.Comment: 35 pages, including 12 figure

    Effects of burn status and conditioning on colonization of wood by stream macroinvertebrates

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    The combination of changing climate and anthropogenic activities is increasing the probability of fire around the world. When fires occur in riparian zones, associated tree mortality can add wood to streams directly, or wood may fall onto the forest floor and remain there for some time before moving into stream channels. Because wood provides critical structure for aquatic macroinvertebrates, our objectives were to assess the effects of wood burn status, conditioning, and their interaction on (1) aquatic taxa community composition; (2) taxa and functional diversities; and (3) trait affinities. We conducted a field experiment using pieces of freshly-cut wood for which we first manipulated the burn status (burned, unburned). We then manipulated conditioning status by placing pieces directly into streams (no conditioning), leaving other pieces in streams for a year (water conditioning), or on the forest floor for a year before submergence (soil conditioning). Analyses included distance based redundancy analysis and linear mixed-effects modeling. Our results demonstrated that changes in wood quality resulting from fire may not alter per se the structure of macroinvertebrate communities. Conditioning status, however, had significant effects on taxonomic composition, taxa and functional diversities, and trait affinities of wood invertebrate communities. The terrestrial legacy of soil conditioning was clearly important in structuring macroinvertebrate assemblages. Furthermore, taxonomic and functional patterns of stream macroinvertebrate colonization were not substantially different between burned and unburned wood, even after a year of incubation in the stream or on the forest floor. This is an important finding for the research topic on how wildfire structures aquatic communities. This information can function as a guide for post-fire stream and riparian management operations taking ecosystem function into account

    Efeitos de sistemas de condução e porta-enxerto sobre a produção e qualidade da uva Cheninc Blanc no Vale do São Francisco: 4. ciclo de produção.

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    Este trabalho teve como objetivo avaliar os efeitos de sistemas de condução e do porta-enxerto sobre os componentes de produção e qualidade da uva Cheninc Blanc durante o 4. ciclo de produção no Vale do São Francisco

    On the low-temperature lattice thermal transport in nanowires

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    We propose a theory of low temperature thermal transport in nano-wires in the regime where a competition between phonon and flexural modes governs the relaxation processes. Starting with the standard kinetic equations for two different types of quasiparticles we derive a general expression for the coefficient of thermal conductivity. The underlying physics of thermal conductance is completely determined by the corresponding relaxation times, which can be calculated directly for any dispersion of quasiparticles depending on the size of a system. We show that if the considered relaxation mechanism is dominant, then at small wire diameters the temperature dependence of thermal conductivity experiences a crossover from T1/2T^{1/2} to T3T^3-dependence. Quantitative analysis shows reasonable agreement with resent experimental results.Comment: 12 pages, 3 eps figure

    Control of Dephasing and Phonon Emission in Coupled Quantum Dots

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    We predict that phonon subband quantization can be detected in the non-linear electron current through double quantum dot qubits embedded into nano-size semiconductor slabs, acting as phonon cavities. For particular values of the dot level splitting Δ\Delta, piezo-electric or deformation potential scattering is either drastically reduced as compared to the bulk case, or strongly enhanced due to phonon van Hove singularities. By tuning Δ\Delta via gate voltages, one can either control dephasing, or strongly increase emission into phonon modes with characteristic angular distributions.Comment: 4 pages, 3 figures, accepted for publication as Rapid Comm. in Phys. Rev.

    Giant Thermoelectric Effect from Transmission Supernodes

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    We predict an enormous order-dependent quantum enhancement of thermoelectric effects in the vicinity of a higher-order `supernode' in the transmission spectrum of a nanoscale junction. Single-molecule junctions based on 3,3'-biphenyl and polyphenyl ether (PPE) are investigated in detail. The nonequilibrium thermodynamic efficiency and power output of a thermoelectric heat engine based on a 1,3-benzene junction are calculated using many-body theory, and compared to the predictions of the figure-of-merit ZT.Comment: 5 pages, 6 figure

    Self-Consistent Relativistic Calculation of Nucleon Mean Free Path

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    We present a fully self-consistent and relativistic calculation of the nucleon mean free path in nuclear matter and finite nuclei. Starting from the Bonn potential, the Dirac-Brueckner-Hartree-Fock results for nuclear matter are parametrized in terms of an effective σ\sigma-ω\omega Lagrangian suitable for the relativistic density-dependent Hartree-Fock (RDHF) approximation. The nucleon mean free path in nuclear matter is derived from this effective Lagrangian taking diagrams up to fourth-order into account. For the nucleon mean free path in finite nuclei, we make use of the density determined by the RDHF calculation in the local density approximation. Our microscopic results are in good agreement with the empirical data and predictions by Dirac phenomenology.Comment: 16 pages RevTex and 6 figures (paper, available upon request from [email protected]) UI-NTH-931

    Fungos associados e fitopatogênicos a espécies florestais nativas e exóticas na Amazônia.

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    Os ambientes amazônicos possuem grande diversidade de fungos fitopatogênicos e associados a manchas foliares em espécies florestais, entretanto são pouco estudados. A importância do levantamento de doenças no viveiro é fundamental para o sucesso na obtenção de mudas sadias, visto que as condições de alta umidade e densidade de plantas, temperatura, substrato e tecido vegetal tenro presentes neste ambiente favorecem o estabelecimento e desenvolvimento de doenças em níveis epidêmicos. Assim a identificação de novos registros e a descoberta de novas espécies de fungos fitopatogênicos contribuem para a ciência da sistemática e filogenia, assim como possibilita a tomada de decisão quanto ao melhor manejo das doenças com menor impacto arnbiental e econômico. Por outro lado, tem-se como limitações a ausência de esporulação desses fungos in vitro, o pequeno numero de micologistas, e a não identificação da espécie hospedeira

    Quantum transport using the Ford-Kac-Mazur formalism

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    The Ford-Kac-Mazur formalism is used to study quantum transport in (1) electronic and (2) harmonic oscillator systems connected to general reservoirs. It is shown that for non-interacting systems the method is easy to implement and is used to obtain many exact results on electrical and thermal transport in one-dimensional disordered wires. Some of these have earlier been obtained using nonequilibrium Green function methods. We examine the role that reservoirs and contacts can have on determining the transport properties of a wire and find several interesting effects.Comment: 10 pages, 4 figure
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