8,979 research outputs found

    Electron scattering in atomic force microscopy experiments

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    It has been shown that electron transitions, as measured in a scanning tunnelling microscope (STM), are related to chemical interactions in a tunnelling barrier. Here, we show that the shape and apparent height of subatomic features in an atomic force microscopy (AFM) experiment on Si(111) depend directly on the available electron states of the silicon surface and the silicon AFM tip. Simulations and experiments confirm that forces and currents show similar subatomic variations for tip-sample distances approaching the bulk bonding length.Comment: 5 pages and 4 figure

    Cellular pattern formation during Dictyostelium aggregation

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    The development of multicellularity in the life cycle of Dictyostelium discoideum provides a paradigm model system for biological pattern formation. Previously, mathematical models have shown how a collective pattern of cell communication by waves of the messenger molecule cyclic adenosine 3′5′-monophosphate (cAMP) arises from excitable local cAMP kinetics and cAMP diffusion. Here we derive a model of the actual cell aggregation process by considering the chemotactic cell response to cAMP and its interplay with the cAMP dynamics. Cell density, which previously has been treated as a spatially homogeneous parameter, is a crucial variable of the aggregation model. We find that the coupled dynamics of cell chemotaxis and cAMP reaction-diffusion lead to the break-up of the initially uniform cell layer and to the formation of the striking cell stream morphology which characterizes the aggregation process in situ. By a combination of stability analysis and two-dimensional simulations of the model equations, we show cell streaming to be the consequence of the growth of a small-amplitude pattern in cell density forced by the large-amplitude cAMP waves, thus representing a novel scenario of spatial patterning in a cell chemotaxis system. The instability mechanism is further analysed by means of an analytic caricature of the model, and the condition for chemotaxis-driven instability is found to be very similar to the one obtained for the standard (non-oscillatory) Keller-Segel system. The growing cell stream pattern feeds back into the cAMP dynamics, which can explain in some detail experimental observations on the time evolution of the cAMP wave pattern, and suggests the characterization of the Dictyostelium aggregation field as a self-organized excitable medium

    Cavity optomechanics with ultra-high Q crystalline micro-resonators

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    We present the first observation of optomechanical coupling in ultra-high Q crystalline whispering-gallery-mode (WGM) resonators. The high purity of the crystalline material enables optical quality factors in excess of 10^{10} and finesse exceeding 10^{6}. Simultaneously, mechanical quality factors greater than 10^{5} are obtained, still limited by clamping losses. Compared to previously demonstrated cylindrical resonators, the effective mass of the mechanical modes can be dramatically reduced by the fabrication of CaF2 microdisc resonators. Optical displacement monitoring at the 10^{-18} m/sqrt{Hz}-level reveals mechanical radial modes at frequencies up to 20 MHz, corresponding to unprecedented sideband factors (>100). Together with the weak intrinsic mechanical damping in crystalline materials, such high sindeband factors render crystalline WGM micro-resonators promising for backaction evading measurements, resolved sideband cooling or optomechanical normal mode splitting. Moreover, these resonators can operate in a regime where optomechanical Brillouin lasing can become accessible

    Spectroscopy of Rb2_{2} dimers in solid 4^{4}He

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    We present experimental and theoretical studies of the absorption, emission and photodissociation spectra of Rb2_{2} molecules in solid helium. We have identified 11 absorption bands of Rb2_{2}. All laser-excited molecular states are quenched by the interaction with the He matrix. The quenching results in efficient population of a metastable (1)3Πu^{3}\Pi_{u} state, which emits fluorescence at 1042 nm. In order to explain the fluorescence at the forbidden transition and its time dependence we propose a new molecular exciplex Rb2(3Πu)_{2}(^{3}\Pi_{u})He2_{2}. We have also found evidence for the formation of diatomic bubble states following photodissociation of Rb2_{2}

    Electron transport in Coulomb- and tunnel-coupled one-dimensional systems

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    We develop a linear theory of electron transport for a system of two identical quantum wires in a wide range of the wire length L, unifying both the ballistic and diffusive transport regimes. The microscopic model, involving the interaction of electrons with each other and with bulk acoustical phonons allows a reduction of the quantum kinetic equation to a set of coupled equations for the local chemical potentials for forward- and backward-moving electrons in the wires. As an application of the general solution of these equations, we consider different kinds of electrical contacts to the double-wire system and calculate the direct resistance, the transresistance, in the presence of tunneling and Coulomb drag, and the tunneling resistance. If L is smaller than the backscattering length l_P, both the tunneling and the drag lead to a negative transresistance, while in the diffusive regime (L >>l_P) the tunneling opposes the drag and leads to a positive transresistance. If L is smaller than the phase-breaking length, the tunneling leads to interference oscillations of the resistances that are damped exponentially with L.Comment: Text 14 pages in Latex/Revtex format, 4 Postscript figure

    Global surfaces of section in the planar restricted 3-body problem

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    The restricted planar three-body problem has a rich history, yet many unanswered questions still remain. In the present paper we prove the existence of a global surface of section near the smaller body in a new range of energies and mass ratios for which the Hill's region still has three connected components. The approach relies on recent global methods in symplectic geometry and contrasts sharply with the perturbative methods used until now.Comment: 11 pages, 1 figur

    Beyond the Heartland : Sentencing under the Advisory Federal Guidelines

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    New obstructions to symplectic embeddings

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    In this paper we establish new restrictions on symplectic embeddings of certain convex domains into symplectic vector spaces. These restrictions are stronger than those implied by the Ekeland-Hofer capacities. By refining an embedding technique due to Guth, we also show that they are sharp.Comment: 80 pages, 3 figures, v2: improved exposition and minor corrections, v3: Final version, expanded and improved exposition and minor corrections. The final publication is available at link.springer.co

    Technology and Teacher Preparation in Exemplary Institutions: 1994 to 2003

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    In a 1994 study commissioned by the Congressional Office of Technology Assessment, Mergendoller, Johnston, Rockman, & Willis (1994) examined four exemplary institutions to identify their approaches to integrating technology into teacher education. A decade later, the field would benefit from a comparison of current approaches of infusing technology into teacher education to the 1994 findings. This study examines the approaches of the first seven teacher education programs to receive the ISTE NETS Distinguished Achievement Award. Current approaches to the process are outlined, including the identification of the key factors impacting their implementation. A comparison of the 1994 and the present study reveals that the systematic coordination of experiences in the teacher preparation program, a unifying theme throughout the program, and a shared vision of technology and teaching are instrumental and may help guide future efforts of technology integration into teacher preparation
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