作者:
Michael S. IsaacsonCornell University
College of Engineering School of Applied and Engineering Physics W.M. Keck Foundation Program in Nanobiotechnology 241 Carpenter Hall Ithaca New York 14853
When a helical bacterial flagellum, clamped at one end, is placed in an external flow, it has been observed that regions of the flagellum transform to the opposite chirality, and travel as pulses down the length of th...
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When a helical bacterial flagellum, clamped at one end, is placed in an external flow, it has been observed that regions of the flagellum transform to the opposite chirality, and travel as pulses down the length of the filament, the process repeating periodically [H. Hotani, J. Mol. Biol. 156, 791 (1982)]. We propose a theory for this phenomenon based on a treatment of the flagellum as an elastic object with multiple stable configurations. The simplest possible implementation of the model accurately reproduces key features seen in experiment.
Electrical resistivity ρ, magnetic susceptibility χ, specific heatC, X-ray photoelectron spectroscopy (XPS) and X-ray and neutron diffraction measurements were made on the compound CeRhSn. The measurements reveal an...
Electrical resistivity ρ, magnetic susceptibility χ, specific heatC, X-ray photoelectron spectroscopy (XPS) and X-ray and neutron diffraction measurements were made on the compound CeRhSn. The measurements reveal an enhancement of the electronic specific heat coefficient γ =C(T)/T≈ 160 mJ mol−1K−2at about 1 K and non-Fermi-liquid-like temperature dependences ρ(T)αT0.75and χ(T)αT−0.5at low temperatures. At high temperatures, the physical properties indicate Kondo-intermediate-valence behaviour. The XPS measurements are consistent with a Ce valence of 3.07. The neutron diffraction measurements reveal anomalies in the planar Ce-Rh and Ce-Ce bond distances in the vicinity of 100 K which are apparently associated with the Ce valence instability in CeRhSn.
The thermoelastic effect was used to produce high frequency, broadband ultrasound in water. A pulsed diode laser, followed by an erbium doped fiber amplifier, was focused onto a light absorbing film deposited on a gla...
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ISBN:
(纸本)0780371771
The thermoelastic effect was used to produce high frequency, broadband ultrasound in water. A pulsed diode laser, followed by an erbium doped fiber amplifier, was focused onto a light absorbing film deposited on a glass substrate. Conversion efficiency was improved by 18 dB using a polydimethylsiloxane (PDMS) film instead of a more commonly used metallic one. Radiation pattern measurements show that considerable energy is radiated at +/-45 degrees for frequencies beyond 75 MHz. These results show that the thermoelastic effect can be used to produce phased arrays for high frequency ultrasound imaging.
The mechanism by which spiral wave patterns appear in populations of Dictyostelium was probed experimentally by external chemical perturbation. Spiral waves, which often arise from the breakup of circular waves driven...
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The mechanism by which spiral wave patterns appear in populations of Dictyostelium was probed experimentally by external chemical perturbation. Spiral waves, which often arise from the breakup of circular waves driven by pacemakers, typically entrain those pacemakers. We studied these processes by resetting the waves with a spatially uniform pulse of extrinsic cyclic AMP. A pattern of spirals reappeared if resetting was early in the signaling stage, but only targets emerged following late resetting, in a manner analogous to cardiac defibrillation. This supports recent hypotheses that wave pattern selection naturally occurs by slow temporal variation of the excitability of the cells.
Summary form only given. We used the finite-difference time-domain method to solve Maxwell equations. More specifically, we applied the non-dissipative Yee's algorithm with a duality relation between the spatial r...
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A complete, three-dimensional theory of Compton scattering is described, which fully takes into account the effects of the electron beam emittance and energy spread upon the scattered x-ray spectral brightness. The ra...
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A complete, three-dimensional theory of Compton scattering is described, which fully takes into account the effects of the electron beam emittance and energy spread upon the scattered x-ray spectral brightness. The radiation scattered by an electron subjected to an arbitrary electromagnetic field distribution in vacuum is first derived in the linear regime, and in the absence of radiative corrections; it is found that each vacuum eigenmode gives rise to a single Doppler-shifted classical dipole excitation. This formalism is then applied to Compton scattering in a three-dimensional laser focus, and yields a complete description of the influence of the electron beam phase-space topology on the x-ray spectral brightness; analytical expressions including the effects of emittance and energy spread are also obtained in the one-dimensional limit. Within this framework, the x-ray brightness generated by a 25 MeV electron beam is modeled, fully taking into account the beam emittance and energy spread, as well as the three-dimensional nature of the laser focus; its application to x-ray protein crystallography is outlined. Finally, coherence, harmonics, and radiative corrections are also briefly discussed.
Summary form only given. We used the finite-difference time-domain method to solve Maxwell equations. More specifically, we applied the non-dissipative Yee's algorithm with a duality relation between the spatial r...
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Summary form only given. We used the finite-difference time-domain method to solve Maxwell equations. More specifically, we applied the non-dissipative Yee's algorithm with a duality relation between the spatial representations of the electric and magnetic fields that represents both the differential and integral forms of Maxwell equations. The computational domain consists of 19 photonic lattice cells in the periodic structure for /spl epsi/ in the x and y direction and 8 mesh cells in the z direction. Each photonic lattice cell has been divided into 40/spl times/40 computational mesh cells, but due to duality of the discretization mesh, effectively we determined each field on only 20/spl times/20 points inside a photonic lattice cell. Periodic boundary conditions were used in all three directions. We observed the spatial distribution of the magnetic and electric fields and the movement of the mode resonance through the photonic band gap as the defect radius is changed. In addition, we determined the dependence of the localization (the field energy confined at the defect cell) on frequency (defect radius). We also found well localized defect modes.A defect in the form of an air hole with a modified radius was introduced in the center of a triangular lattice.
We present a numerical study of the localized transverse magnetic (TM) defect modes in a two-dimensional, triangular-lattice photonic crystal. The sample consists of an array of circular, air cylinders in a dielectric...
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We present a numerical study of the localized transverse magnetic (TM) defect modes in a two-dimensional, triangular-lattice photonic crystal. The sample consists of an array of circular, air cylinders in a dielectric medium (GaAs). The defect modes were calculated by using a parallel version of the finite-difference time-domain method on the Yee mesh. To validate our computations the results for the transverse electric case were checked against experimental results and the numerical results using a different method. We study the spatial symmetry for TM modes, obtained by changing the dipole excitation frequency. Also, we vary the defect-cylinder radius to tune the resonant frequency across the band gap. The TM mode is found to be highly localized at the defect in the photonic lattice.
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