17,590 research outputs found
Limiting stable currents in bounded electron and ion streams
The classical static analysis of the infinite planar diode has been extended to include the effects of finite transverse beam size. Simple expressions have been found for the increase in maximum stable current density over that of an infinite stream for finite cylindrical and strip streams flowing between plates of infinite diodes. The results are also given in terms of stream perveance. The effect of a nonuniform distribution of current across the stream is shown to be relatively small. Experimental values of maximum stable current agree with those obtained from the analysis. A further extension of the static analysis has been made to include the effects of additional conducting plane boundaries parallel to the stream motion. For length-to-width ratios L/D less than 0.25 the tube is adequately described by the results for the infinite planar diode and for L/D greater than 4, the infinitely-long drift tube theory suffices. At intermediate values of L/D, the maximum amount of current that can be stably passed through the tube is greater than that predicted by either asymptotic theory
Floppy modes and non-affine deformations in random fiber networks
We study the elasticity of random fiber networks. Starting from a microscopic
picture of the non-affine deformation fields we calculate the macroscopic
elastic moduli both in a scaling theory and a self-consistent effective medium
theory. By relating non-affinity to the low-energy excitations of the network
(``floppy-modes'') we achieve a detailed characterization of the non-affine
deformations present in fibrous networks.Comment: 4 pages, 2 figures, new figure
Evaluation of solar cell welds by scanning acoustic microscopy
Scanning laser acoustic microscopy was used to nondestructively evaluate solar cell interconnect bonds made by resistance welding. Both copper-silver and silver-silver welds were analyzed. The bonds were produced either by a conventional parallel-gap welding technique using rectangular electrodes or new annular gap design with a circular electrode cross section. With the scanning laser acoustic microscope, it was possible to produce a real time television image which reveales the weld configuration as it relates to electrode geometry. The effect of electrode misalinement with the surface of the cell was also determined. A preliminary metallographic analysis was performed on selected welds to establish the relationship between actual size and shape of the weld area and the information available from acoustic micrographs
Electron-Hole Crossover in Graphene Quantum Dots
We investigate the addition spectrum of a graphene quantum dot in the
vicinity of the electron-hole crossover as a function of perpendicular magnetic
field. Coulomb blockade resonances of the 50 nm wide dot are visible at all
gate voltages across the transport gap ranging from hole to electron transport.
The magnetic field dependence of more than 50 states displays the unique
complex evolution of the diamagnetic spectrum of a graphene dot from the
low-field regime to the Landau regime with the n=0 Landau level situated in the
center of the transport gap marking the electron-hole crossover. The average
peak spacing in the energy region around the crossover decreases with
increasing magnetic field. In the vicinity of the charge neutrality point we
observe a well resolved and rich excited state spectrum.Comment: 4 pages, 3 figure
Spin States in Graphene Quantum Dots
We investigate ground and excited state transport through small (d = 70 nm)
graphene quantum dots. The successive spin filling of orbital states is
detected by measuring the ground state energy as a function of a magnetic
field. For a magnetic field in-plane of the quantum dot the Zemann splitting of
spin states is measured. The results are compatible with a g-factor of 2 and we
detect a spin-filling sequence for a series of states which is reasonable given
the strength of exchange interaction effects expected for graphene
Three-fold way to extinction in populations of cyclically competing species
Species extinction occurs regularly and unavoidably in ecological systems.
The time scales for extinction can broadly vary and inform on the ecosystem's
stability. We study the spatio-temporal extinction dynamics of a paradigmatic
population model where three species exhibit cyclic competition. The cyclic
dynamics reflects the non-equilibrium nature of the species interactions. While
previous work focusses on the coarsening process as a mechanism that drives the
system to extinction, we found that unexpectedly the dynamics to extinction is
much richer. We observed three different types of dynamics. In addition to
coarsening, in the evolutionary relevant limit of large times, oscillating
traveling waves and heteroclinic orbits play a dominant role. The weight of the
different processes depends on the degree of mixing and the system size. By
analytical arguments and extensive numerical simulations we provide the full
characteristics of scenarios leading to extinction in one of the most
surprising models of ecology
Nonlinear ptychographic coherent diffractive imaging
Ptychographic Coherent diffractive imaging (PCDI) is a significant advance in imaging allowing the measurement of the full electric field at a sample without use of any imaging optics. So far it has been confined solely to imaging of linear optical responses. In this paper we show that because of the coherence-preserving nature of nonlinear optical interactions, PCDI can be generalised to nonlinear optical imaging. We demonstrate second harmonic generation PCDI, directly revealing phase information about the nonlinear coefficients, and showing the general applicability of PCDI to nonlinear interactions
Nonaffine rubber elasticity for stiff polymer networks
We present a theory for the elasticity of cross-linked stiff polymer
networks. Stiff polymers, unlike their flexible counterparts, are highly
anisotropic elastic objects. Similar to mechanical beams stiff polymers easily
deform in bending, while they are much stiffer with respect to tensile forces
(``stretching''). Unlike in previous approaches, where network elasticity is
derived from the stretching mode, our theory properly accounts for the soft
bending response. A self-consistent effective medium approach is used to
calculate the macroscopic elastic moduli starting from a microscopic
characterization of the deformation field in terms of ``floppy modes'' --
low-energy bending excitations that retain a high degree of non-affinity. The
length-scale characterizing the emergent non-affinity is given by the ``fiber
length'' , defined as the scale over which the polymers remain straight.
The calculated scaling properties for the shear modulus are in excellent
agreement with the results of recent simulations obtained in two-dimensional
model networks. Furthermore, our theory can be applied to rationalize bulk
rheological data in reconstituted actin networks.Comment: 12 pages, 10 figures, revised Section II
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