101 research outputs found
Minimal domain size necessary to simulate the field enhancement factor numerically with specified precision
In the literature about field emission, finite elements and finite
differences techniques are being increasingly employed to understand the local
field enhancement factor (FEF) via numerical simulations. In theoretical
analyses, it is usual to consider the emitter as isolated, i.e, a single tip
field emitter infinitely far from any physical boundary, except the substrate.
However, simulation domains must be finite and the simulation boundaries
influences the electrostatic potential distribution. In either finite elements
or finite differences techniques, there is a systematic error () in
the FEF caused by the finite size of the simulation domain. It is attempting to
oversize the domain to avoid any influence from the boundaries, however, the
computation might become memory and time consuming, especially in full three
dimensional analyses. In this work, we provide the minimum width and height of
the simulation domain necessary to evaluate the FEF with at the
desired tolerance. The minimum width () and height () are given relative
to the height of the emitter (), that is,
necessary to simulate isolated emitters on a substrate. We also provide the
to simulate arrays and the to simulate an emitter
between an anode-cathode planar capacitor. At last, we present the formulae to
obtain the minimal domain size to simulate clusters of emitters with precision
. Our formulae account for ellipsoidal emitters and hemisphere
on cylindrical posts. In the latter case, where an analytical solution is not
known at present, our results are expected to produce an unprecedented
numerical accuracy in the corresponding local FEF
Physics-based derivation of a formula for the mutual depolarization of two post-like field emitters
Recent analyses of the field enhancement factor (FEF) from multiple emitters
have revealed that the depolarization effect is more persistent with respect to
the separation between the emitters than originally assumed. It has been shown
that, at sufficiently large separations, the fractional reduction of the FEF
decays with the inverse cube power of separation, rather than exponentially.
The behavior of the fractional reduction of the FEF encompassing both the range
of technological interest ( being the separation and is
the height of the emitters) and , has not been predicted by
the existing formulas in field emission literature, for post-like emitters of
any shape. In this letter, we use first principles to derive a simple
two-parameter formula for fractional reduction that can be of interest for
experimentalists to modeling and interpret the FEF from small clusters of
emitters or arrays in small and large separations. For the structures tested,
the agreement between numerical and analytical data is
Zero-energy states in graphene quantum dots and rings
We present exact analytical zero-energy solutions for a class of smooth
decaying potentials, showing that the full confinement of charge carriers in
electrostatic potentials in graphene quantum dots and rings is indeed possible
without recourse to magnetic fields. These exact solutions allow us to draw
conclusions on the general requirements for the potential to support fully
confined states, including a critical value of the potential strength and
spatial extent.Comment: 8 pages, 3 figures, references added, typos corrected, discussion
section expande
Method to obtain nonuniformity information from field emission behavior
Copyright © 2010 American Vacuum Society / American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Journal of Vacuum Science and Technology Part B: Microelectronics and Nanometer Structures, 28(3), Article number 441 and may be found at http://scitation.aip.org/content/avs/journal/jvstb/28/3/10.1116/1.3327928.This article describes the characterization of field emission from a planar cathode to a spherical anode with the approach curve method (ACM). In such a diode configuration the electric field strength at the cathode surface is nonuniform. This nonuniformity gives an extra degree of freedom and it allows the interpretation of the current-voltage and voltage-distance (VĂd) curves in terms of nonuniformity. The authors apply the ACM to Cu emitters to explain the nonlinearity of the VĂd curve in ACM measurements. This analysis provides a good insight into field emission phenomena, supporting a method for nonuniformity characterization based on field emission behavior
Diversidade e filogenia de estirpes de Rhizobium pela metodologia de MLSA.
O nitrogĂȘnio (N) Ă© um dos elementos mais requisitados pelas culturas, sendo um fator determinante na produtividade agrĂcola. Sabe-se que o grupo de bactĂ©rias conhecidas como rizĂłbios pode fixar o N2 atmosfĂ©rico, suprindo total ou parcialmente a carĂȘncia do mesmo no solo. Dessa forma, a caracterização molecular e filogenĂ©tica de rizĂłbios nativos de solos brasileiros Ă© importante para futuros estudos. O objetivo deste trabalho foi caracterizar a diversidade filogenĂ©tica de cinco estirpes de Rhizobium, que apresentam fortes indicaçÔes de representarem novas espĂ©cies por estudos filogenĂ©ticos anteriores. Todas as estirpes foram crescidas em meio de cultura especĂfico e submetidas Ă anĂĄlise por MLSA (Multi-Locus Sequence Analysis) e BOX-PCR. Os resultados de ambas anĂĄlises agruparam as estirpes em clusters separados das demais espĂ©cies do gĂȘnero, reforçando a hipĂłtese de que tais bactĂ©rias podem representar uma nova espĂ©cie dentro do gĂȘnero Rhizobium.Fertbio
Produtividade de linhagens de Dactylis glomerata na regiĂŁo da campanha gaĂșcha.
Resumo.Fernando Flores Cardoso, Daniel Portella Montardo, José Carlos Ferrugem Moraes, Marcos Flåvio Silva Borba, Sandro da Silva Camargo, editores técnicos
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