Large Surface Field Research Articles

PP 1/301 (1) Ceratitis capitata – bait application

Specific scopeThis Standard describes the conduct of trials for the efficacy evaluation of bait insecticides against Ceratitis capitata on citrus and top fruits. For foliar spray application see EPPO Standard PP 1/302. Together they replace EPPO Standard PP 1/106 Ceratitis capitata.Specific approval and amendmentFirst approved in 2017–09.Special remarksThis Standard should be used for the evaluation of insecticides applied with baits to citrus or other fruit crops, usually covering a small part of the foliage. Trials can only be carried out under natural infestations on large surface field plots.

Filling and wetting transitions of nematic liquid crystals on sinusoidal substrates

Close to sinusoidal substrates, simple fluids may undergo a filling transition, in which the fluid passes from a dry to a filled state, where the interface remains unbent but bound to the substrate. Increasing the surface field, the interface unbinds and a wetting transition occurs. We show that this double-transition sequence may be strongly modified in the case of ordered fluids, such as nematic liquid crystals. Depending on the preferred orientation of the nematic molecules at the structured substrate and at the isotropic-nematic interface, the filling transition may not exist, and the fluid passes directly from a dry to a complete-wet state, with the interface far from the substrate. More interestingly, in other situations, the complete wetting transition may be prevented, and the fluid passes from a dry to a filled state, and remains in this configuration, with the interface always attached to the substrate, even for very large surface fields. Both transitions are observed only for a same substrate in a narrow range of amplitudes.

Mechanism of field electron emission from carbon nanotubes

Field electron emission (FE) is a quantum tunneling process in which electrons are injected from materials (usually metals) into a vacuum under the influence of an applied electric field. In order to obtain usable electron current, the conventional way is to increase the local field at the surface of an emitter. For a plane metal emitter with a typical work function of 5 eV, an applied field of over 1 000 V/μm is needed to obtain a significant current. The high working field (and/or the voltage between the electrodes) has been the bottleneck for many applications of the FE technique. Since the 1960s, enormous effort has been devoted to reduce the working macroscopic field (voltage). A widely adopted idea is to sharpen the emitters to get a large surface field enhancement. The materials of emitters should have good electronic conductivity, high melting points, good chemical inertness, and high mechanical stiffness. Carbon nanotubes (CNTs) are built with such needed properties. As a quasi-one-dimensional material, the CNT is expected to have a large surface field enhancement factor. The experiments have proved the excellent FE performance of CNTs. The turn-on field (the macroscopic field for obtaining a density of 10 μA/cm2) of CNT based emitters can be as low as 1 V/μm. However, this turn-on field is too good to be explained by conventional theory. There are other observations, such as the non-linear Fowler-Nordheim plot and multi-peaks field emission energy distribution spectra, indicating that the field enhancement is not the only story in the FE of CNTs. Since the discovery of CNTs, people have employed more serious quantum mechanical methods, including the electronic band theory, tight-binding theory, scattering theory and density function theory, to investigate FE of CNTs. A few theoretical models have been developed at the same time. The multi-walled carbon nanotubes (MWCNTs) should be assembled with a sharp metal needle of nano-scale radius, for which the FE mechanism is more or less clear. Although MWCNTs are more common in present FE applications, the single-walled carbon nanotubes (SWCNTs) are more interesting in the theoretical point of view since the SWCNTs have unique atomic structures and electronic properties. It would be very interesting if people can predict the behavior of the well-defined SWCNTs quantitatively (for MWCNTs, this is currently impossible). The FE as a tunneling process is sensitive to the apex-vacuum potential barrier of CNTs. On the other hand, the barrier could be significantly altered by the redistribution of excessive charges in the micrometer long SWCNTs, which have only one layer of carbon atoms. Therefore, the conventional theories based upon the hypothesis of fixed potential (work function) would not be valid in this quasi-one-dimensional system. In this review, we shall focus on the mechanism that would be responsible for the superior field emission characteristics of CNTs. We shall introduce a multi-scale simulation algorithm that deals with the entire carbon nanotube as well as the substrate as a whole. The simulation for (5, 5) capped SWCNTs with lengths in the order of micrometers is given as an example. The results show that the field dependence of the apex-vacuum electron potential barrier of a long carbon nanotube is a more pronounced effect, besides the local field enhancement phenomenon.

Effect of the Pekar mechanism on the phonon-drag thermopower inp-typeSi∕Si1−xGexheterostructures

We examine the effect of the Pekar mechanism on the coupling of two-dimensional (2D) holes with 3D acoustic phonons in $p$-type $\mathrm{Si}∕{\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ heterostructures by using the phonon-drag thermopower, ${S}^{g}$, as a probe. The origin of the Pekar coupling is due to the variation of the dielectric permittivity under strain in nanostructures characterized by strong confining electric fields. The hole-phonon interaction via the Pekar mechanism resembles that of piezoelectric coupling in polar materials and dominates over the deformation potential coupling at low temperatures. It is found that ${S}^{g}$ shows a ${T}^{4}$ temperature dependence at low $T$ (Bloch-Gruneisen regime) which is in good agreement with the experimental data reported recently by Possanzini et al. [Phys. Rev. B 69, 195306 (2004)]. We perform detailed calculations of ${S}^{g}$ by considering both a screened Pekar coupling and a screened deformation potential coupling in $p$-type $\mathrm{Si}∕\mathrm{Si}\mathrm{Ge}$ structures in the temperature range $0.25lTl6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The value of the deformation potential constant is taken to be $4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, in good agreement with previous studies. The strength of the Pekar coupling is determined by the magnitude of the surface electric field ${E}_{s}$. We treat ${E}_{s}$ as an adjustable parameter in our calculations and we find good agreement with all the experimental data of ${S}^{g}$, available at the moment, by using a value for ${E}_{s}$ of the order of ${10}^{9}\phantom{\rule{0.3em}{0ex}}\mathrm{V}∕\mathrm{m}$. The possibility for the existence of such a large surface field in $\mathrm{Si}∕\mathrm{Si}\mathrm{Ge}$ is discussed.

Strain-induced incomplete wetting at CuAu(001) surfaces

X-ray reflectivity measurements, modeled on atomic scale by a dynamic approach, reveal a smooth Au top layer and subsequent $\mathrm{Cu}∕\mathrm{Au}$ layering at the (001) surface of CuAu crystals at high temperatures, enforced by a large surface field ${h}_{1}$. Approaching the first order bulk transition the ordered near surface film grows but does not completely wet its disordered bulk phase as predicted. In contrast to earlier wetting experiments on heterogeneous systems which were dominated by surface roughness, we found strong lattice relaxations and incomplete wetting due to strain.

Mechanisms of electron field emission from diamond, diamond-like carbon, and nanostructured carbon

It is argued that the facile electron field emission from carbon systems occurs primarily because surface groups such as C–H can produce large changes in local electron affinity, so that electric fields from the anode can be focused toward unhydrogenated surface areas of high affinity, the fields ending on negative charges in an underlying depletion layer. The resulting downward band bending creates large surface fields which allow Fowler–Nordheim emission, while not exceeding the material’s breakdown field.

Effect of Gravity and Confinement on Phase Equilibria: A Density Matrix Renormalization Approach

The phase diagram of the 2D Ising model confined between two infinite walls and subject to opposing surface fields and to a bulk "gravitational" field is calculated by means of density matrix renormalization methods. In absence of gravity two phase coexistence is restricted to temperatures below the wetting temperature. We find that gravity restores the two phase coexistence up to the bulk critical temperature, in agreement with previous mean-field predictions. We calculate the exponents governing the finite size scaling in the temperature and in the gravitational field directions. The former is the exponent which describes the shift of the critical temperature in capillary condensation. The latter agrees, for large surface fields, with a scaling assumption of Van Leeuwen and Sengers. Magnetization profiles in the two phase and in the single phase region are calculated. The profiles in the single phase region, where an interface is present, agree well with magnetization profiles calculated from a simple solid-on-solid interface hamiltonian.

Strong surface field enhancements in the scattering of p-polarized light from fractal metal surfaces

We study the light scattering from a one-dimensional rough surface characterized by a self-affine fractal structure. The rigorous Green theorem, integral equations formulation is employed to calculate the scattered electromagnetic field. An increase in the roughness-induced excitation of p-polarized surface plasmon polaritons is found with increasing fractal dimension, leading to very large surface field enhancements.

A search for magnetic fields in Be stars

view Abstract Citations (25) References (44) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A search for magnetic fields in Be stars. Barker, P. K. ; Landstreet, J. D. ; Marlborough, J. M. ; Thompson, I. B. Abstract No mean longitudinal magnetic fields have been detected in a sample of 15 Be and related stars, with sigma approximately 70-250 gauss. These objects are thus magnetically indistinguishable from normal upper main-sequence stars. However, large surface fields could escape detection, depending upon field geometry and orientation. Models for Be envelopes which require predominantly toroidal fields are not seriously constrained by these observed limits on effective field strength. Circumstantial evidence suggests that many Be stars may indeed be magnetic; accordingly, future more rigorous searches for fields are desirable. Publication: The Astrophysical Journal Pub Date: January 1985 DOI: 10.1086/162841 Bibcode: 1985ApJ...288..741B Keywords: B Stars; Magnetic Measurement; Stellar Magnetic Fields; Astronomical Catalogs; H Lines; Stellar Envelopes; Stellar Models; Astrophysics full text sources ADS | data products SIMBAD (15)

Starspots on flare stars.

Sizes of starspots on flare stars can be derived from the author's convection-cell hypothesis. The sizes are in fair agreement with those observed on YY Gem, CC Eri, and BY Dra by Bopp and Evans (1973). The hypothesis predicts that periodic brightness variations due to starspots are restricted to stars brighter than a critical absolute visual magnitude. A convective model of a starspot on YY Gem has been computed, assuming that the missing flux is in the form of Alfven waves. It is found that the surface field must exceed 10,000 G, and is probably less than about 30,000 G. With a surface field of 20,000 G, the effective temperature of the spot is in the range from 1590 to 1890 K, depending on the field gradient. These figures are to be compared with an effective temperature of 2000 K estimated from observations by Bopp and Evans. Efficient dynamo action is shown to be a possible mechanism for generating such large surface fields. There is a possibility that tidal effects may influence starspot formation.