Transport properties of nano-devices: One-dimensional model study

A 1D model study of charge transport in nano-devices is made by comparing multi-configuration time dependent Hartree-Fock and frozen core calculations. The influence of exchange and Coulomb correlation on the tunneling current is determined. We identify the shape of the tunneling barrier and the resonance structure of the nano-device as the two dominant parameters determining the electron transport. Whereas the barrier shape determines the size of the tunneling current, the resonances determine the structure of the current.Comment: 4 page

Testing wheat for internal infesting insects with an electrically conductive roller mill: Presentation

Although grain is always inspected for adult insects and insect damaged kernels upon shipping and receiving, immature insects living inside the kernels of grain cannot be readily detected. A laboratory roller mill was modified to measure and analyze the electrical conductance of wheat as it was crushed. The electrical conductance of normal wheat kernels is low and fairly constant. In contrast, the electrical conductance of infested wheat kernels produces a sudden change in the electrical signal. The peak height of the electrical spike depends on the size of the larvae and the resulting contact of the crushed larvae between the rolls. This instrument was designed to test wheat with moisture content of 13.5% or less. The laboratory mill can test a kilogram of wheat in less than 2 min. Hard red winter and soft red winter wheat samples were used in experiments. Known numbers of infested kernels were added to the wheat samples. The infested kernels contained larvae of rice weevils and lesser grain borers sorted into large, medium, and small size groups. The instrument detected ~8 of 10 infested kernels per 100 g of wheat with large-larvae (fourth instar or pupae). It detected ~7 of 10 infested kernels with medium-larvae (second or third instar) and ~5 of 10 infested kernels infested with the small-larvae (first or second instar). Under reasonable grain moisture and careful sample handling, there were no non-infested kernels classified as insect infested. The mill can lead to rapid and automated detection of infested wheat.Although grain is always inspected for adult insects and insect damaged kernels upon shipping and receiving, immature insects living inside the kernels of grain cannot be readily detected. A laboratory roller mill was modified to measure and analyze the electrical conductance of wheat as it was crushed. The electrical conductance of normal wheat kernels is low and fairly constant. In contrast, the electrical conductance of infested wheat kernels produces a sudden change in the electrical signal. The peak height of the electrical spike depends on the size of the larvae and the resulting contact of the crushed larvae between the rolls. This instrument was designed to test wheat with moisture content of 13.5% or less. The laboratory mill can test a kilogram of wheat in less than 2 min. Hard red winter and soft red winter wheat samples were used in experiments. Known numbers of infested kernels were added to the wheat samples. The infested kernels contained larvae of rice weevils and lesser grain borers sorted into large, medium, and small size groups. The instrument detected ~8 of 10 infested kernels per 100 g of wheat with large-larvae (fourth instar or pupae). It detected ~7 of 10 infested kernels with medium-larvae (second or third instar) and ~5 of 10 infested kernels infested with the small-larvae (first or second instar). Under reasonable grain moisture and careful sample handling, there were no non-infested kernels classified as insect infested. The mill can lead to rapid and automated detection of infested wheat

Fiber-optical analogue of the event horizon

The physics at the event horizon resembles the behavior of waves in moving media. Horizons are formed where the local speed of the medium exceeds the wave velocity. We use ultrashort pulses in microstructured optical fibers to demonstrate the formation of an artificial event horizon in optics. We observed a classical optical effect, the blue-shifting of light at a white-hole horizon. We also show by theoretical calculations that such a system is capable of probing the quantum effects of horizons, in particular Hawking radiation.Comment: MEDIA EMBARGO. This paper is subject to the media embargo of Scienc

Temporal and spatial patterns in aerosol insecticide droplet distribution: Modifying application strategies to improve coverage and efficacy

With the phase-out of methyl bromide, treatment of food facilities with aerosol insecticides as part of management programs has increased. The physical layout of the structure, the distribution of equipment and other items within the space, and the application method and location may all cause spatial variation in how the insecticide is deposited, which can result in areas with insufficient or excessive amounts of insecticide applied. The impact of aerosol insecticide application position and dispersal method/formulation on the distribution of droplets was evaluated using a series of applications within the same flour mill room. The spatial pattern of droplet distribution and the effect of treatment on bioassay insects (Tribolium confusum Jacquelin DuVal) was evaluated. There was variation in aerosol concentration and droplet size distributions within room and application position had an impact on the spatial pattern of aerosol droplets. The further away and more obstructed by structural features a location was the lower the aerosol concentration, but concentration was also lower to the side and behind the release point. Evaluation of the temporal pattern in droplet deposition shows that most larger droplets settle out of the air relatively quickly, supporting that idea that shorter shutdown times are be possible. Efficacy was correlated with droplet concentration. The overall conclusion is that there can be considerable variation in distribution of aerosol insecticides and as a result considerable potential for improvement in the effectiveness of these applications.With the phase-out of methyl bromide, treatment of food facilities with aerosol insecticides as part of management programs has increased. The physical layout of the structure, the distribution of equipment and other items within the space, and the application method and location may all cause spatial variation in how the insecticide is deposited, which can result in areas with insufficient or excessive amounts of insecticide applied. The impact of aerosol insecticide application position and dispersal method/formulation on the distribution of droplets was evaluated using a series of applications within the same flour mill room. The spatial pattern of droplet distribution and the effect of treatment on bioassay insects (Tribolium confusum Jacquelin DuVal) was evaluated. There was variation in aerosol concentration and droplet size distributions within room and application position had an impact on the spatial pattern of aerosol droplets. The further away and more obstructed by structural features a location was the lower the aerosol concentration, but concentration was also lower to the side and behind the release point. Evaluation of the temporal pattern in droplet deposition shows that most larger droplets settle out of the air relatively quickly, supporting that idea that shorter shutdown times are be possible. Efficacy was correlated with droplet concentration. The overall conclusion is that there can be considerable variation in distribution of aerosol insecticides and as a result considerable potential for improvement in the effectiveness of these applications

Harmonic generation by atoms in circularly polarized two-color laser fields with coplanar polarizations and commensurate frequencies

The generation of harmonics by atoms or ions in a two-color, coplanar field configuration with commensurate frequencies is investigated through both, an analytical calculation based on the Lewenstein model and the numerical ab initio solution of the time-dependent Schroedinger equation of a two-dimensional model ion. Through the analytical model, selection rules for the harmonic orders in this field configuration, a generalized cut-off for the harmonic spectra, and an integral expression for the harmonic dipole strength is provided. The numerical results are employed to test the predictions of the analytical model. The scaling of the cut-off as a function of both, one of the laser intensities and frequency ratio $\eta$, as well as entire spectra for different $\eta$ and laser intensities are presented and analyzed. The theoretical cut-off is found to be an upper limit for the numerical results. Other discrepancies between analytical model and numerical results are clarified by taking into account the probabilities of the absorption processes involved.Comment: 8 figure

Scaling laws for soliton pulse compression by cascaded quadratic nonlinearities

We present a detailed study of soliton compression of ultra-short pulses based on phase-mismatched second-harmonic generation (\textit{i.e.}, the cascaded quadratic nonlinearity) in bulk quadratic nonlinear media. The single-cycle propagation equations in the temporal domain including higher-order nonlinear terms are presented. The balance between the quadratic (SHG) and the cubic (Kerr) nonlinearity plays a crucial role: we define an effective soliton number -- related to the difference between the SHG and the Kerr soliton numbers -- and show that it has to be larger than unity for successful pulse compression to take place. This requires that the phase mismatch be below a critical level, which is high in a material where the quadratic nonlinearity dominates over the cubic Kerr nonlinearity. Through extensive numerical simulations we find dimensionless scaling laws, expressed through the effective soliton number, which control the behaviour of the compressed pulses. These laws hold in the stationary regime, in which group-velocity mismatch effects are small, and they are similar to the ones observed for fiber soliton compressors. The numerical simulations indicate that clean compressed pulses below two optical cycles can be achieved in a $\beta$-barium borate crystal at appropriate wavelengths, even for picosecond input pulses.Comment: 11 pages, 8 figures, resubmitted version, to appear in October issue of J. Opt. Soc. Am. B. Substantially revised, updated mode

Two-color ionization of hydrogen by short intense pulses

Photoelectron energy spectra resulting by the interaction of hydrogen with two short pulses having carrier frequencies, respectively, in the range of the infrared and XUV regions have been calculated. The effects of the pulse duration and timing of the X-ray pulse on the photoelectron energy spectra are discussed. Analysis of the spectra obtained for very long pulses show that certain features may be explained in terms of quantum interferences in the time domain. It is found that, depending on the duration of the X-ray pulse, ripples in the energy spectra separated by the infrared photon energy may appear. Moreover, the temporal shape of the low frequency radiation field may be inferred by the breadth of the photoelectron energy spectra.Comment: 12 pages, 8 figure