17 research outputs found
Tunable asymmetric reflectance in silver films near the percolation threshold
We report on the optical characterization of semicontinuous nanostructured
silver films exhibiting tunable optical reflectance asymmetries. The films are
obtained using a multi-step process, where a nanocrystalline silver film is
first chemically deposited on a glass substrate and then subsequently coated
with additional silver via thermal vacuum-deposition. The resulting films
exhibit reflectance asymmetries whose dispersions may be tuned both in sign and
in magnitude, as well as a universal, tunable spectral crossover point. We
obtain a correlation between the optical response and charge transport in these
films, with the spectral crossover point indicating the onset of charge
percolation. Such broadband, dispersion-tunable asymmetric reflectors may find
uses in future light-harvesting systems.Comment: 18 pages, 5 figures, accepted by Journal of Applied Physic
Morphology and foraging behavior of Oklahoma's grass-feeding termite: Gnathamitermes tubiformans (Isoptera: Termitidae)
Grass-feeding termites, Gnathamitermes sp., can be considered beneficial in pastureland, where they aerate topsoil, recycle nutrients from grasses and forbs, and excavate and redistribute soil. The primary objective of this report is to illustrate and determine the average physical dimensions of Gnathamitermes tubiformans soldiers by measuring key body parts, including left mandible length, as well as discuss observed foraging behavior. G.Tubiformans soldiers have unique mandibles that can be used for accurate identification. Workers are morphologically similar compared with some other arid-land termite species, thus soldier characteristics are used for identification. The secondary objective is to describe observations of G.Tubiformans foraging behavior. This report also provides detailed line drawings and a watercolor illustration for aid in identification. Termites were collected near Temple, Cotton County, Oklahoma, and measured with high precision microscopy. To ensure accurate identification, fourteen different soldier body measurements were determined. The ability to identify termites that are potentially beneficial or detrimental to forage grass biomass production may aid efforts to enhance livestock production on pastures.Peer reviewedEntomology and Plant PathologyPsycholog
Electron transmission and phase time in semiconductor superlattices
We discuss the time spent by an electron propagating through a finite
periodic system such as a semiconductor superlattice. The relation between
dwell-time and phase-time is outlined. The envelopes of phase-time at maximum
and minimum transmission are derived, and it is shown that the peaks and
valleys of phase-time can be well described by parameters fitted at the
extrema. For a many-period system this covers most of the allowed band.
Comparison is made to direct numerical solutions of the time-dependent
Schr\"odinger equation by Veenstra et al. [cond-mat/0411118] who compared
systems with and without addition of an anti-reflection coating (ARC). With an
ARC, the time delay is consistent with propagation at the Bloch velocity of the
periodic system, which significantly reduces the time delay, in addition to
increasing the transmissivity.Comment: 8 pages, 10 figures, based on a talk at Theory-Canada 3 in Edmonton
AB, June 200
Antireflection coatings from analogy between electron scattering and spin precession
We use the analogy between scattering of a wave from a potential, and the precession of a spin-half particle in a magnetic field, to gain insight into the design of an antireflection coating for electrons in a semiconductor superlattice. It is shown that the classic recipes derived for optics are generally not applicable due to the different dispersion law for electrons. Using the stability conditions we show that a Poisson distribution of impedance steps is a better approximation than is a Gaussian distribution. Examples are given of filters with average transmissivity exceeding 95% over an allowed band