1,965 research outputs found
Absence of localization in a disordered one-dimensional ring threaded by an Aharonov-Bohm flux
Absence of localization is demonstrated analytically to leading order in weak
disorder in a one-dimensional Anderson model of a ring threaded by an
Aharonov-Bohm (A-B) flux. The result follows from adapting an earlier
perturbation treatment of disorder in a superconducting ring subjected to an
imaginary vector potential proportional to a depinning field for flux lines
bound to random columnar defects parallel to the axis of the ring. The absence
of localization in the ring threaded by an A-B flux for sufficiently weak
disorder is compatible with large free electron type persistent current
obtained in recent studies of the above model
Transmission, reflection and localization in a random medium with absorption or gain
We study reflection and transmission of waves in a random tight-binding
system with absorption or gain for weak disorder, using a scattering matrix
formalism. Our aim is to discuss analytically the effects of absorption or gain
on the statistics of wave transport. Treating the effects of absorption or gain
exactly in the limit of no disorder, allows us to identify short- and long
lengths regimes relative to absorption- or gain lengths, where the effects of
absorption/gain on statistical properties are essentially different. In the
long-lengths regime we find that a weak absorption or a weak gain induce
identical statistical corrections in the inverse localization length, but lead
to different corrections in the mean reflection coefficient. In contrast, a
strong absorption or a strong gain strongly suppress the effect of disorder in
identical ways (to leading order), both in the localization length and in the
mean reflection coefficient.Comment: Important revisions and expansion caused by a crucial property of
$\hat Q
Sol-gel preparation of pure and doped TiOâ‚‚ films for the photocatalytic oxidation of ethanol in air
Stable sols of TiO2 were synthesized by a non-aqueous sol-gel process using titanium (IV) isopropoxide as precursor. The microstructure, optical and morphological properties of the films obtained by spin-coating from the sol, and annealed at different temperatures, were investigated using scanning electron microscopy, transmission electron microscopy, diffuse reflectance spectroscopy and ellipsometry. The crystalline structure of the films was characterized by X-ray diffraction and their photocatalytic activity was evaluated for the oxidation of ethanol in air. The influence of the calcination temperature, pre-heat treatment and the number of layers was studied. Simultaneous thermo-gravimetric and differential thermal analysis measurements were carried out to ascertain the thermal decomposition behavior of the precursors. In order to obtain a higher photoresponse in the visible region, a series of vanadium-, niobium- and tantalum-doped TiO2 catalysts was synthesized by the same sol-gel method. For V doping two different precursors, a vanadium alkoxide and V2O5, were used. The effect on the crystallization and photocatalytic activity of the doped TiO2 films was investigated. Furthermore, to identify the effective composition of the samples, they were characterized by X-ray photoelectron spectroscopy and the surface area of the powders was measured by N-2 adsorption. The 10 wt.% doped catalysts exhibit high photocatalytic activity under visible light and among them the best performance was obtained for the sample containing Ta as dopant. The crystallite sizes are closely related to the photocatalytic activity
Conductance and localization in disordered wires: role of evanescent states
This paper extends an earlier analytical scattering matrix treatment of
conductance and localization in coupled two- and three Anderson chain systems
for weak disorder when evanescent states are present at the Fermi level. Such
states exist typically when the interchain coupling exceeds the width of
propagating energy bands associated with the various transverse eigenvalues of
the coupled tight-binding systems. We calculate reflection- and transmission
coefficients in cases where, besides propagating states, one or two evanescent
states are available at the Fermi level for elastic scattering of electrons by
the disordered systems. We observe important qualitative changes in these
coefficients and in the related localization lengths due to ineffectiveness of
the evanescent modes for transmission and reflection in the various scattering
channels. In particular, the localization lengths are generally significantly
larger than the values obtained when evanescent modes are absent. Effects
associated with disorder mediated coupling between propagating and evanescent
modes are shown to be suppressed by quantum interference effects, in lowest
order for weak disorder
Mean Free Path in Disordered Multichannel Tight-Binding Wires
Transport in a disordered tight-binding wire involves a collection of
different mean free paths resulting from the distinct fermi points, which
correspond to the various scattering channels of the wire. The generalization
of Thouless' relation between the mean free path and the localization length
permits to define an average channel mean free path,, such that
in an -channel system. The averaged mean free path
is expressed exactly in terms of the total reflection coefficient of
the wire and compared with the mean free path defined in the maximum entropy
approach
Exact transmission moments in one-dimensional weak localization and single-parameter scaling
We obtain for the first time the expressions for the mean and the variance of
the transmission coefficient for an Anderson chain in the weak localization
regime, using exact expansions of the complex transmission- and reflection
coefficients to fourth order in the weakly disordered site energies. These
results confirm the validity of single-parameter scaling theory in a domain
where the higher transmission cumulants may be neglected. We compare our
results with earlier results for transmission cumulants in the weak
localization domain based on the phase randomization hypothesis
Estimation of Rice Yield Losses Due to the African Rice Gall Midge, \u3ci\u3eOrseolia oryzivora\u3c/i\u3e Harris and Gagne
The African rice gall midge, Orseolia oryzivora Harris and Gagne (Diptera: Cecidomyiidae), is an important pest of rice, Oryza sativa, in Burkina Faso as well as other countries in West and East Africa. In spite of its importance, little is known regarding the relationship between gall midge populations and grain yield losses. To determine yield losses, the gall midge was reared in cages, and adult midges were placed on caged plants of the rice variety ITA 123 at different population levels. The seven treatments consisted of different numbers of insects infested on the plants: 0 insect pairs (noninfested check), and 1, 5, 10, 15, 20, or 25 midge pairs/cage. The loss in yield in relation to the noninfested control was highly positively correlated (R2 = 0.81) with the percentage of gall midge damaged tillers. The infestation by the insect on the plants resulted in the compensatory production of tillers which developed in response to the gall midge damage, but the compensation was not sufficient to make up for the loss of yield due to the damaged tillers. Yield loss ranged from 0% in the control to 65.3% in the treatment with 25 pairs of adults. One percent of tillers damaged resulted in 2% grain yield loss
Rice Pests in the Ivory Coast, West Africa: Farmers’ Perceptions and Management Strategies
An understanding of farmers’ perceptions of pests and their existing pest control methods can make an important contribution to the development of effective integrated pest management strategies. While the relatively successful rice IPM technologies in Asia have been extensively documented in studies examining farmers’ pest perceptions and management practices, studies examining, specifically, perceptions and management practices of West African rice farmers are rare. This may reflect the more limited success to date in the implementation of rice IPM strategies in the region. This paper describes rice pests, pest perceptions, and management practices of smallholder rice farmers in the Ivory Coast. It was found that the major rice pests, as perceived by farmers, are weeds, vertebrates (birds and rodents), and insects. Farmers are generally unable to diagnose rice plant diseases and thus did not consider them as important production constraints. Implications are derived for an integrated pest management strategy that incorporates farmers’ perceptions and builds on their existing cultural control practices
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