1,312 research outputs found
Universality of ac-conduction in anisotropic disordered systems: An effective medium approximation study
Anisotropic disordered system are studied in this work within the random
barrier model. In such systems the transition probabilities in different
directions have different probability density functions. The
frequency-dependent conductivity at low temperatures is obtained using an
effective medium approximation. It is shown that the isotropic universal
ac-conduction law, , is recovered if properly scaled
conductivity () and frequency () variables are used.Comment: 5 pages, no figures, final form (with corrected equations
British Bryological Society expedition to Mulanje Mountain, Malawi : 13., new and other unpublished records
Further results are provided of the 1991 British Bryological Society Expedition to Mulanje Mt., Malawi including 168 taxa of bryophyte, comprising 72 taxa of liverwort (38 new to Malawi) and 96 taxa of mosses (45 new to Malawi)
Signatures of superconducting gap inhomogeneities in optical properties
Scanning tunneling spectroscopy applied to the high- cuprates has
revealed significant spatial inhomogeneity on the nanoscale. Regions on the
order of a coherence length in size show variations of the magnitude of the
superconducting gap of order or more. An important unresolved question
is whether or not these variations are also present in the bulk, and how they
influence superconducting properties. As many theories and data analyses for
high- superconductivity assume spatial homogeneity of the gap magnitude,
this is a pressing question. We consider the far-infrared optical conductivity
and evaluate, within an effective medium approximation, what signatures of
spatial variations in gap magnitude are present in various optical quantities.
In addition to the case of d-wave superconductivity, relevant to the high-
cuprates, we have also considered s-wave gap symmetry in order to provide
expected signatures of inhomogeneities for superconductors in general. While
signatures of gap inhomogeneities can be strongly manifested in s-wave
superconductors, we find that the far-infrared optical conductivity in d-wave
is robust against such inhomogeneity.Comment: 8 pages, 7 figure
Anisotropic dielectric function in polar nano-regions of relaxor ferroelectrics
The paper suggests to treat the infrared reflectivity spectra of single
crystal perovskite relaxors as fine-grained ferroelectric ceramics: locally
frozen polarization makes the dielectric function strongly anisotropic in the
phonon frequency range and the random orientation of the polarization at
nano-scopic scale requires to take into account the inhomogeneous
depolarization field. Employing a simple effective medium approximation
(Bruggeman symmetrical formula) to dielectric function describing the polar
optic modes as damped harmonic oscillators turns out to be sufficient for
reproducing all principal features of room temperature reflectivity of PMN. One
of the reflectivity bands is identified as a geometrical resonance entirely
related to the nanoscale polarization inhomogeneity. The approach provides a
general guide for systematic determination of the polar mode frequencies split
by the inhomogeneous polarization at nanometer scale.Comment: 5 pages, 2 figure
Spectral Representation for the Effective Macroscopic Response of a Polycrystal: Application to Third-Order Nonlinear Susceptibility
Erratum:
In our paper, we show that the spectral representation for isotropic
two-component composites also applies to uniaxial polycrystals. We have learned
that this result was, in fact, first conjectured by G.W. Milton. While our
derivation is more detailed, our result for the spectral function is the same
as Milton's. We very much regret not having been aware of this work at the time
of writing our paper.
Original abstract:
We extend the spectral theory used for the calculation of the effective
linear response functions of composites to the case of a polycrystalline
material with uniaxially anisotropic microscopic symmetry. As an application,
we combine these results with a nonlinear decoupling approximation as modified
by Ma et al., to calculate the third-order nonlinear optical susceptibility of
a uniaxial polycrystal, assuming that the effective dielectric function of the
polycrystal can be calculated within the effective-medium approximation.Comment: v2 includes erratum and the original preprin
Potential dietary feed additives with antibacterial effects and their impact on performance of weaned piglets: A meta-analysis
This meta-analysis evaluated the use of potential dietary feed additives (pDFA) with antibacterial effects and their impact on the perfomance of weaned piglets. Twenty-three peer-reviewed in vivo studies, comprising 50 trials, were identified between January 2010 and January 2017. The pDFA in these studies could be grouped in 5 classes: antimicrobial peptides, chitosan, lysozyme, medium chain fatty acids/ triglycerides and plant extracts. Mixed-effect meta-analyses with type of pDFA as fixed effect were performed for the growth parameters âaverage daily gainâ (ADG) and âfeed conversion ratioâ (FCR), which are the two most important and used economic performance parameters for farmers.
For each class of pDFA, results of the meta-analysis showed significantly higher average daily gain in the group with pDFA compared to the negative control group, while no significant difference with the positive control group was observed. Furthermore, a positive effect on FCR was found, i.e. significantly less feed was needed to gain 1âkg of body weight in the group with pDFA compared to the negative control group. No significant differences with positive control groups were observed for each class of pDFA, except for plant extracts, where the FCR was also significantly reduced in the treatment group. These results suggest that pDFA could reduce the use of antimicrobials without significant negative effects on performance indicators
Potential dietary feed additives with antibacterial effects and their impact on performance of weaned piglets : a meta-analysis
This meta-analysis evaluated the use of potential dietary feed additives (pDFA) with antibacterial effects and their impact on the perfomance of weaned piglets. Twenty-three peer-reviewed in vivo studies, comprising 50 trials, were identified between January 2010 and January 2017. The pDFA in these studies could be grouped in 5 classes: antimicrobial peptides, chitosan, lysozyme, medium chain fatty acids/triglycerides and plant extracts. Mixed-effect meta-analyses with type of pDFA as fixed effect were performed for the growth parameters 'average daily gain' (ADG) and 'feed conversion ratio' (FCR), which are the two most important and used economic performance parameters for farmers.
For each class of pDFA, results of the meta-analysis showed significantly higher average daily gain in the group with pDFA compared to the negative control group, while no significant difference with the positive control group was observed. Furthermore, a positive effect on FCR was found, i.e. significantly less feed was needed to gain 1 kg of body weight in the group with pDFA compared to the negative control group. No significant differences with positive control groups were observed for each class of pDFA, except for plant extracts, where the FCR was also significantly reduced in the treatment group. These results suggest that pDFA could reduce the use of antimicrobials without significant negative effects on performance indicators
Community detection in networks with positive and negative links
Detecting communities in complex networks accurately is a prime challenge,
preceding further analyses of network characteristics and dynamics. Until now,
community detection took into account only positively valued links, while many
actual networks also feature negative links. We extend an existing Potts model
to incorporate negative links as well, resulting in a method similar to the
clustering of signed graphs, as dealt with in social balance theory, but more
general. To illustrate our method, we applied it to a network of international
alliances and disputes. Using data from 1993--2001, it turns out that the world
can be divided into six power blocs similar to Huntington's civilizations, with
some notable exceptions.Comment: 7 pages, 2 figures. Revised versio
Plasmon tunability in metallodielectric metamaterials
The dielectric properties of metamaterials consisting of periodically
arranged metallic nanoparticles of spherical shape are calculated by rigorously
solving Maxwell's equations. Effective dielectric functions are obtained by
comparing the reflectivity of planar surfaces limiting these materials with
Fresnel's formulas for equivalent homogeneous media, showing mixing and
splitting of individual-particle modes due to inter-particle interaction.
Detailed results for simple cubic and fcc crystals of aluminum spheres in
vacuum, silver spheres in vacuum, and silver spheres in a silicon matrix are
presented. The filling fraction of the metal f is shown to determine the
position of the plasmon modes of these metamaterials. Significant deviations
are observed with respect to Maxwell-Garnett effective medium theory for large
f, and multiple plasmons are predicted to exist in contrast to Maxwell-Garnett
theory.Comment: 6 pages, 4 figure
Bryophytes of Uganda : 6., new and additional records, 3.
12 hepatics and 32 mosses are reported new to Uganda, 1 moss being also new to Africa, and 1 liverwort new to mainland Africa. Ectropothecium plumigerum (Broth.) HedenĂ€s is a new combination (basionym: Isopterygium plumigerum Broth.) with a new synonym Taxicaulis plumirameus MĂŒll.Hal. nom. nud., and Taxiphyllum maniae (Renauld & Paris) M. Fleisch. is a new synonym of Taxiphyllum taxirameum (Mitt.) M.Fleisch. Three mosses are removed from the Uganda list
- âŠ