705 research outputs found
General Localization Lengths for Two Interacting Particles in a Disordered Chain
The propagation of an interacting particle pair in a disordered chain is
characterized by a set of localization lengths which we define. The
localization lengths are computed by a new decimation algorithm and provide a
more comprehensive picture of the two-particle propagation. We find that the
interaction delocalizes predominantly the center-of-mass motion of the pair and
use our approach to propose a consistent interpretation of the discrepancies
between previous numerical results.Comment: 4 pages, 2 epsi figure
Synchronization in networks of mobile oscillators
We present a model of synchronization in networks of autonomous agents where
the topology changes due to agents motion. We introduce two time scales, one
for the topological change and another one for local synchronization. If the
former scale is much shorter, an approximation that averages out the effect of
motion is available. Here we show, however, that the time required for
synchronization achievement increases with respect to that approximation in the
opposite case. We find that this effect is more important close to the
continuum percolation transition point. The simulation results are confirmed by
means of spectral analysis of the time dependent Laplacian matrix. Our results
show that the trade-off between these two time scales, which have opposite
effects on synchronization, should be taken into account for the design of
mobile device networks.Comment: 4 pages, 2 figure
Absence of backscattering at integrable impurities in one-dimensional quantum many-body systems
We study interacting one dimensional (1D) quantum lattice gases with
integrable impurities. These model Hamiltonians can be derived using the
quantum inverse scattering method for inhomogeneous models and are by
construction integrable. Absence of backscattering at the impurities is shown
to be the characteristic feature of these disordered systems. The value of the
effective carrier charge and the Sutherland-Shastry relation are derived for
the half-filled XXX model and are shown to be independent of the impurity
concentration and strength. For the half-filled XXZ model we show that there is
no enhancement of the persistent currents for repulsive interactions. For
attractive interactions we identify a crossover regime beyond which enhancement
of the currents is observed.Comment: 14 RevTeX 3.0 pages with 1 PS-figure include
The Aharonov-Bohm effect for an exciton
We study theoretically the exciton absorption on a ring shreded by a magnetic
flux. For the case when the attraction between electron and hole is
short-ranged we get an exact solution of the problem. We demonstrate that,
despite the electrical neutrality of the exciton, both the spectral position of
the exciton peak in the absorption, and the corresponding oscillator strength
oscillate with magnetic flux with a period ---the universal flux
quantum. The origin of the effect is the finite probability for electron and
hole, created by a photon at the same point, to tunnel in the opposite
directions and meet each other on the opposite side of the ring.Comment: 13 RevTeX 3.0 pages plus 4 EPS-figures, changes include updated
references and an improved chapter on possible experimental realization
Influence of Different Envelope Maskers on Signal Recognition and Neuronal Representation in the Auditory System of a Grasshopper
Background: Animals that communicate by sound face the problem that the signals arriving at the receiver often are degraded and masked by noise. Frequency filters in the receiver’s auditory system may improve the signal-to-noise ratio (SNR) by excluding parts of the spectrum which are not occupied by the species-specific signals. This solution, however, is hardly amenable to species that produce broad band signals or have ears with broad frequency tuning. In mammals auditory filters exist that work in the temporal domain of amplitude modulations (AM). Do insects also use this type of filtering? Principal Findings: Combining behavioural and neurophysiological experiments we investigated whether AM filters may improve the recognition of masked communication signals in grasshoppers. The AM pattern of the sound, its envelope, is crucial for signal recognition in these animals. We degraded the species-specific song by adding random fluctuations to its envelope. Six noise bands were used that differed in their overlap with the spectral content of the song envelope. If AM filters contribute to reduced masking, signal recognition should depend on the degree of overlap between the song envelope spectrum and the noise spectra. Contrary to this prediction, the resistance against signal degradation was the same for five of six masker bands. Most remarkably, the band with the strongest frequency overlap to the natural song envelope (0–100 Hz) impaired acceptance of degraded signals the least. To assess the noise filter capacities of singl
Multifractal analysis of the metal-insulator transition in anisotropic systems
We study the Anderson model of localization with anisotropic hopping in three
dimensions for weakly coupled chains and weakly coupled planes. The eigenstates
of the Hamiltonian, as computed by Lanczos diagonalization for systems of sizes
up to , show multifractal behavior at the metal-insulator transition even
for strong anisotropy. The critical disorder strength determined from the
system size dependence of the singularity spectra is in a reasonable agreement
with a recent study using transfer matrix methods. But the respective spectrum
at deviates from the ``characteristic spectrum'' determined for the
isotropic system. This indicates a quantitative difference of the multifractal
properties of states of the anisotropic as compared to the isotropic system.
Further, we calculate the Kubo conductivity for given anisotropies by exact
diagonalization. Already for small system sizes of only sites we observe
a rapidly decreasing conductivity in the directions with reduced hopping if the
coupling becomes weaker.Comment: 25 RevTeX pages with 10 PS-figures include
Sounds, Behaviour, and Auditory Receptors of the Armoured Ground Cricket, Acanthoplus longipes
The auditory sensory system of the taxon Hetrodinae has not been studied previously. Males of the African armoured ground cricket, Acanthoplus longipes (Orthoptera: Tettigoniidae: Hetrodinae) produce a calling song that lasts for minutes and consists of verses with two pulses. About three impulses are in the first pulse and about five impulses are in the second pulse. In contrast, the disturbance stridulation consists of verses with about 14 impulses that are not separated in pulses. Furthermore, the inter-impulse intervals of both types of sounds are different, whereas verses have similar durations. This indicates that the neuronal networks for sound generation are not identical. The frequency spectrum peaks at about 15 kHz in both types of sounds, whereas the hearing threshold has the greatest sensitivity between 4 and 10 kHz. The auditory afferents project into the prothoracic ganglion. The foreleg contains about 27 sensory neurons in the crista acustica; the midleg has 18 sensory neurons, and the hindleg has 14. The auditory system is similar to those of other Tettigoniidae
Luminescence from highly excited nanorings: Luttinger liquid description
We study theoretically the luminescence from quantum dots of a ring geometry.
For high excitation intensities, photoexcited electrons and holes form Fermi
seas. Close to the emission threshold, the single-particle spectral lines
aquire weak many-body satellites. However, away from the threshold, the
discrete luminescence spectrum is completely dominated by many-body
transitions. We employ the Luttinger liquid approach to exactly calculate the
intensities of all many-body spectral lines. We find that the transition from
single-particle to many-body structure of the emission spectrum is governed by
a single parameter and that the distribution of peaks away from the threshold
is universal.Comment: 10 pages including 2 figure
Virulence-associated genes, resistance genes and adhesion and probiotic activity tested by a new screening method
We established an automated screening method to characterize adhesion of
Escherichia coli to intestinal porcine epithelial cells (IPEC-J2) and their
probiotic activity against infection by enteropathogenic E. coli (EPEC). 104
intestinal E. coli isolates from domestic pigs were tested by PCR for the
occurrence of virulence-associated genes, genes coding for resistances to
antimicrobial agents and metals, and for phylogenetic origin by PCR. Adhesion
rates and probiotic activity were examined for correlation with the presence
of these genes. Finally, data were compared with those from 93 E. coli
isolates from wild boars. Isolates from domestic pigs carried a broad variety
of all tested genes and showed great diversity in gene patterns. Adhesions
varied with a maximum of 18.3 or 24.2 mean bacteria adherence per epithelial
cell after 2 or 6 hours respectively. Most isolates from domestic pigs and
wild boars showed low adherence, with no correlation between
adhesion/probiotic activity and E. coli genes or gene clusters. The gene
sfa/foc, encoding for a subunit of F1C fimbriae did show a positive
correlative association with adherence and probiotic activity; however E. coli
isolates from wild boars with the sfa/foc gene showed less adhesion and
probiotic activity than E. coli with the sfa/foc gene isolated from domestic
pigs after 6 hour incubation. In conclusion, screening porcine E. coli for
virulence associated genes genes, adhesion to intestinal epithelial cells, and
probiotic activity revealed a single important adhesion factor, several
probiotic candidates, and showed important differences between E. coli of
domestic pigs and wild boars
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