1,214 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
No L1 privilege in talker adaptation
As a rule, listening is easier in first (L1) than second languages (L2); difficult L2 listening can challenge even highly proficient users. We here examine one particular listening function, adaptation to novel talkers, in such a high-proficiency population: Dutch emigrants to Australia, predominantly using English outside the family, but all also retaining L1 proficiency. Using lexically-guided perceptual learning (Norris, McQueen & Cutler, 2003), we investigated these listenersâ adaptation to an ambiguous speech sound, in parallel experiments in both their L1 and their L2. A control study established that perceptual learning outcomes were unaffected by the procedural measures required for this double comparison. The emigrants showed equivalent proficiency in tests in both languages, robust perceptual adaptation in their L2, English, but no adaptation in L1. We propose that adaptation to novel talkers is a language-specific skill requiring regular novel practice; a limited set of known (family) interlocutors cannot meet this requirement
Listener adjustment of stress cue use to fit language vocabulary structure
In lexical stress languages, phonemically identical syllables can differ suprasegmentally (in duration, amplitude, F0). Such stress cues allow listeners to speed spoken-word recognition by rejecting mismatching competitors (e.g., unstressed set- in settee rules out stressed set- in setting, setter, settle). Such processing effects have indeed been observed in Spanish, Dutch and German, but English listeners are known to largely ignore stress cues. Dutch and German listeners even outdo English listeners in distinguishing stressed versus unstressed English syllables. This has been attributed to the relative frequency across the stress languages of unstressed syllables with full vowels; in English most unstressed syllables contain schwa, instead, and stress cues on full vowels are thus least often informative in this language. If only informativeness matters, would English listeners who encounter situations where such cues would pay off for them (e.g., learning one of those other stress languages) then shift to using stress cues? Likewise, would stress cue users with English as L2, if mainly using English, shift away from using the cues in English? Here we report tests of these two questions, with each receiving a yes answer. We propose that English listenersâ disregard of stress cues is purely pragmatic
Effective Elastic Moduli in Solids with High Crack Density
We investigate the weakening of elastic materials through randomly
distributed circles and cracks numerically and compare the results to
predictions from homogenization theories. We find a good agreement for the case
of randomly oriented cracks of equal length in an isotropic plane-strain medium
for lower crack densities; for higher densities the material is weaker than
predicted due to precursors of percolation. For a parallel alignment of cracks,
where percolation does not occur, we analytically predict a power law decay of
the effective elastic constants for high crack densities, and confirm this
result numerically.Comment: 8 page
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
The dynamics of lexical activation and competition in bilingualsâ first versus second language
Speech input causes listeners to activate multiple candidate words which then compete with one another. These include onset competitors, that share a beginning (bumper, butter), but also, counterintuitively, rhyme competitors, sharing an ending (bumper, jumper). In L1, competition is typically stronger for onset than for rhyme. In L2, onset competition has been attested but rhyme competition has heretofore remained largely unexamined. We assessed L1 (Dutch) and L2 (English) word recognition by the same late-bilingual individuals. In each language, eye gaze was recorded as listeners heard sentences and viewed sets of drawings: three unrelated, one depicting an onset or rhyme competitor of a word in the input. Activation patterns revealed substantial onset competition but no significant rhyme competition in either L1 or L2. Rhyme competition may thus be a âluxuryâ feature of maximally efficient listening, to be abandoned when resources are scarcer, as in listening by late bilinguals, in either language
Optical conductivity of a granular metal at not very low temperatures
We study the finite-temperature optical conductivity, sigma(omega,T), of a
granular metal using a simple model consisting of a array of spherical metallic
grains. It is necessary to include quantum tunneling and Coulomb blockade
effects to obtain the correct temperature dependence of sigma(omega, T), and to
consider polarization oscillations to obtain the correct frequency dependence.
We have therefore generalized the Ambegaokar-Eckern-Schoen (AES) model for
granular metals to obtain an effective field theory incorporating the
polarization fluctuations of the individual metallic grains. In contrast to the
DC conductivity, which is determined by inter-grain charge transfer and obeys
an Arrhenius law at low temperature, the AC conductivity is dominated by a
resonance peak for intra-grain polarization oscillations, which has a power-law
tail at low frequencies. More importantly, although the resonance frequency
agrees with the classical prediction, the resonance width depends on intergrain
quantum tunneling and Coulomb blockade parameters, in addition to the classical
Drude relaxation within the grain. This additional damping is due to inelastic
cotunneling of polarization fluctuations to neighbouring grains and it
qualitatively differs from the DC conductivity in its temperature dependence
quite unlike the expectation from Drude theory.Comment: Added figures, published version, 16 pages, REVTe
Vanishing Loss Effect on the Effective ac Conductivity behavior for 2D Composite Metal-Dielectric Films At The Percolation Threshold
We study the imaginary part of the effective conductivity as well as its
distribution probability for vanishing losses in 2D composites. This
investigation showed that the effective medium theory provides only
informations about the average conductivity, while its fluctuations which
correspond to the field energy in this limit are neglected by this theory.Comment: 6 pages, 2 figures, submitted to Phys.Rev.
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
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
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