1,305 research outputs found
Improving the outcome of infants born at \u3c30 weeks\u27 gestation - a randomized controlled trial of preventative care at home
Barrier-controlled carrier transport in microcrystalline semiconducting materials: Description within a unified model
A recently developed model that unifies the ballistic and diffusive transport
mechanisms is applied in a theoretical study of carrier transport across
potential barriers at grain boundaries in microcrystalline semiconducting
materials. In the unified model, the conductance depends on the detailed
structure of the band edge profile and in a nonlinear way on the carrier mean
free path. Equilibrium band edge profiles are calculated within the trapping
model for samples made up of a linear chain of identical grains. Quantum
corrections allowing for tunneling are included in the calculation of electron
mobilities. The dependence of the mobilities on carrier mean free path, grain
length, number of grains, and temperature is examined, and appreciable
departures from the results of the thermionic-field-emission model are found.
Specifically, the unified model is applied in an analysis of Hall mobility data
for n-type microcrystalline Si thin films in the range of thermally activated
transport. Owing mainly to the effect of tunneling, potential barrier heights
derived from the data are substantially larger than the activation energies of
the Hall mobilities. The specific features of the unified model, however,
cannot be resolved within the rather large uncertainties of the analysis.Comment: REVTex, 19 pages, 9 figures; to appear in J. Appl. Phy
Maser Oscillation in a Whispering-Gallery-Mode Microwave Resonator
We report the first observation of above-threshold maser oscillation in a
whispering-gallery(WG)-mode resonator, whose quasi-transverse-magnetic, 17th
azimuthal-order WG mode, at a frequency of approx. 12.038 GHz, with a loaded Q
of several hundred million, is supported on a cylinder of mono-crystalline
sapphire. An electron spin resonance (ESR) associated with Fe3+ ions, that are
substitutively included within the sapphire at a concentration of a few parts
per billion, coincides in frequency with that of the (considerably narrower) WG
mode. By applying a c.w. `pump' to the resonator at a frequency of approx.
31.34 GHz, with no applied d.c. magnetic field, the WG (`signal') mode is
energized through a three-level maser scheme. Preliminary measurements
demonstrate a frequency stability (Allan deviation) of a few times 1e-14 for
sampling intervals up to 100 s.Comment: REVTeX v.4, 3 pages, with a separate .bbl file and 3 .eps figure
Effect of doping-- and field--induced charge carrier density on the electron transport in nanocrystalline ZnO
Charge transport properties of thin films of sol--gel processed undoped and
Al-doped zinc oxide nanoparticles with variable doping level between 0.8 at%
and 10 at% were investigated. The X-ray diffraction studies revealed a decrease
of the average crystallite sizes in highly doped samples. We provide estimates
of the conductivity and the resulting charge carrier densities with respect to
the doping level. The increase of charge carrier density due to extrinsic
doping were compared to the accumulation of charge carriers in field effect
transistor structures. This allowed to assess the scattering effects due to
extrinsic doping on the electron mobility. The latter decreases from 4.6*10^-3
cm^2/Vs to 4.5*10^-4 cm^2/Vs with increasing doping density. In contrast, the
accumulation leads to an increasing mobility up to 1.5*10^-2 cm^2/Vs. The
potential barrier heights related to grain boundaries between the crystallites
were derived from temperature dependent mobility measurements. The extrinsic
doping initially leads to a grain boundary barrier height lowering, followed by
an increase due to doping-induced structural defects. We conclude that the
conductivity of sol--gel processed nanocrystalline ZnO:Al is governed by an
interplay of the enhanced charge carrier density and the doping-induced charge
carrier scattering effects, achieving a maximum at 0.8 at% in our case.Comment: 8 pages, 7 figure
Holistic corpus-based dialectology
This paper is concerned with sketching future directions for corpus-based dialectology. We advocate a holistic approach to the study of geographically conditioned linguistic variability, and we present a suitable methodology, 'corpusbased dialectometry', in exactly this spirit. Specifically, we argue that in order to live up to the potential of the corpus-based method, practitioners need to (i) abandon their exclusive focus on individual linguistic features in favor of the study of feature aggregates, (ii) draw on computationally advanced multivariate analysis techniques (such as multidimensional scaling, cluster analysis, and principal component analysis), and (iii) aid interpretation of empirical results by marshalling state-of-the-art data visualization techniques. To exemplify this line of analysis, we present a case study which explores joint frequency variability of 57 morphosyntax features in 34 dialects all over Great Britain
Time invariance violating nuclear electric octupole moments
The existence of a nuclear electric octupole moment (EOM) requires both
parity and time invariance violation. The EOMs of odd nuclei that are
induced by a particular T- and P-odd interaction are calculated. We compare
such octupole moments with the collective EOMs that can occur in nuclei having
a static octupole deformation. A nuclear EOM can induce a parity and time
invariance violating atomic electric dipole moment, and the magnitude of this
effect is calculated. The contribution of a nuclear EOM to such a dipole moment
is found, in most cases, to be smaller than that of other mechanisms of atomic
electric dipole moment production.Comment: Uses RevTex, 25 page
Adolescents with Learning Disabilities
The subjective experiences of adolescents with learning disabilities (LD) were compared to those of their low-achieving and normally achieving peers. Three groups of juniors and seniors from one suburban high school district, 18 students with learning disabilities (15 males, 3 females), 17 low-achieving students (12 males, 5 females), and 20 average-achieving students (12 males, 8 females), were given electronic pagers and booklets for 1 week. They were signaled every 40 minutes during school hours, and every 2 hours after school. As soon as possible after receiving a signal, they responded to questions in their booklets. The questions provided subjective measures on levels of affect, activation, cognitive efficiency, self-esteem, motivation, and feedback from others. The students with learning disabilities reported feeling more positive and active than either of the other groups during school hours, while after school there were no differences on any of the subjective measures for the three groups. Specific LD school practices are highlighted for their probable impact on the heightened affect and activation of the students with learning disabilities.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68706/2/10.1177_002221949302600912.pd
Fabrication and characterization of dual function nanoscale pH-scanning ion conductance microscopy (SICM) probes for high resolution pH mapping
The easy fabrication and use of nanoscale dual function pH-scanning ion conductance microscopy (SICM) probes is reported. These probes incorporate an iridium oxide coated carbon electrode for pH measurement and an SICM barrel for distance control, enabling simultaneous pH and topography mapping. These pH-SICM probes were fabricated rapidly from laser pulled theta quartz pipets, with the pH electrode prepared by in situ carbon filling of one of the barrels by the pyrolytic decomposition of butane, followed by electrodeposition of a thin layer of hydrous iridium oxide. The other barrel was filled with an electrolyte solution and Ag/AgCl electrode as part of a conductance cell for SICM. The fabricated probes, with pH and SICM sensing elements typically on the 100 nm scale, were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and various electrochemical measurements. They showed a linear super-Nernstian pH response over a range of pH (pH 2â10). The capability of the pH-SICM probe was demonstrated by detecting both pH and topographical changes during the dissolution of a calcite microcrystal in aqueous solution. This system illustrates the quantitative nature of pH-SICM imaging, because the dissolution process changes the crystal height and interfacial pH (compared to bulk), and each is sensitive to the rate. Both measurements reveal similar dissolution rates, which are in agreement with previously reported literature values measured by classical bulk methods
Surface Structure of Liquid Metals and the Effect of Capillary Waves: X-ray Studies on Liquid Indium
We report x-ray reflectivity (XR) and small angle off-specular diffuse
scattering (DS) measurements from the surface of liquid Indium close to its
melting point of C. From the XR measurements we extract the surface
structure factor convolved with fluctuations in the height of the liquid
surface. We present a model to describe DS that takes into account the surface
structure factor, thermally excited capillary waves and the experimental
resolution. The experimentally determined DS follows this model with no
adjustable parameters, allowing the surface structure factor to be deconvolved
from the thermally excited height fluctuations. The resulting local electron
density profile displays exponentially decaying surface induced layering
similar to that previously reported for Ga and Hg. We compare the details of
the local electron density profiles of liquid In, which is a nearly free
electron metal, and liquid Ga, which is considerably more covalent and shows
directional bonding in the melt. The oscillatory density profiles have
comparable amplitudes in both metals, but surface layering decays over a length
scale of \AA for In and \AA for Ga. Upon controlled
exposure to oxygen, no oxide monolayer is formed on the liquid In surface,
unlike the passivating film formed on liquid Gallium.Comment: 9 pages, 5 figures; submitted to Phys. Rev.
A VinÄa potscape: formal chronological models for the use and development of VinÄa ceramics in south-east Europe.
Recent work at VinÄa-Belo Brdo has combined a total of more than 200 radiocarbon dates with an array of other information to construct much more precise narratives for the structural history of the site and the cultural materials recovered from it. In this paper, we present the results of a recent attempt to construct formal models for the chronology of the wider VinÄa potscape, so that we can place our now detailed understanding of changes at Belo Brdo within their contemporary contexts. We present our methodology for assessing the potential of the existing corpus of more than 600 radiocarbon dates for refining the chronology of the five phases of VinÄa ceramics proposed by MilojÄiÄ across their spatial ranges, including a total of 490 of them in a series of Bayesian chronological models. Then we outline our main results for the development of VinÄa pottery. Finally, we discuss some of the major implications for our understanding of the source, character and tempo of material change
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