X-ray polarimetry with an active-matrix pixel proportional counter

We report the first results from an X-ray polarimeter with a micropattern gas proportional counter using an amorphous silicon active matrix readout. With 100% polarized X-rays at 4.5 keV, we obtain a modulation factor of 0.33 +/- 0.03, confirming previous reports of the high polarization sensitivity of a finely segmented pixel proportional counter. The detector described here has a geometry suitable for the focal plane of an astronomical X-ray telescope. Amorphous silicon readout technology will enable additional extensions and improvements.Comment: 4 pages, 4 figures, 1 tabl

Evidence of pachyostosis in the cryptocleidoid plesiosaur Tatenectes laramiensis from the Sundance Formation of Wyoming

In this paper we present evidence for pachyostosis in the cryptocleidoid plesiosaur Tatenectes laramiensis Knight, 1900 (O\u27Keefe and Wahl, 2003a). Pachyostosis is not common in plesiosaurs and is particularly rare in non-pliosaurian plesiosaurs, although enlarged gastralia were first recognized in Tatenectes by Wahl (1999). This study aims to investigate the nature of the disproportionately large gastralia of Tatenectes m greater depth, based on new material. A recently discovered partial skeleton consisting of a dorsal vertebral series, ribs, gastralia, and a complete pelvic girdle was collected from the Jurassic-aged Sundance Formation of the Bighorn Basin in Wyoming during the summer of 2006. The gastralia of this specimen are disproportionately large considering the small size of the taxon (about 3 meters total length), and we therefore investigated the size of these elements quantitatively. Polished cross-sections were also prepared to explore the histology of the ribs and gastralia. The ribs of Tatenectes are not pachyostotic, whereas the gastralia exhibit a novel condition of pachyostosis while lacking osteosclerosis

Recombination in polymer-fullerene bulk heterojunction solar cells

Recombination of photogenerated charge carriers in polymer bulk heterojunction (BHJ) solar cells reduces the short circuit current (Jsc) and the fill factor (FF). Identifying the mechanism of recombination is, therefore, fundamentally important for increasing the power conversion efficiency. Light intensity and temperature dependent current-voltage measurements on polymer BHJ cells made from a variety of different semiconducting polymers and fullerenes show that the recombination kinetics are voltage dependent and evolve from first order recombination at short circuit to bimolecular recombination at open circuit as a result of increasing the voltage-dependent charge carrier density in the cell. The "missing 0.3V" inferred from comparison of the band gaps of the bulk heterojunction materials and the measured open circuit voltage at room temperature results from the temperature dependence of the quasi-Fermi-levels in the polymer and fullerene domains - a conclusion based upon the fundamental statistics of Fermions.Comment: Accepted for publication in Physical Review B. http://prb.aps.org/accepted/B/6b07cO3aHe71bd1b149e1425e58bf2868cda2384d?ajax=1&height=500&width=50

Intrinsic hole mobility and trapping in a regio-regular poly(thiophene)

The transport properties of high-performance thin-film transistors (TFT) made with a regio-regular poly(thiophene) semiconductor (PQT-12) are reported. The room-temperature field-effect mobility of the devices varied between 0.004 cm2/V s and 0.1 cm2/V s and was controlled through thermal processing of the material, which modified the structural order. The transport properties of TFTs were studied as a function of temperature. The field-effect mobility is thermally activated in all films at T<200 K and the activation energy depends on the charge density in the channel. The experimental data is compared to theoretical models for transport, and we argue that a model based on the existence of a mobility edge and an exponential distribution of traps provides the best interpretation of the data. The differences in room-temperature mobility are attributed to different widths of the shallow localized state distribution at the edge of the valence band due to structural disorder in the film. The free carrier mobility of the mobile states in the ordered regions of the film is the same in all structural modifications and is estimated to be between 1 and 4 cm2/V s.Comment: 20 pages, 8 figure

Training and orthotic effects related to functional electrical stimulation of the peroneal nerve in stroke.

OBJECTIVE: To examine the evidence for a training effect on the lower limb of functional electrical stimulation. DESIGN: Cohort study. PATIENTS: A total of 133 patients >6 months post-stroke. METHODS: Training and orthotic effects were determined from walking speed over 10 m, associated minimal and substantial clinically important differences (i.e. >0.05 and >0.10 m/s), and Functional Ambulation Category (FAC), ranging from household walking to independent walking in the community. RESULTS: An overall significant (p < 0.01) training effect was found that was not a clinically important difference (0.02 m/s); however, "community" FAC (≥ 0.8 m/s) and "most limited community walkers" FAC (0.4-0.58 m/s), but not "household walkers" (< 0.4 m/s), benefitted from a clinically important difference. A highly significant (p< 0.001), substantial clinically important orthotic effect (0.10 m/s) was found. In terms of overall improvement of one or more FACs, 23% achieved this due to a training effect, compared with 43% due to an orthotic effect. CONCLUSION: The findings suggest that functional electrical stimulation provides a training effect in those who are less impaired. Further work, which optimizes the use of the device for restoration of function, rather than as an orthotic device, will provide greater clarity on the effectiveness of functional electrical stimulation for eliciting a training effect

The Value of Literacy Practices

The concepts of literacy events and practices have received considerable attention in educational research and policy. In comparison, the question of value, that is, ‘which literacy practices do people most value?’ has been neglected. With the current trend of cross-cultural adult literacy assessment, it is increasingly important to recognise locally valued literacy practices. In this paper we argue that measuring preferences and weighting of literacy practices provides an empirical and democratic basis for decisions in literacy assessment and curriculum development and could inform rapid educational adaptation to changes in the literacy environment. The paper examines the methodological basis for investigating literacy values and its potential to inform cross-cultural literacy assessments. The argument is illustrated with primary data from Mozambique. The correlation between individual values and respondents’ socio-economic and demographic characteristics is explored

Thermally stimulated H emission and diffusion in hydrogenated amorphous silicon

We report first principles ab initio density functional calculations of hydrogen dynam- ics in hydrogenated amorphous silicon. Thermal motion of the host Si atoms drives H diffusion, as we demonstrate by direct simulation and explain with simple models. Si-Si bond centers and Si ring centers are local energy minima as expected. We also describe a new mechanism for break- ing Si-H bonds to release free atomic H into the network: a fluctuation bond center detachment (FBCD) assisted diffusion. H dynamics in a-Si:H is dominated by structural fluctuations intrinsic to the amorphous phase not present in the crystal.Comment: 4 pages, 5 figures, In press EPL (Jun. 2007

Higher Descent Data as a Homotopy Limit

We define the 2-groupoid of descent data assigned to a cosimplicial 2-groupoid and present it as the homotopy limit of the cosimplicial space gotten after applying the 2-nerve in each cosimplicial degree. This can be applied also to the case of $n$-groupoids thus providing an analogous presentation of "descent data" in higher dimensions.Comment: Appeared in JHR

Multiple minimum coverings of Kn with copies of K4 - e

This paper is the last of a trilogy completely solving the maximum packing and minimum covering problems for the complete graph on n vertices, Kn, with copies of K4 - e, that is, the complete graph on four vertices with one edge missing

Pre-torsors and Galois comodules over mixed distributive laws

We study comodule functors for comonads arising from mixed distributive laws. Their Galois property is reformulated in terms of a (so-called) regular arrow in Street's bicategory of comonads. Between categories possessing equalizers, we introduce the notion of a regular adjunction. An equivalence is proven between the category of pre-torsors over two regular adjunctions $(N_A,R_A)$ and $(N_B,R_B)$ on one hand, and the category of regular comonad arrows $(R_A,\xi)$ from some equalizer preserving comonad ${\mathbb C}$ to $N_BR_B$ on the other. This generalizes a known relationship between pre-torsors over equal commutative rings and Galois objects of coalgebras.Developing a bi-Galois theory of comonads, we show that a pre-torsor over regular adjunctions determines also a second (equalizer preserving) comonad ${\mathbb D}$ and a co-regular comonad arrow from ${\mathbb D}$ to $N_A R_A$, such that the comodule categories of ${\mathbb C}$ and ${\mathbb D}$ are equivalent.Comment: 34 pages LaTeX file. v2: a few typos correcte