537 research outputs found
On the field cooled susceptibility of YBaCuO sample
No full textUsing a SQUID magnetometer, we have studied the magnetization of different samples of high Tc sintered superconductors in field cooled (FC) conditions. Depending on the sample, very different trends in the variation of the saturation value of the FC magnetization with the field have been observed ; in particular for good samples it increases progressively with decreasing H. We ascribe the difference in behaviour to different strengths of coupling between grains, depending on the preparation conditions. Data obtained by complementary techniques support this explanation
The star formation rate of the Universe atz≈ 6 from the Hubble ultra-Deep Field
No full textWe determine the abundance of i′-band dropouts in the recently released HST/ACS Hubble Ultra-Deep Field (UDF). Because the majority of these sources are likely to be z≈ 6 galaxies whose flux decrement between the F775W i′-band and F850LP z′-band arises from Lyman-α absorption, the number of detected candidates provides a valuable upper limit to the unextincted star formation rate at this redshift. We demonstrate that the increased depth of UDF enables us to reach an 8 σ limiting magnitude of z′AB= 28.5 (equivalent to 1.5 h−270 M⊙ yr−1 at z= 6.1, or 0.1 L⋆UV for the z≈ 3U-drop population), permitting us to address earlier ambiguities arising from the unobserved form of the luminosity function. We identify 54 galaxies (and only one star) at z′AB 1.3 over the deepest 11-arcmin2 portion of the UDF. The characteristic luminosity (L⋆) is consistent with values observed at z≈ 3. The faint end slope (α) is less well constrained, but is consistent with only modest evolution. The main change appears to be in the number density (Φ*). Specifically, and regardless of possible contamination from cool stars and lower-redshift sources, the UDF data support our previous result that the star formation rate at z≈ 6 was approximately six times less than at z≈ 3. This declining comoving star formation rate [0.005 h70 M⊙ yr−1 Mpc−3 at z≈ 6 at LUV > 0.1 L⋆ for a Salpeter initial mass function (IMF)] poses an interesting challenge for models which suggest that LUV > 0.1 L⋆ star-forming galaxies at z≃ 6 reionized the Universe. The short-fall in ionizing photons might be alleviated by galaxies fainter than our limit, or a radically different IMF. Alternatively, the bulk of reionization might have occurred at z≫ 6
Super-resolution of remotely sensed images with variable-pixel linear reconstruction
No full textThis paper describes the development and applications of a super-resolution method, known as Super-Resolution Variable-Pixel Linear Reconstruction. The algorithm works combining different lower resolution images in order to obtain, as a result, a higher resolution image. We show that it can make significant spatial resolution improvements to satellite images of the Earth¿s surface allowing recognition of objects with size approaching the limiting spatial resolution of the lower resolution images. The algorithm is based on the Variable-Pixel Linear Reconstruction algorithm developed by Fruchter and Hook, a well-known method in astronomy but never used for Earth remote sensing purposes. The algorithm preserves photometry, can weight input images according to the statistical significance of each pixel, and removes the effect of geometric distortion on both image shape and photometry. In this paper, we describe its development for remote sensing purposes, show the usefulness of the algorithm working with images as different to the astronomical images as the remote sensing ones, and show applications to: 1) a set of simulated multispectral images obtained from a real Quickbird image; and 2) a set of multispectral real Landsat Enhanced Thematic Mapper Plus (ETM+) images. These examples show that the algorithm provides a substantial improvement in limiting spatial resolution for both simulated and real data sets without significantly altering the multispectral content of the input low-resolution images, without amplifying the noise, and with very few artifacts
Torque measurements of textured YBaCuO sintered pellets
No full textThe authors present at 4.2 K magnetic torque balance study of a textured sintered YBCO sample produced by pyrolysis using in the thermal treatments an ozone-enriched oxygen atmosphere. The shape of the torque signal as a function of the angle and of the measuring field shows features that are intermediate between those of a standard sintered pellet and those of a single crystal. The anisotropic experimental values of the lower critical field Hc1 compare rather well to those previously found on YBCO single crystals. The intensity of the remaining magnetization, measured after a field cycling procedure, suggests the existence of a strong pinning for H⩾Hc1(c) along the c-axi
Co-ordinating distributed knowledge: An investigation into the use of an organisational memory
No full textThis paper presents an ethnographically informed investigation into the use of an organisational memory, focusing in particular on how information was used in the performance of work. We argue that understanding how people make use of distributed knowledge is crucial to the design of an organisational memory. However, we take the perspective that an ‘organisational memory’ is not technology dependant, but is an emergent property of group interaction. In this sense, the technology does not form the organisational memory, but provides a novel means of augmenting the co-ordination of collaborative action. The study examines the generation, development and maintenance of knowledge repositories and archives. The knowledge and information captured in the organisational memory enabled the team members to establish a common understanding of the design and to gain an appreciation of the issues and concerns of the other disciplines. The study demonstrates why technology should not be thought of in isolation from its contexts of use, but also how designers can make use of the creative flexibility that people employ in their everyday activities. The findings of the study are therefore of direct relevance to both the design of knowledge archives and to the management of this information within organisations
Lyman break galaxies and the star formation rate of the Universe at z ~ 6
No full textWe determine the space density of UV-luminous starburst galaxies at z≈ 6 using deep HST ACS SDSS-i′ (F775W) and SDSS-z′ (F850LP) and VLT ISAAC J and Ks band imaging of the Chandra Deep Field South. We find eight galaxies and one star with (i′−z′) > 1.5 to a depth of z′AB= 25.6 (an 8σ detection in each of the 3 available ACS epochs). This corresponds to an unobscured star formation rate of ≈15 h−270 M⊙ yr−1 at z= 5.9, equivalent to L* for the Lyman-break population at z= 3–4 (ΩΛ= 0.7, ΩM= 0.3). We are sensitive to star-forming galaxies at 5.6 ≲z≲ 7.0 with an effective comoving volume of ≈1.8 × 105h−370 Mpc3 after accounting for incompleteness at the higher redshifts due to luminosity bias. This volume should encompass the primeval subgalactic-scale fragments of the progenitors of about a thousand L* galaxies at the current epoch. We determine a volume-averaged global star formation rate of (6.7 ± 2.7) × 10−4h70 M⊙ yr−1 Mpc−3 at z∼ 6 from rest-frame UV selected starbursts at the bright end of the luminosity function: this is a lower limit because of dust obscuration and galaxies below our sensitivity limit. This measurement shows that at z∼ 6 the star formation density at the bright end is a factor of ∼6 times less than that determined by Steidel et al. for a comparable sample of UV-selected galaxies at z= 3–4, and so extends our knowledge of the star formation history of the Universe to earlier times than previous work and into the epoch where reionization may have occurred
Continuous melting of a driven two-dimensional flux lattice with strong pins
No full textThe phase diagram of a driven two-dimensional vortex lattice in the presence of dense quasi-point pins is investigated. The transition from the crystal to the liquid is found continuous at intermediate inductions. The correlations in the pseudo random force that allow for an uncomplete unbinding of the dislocations is proposed as a key mechanism to account for the continuous transition. Copyright Springer-Verlag Berlin/Heidelberg 2003
Transition to plastic motion as a critical phenomenon and anomalous interface layer of a 2D driven vortex lattice
No full textThe dynamic transition between ordered flow and
plastic flow is studied for a two-dimensional driven vortex
lattice, in the presence of sharp and dense pinning centers, from
numerical simulations. For this system, which does not show
smectic ordering, the lattice exhibits a first order transition
from a crystal to a liquid, shortly followed by the dynamical
transition to plastic flow. The resistivity provides a critical
order parameter for the latter, and critical exponents are
determined in analogy with a percolation transition. At the
boundary between a pinned region and an unpinned one, an
anomalous layer is observed, where the vortices are more strongly
pinned than in the bulk
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No full textMagnetothermal instabilities in an organic superconductor
No full textWe have studied the occurrence of magnetothermal instabilities in a
single crystal during field sweep magnetization experiments,
equivalent to short time relaxation studies. We find instability behaviour in good agreement
with a recent model by Mints, for a non-linear E(J) characteristic. In particular, we find that a
decrease of the dynamic relaxation rate, characterizing the effective activation energy,
precedes the unstable regime. We point out formal analogies between such instabilities and
the general predictions for flux avalanches
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