132 research outputs found
Electron Spin Dynamics of the Superconductor CaC6 probed by ESR
Conduction Electron Spin Resonance (CESR) was measured on a thick slab of
CaC6 in the normal and superconducting state. A surprising increase of the CESR
intensity below Tc can not be explained by the theoretically predicted change
in spin susceptibility. It is interpreted as a vortex enhanced increase of the
effective skin depth. Non-linear microwave absorption measurements in the
superconducting state describe CaC6 as an anisotropic BCS superconductor. The
study of the spin dynamics in the superconducting state and the discovery of
the vortex enhanced increase of the skin depth poses a challenge to theory to
provide a comprehensive description of the observed phenomena. CESR data in the
normal state characterize CaC6 as a three-dimensional (3D) metal. The analysis
suggests that the scattering of conduction electrons is dominated by impurities
and supports the description of superconductivity in the dirty limit.Comment: 4 pages, 3 figure
Antibacterial Activity of Gentamicin-bonded Gelatin-sealed Polyethylene Terephthalate Vascular Prostheses
AbstractObjectivesTo create an antibiotic-modified vascular prosthesis with a prolonged bactericidal activity, susceptible to endothelialisation.MethodsWe used a covalent method of gentamicin sulphate immobilisation to polyethylene terephthalate prosthesis sealed with gelatin. Antibacterial activity was assayed in Luria-Bertani medium against Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa strains. Prosthesis endothelialisation was performed using bovine aorta endothelial cells (BAEC).ResultsGentamicin was bound to vascular prostheses in the amount of 12g per kg of prosthesis. Ninety-seven percent of antibiotic bound in covalent way and remained on the biomaterial for at least 30 days during shaking in PBS solution. Gentamicin-modified prostheses exerted bactericidal or bacteriostatic effect on growth of clinical and reference bacterial strains, prevented biofilm formation and were highly susceptible to endothelialisation. BAEC viability exceeded 90%, which indicated that gentamicin-vascular prostheses were not toxic for these cells.ConclusionsCovalent gentamicin immobilisation resulted in effective antibacterial protection of vascular prostheses against clinical and reference strains of S. aureus, E. coli and P. aeruginosa and allowed for a strong adherence of endothelial cells to antibiotic-modified prostheses
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Surface of underdoped YBa2Cu3O7- δ as revealed by STM/STS
We performed scanning tunneling microscopy and spectroscopy on untwinned crystals of underdoped YBa2Cu3O7- δ at δ = 0.4. A comprehensive statistical analysis of our topographic data indicates a doping dependent cleaving behavior of this material. We find in particular that at δ = 0.4 the material primarily cleaves in multiples of one unit cell along the c-axis with a high corrugation of the topmost layer. Our data suggest that the low temperature cleaving mainly results in a disruption of the CuO chain layers involving a redistribution of the layer atoms onto the two cleaving planes. In a few instances, fractional step heights (in terms of the c-axis lattice constant) are observed as well. Scanning tunneling spectroscopy reveals that such fractional steps connect surfaces which differ significantly in their tunneling conductance
The Nature of Composite LINER/HII Galaxies, As Revealed from High-Resolution VLA Observations
A sample of 37 nearby galaxies displaying composite LINER/HII and pure HII
spectra was observed with the VLA in an investigation of the nature of their
weak radio emission. The resulting radio contour maps overlaid on optical
galaxy images are presented here, together with an extensive literature list
and discussion of the individual galaxies. Radio morphological data permit
assessment of the ``classical AGN'' contribution to the global activity
observed in these ``transition'' LINER galaxies. One in five of the latter
objects display clear AGN characteristics: these occur exclusively in
bulge-dominated hosts.Comment: 31 pages, 27 figures, accepted by ApJ
The Radio Properties of Composite LINER/HII Galaxies
Arcsec-resolution VLA observations -- newly obtained as well as published --
of 40 nearby galaxies are discussed, completing a study of the radio properties
of a magnitude-limited sample of nearby galaxies of the composite LINER/HII
type. Our results reveal an overall detection rate of at least 25% AGN
candidates among these composite sources. The general properties of these AGN
candidates, as compared to non-AGN composite sources and HII galaxies, are
discussed.Comment: Accepted for publication in ApJ
V*-algebras, independence algebras and logic
Independence algebras were introduced in the early 1990s by specialists in semigroup theory, as a tool to explain similarities between the transformation monoid on a set and the endomorphism monoid of a vector space. It turned out that these algebras had already been defined and studied in the 1960s, under the name of v*-algebras, by specialists in universal algebra (and statistics). Our goal is to complete this picture by discussing how, during the middle period, independence algebras began to play a very important role in logic
Bridging charge-orbital ordering and Fermi surface instabilities in half-doped single-layered manganite La_0.5Sr_1.5MnO_4
Density waves are inherent to the phase diagrams of materials that exhibit
unusual, and sometimes extraordinarily useful properties, such as
superconductivity and colossal magnetoresistance. While the pure charge density
waves (CDW) are well described by an itinerant approach, where electrons are
treated as waves propagating through the crystal, the charge-orbital ordering
(COO) is usually explained by a local approach, where the electrons are treated
as localized on the atomic sites. Here we show that in the half-doped manganite
La0.5Sr1.5MnO4 (LSMO) the electronic susceptibility, calculated from the
angle-resolved photoemission spectra (ARPES), exhibits a prominent
nesting-driven peak at one quarter of the Brillouin zone diagonal, that is
equal to the reciprocal lattice vector of the charge-orbital pattern. Our
results demonstrate that the Fermi surface geometry determines the propensity
of the system to form a COO state which, in turn, implies the applicability of
the itinerant approach also to the COO
Images of very high energy cosmic ray sources in the Galaxy: I. A source towards the Galactic Centre
Recent analyses of the anisotropy of cosmic rays at eV (the AGASA
and SUGAR data) show significant excesses from regions close to the Galactic
Centre and Cygnus. Our aim is to check whether such anisotropies can be caused
by single sources of charged particles. We investigate propagation of protons
in two models of the Galactic regular magnetic field (with the irregular
component included) assuming that the particles are injected by a short lived
discrete source lying in the direction of the Galactic Centre. We show that
apart from a prompt image of the source, the regular magnetic field may cause
delayed images at quite large angular distances from the actual source
direction. The image is strongly dependent on the time elapsed after ejection
of particles and it is also very sensitive to their energy. For the most
favourable conditions for particle acceleration by a young pulsar the predicted
fluxes are two to four order of magnitudes higher than that observed. The
particular numbers depend strongly on the Galactic magnetic field model adopted
but it looks that a single pulsar in the Galactic Centre could be responsible
for the observed excess.Comment: 20 pages, 7 figures, accepted to J. Phys.
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