76 research outputs found
B NMR detection of the magnetic field distribution in the mixed superconducting state of MgB
The temperature dependence of the magnetic field distribution in the mixed
superconducting phase of randomly oriented MgB powder was probed by B NMR spectroscopy. Below the temperature of the second critical () field, K, our spectra reveal two NMR signal
components, one mapping the magnetic field distribution in the mixed
superconducting state and the other one arising from the normal state. The
complementary use of bulk magnetization and NMR measurements reveals that
MgB is an anisotropic superconductor with a Tesla
anisotropy parameter
The biology and ecology of juvenile pilotfish (Naucrates ductor) associated with Fish Aggregating Devices (FADs) in eastern Mediterranean waters
The pilotfish (Naucrates ductor) is an epipelagic oceanic species, whose 0-group specimens tend to aggregate below flotsam, constituting a principal by-catch of the western Mediterranean FAD fishery. During a one-year survey sampling, monthly experimental hauls using surrounding nets were conducted at sites in south Peloponnesian waters, where FADs were moored; during the summer-early autumn months bongo tows were also carried out. 0-group pilotfish appeared beneath FADs from mid-summer till early winter and the total length of the collected specimens ranged between 120 and 330 mm. Pilotfish larvae, measuring 2.0- 3.9 mm, were found at FAD sites in mid-summer and in early autumn. The age of juvenile specimens, determined by counting daily increments on their sagittae, ranged between 50-141 increments (days) for males, and 51-131 increments (days) for females. The predicted asymptotic length was found to be 412.3mm TL for males and 435.2 mm TL for females. The onset of sexual maturity appeared to occur when specimens were a few months old. Stomach content analysis suggested that the diet of 0-group pilotfish associated to FADs in Greek waters depended mainly on decapod larvae, hyperiid amphipods, and alciopid polychaetes
ROTATIONAL-DYNAMICS OF SOLID C-70 - A NEUTRON-SCATTERING STUDY
PMID: 10011126PMID: 10011126 This work at the University of Sussex at supported by the Science and Engineering Research Council, U.K.PMID: 10011126 This work at the University of Sussex at supported by the Science and Engineering Research Council, U.K.PMID: 10011126 This work at the University of Sussex at supported by the Science and Engineering Research Council, U.K.We report the results of neutron-diffraction and low-energy neutron-inelastic-scattering experiments on high-purity solid C-70 between 10 and 640 K. Thermal hysteresis effects are found to accompany structural changes both on cooling and on heating. The observed diffuse scattering intensity does not change with temperature. At 10 K broad librational peaks are observed at 1.82(16) meV [full width at half maximum=1.8(5) meV]. The peaks soften and broaden further with increasing temperature. At and above room temperature, they collapse into a single quasielastic line. At 300 K, the diffusive reorientational motion appears to be somewhat anisotropic, becoming less so with increasing temperature. An isotropic rotational diffusion model, in which the motions of adjacent molecules are uncorrelated, describes well the results at 525 K. The temperature dependence of the rotational diffusion constants is consistent with a thermally activated process having an activation energy of 32(7) meV.This work at the University of Sussex at supported by the Science and Engineering Research Council, U.K
The diet and feeding ecology of Conger conger (L. 1758) in the deep waters of the Eastern Ionian Sea
The diet of the European conger eel Conger conger was investigated for the first time in the Eastern Mediterranean. Fish dominated the European conger eel diet in the deep waters of E. Ionian Sea. All other prey taxa were identified as accidental preys. However, intestine analysis showed that Natantia, Brachyura and Cephalopoda might have a more important contribution in the diet of the species. C. conger exhibited a benthopelagic feeding behavior as it preyed upon both demersal and mesopelagic taxa. The high vacuity index and the low stomach and intestine fullness indicated that the feeding intensity of the species in the deep waters of Eastern Ionian Sea was quite low. C. conger feeding strategy was characterised by specialisation in various resource items. A between-phenotype contribution to niche width was observed for some prey categories. European Conger eel feeding specialisation seemed to be an adaptation to a food-scarce environment, as typified in deep-water habitat
Distribution of trace metals in the sediments of Elefsis Gulf
The present work examines the temporal evolution of industrial contamination in sediments caused by trace metals in the Elefsis Gulf, Greece. The current state of contamination as well as the related sources can be obtained from the study of the sediments. Trace metals, namely Ni, Cu, Fe, and Mn, were collected during 1984-2003 by the local monitoring network. The statistical analysis of the data and the contaminants isopleths patterns support the following conclusions: There is a strong correlation between the nearby coastal industrial activities and the distribution of the trace metals’ concentration in sediments. There is no seasonal or annual variation of the distribution. The sediments’ concentration of Mn in all sampling points and Cu in the center of the Gulf has decreased during the last years
Low-temperature structural model of hcp solid C
We report intermolecular potential-energy calculations for solid C_ and
determine the optimum static orientations of the molecules at low temperature;
we find them to be consistent with the monoclinic structural model proposed by
us in an earlier report [Solid State Commun. {\bf 105), 247 (1998)]. This model
indicates that the C_5 axis of the molecule is tilted by an angle 18^o
from the monoclinic b axis in contrast with the molecular orientation proposed
by Verheijen {\it et al.} [J. Chem. Phys. {\bf 166}, 287 (1992)] where the C_5
axis is parallel to the monoclinic b axis. In this calculation we have
incorporated the effective bond charge Coulomb potential together with the
Lennard-Jones potential between the molecule at the origin of the monoclinic
unit cell and its six nearest neighbours, three above and three below. The
minimum energy configuration for the molecular orientations turns out to be at
=18^o, =8^o, and =5^o, where , , and
define the molecular orientations.Comment: ReVTeX (4 pages) + 2 PostScript figure
Optical absorption in the strong coupling limit of Eliashberg theory
We calculate the optical conductivity of superconductors in the
strong-coupling limit. In this anomalous limit the typical energy scale is set
by the coupling energy, and other energy scales such as the energy of the
bosons mediating the attraction are negligibly small. We find a universal
frequency dependence of the optical absorption which is dominated by bound
states and differs significantly from the weak coupling results. A comparison
with absorption spectra of superconductors with enhanced electron-phonon
coupling shows that typical features of the strong-coupling limit are already
present at intermediate coupling.Comment: 10 pages, revtex, 4 uuencoded figure
A magnetization and B NMR study of MgAlB superconductors
We demonstrate for the first time the magnetic field distribution of the pure
vortex state in lightly doped MgAlB () powder
samples, by using B NMR in magnetic fields of 23.5 and 47 kOe. The
magnetic field distribution at T=5 K is Al-doping dependent, revealing a
considerable decrease of anisotropy in respect to pure MgB. This result
correlates nicely with magnetization measurements and is consistent with
-band hole driven superconductivity for MgB
Reversible Pressure-Induced Amorphization in Solid C70 : Raman and Photoluminescence Study
We have studied single crystals of by Raman scattering and
photoluminescence in the pressure range from 0 to 31.1 GPa. The Raman spectrum
at 31.1 GPa shows only a broad band similar to that of the amorphous carbon
without any trace of the Raman lines of . After releasing the pressure
from 31.1 GPa, the Raman and the photoluminescence spectra of the recovered
sample are that of the starting crystal. These results indicate that
the molecules are stable upto 31.1 GPa and the amorphous carbon high
pressure phase is reversible, in sharp contrast to the results on solid
. A qualitative explaination is suggested in terms of inter- versus
intra-molecular interactions.Comment: To appear in Phys. Rev. Lett., 12 pages, RevTeX (preprint format), 3
figures available upon reques
Superconductivity in Fullerides
Experimental studies of superconductivity properties of fullerides are
briefly reviewed. Theoretical calculations of the electron-phonon coupling, in
particular for the intramolecular phonons, are discussed extensively. The
calculations are compared with coupling constants deduced from a number of
different experimental techniques. It is discussed why the A_3 C_60 are not
Mott-Hubbard insulators, in spite of the large Coulomb interaction. Estimates
of the Coulomb pseudopotential , describing the effect of the Coulomb
repulsion on the superconductivity, as well as possible electronic mechanisms
for the superconductivity are reviewed. The calculation of various properties
within the Migdal-Eliashberg theory and attempts to go beyond this theory are
described.Comment: 33 pages, latex2e, revtex using rmp style, 15 figures, submitted to
Review of Modern Physics, more information at
http://radix2.mpi-stuttgart.mpg.de/fullerene/fullerene.htm
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