75 research outputs found
Status of Acropora palmata Populations off the Coast of South Caicos, Turks and Caicos Islands
This study is the first detailed assessment of A. palmata populations of the Turks and Caicos Islands. A total of 203 individual colonies and 62 thickets were tagged on five shallow reefs. Depth, percentages of living tissue, recent mortality and old skeleton were estimated. Presence of disease and predatory snails was noted, and disease spread and grazing rates of the snails estimated. Colonies were found in depths of 0.2 - 4 m. Living tissue for individual colonies (75.9% ± 2.2 SE) was significantly greater than for thickets (58.6% ± 3.6) and in both cases exceeded old skeleton (individuals: 22.7% ± 2.1 SE, thickets: 38.0% ± 3.4 SE). Percentage of recent mortality was very low (individuals: 1.3% ± 0.3 SE, thickets: 3.4% ± 0.7%). We found WBD (n = 2), white pox disease a (WPDa) (n = 7) and white pox disease b (WPDb) (n = 14) with greatly varying spreading rates. The WBD infected colonies showed an atypical spread from the top of the branch towards the base. Coralliophila abbreviata and C. caribaea affected 3 .7 54.7% of the populations (grazing rate: 4.29 cm 2 /day/snail ± 1.16 SE). South Caicos’ A. palmata populations are still in good condition, though increasing human disturbances combined with disease and predatory snails may threaten these populations
The Phase Transition to a Square Vortex Lattice in Type-II Superconductors with Fourfold Anisotropy
We investigate the stability of the square vortex lattice which has been
recently observed in experiments on the borocarbide family of superconductors.
Taking into account the tetragonal symmetry of these systems, we add fourfold
symmetric fourth-derivative terms to the Ginzburg-Landau(GL) free energy. At
these terms may be treated perturbatively to lowest order to locate
the transition from a distorted hexagonal to a square vortex lattice. We also
solve for this phase boundary numerically in the strongly type-II limit,
finding large corrections to the lowest-order perturbative results. We
calculate the relative fourfold anisotropy for field in the plane
to be 4.5% at the temperature, , where the transition occurs at
for field along the axis. This is to be compared to the 3.6%
obtained in the perturbative calculation. Furthermore, we find that the phase
boundary in the phase diagram has positive slope near .Comment: 15 pages including 2 figures, LaTe
Why pinning by surface irregularities can explain the peak effect in transport properties and neutron diffraction results in NbSe2 and Bi-2212 crystals?
The existence of a peak effect in transport properties (a maximum of the
critical current as function of magnetic field) is a well-known but still
intriguing feature of type II superconductors such as NbSe2 and Bi-2212. Using
a model of pinning by surface irregularities in anisotropic superconductors, we
have developed a calculation of the critical current which allows estimating
quantitatively the critical current in both the high critical current phase and
in the low critical current phase. The only adjustable parameter of this model
is the angle of the vortices at the surface. The agreement between the
measurements and the model is really very impressive. In this framework, the
anomalous dynamical properties close to the peak effect is due to co-existence
of two different vortex states with different critical currents. Recent neutron
diffraction data in NbSe2 crystals in presence of transport current support
this point of view
Elasticity-driven interaction between vortices in type-II superconductors
The contribution to the vortex lattice energy which is due to the
vortex-induced strains is calculated covering all the magnetic field range
which defines the vortex state. This contribution is compared with previously
reported ones what shows that, in the most part of the vortex state, it has
been notably underestimated until now. The reason of such underestimation is
the assumption that only the vortex cores induce strains. In contrast to what
is generally assumed, both core and non-core regions are important sources of
strains in high- superconductors.Comment: 10 pages, 1 figure, revtex
Temperature Dependence of the Flux Line Lattice Transition into Square Symmetry in Superconducting LuNiBC
We have investigated the temperature dependence of the H || c flux line
lattice structural phase transition from square to hexagonal symmetry, in the
tetragonal superconductor LuNi_2B_2C (T_c = 16.6 K). At temperatures below 10 K
the transition onset field, H_2(T), is only weakly temperature dependent. Above
10 K, H_2(T) rises sharply, bending away from the upper critical field. This
contradicts theoretical predictions of H_2(T) merging with the upper critical
field, and suggests that just below the H_c2(T)-curve the flux line lattice
might be hexagonal.Comment: 4 pages, 3 figure
Theory of superconductor with kappa close to 1/sqrt{2}
As was firstly shown by E. Bogomolny, the critical Ginzburg-Landay (GL)
parameter kappa =1/sqrt{2} at which a superconductor changes its behavior from
type-I to type-II, is the special highly degenerate point where Abrikosov
vortices do not interact and therefore all vortex states have the same energy.
Developing a secular perturbation theory we studied how this degeneracy is
lifted when kappa is slightly different from 1\sqrt{2} or when the GL theory is
extended to the higher in T-Tc terms. We constructed a simple secular
functional, that depends only on few experimentally measurable phenomenological
parameters and therefore is quite efficient to study the vortex state of
superconductor in this transitional region of kappa. Basing on this, we
calculated such vortex state properties as: critical fields, energy of the
normal-superconductor interface, energy of the vortex lattice, vortex
interaction energy etc. and compared them with previous results that were based
on bulky solutions of GL equations.Comment: Revtex, 14 pages, 4 postscript pictures embedded in the tex
Distribution of Transport Current in a Type II Superconductor Studied by Small Angle Neutron Scattering
We report Small-Angle Neutron Scattering (SANS) measurements on the vortex
lattice in a PbIn polycrystal in the presence of an applied current. Using the
rocking curves as a probe of the distribution of current in the sample, we
observe that vortex pinning is due to the surface roughness. This leads to a
surface current that persists in the flux flow region. We show the influence of
surface treatments on the distribution of this current.Comment: 5 pages 5 figures. accepted for publication in Phys Rev Let
Tunnel magnetoresistance devices processed by oxidation in air and UV assisted oxidation in oxygen
Tunnel magnetoresistance (TMR) devices were processed by sputter deposition of Co, Al and NiFe on oxidized Si wafers. After the Al deposition, an ex-situ oxidation in air at room temperature or an in-situ oxidation enhanced by ultraviolet (UV) irradiation in high purity oxygen at 100 mbar follows. The electrical and magnetic properties of the junctions are measured and discussed concerning specific junction resistance, magnetoresistance ratio, long time stability of the junctions, and failure rate of the processes. Some microscopic experiments provided consistent information of the tunnel barrier. MR ratios between 15% and 20% were measured for the different oxidation processes
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