1,148 research outputs found
A stratified transect approach captures reef complexity with canopy-forming organisms
On the Great Barrier Reef (GBR), persistent changes to reef communities have begun to be documented, and on inshore reefs these shifts may favour the proliferation of macroalgae. Critical to understanding changes to reef community structure in response to anthropogenic impacts is developing effective methods to accurately document the abundance of different reef organisms. Effective monitoring must be time and cost efficient, replicable, and able to sufficiently and accurately detect disturbances to allow development of strategies to mitigate their impacts. Traditional techniques to document coral reef communities (i.e. photo-quadrats, benthic intercept transects) rely on planar views, which tend to either over- or under-represent canopy-forming organisms. As canopy-forming organisms are likely to be affected by anthropogenic influences (corals negatively, algae positively), it is essential for monitoring programs to implement methods sufficient to document changes to the vertical dimension of coral reefs. Here we build on previous work to document the canopy effect in coral-dominated ecosystems and propose a new survey approach suitable for implementation in algal-dominated systems. A vertically stratified transect, modified from a traditional point intercept transect, captures benthic and canopy-forming members of reef communities and provides information on three-dimensional complexity. To test the capability of the new method to detect changes in vertical reef structure, seaweed was removed from experimental quadrats and monitoring techniques were applied before and after four months of regrowth. A stratified method more accurately captured the three-dimensional change resulting from algal canopy growth, while resolving the over- and under-representation of algal biomass in two traditional techniques. We propose that a stratified transect method improves abundance estimates of canopy-forming organisms whilst maintaining data compatibility with traditional methods
Modular termination verification for non-blocking concurrency
© Springer-Verlag Berlin Heidelberg 2016.We present Total-TaDA, a program logic for verifying the total correctness of concurrent programs: that such programs both terminate and produce the correct result. With Total-TaDA, we can specify constraints on a thread’s concurrent environment that are necessary to guarantee termination. This allows us to verify total correctness for nonblocking algorithms, e.g. a counter and a stack. Our specifications can express lock- and wait-freedom. More generally, they can express that one operation cannot impede the progress of another, a new non-blocking property we call non-impedance. Moreover, our approach is modular. We can verify the operations of a module independently, and build up modules on top of each other
Ultrathin Metallic Coatings Can Induce Quantum Levitation between Nanosurfaces
There is an attractive Casimir-Lifshitz force between two silica surfaces in
a liquid (bromobenze or toluene). We demonstrate that adding an ultrathin
(5-50{\AA}) metallic nanocoating to one of the surfaces results in repulsive
Casimir-Lifshitz forces above a critical separation. The onset of such quantum
levitation comes at decreasing separations as the film thickness decreases.
Remarkably the effect of retardation can turn attraction into repulsion. From
that we explain how an ultrathin metallic coating may prevent
nanoelectromechanical systems from crashing together.Comment: 4 pages, 5 figure
Duplex Scanning as the Sole Preoperative Imaging Method for Infrainguinal Arterial Surgery
AbstractObjective: to evaluate preoperative duplex as the sole investigation prior to lower limb reconstruction. Design retrospective analysis. Material and methods: between January 1995 and December 1999, 157 of 329 surgical interventions for chronic infrainguinal arterial or aneurysmal disease were performed without preoperative angiography.Results : in patients undergoing femoral artery endarterectomy, the extent of the stenosis and the status of the distal deep femoral artery were correctly diagnosed with duplex scanning in all but one patient. Duplex scan findings in patients undergoing infrainguinal bypass procedures were in agreement with the findings obtained from on-table angiography in regard to the selection of optimal outflow anastomotic sites in 123 (98%). Duplex scanning correctly evaluated the status of runoff in 113 (90%). There were no significant differences in 30-day occlusion rate and patency at 12 months between reconstructions performed with and without preoperative angiography. Conclusion: in patients with conclusive duplex scan findings there is no need to perform angiography prior to lower limb reconstruction
Consistency of shared reference frames should be reexamined
In a recent Letter [G. Chiribella et al., Phys. Rev. Lett. 98, 120501
(2007)], four protocols were proposed to secretly transmit a reference frame.
Here We point out that in these protocols an eavesdropper can change the
transmitted reference frame without being detected, which means the consistency
of the shared reference frames should be reexamined. The way to check the above
consistency is discussed. It is shown that this problem is quite different from
that in previous protocols of quantum cryptography.Comment: 3 pages, 1 figure, comments are welcom
Fisher Information for Inverse Problems and Trace Class Operators
This paper provides a mathematical framework for Fisher information analysis
for inverse problems based on Gaussian noise on infinite-dimensional Hilbert
space. The covariance operator for the Gaussian noise is assumed to be trace
class, and the Jacobian of the forward operator Hilbert-Schmidt. We show that
the appropriate space for defining the Fisher information is given by the
Cameron-Martin space. This is mainly because the range space of the covariance
operator always is strictly smaller than the Hilbert space. For the Fisher
information to be well-defined, it is furthermore required that the range space
of the Jacobian is contained in the Cameron-Martin space. In order for this
condition to hold and for the Fisher information to be trace class, a
sufficient condition is formulated based on the singular values of the Jacobian
as well as of the eigenvalues of the covariance operator, together with some
regularity assumptions regarding their relative rate of convergence. An
explicit example is given regarding an electromagnetic inverse source problem
with "external" spherically isotropic noise, as well as "internal" additive
uncorrelated noise.Comment: Submitted to Journal of Mathematical Physic
Sign of the Casimir-Polder interaction between atoms and oil-water interfaces: Subtle dependence on dielectric properties
We demonstrate that Casimir-Polder energies between noble gas atoms
(dissolved in water) and oil-water interfaces are highly surface specific. Both
repulsion (e.g. hexane) and attraction (e.g. glycerine and cyclodecane) is
found with different oils. For several intermediate oils (e.g. hexadecane,
decane, and cyclohexane) both attraction and repulsion can be found in the same
system. Near these oil-water interfaces the interaction is repulsive in the
non-retarded limit and turns attractive at larger distances as retardation
becomes important. These highly surface specific interactions may have a role
to play in biological systems where the surface may be more or less accessible
to dissolved atoms.Comment: 5 pages, 6 figure
2-D Elastodynamic Scattering from a Finite Closed Crack
In the present paper the problem of 2-D elastodynamic scattering of horizontally polarized transverse waves from a finite planar or nonplanar closed crack is studied. The boundary conditions on the flaw are of a type which incorporate restoring forces (as well as energy dissipation), and this enables the modelling of a crack which is partly closed under a static background pressure. Given an incident plane wave and the crack geometry we calculate the backscattered far field in the time-harmonic case. In this study there are also a numerical comparison between two well known theoretical methods for 2-D scattering of ultrasonic sound by flaws in elastic solids. The methods are the GTD (Geometrical Theory of Diffraction) method that gives an asymptotic solution for high frequencies and the nullfield approach that yields an “exact” numerical solution. The boundary conditions for the partly closed crack are proposed by Boström and Wickham [1]. For a thorough description of the details of the GTD method as applied to scattering problems in elastodynamics, the reader is referred to the book by Achenbach, Gautesen, and McMaken [2]. The nullfield approach has previously been used for treating 3-D planar and nonplanar cracks with similar boundary conditions [3], the same ideas will here be used in the treatment of the 2-D case. The backscattered far field amplitude is numerically calculated and compared between the two methods
Thermodynamical aspects of the Casimir force between real metals at nonzero temperature
We investigate the thermodynamical aspects of the Casimir effect in the case
of plane parallel plates made of real metals. The thermal corrections to the
Casimir force between real metals were recently computed by several authors
using different approaches based on the Lifshitz formula with diverse results.
Both the Drude and plasma models were used to describe a real metal. We
calculate the entropy density of photons between metallic plates as a function
of the surface separation and temperature. Some of these approaches are
demonstrated to lead to negative values of entropy and to nonzero entropy at
zero temperature depending on the parameters of the system. The conclusion is
that these approaches are in contradiction with the third law of thermodynamics
and must be rejected. It is shown that the plasma dielectric function in
combination with the unmodified Lifshitz formula is in perfect agreement with
the general principles of thermodynamics. As to the Drude dielectric function,
the modification of the zero-frequency term of the Lifshitz formula is outlined
that not to violate the laws of thermodynamics.Comment: 8pages, 4 figures; Phys. Rev. A, to appea
Thermal correction to the Casimir force, radiative heat transfer, and an experiment
The low-temperature asymptotic expressions for the Casimir interaction
between two real metals described by Leontovich surface impedance are obtained
in the framework of thermal quantum field theory. It is shown that the Casimir
entropy computed using the impedance of infrared optics vanishes in the limit
of zero temperature. By contrast, the Casimir entropy computed using the
impedance of the Drude model attains at zero temperature a positive value which
depends on the parameters of a system, i.e., the Nernst heat theorem is
violated. Thus, the impedance of infrared optics withstands the thermodynamic
test, whereas the impedance of the Drude model does not. We also perform a
phenomenological analysis of the thermal Casimir force and of the radiative
heat transfer through a vacuum gap between real metal plates. The
characterization of a metal by means of the Leontovich impedance of the Drude
model is shown to be inconsistent with experiment at separations of a few
hundred nanometers. A modification of the impedance of infrared optics is
suggested taking into account relaxation processes. The power of radiative heat
transfer predicted from this impedance is several times less than previous
predictions due to different contributions from the transverse electric
evanescent waves. The physical meaning of low frequencies in the Lifshitz
formula is discussed. It is concluded that new measurements of radiative heat
transfer are required to find out the adequate description of a metal in the
theory of electromagnetic fluctuations.Comment: 19 pages, 4 figures. svjour.cls is used, to appear in Eur. Phys. J.
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