6,423 research outputs found
Surface optical vortices
It is shown how the total internal reflection of orbital-angular-momentum-endowed light can lead to the generation of evanescent light possessing rotational properties in which the intensity distribution is firmly localized in the vicinity of the surface. The characteristics of these surface optical vortices depend on the form of the incident light and on the dielectric mismatch of the two media. The interference of surface optical vortices is shown to give rise to interesting phenomena, including pattern rotation akin to a surface optical Ferris wheel. Applications are envisaged to be in atom lithography, optical surface tweezers, and spanners
On the conveyance of angular momentum in electronic energy transfer
When electronic excitation transfer occurs, it is of considerable interest to establish whether angular momentum can also be conveyed in the process. The question is prompted by a consideration that when the participating chromophores are atoms, ions, or molecular systems having high local symmetry, the electronic excited states that are involved are generally characterized not only by energy, but by angular momentum properties. Moreover, it is known that electron spin can be communicated between quantum dot exciton states. Resolving the general issue entails an electrodynamic representation exploiting irreducible tensor methods, the analysis being illustrated by application to energy transfer associated with a variety of multipolar transitions. The results exhibit novel connections between an angular momentum content of the electromagnetic coupling and a strongly varying distance dependence. It is concluded that the communication of angular momentum does not in general map unambiguously between a donor and energy acceptor
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Fungal community assembly in soils and roots under plant invasion and nitrogen deposition
Quantised orbital angular momentum transfer and magnetic dichroism in the interaction of electron vortices with matter
Following the very recent experimental realisation of electron vortices, we
consider their interaction with matter, in particular the transfer of orbital
angular momentum in the context of electron energy loss spectroscopy, and the
recently observed dichroism in thin film magnetised iron samples. We show here
that orbital angular momentum exchange does indeed occur between electron
vortices and the internal electronic-type motion, as well as center of mass
motion of atoms in the electric dipole approximation. This contrasts with the
case of optical vortices where such transfer only occurs in transitions
involving multipoles higher than the dipole. The physical basis of the observed
dichroism is explained
Earthquake Prediction and Hazards Evaluation in the Year 2000 -- A Dialogue
Decisionmakers have different perspectives about geologic hazards than
scientists and engineers.
These differences, which have been summarized by
Szanton (1981, table 3-1), are the reasons that implementation of loss reduction
measures are difficult. The differences are:
The ultimate objective of the decisionmaker is the approval of the
electorate; it is the respect of peers for the scientist/engineer;
The time horizon for the decisionmaker is short; it is long for the
scientist/engineer;
The focus of the decisionmaker is on the external logic of the problem;
it is on the internal logic for the scientist/engineer;
The mode of thought for the decisionmaker is deductive and particular;
it is inductive and generic for the scientist/engineer;
The most valued outcome for the decisionmaker is a reliable solution; it
is original insight for the scientist/engineer;
The mode of expression is simple and absolute for the decisionmaker; it
is abstruse and qualified for the scientist/engineer, and;
The preferred form of conclusion for the decisionmaker is one of "best
solution" with uncertainties submerged; it is multiple possibilities with
uncertainties emphasized for the scientist/engineer.
With these principles in mind, let us now turn the clock forward to the year 2000
and a discussion between a decisionmaker and a scientist as they seek to resolve
their philosophical differences and reach solutions to problems of earthquake-hazards
reduction
Bomb radiocarbon and tag-recapture dating of sandbar shark (Carcharhinus plumbeus)
The sandbar shark (Carcharhinus plumbeus) was the cornerstone species of western North Atlantic and Gulf of Mexico large coastal shark fisheries until 2008 when they
were allocated to a research-only fishery. Despite decades of fishing on this species, important life history
parameters, such as age and growth, have not been well known. Some validated age and growth information exists for sandbar shark, but more comprehensive life history information is needed. The complementary application of bomb radiocarbon and tag-recapture dating was used in this
study to determine valid age-estimation criteria and longevity estimates for this species. These two methods
indicated that current age interpretations based on counts of growth bands in vertebrae are accurate to 10 or 12 years. Beyond these years, we could not determine with certainty when such an underestimation of age begins; however, bomb radiocarbon and tag-recapture data indicated that large adult sharks were considerably older than the estimates derived from counts of growth bands. Three adult sandbar sharks were 20 to 26 years old based on bomb radiocarbon results and were a 5- to 11-year increase over the previous age estimates for these sharks. In support of
these findings, the tag-recapture data provided results that were consistent with bomb radiocarbon dating and
further supported a longevity that exceeds 30 years for this species
The Circumstellar Disk Mass Distribution in the Orion Trapezium Cluster
We present the results of a submillimeter interferometric survey of
circumstellar disks in the Trapezium Cluster of Orion. We observed the 880
micron continuum emission from 55 disks using the Submillimeter Array, and
detected 28 disks above 3sigma significance with fluxes between 6-70 mJy and
rms noise between 0.7-5.3 mJy. Dust masses and upper limits are derived from
the submillimeter excess above free-free emission extrapolated from longer
wavelength observations. Above our completeness limit of 0.0084 solar masses,
the disk mass distribution is similar to that of Class II disks in
Taurus-Auriga and rho Ophiuchus but is truncated at 0.04 solar masses. We show
that the disk mass and radius distributions are consistent with the formation
of the Trapezium Cluster disks ~1 Myr ago and subsequent photoevaporation by
the ultraviolet radiation field from Theta-1 Ori C. The fraction of disks which
contain a minimum mass solar nebula within 60 AU radius is estimated to be
11-13% in both Taurus and the Trapezium Cluster, which suggests the potential
for forming Solar Systems is not compromised in this massive star forming
region.Comment: Accepted for publication in ApJL (2009 Feb 3
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