1,113 research outputs found
Exciton resonances quench the photoluminescence of zigzag carbon nanotubes
We show that the photoluminescence intensity of single-walled carbon
nanotubes is much stronger in tubes with large chiral angles - armchair tubes -
because exciton resonances make the luminescence of zigzag tubes intrinsically
weak. This exciton-exciton resonance depends on the electronic structure of the
tubes and is found more often in nanotubes of the +1 family. Armchair tubes do
not necessarily grow preferentially with present growth techniques; they just
have stronger luminescence. Our analysis allows to normalize photoluminescence
intensities and find the abundance of nanotube chiralities in macroscopic
samples.Comment: 4 pages and 2 supplementary pages; 6 figure
Porosity Results for Sets of Strict Contractions on Geodesic Metric Spaces
We consider a large class of geodesic metric spaces, including Banach spaces,
hyperbolic spaces and geodesic -spaces, and investigate
the space of nonexpansive mappings on either a convex or a star-shaped subset
in these settings. We prove that the strict contractions form a negligible
subset of this space in the sense that they form a -porous subset. For
separable metric spaces we show that a generic nonexpansive mapping has
Lipschitz constant one at typical points of its domain. These results contain
the case of nonexpansive self-mappings and the case of nonexpansive set-valued
mappings as particular cases.Comment: 35 pages; accepted version of the manuscript; accepted for
publication in Topological Methods in Nonlinear Analysi
Optimization of femtosecond laser plasma KĪ± sources
Since the night when Conrad RĆontgen ĀÆrst saw his hand's bones on a Ā°uorescence screen [71] x rays have been one of the most important tools for revealing the inner structure of matter. Beside x-ray imaging for medical diagnosis, the study of x-ray diĀ®raction patterns in chemistry and physics has become a major application of x rays, because it allows the structure of crystalline solids with atomic spatial resolution to be analyzed. Processes which are determined by atomic motion, like chemical reactions and phase transitions, happen on a timescale comparable to the vibrational period of the atoms, which is typically 10Ā”13 s or 100 fs [76]. The ideal tool to investigate such a process would simultaneously oĀ®er a temporal resolution smaller than this time scale and the ability to resolve the interatomic distances. Until recently, no such tool existed and the investigator had to choose between the femtosecond time resolution of optical spectroscopy with femtosecond lasers and the spatial resolving power of x-ray studies. The experimental method which came closest to the `ideal tool' was time-resolved x-ray diĀ®raction using a synchrotron radiation source. It was especially used to study biologically important processes like the photoinduced trans-cis isomerization of a part of the protein rhodopsin which forms the light detecting process in the human eye [64] or the binding of oxygen and carbon monoxide to myoglobin [94]. These experiments had a nanosecond time resolution which proved to be accurate for the processes under investigation, but all-optical experiments indicate that the initial structural changes, which precede the studied ones, happen much faster, within several ten or hundred femtoseconds [34, 35]. Moreover, optical measurements have revealed that femtosecond time scales are also relevant to processes in solid state physics, like the atomic rearrangement during transitions from one solid phase to another [84] and the disordering of semiconductors after strong electronic excitation by a laser pulse [85]. State-of-the-art synchrotron/streak camera setups achieve a time-resolution of about 1 ps [52].
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Thermal expansion behaviour of unidirectionally SiC fibre-reinforced MAS and BMAS glass-ceramics
The thermal expansion behaviour of SiC fibre-reinforced Magnesium-Aluminium-Silicate (MAS) and Barium-Magnesium- Aluminium-Silicate (BMAS) glass-ceramics was studied in air as well as in argon furnace atmosphere. The results show that the coefficients of thermal expansion up to a temperature of 800 C are influenced to a minor degree by the furnace atmosphere and to a major degree by the phase composition of the matrices and the fibre concentration. The largest values in air and argon atmosphere are obtained from BMAS(14)/SiC fibre composites with Ī±āā
ā/āāā = 3.92 * 10ā»ā¶ Kā»Ā¹ and 3.87 * 10ā»ā¶ Kā»Ā¹ respectively, at a fibre content of 52 vol.%, while the lowest values are found for BMAS(9)/SiC fibre composites in air with Ī±āā
ā/āāā = 3.24 * 10ā»ā¶ Kā»Ā¹ and in argon with Ī±āā
ā/āāā = 3.11 * 10ā»ā¶ Kā»Ā¹ at a fibre content of about 25 vol.%. Increasing fibre concentration leads to increasing thermal expansion. The experimentally determined values were compared with calculated ones by the mixing rule and a good agreement was found, particularly for the BMAS(14) composites
Glowing Glass
Defined by the building law in each publicly accessible building (e.g. schools, administration etc.) emergency exit routes have to be marked usually by means of active or passive lightening systems. The use of passive lightening systems require comprehensive components with an independent, battery-powered energy-supply that produces light even in the case of an energy black-out (e.g. disaster situations). The use of powerlines plus the frequently service of battery-powered systems is complicated and expansive. Alternatively after-glowing, phosphorescent signs, attached on walls, wallpapers or doors are an existing alternative. Mostly known to everybody are the green emergency exit signs. Furthermore phosphorescent paints on floors or walls are also used to guide people on the quickest escape way. Used inside of buildings their appearance has mostly a disturbing and negative attitude, even more at premium interior designs. Therefore, the composition of passive lightened systems with premium-quality surfaces leads to a widely usable product phosphorescent glass. This glass consists of laminated glass with a phosphorescent paint application within the glass interlayer. The paper describes the research and development of phosphorescent glass with a strong emphasis on materials testing, application technics and the behavior as laminated safety glass
Generic properties of nonexpansive mappings on unbounded domains
We investigate typical properties of nonexpansive mappings on unbounded,
closed and convex subsets of hyperbolic metric spaces. For a metric of uniform
convergence on bounded sets, we show that the typical nonexpansive mapping is a
contraction in the sense of Rakotch on every bounded subset and there is a
bounded set which is mapped into itself by this mapping. In particular, we
obtain that the typical nonexpansive mapping in this setting has a unique fixed
point. Nevertheless, it turns out that the typical mapping is not a Rakotch
contraction on the whole space and that it has the maximal possible Lipschitz
constant of one on a residual subset of its domain. By typical we mean that the
complement of the set of mappings with this property is -porous, that
is, small in a topological sense. For a metric of pointwise convergence, we
show that the set of Rakotch contractions is meagre.Comment: 21 page
The toxicity of cadmium and resulting hazards for human health
Cadmium (Cd) has been in industrial use for a long period of time. Its serious toxicity moved into scientific focus during the middle of the last century. In this review, we discuss historic and recent developments of toxicological and epidemiological questions, including exposition sources, resorption pathways and organ damage processes
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