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 $\mathrm{CAT}(\kappa)$-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 $\sigma$-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 Kα sources

Since the night when Conrad RÄontgen ¯rst saw his hand's bones on a °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 di®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 10¡13 s or 100 fs [76]. The ideal tool to investigate such a process would simultaneously o®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 di®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]. LO

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 α₁₅₀/₈₀₀ = 3.92 * 10⁻⁶ K⁻¹ and 3.87 * 10⁻⁶ K⁻¹ respectively, at a fibre content of 52 vol.%, while the lowest values are found for BMAS(9)/SiC fibre composites in air with α₁₅₀/₈₀₀ = 3.24 * 10⁻⁶ K⁻¹ and in argon with α₁₅₀/₈₀₀ = 3.11 * 10⁻⁶ K⁻¹ 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 $\sigma$-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