Optical Storage in Side-Chain Liquid Crystalline Polyesters

Long-term, high resolution holographic storage has been achieved in side-chain liquid crystalline polyesters with high diffraction efficiency. The information can be completely erased by heating the film to about 80°C. The modular architecture of the polyesters permits a varition in the length of the main- and side-chains and the substituents of the azo chromophore. Each of these has a profound influence on the storage properties of the polyesters. In this article, we examine the influence of the length of the side-chains, the probe-beam-polarization dependence of the observed signal on the interface and the crystalline vs. amorphous nature of the films for optical storage.     

Superferromagnetic nanostructures

Interactions between magnetic nanoparticles can lead to superferromagnetic ordering, i.e. ordering of their magnetic moments at low temperatures. The use of a simple mean field theory, describing the temperature dependence of the order parameter is discussed. This model is found to give excellent fits to experimental results. In systems of particles with pure dipole interactions, the degree of ordering depends critically on the geometrical configuration of the particles. The application of superferromagnetic nanostructures for magnetic refrigeration is also discussed.     

Scars of discrete rotational bands in hot nuclei

Rotational motion loses its coherence as a function of the nuclear internal excitation energy. The damping process does not proceed in a continuous fashion and scars of discrete rotational bands are found, inbedded in a background of damped rotational states, regardless whether the calculations are carried out using effective or “random” forces. The complexity of the damping mechanism is revealed in the lineshape of the ridges in the γ-γ correlation spectrum.     

Dynamical event during slow crack propagation

We address the role of material heterogeneities on the propagation of a slow rupture at laboratory scale. With a high speed camera, we follow an in-plane crack front during its propagation through a transparent heterogeneous Plexiglas block. We obtain two major results. First, the slip along the interface is strongly correlated over scales much larger than the asperity sizes. Second, the dynamics is scale dependent. Locally, mechanical instabilities are triggered during asperity depinning and propagate along the front. The intermittent behavior at the asperity scale is in contrast with the large scale smooth creeping evolution of the average crack position. The dynamics is described on the basis of a Family-Vicsek scaling.     

New source of dense, cryogenic positron plasmas

We have developed a new method, based on the ballistic transfer of preaccumulated plasmas, to obtain large and dense positron plasmas in a cryogenic environment. The method involves transferring plasmas emanating from a region with a low magnetic field (0.14 T) and relatively high pressure (10(-9) mbar) into a 15 K Penning-Malmberg trap immersed in a 3 T magnetic field with a base pressure better than 10(-13) mbar. The achieved positron accumulation rate in the high field cryogenic trap is more than one and a half orders of magnitude higher than the previous most efficient UHV compatible scheme. Subsequent stacking resulted in a plasma containing more than 1.2 x 10(9) positrons, which is a factor 4 higher than previously reported. Using a rotating wall electric field, plasmas containing about 20 x 10(6) positrons were compressed to a density of 2.6 x 10(10) cm(-3). This is a factor of 6 improvement over earlier measurements.     

Evidence for the production of slow antiprotonic hydrogen in vacuum

We present evidence showing how antiprotonic hydrogen, the quasistable antiproton (p)-proton bound system, has been synthesized following the interaction of antiprotons with the molecular ion H2+ in a nested Penning trap environment. From a careful analysis of the spatial distributions of antiproton annihilation events, evidence is presented for antiprotonic hydrogen production with sub-eV kinetic energies in states around n=70, and with low angular momenta. The slow antiprotonic hydrogen may be studied using laser spectroscopic techniques.     

Intratumor heterogeneity in perfusion in human melanoma xenografts measured by contrast-enhanced magnetic resonance imaging

The perfusion in tumors shows substantial spatial heterogeneity compared to that in normal tissues. The aim of the present study was to evaluate the intratumor heterogeneity in perfusion in tumors of two amelanotic human melanoma xenograft lines, A-07 and R-18, grown intradermally in Balb/c nu/nu mice. A non-invasive contrast-enhanced magnetic resonance imaging method yielding results in absolute values was applied. The perfusion was determined in manually defined regions of interest, corresponding to a whole tumor or to subregions of a tumor. The mean perfusion and the intertumor heterogeneity in perfusion were similar for the two tumor lines. For whole A-07 tumors, the perfusion ranged from 0.089 mL/(g . min) to 0.20 mL/(g . min) [mean: 0.15 mL/(g . min)], and for whole R-18 tumors, from 0.030 mL/(g . min) to 0.17 mL/(g . min) [mean: 0.13 mL/(g . min)]. The intratumor heterogeneity, on the other hand, was estimated to be 6.4 times larger in A-07 tumors than in R-18 tumors. The highest perfusion values, up to 0.69 mL/(g . min), were found in subregions of A-07 tumors. The intratumor heterogeneity was substantially larger than the intertumor heterogeneity in A-07 tumors, whereas in R-18 tumors, the intratumor heterogeneity was similar to the intertumor heterogeneity. These observations imply that measurements of mean tumor perfusion may have limited value as a predictive assay for outcome of treatment.     

Stereocontrolled Organocatalytic Strategy for the Synthesis of Optically Active 2,3‐Disubstituted cis‐2,3‐Dihydrobenzofurans

An intramolecular, organocatalyzed Michael addition has been developed to obtain biologically important 2,3‐disubstituted cis‐2,3‐dihydrobenzofurans. By using mandelic acid salts of primary aminocatalysts, derived from cinchona alkaloids, the intramolecular cyclization reaction has been developed to proceed in high yield, with moderate to good diastereoselectivity, and up to 99 % ee. Based on the absolute configuration of the formed 2,3‐disubstituted‐cis‐2,3‐dihydrobenzofurans and by considering the observed substrate scope restrictions, a mechanistic rationalization has been presented.