N,N'-dimethylperylene-3,4,9,10-bis(dicarboximide) on alkali halide(001) surfaces

The growth of N,N'-dimethylperylene-3,4,9,10-bis(dicarboximide) (DiMe-PTCDI) on KBr(001) and NaCl(001) surfaces has been studied. Experimental results have been achieved using frequency modulation atomic force microscopy at room temperature under ultra-high vacuum conditions. On both substrates, DiMe-PTCDI forms molecular wires with a width of 10 nm, typically, and a length of up to 600 nm at low coverages. All wires grow along the [110] direction (or [1$\bar{1}$0] direction, respectively) of the alkali halide (001) substrates. There is no wetting layer of molecules: Atomic resolution of the substrates can be achieved between the wires. The wires are mobile on KBr surface but substantially more stable on NaCl. A p(2 x 2) superstructure in brickwall arrangement on the ionic crystal surfaces is proposed based on electrostatic considerations. Calculations and Monte-Carlo simulations using empirical potentials reveal possible growth mechanisms for molecules within the first layer for both substrates, also showing a significantly higher binding energy for NaCl(001). For KBr, the p(2 x 2) superstructure is confirmed by the simulations, for NaCl, a less dense, incommensurate superstructure is predicted.Comment: 5 pages, 5 figure

The ionization rates of galactic nuclei and disks from Herschel/HIFI observations of water and its associated ions

(Abridged) We present Herschel/HIFI spectra of the H2O 1113 GHz and H2O+ 1115 GHz lines toward five nearby prototypical starburst/AGN systems, and OH+ 971 GHz spectra toward three of these. The beam size of 20" corresponds to resolutions between 0.35 and 7 kpc. The observed line profiles range from pure absorption (NGC 4945, M82) to P-Cygni indicating outflow (NGC 253, Arp 220) and inverse P-Cygni indicating infall (Cen A). The similarity of the H2O, OH+, and H2O+ profiles to each other and to HI indicates that diffuse and dense gas phases are well mixed. We estimate column densities assuming negligible excitation (for absorption features) and using a non-LTE model (for emission features), adopting calculated collision data for H2O and OH+, and rough estimates for H2O+. Column densities range from ~10^13 to ~10^15 cm^-2 for each species, and are similar between absorption and emission components, indicating that the nuclear region does not contribute much to the emission in these ground-state lines. The N(H2O)/N(H2O+) ratios of 1.4-5.6 indicate an origin of the lines in diffuse gas, and the N(OH+)/N(H2O+) ratios of 1.6-3.1 indicate a low H2 fraction (~11%) in the gas. Adopting recent Galactic values for the average gas density and the ionization efficiency, we find ionization rates for our sample galaxies of ~3x10^-16 s^-1 which are similar to the value for the Galactic disk, but ~10x below that of the Galactic Center and ~100x below estimates for AGN from excited-state H3O+ lines. We conclude that the ground-state lines of water and its associated ions probe primarily non-nuclear gas in the disks of these centrally active galaxies. Our data thus provide evidence for a decrease in ionization rate by a factor of ~10 from the nuclei to the disks of galaxies, as found before for the Milky Way.Comment: Accepted for publication in Astronomy & Astrophysics; 8 pages, 8 figure

The Hofstadter Energy Spectrum for an Interacting 2DEG

We study the effects of the Coulomb interactions between electrons on the Hofstadter butterfly, which characterizes the subband structure of the Landau levels of a two-dimensional electron gas in a perpendicular homogeneous magnetic field and a periodic lateral superlattice potential. The interactions essentially preserve the intricate gap structure of the Hofstadter spectra, but with a lower symmetry that depends on the filling of the Landau bands. For short enough periods and strong enough modulation the miniband structure can be resolved in the thermodynamic density of states.Comment: LaTeX 4 pages with 3 PostScript figures, Contribution to EP2DSXI Nottingham August 95 to appear in Surface Scienc

Submillimeter Polarimetry with PolKa, a reflection-type modulator for the APEX telescope

Imaging polarimetry is an important tool for the study of cosmic magnetic fields. In our Galaxy, polarization levels of a few up to $\sim$10\% are measured in the submillimeter dust emission from molecular clouds and in the synchrotron emission from supernova remnants. Only few techniques exist to image the distribution of polarization angles, as a means of tracing the plane-of-sky projection of the magnetic field orientation. At submillimeter wavelengths, polarization is either measured as the differential total power of polarization-sensitive bolometer elements, or by modulating the polarization of the signal. Bolometer arrays such as LABOCA at the APEX telescope are used to observe the continuum emission from fields as large as \sim0\fdg2 in diameter. %Here we present the results from the commissioning of PolKa, a polarimeter for Here we present PolKa, a polarimeter for LABOCA with a reflection-type waveplate of at least 90\% efficiency. The modulation efficiency depends mainly on the sampling and on the angular velocity of the waveplate. For the data analysis the concept of generalized synchronous demodulation is introduced. The instrumental polarization towards a point source is at the level of $\sim0.1$\%, increasing to a few percent at the $-10$db contour of the main beam. A method to correct for its effect in observations of extended sources is presented. Our map of the polarized synchrotron emission from the Crab nebula is in agreement with structures observed at radio and optical wavelengths. The linear polarization measured in OMC1 agrees with results from previous studies, while the high sensitivity of LABOCA enables us to also map the polarized emission of the Orion Bar, a prototypical photon-dominated region

Wachstum im asiatisch-pazifischen Raum : Implikationen für die internationale Arbeitsteilung.

Wirtschaftswachstum; Internationale Arbeitsteilung; Internationaler Wettbewerb; Asiatisch-pazifischer Raum;

Physical determinants of asymmetric cell divisions in the early development of Caenorhabditis elegans

Abstract Asymmetric cell divisions are of fundamental importance for the development of multicellular organisms, e.g. for the generation of founder cells. Prime examples are asymmetric cell divisions in germline precursors during the early embryogenesis of the transparent roundworm Caenorhabditis elegans, one of the major developmental model organisms. However, due to a lack of quantitative data it has remained unclear how frequent unequal daughter cell sizes emerge in the worm’s early embryogenesis, and whether these originate from sterical or biochemical cues. Using quantitative light-sheet microscopy, we have found that about 40% of all cell divisions in C. elegans until gastrulation generate daughter cells with significantly different volumes. Removing the embryo’s rigid eggshell revealed asymmetric divisions in somatic cells to be primarily induced by steric effects. Division asymmetries in the germline remained unaltered and were correctly reproduced by a model based on a cell-size independent, eccentric displacement of the metaphase plate. Our data suggest that asymmetric cell divisions, imposed by physical determinants, are essential for establishing important cell-cell interactions that eventually fuel a successful embryogenesis

Guiding center picture of magnetoresistance oscillations in rectangular superlattices

We calculate the magneto-resistivities of a two-dimensional electron gas subjected to a lateral superlattice (LSL) of rectangular symmetry within the guiding-center picture, which approximates the classical electron motion as a rapid cyclotron motion around a slowly drifting guiding center. We explicitly evaluate the velocity auto-correlation function along the trajectories of the guiding centers, which are equipotentials of a magnetic-field dependent effective LSL potential. The existence of closed equipotentials may lead to a suppression of the commensurability oscillations, if the mean free path and the LSL modulation potential are large enough. We present numerical and analytical results for this suppression, which allow, in contrast to previous quantum arguments, a classical explanation of similar suppression effects observed experimentally on square-symmetric LSL. Furthermore, for rectangular LSLs of lower symmetry they lead us to predict a strongly anisotropic resistance tensor, with high- and low-resistance directions which can be interchanged by tuning the externally applied magnetic field.Comment: 12 pages, 9 figure

Novel Technique of Craniospinal Axis Proton Therapy with the Spot-Scanning System: Avoidance of Patching Multiple Fields and Optimized Ventral Dose Distribution

Background and Purpose:: Conventional craniospinal irradiation (CSI) is a complex procedure carrying a high risk of adverse side effects. Still, it is indispensable for cure in a number of pediatric brain tumors. In this study, the feasibility and the potential advantage of spot-scanning proton therapy for CSI are investigated. Material and Methods:: A boy (5.5 years of age) with a recurrent medulloblastoma received CSI with a single posterior field using the spot-scanning system at Paul Scherrer Institute. Dose distribution to the targets and the organs at risk, treatment time, reproducibility of patient positioning, toxicity (according to EORTC/RTOG score), and treatment outcome were evaluated. Results:: The plan achieved a homogeneous coverage of the target volume, even using a single field. The doses to the organs ventral to the target were minimized. During treatment, grade 1 skin reaction and grade 2 central nervous system toxicity were observed. After 2 months, the boy presented with a transitory fatigue. After 24 months, he is alive and free of disease. Growth hormones and thyroid hormones are reduced. Conclusion:: These results, based on a single patient, suggest that spot-scanning proton therapy for craniospinal treatment is feasible and safe. By applying a single dorsal field, difficulties of multiple-field patching can be avoided and the ventral dose spread can be minimize