Can we do better than Hybrid Monte Carlo in Lattice QCD?

The Hybrid Monte Carlo algorithm for the simulation of QCD with dynamical staggered fermions is compared with Kramers equation algorithm. We find substantially different autocorrelation times for local and nonlocal observables. The calculations have been performed on the parallel computer CRAY T3D.Comment: Talk presented at LATTICE96(algorithms), LaTeX 3 pages, uses espcrc2, epsf, 2 postscript figure

What is the Entanglement Length in a Polymer Melt ?

We present results of molecular dynamics simulations of very long model polymer chains analyzed by various experimentally relevant techniques. The segment motion of the chains is found to be in very good agreement with the repatation model. We also calculated the plateau-modulus G_N. The predicitions of the entanglement length N_e from G_N and from the mean square displacements of the chains segments disagree by a factor of about 2.2(2), indicating an error in the prefactor in the standard formula for G_N. We show that recent neutron spin echo measurements were carried out for chain lengths which are too small for a correct determination of N_e.Comment: 5 pages, 4 figures, RevTe

The role of the organic layer for phosphorus nutrition of young beech trees (Fagus sylvatica L.) revealed by multi-isotopic labelling (P-33; H2O-18) at two sites differing in soil phosphorus availability

The accumulation of an organic layer in forests is linked to the ratio between litterfall and decomposition rates, the latter being decelerated due to acidification and associated nutrient depletion with proceeding ecosystem development. Nevertheless, the nutrient pool in the organic layer might still represent an important interim storage and source for phosphorus (P) nutrition of forests on nutrient-poor soils. Due to the retention of P in soil e.g., by sorption to sesquioxides, P-poor ecosystems tend to show P recycling by organic matter decomposition. Our objective was to assess the importance of the organic layer to P nutrition of young beech trees. We established a mesocosm experiment including plants and soil from two forest sites differing in P availability. In half of all pots comprising both sites, the organic layer was present while the organic layer was lacking in the other half. We applied P-33 and H2O-18 to the pots. After 0h, 24h, 48h, 96h and 192h we destructively harvested the young beech trees, sampled the organic layer and mineral soil. P-33 activity was measured for every compartment in soil and plant (xylem, leaves, branches, stems) whereas δO-18 values in phosphate(δO-18P) were assessed for soil only. For both sites, δO-18P values in resin-extractable P in soil were close to those expected if isotope fractionation during intracellular pyrophosphate storage and subsequent release takes place. Therefore, δO-18P values indicate that bioavailable P in both soils has been cycled through microorganisms. However, the absence of an organic layer at the P-poor site resulted in a considerable shift of δO-18P values from those to be expected if P has been cycled through microorganisms. For both sites, the presence of the organic layer increased P-33 activity in xylem sap compared to the treatment without (104% P-poor site, 700% P-rich site). The total P-33 activity in plant tissue in pots from the P-rich site was not affected by the presence or absence of an organic layer after 192h, whereas a strong increase of 155 kBq/g DM was recorded for the P-poor site if an organic layer was present. Therefore, the key role of the organic layer for plant P nutrition on a P-depleted site was highlighted by our multi-isotopic labelling approach. In conclusion, our results suggest that P mobilization strategies differ among sites i.e., a P recycling vs. a P acquiring strategy

The fine structure line deficit in S 140

We try to understand the gas heating and cooling in the S 140 star forming region by spatially and spectrally resolving the distribution of the main cooling lines with GREAT/SOFIA. We mapped the fine structure lines of [OI] (63 {\mu}m) and [CII] (158 {\mu}m) and the rotational transitions of CO 13-12 and 16-15 with GREAT/SOFIA and analyzed the spatial and velocity structure to assign the emission to individual heating sources. We measure the optical depth of the [CII] line and perform radiative transfer computations for all observed transitions. By comparing the line intensities with the far-infrared continuum we can assess the total cooling budget and measure the gas heating efficiency. The main emission of fine structure lines in S 140 stems from a 8.3'' region close to the infrared source IRS 2 that is not prominent at any other wavelength. It can be explained by a photon-dominated region (PDR) structure around the embedded cluster if we assume that the [OI] line intensity is reduced by a factor seven due to self-absorption. The external cloud interface forms a second PDR at an inclination of 80-85 degrees illuminated by an UV field of 60 times the standard interstellar radiation field. The main radiation source in the cloud, IRS 1, is not prominent at all in the fine structure lines. We measure line-to-continuum cooling ratios below 10^(-4), i.e. values lower than in any other Galactic source, rather matching the far-IR line deficit seen in ULIRGs. In particular the low intensity of the [CII] line can only be modeled by an extreme excitation gradient in the gas around IRS 1. We found no explanation why IRS 1 shows no associated fine-structure line peak, while IRS 2 does. The inner part of S 140 mimics the far-IR line deficit in ULIRGs thereby providing a template that may lead to a future model.Comment: Accepted for publication in Astronomy & Astrophysic

GREAT: the SOFIA high-frequency heterodyne instrument

We describe the design and construction of GREAT, the German REceiver for Astronomy at Terahertz frequencies operated on the Stratospheric Observatory for Infrared Astronomy (SOFIA). GREAT is a modular dual-color heterodyne instrument for highresolution far-infrared (FIR) spectroscopy. Selected for SOFIA's Early Science demonstration, the instrument has successfully performed three Short and more than a dozen Basic Science flights since first light was recorded on its April 1, 2011 commissioning flight. We report on the in-flight performance and operation of the receiver that - in various flight configurations, with three different detector channels - observed in several science-defined frequency windows between 1.25 and 2.5 THz. The receiver optics was verified to be diffraction-limited as designed, with nominal efficiencies; receiver sensitivities are state-of-the-art, with excellent system stability. The modular design allows for the continuous integration of latest technologies; we briefly discuss additional channels under development and ongoing improvements for Cycle 1 observations. GREAT is a principal investigator instrument, developed by a consortium of four German research institutes, available to the SOFIA users on a collaborative basis

Prüfung von Perkolations- und Elutionsversuchen zur Schadstofffreisetzung aus mineralischen Ersatzbaustoffen

In einem groß angelegten Ringversuch wurden die DIN-Normen DIN 19528 und DIN 19529 verifiziert. In dieser Arbeit haben wir die darin vorgesehenen Experimente durchgeführt, das Versuchsprogramm erweitert und die Ergebnisse mittels verschiedener mathematischer Verfahren ausgewertet. Schüttelversuche, wie in DIN 19529 vorgesehen, haben wir durch Ansätze mit weiteren Wasser-/Feststoffverhältnissen erweitert. Diese Ergebnisse wurden zur Ermittlung der das Verteilungsverhalten bestimmenden Parameter mittels MuBaX (Iden und Durner, 2007) ausgewertet. Die Säulenversuche nach DIN 19528 wurden ebenfalls in ihrer zeitlichen Dauer erweitert und durch Fließunterbrechungen ergänzt. Für die Abschätzung der kurz- bis mittelfristigen Freisetzungen wurden reproduzierbare und zwischen den unterschiedlichen Methoden vergleichbare Ergebnisse erzielt

Self-similar chain conformations in polymer gels

We use molecular dynamics simulations to study the swelling of randomly end-cross-linked polymer networks in good solvent conditions. We find that the equilibrium degree of swelling saturates at Q_eq = N_e**(3/5) for mean strand lengths N_s exceeding the melt entanglement length N_e. The internal structure of the network strands in the swollen state is characterized by a new exponent nu=0.72. Our findings are in contradiction to de Gennes' c*-theorem, which predicts Q_eq proportional N_s**(4/5) and nu=0.588. We present a simple Flory argument for a self-similar structure of mutually interpenetrating network strands, which yields nu=7/10 and otherwise recovers the classical Flory-Rehner theory. In particular, Q_eq = N_e**(3/5), if N_e is used as effective strand length.Comment: 4 pages, RevTex, 3 Figure