Small-Angle Excess Scattering: Glassy Freezing or Local Orientational Ordering?

We present Monte Carlo simulations of a dense polymer melt which shows glass-transition-like slowing-down upon cooling, as well as a build up of nematic order. At small wave vectors q this model system shows excess scattering similar to that recently reported for light-scattering experiments on some polymeric and molecular glass-forming liquids. For our model system we can provide clear evidence that this excess scattering is due to the onset of short-range nematic order and not directly related to the glass transition.Comment: 3 Pages of Latex + 4 Figure

Interface localisation-delocalisation transition in a symmetric polymer blend: a finite-size scaling Monte Carlo study

Using extensive Monte Carlo simulations we study the phase diagram of a symmetric binary (AB) polymer blend confined into a thin film as a function of the film thickness D. The monomer-wall interactions are short ranged and antisymmetric, i.e, the left wall attracts the A-component of the mixture with the same strength as the right wall the B-component, and give rise to a first order wetting transition in a semi-infinite geometry. The phase diagram and the crossover between different critical behaviors is explored. For large film thicknesses we find a first order interface localisation/delocalisation transition and the phase diagram comprises two critical points, which are the finite film width analogies of the prewetting critical point. Using finite size scaling techniques we locate these critical points and present evidence of 2D Ising critical behavior. When we reduce the film width the two critical points approach the symmetry axis $\phi=1/2$ of the phase diagram and for $D \approx 2 R_g$ we encounter a tricritical point. For even smaller film thickness the interface localisation/delocalisation transition is second order and we find a single critical point at $\phi=1/2$. Measuring the probability distribution of the interface position we determine the effective interaction between the wall and the interface. This effective interface potential depends on the lateral system size even away from the critical points. Its system size dependence stems from the large but finite correlation length of capillary waves. This finding gives direct evidence for a renormalization of the interface potential by capillary waves in the framework of a microscopic model.Comment: Phys.Rev.

Path integral for half-binding potentials as quantum mechanical analog for black hole partition functions

The semi-classical approximation to black hole partition functions is not well-defined, because the classical action is unbounded and the first variation of the uncorrected action does not vanish for all variations preserving the boundary conditions. Both problems can be solved by adding a Hamilton-Jacobi counterterm. I show that the same problem and solution arises in quantum mechanics for half-binding potentials.Comment: 6 pages, proceedings contribution to "Path integrals - New Trends and Perspectives", Dresden, September 200

Photoproduction of eta-mesons near-threshold - Reply

The Comment by Mukhopadhyay, Zhang, and Benmerrouche [1] does not challenge any conclusion drawn in the interpretation of our data [2] but even strengthens some of them

Study of N* resonances by eta photoproduction

Photoproduction of neutral mesons is very well suited to investigate the excitation of baryon resonances on free and bound nucleons. The isoscalar eta-meson does not couple to Delta-resonances and therefore provides a very selective tool for the study os isospin T=1/2 N* resonances. The eta photoproduction from protons in the threshold region is strongly dominated by the excitations of the S11(1535) resonance. The electromagnetic coupling gamma p-> S11 as well as contributions from other resonances (P11, Roper)are of fundamental interest for hadron models. We have studied eta photoproduction form the proton, deuteron, and some complex nuclei (C, Ca,Nb,Pb) at the Mainz microtron using the TAPS detector system

Integrative carbonate budget model of the Great Barrier Reef

This project synthesizes and refines the carbonate budget produced for the Great Barrier Reef (GBR) in Wolfe et al. (2019). Here, we present robust methodological advances to facilitate the calculation of carbonate budgets for the GBR, including:1. The first method to quantify carbonate budgets using percent-cover data, alleviating the requirement for detailed in-situ census-based approaches to quantifying structural complexity and carbonate production (e.g., rugosity).2. Development of a carbonate budget model for the GBR that allows for site-specific conversions of coral cover to produce estimates of carbonate production.3. Inclusion of ecologically-relevant parameterisation of parrotfish bioerosion using an integrative modelling approach and GBR-specific parameter estimates.4. Explicit consideration of uncertainty surrounding production and erosion processes across spatial and temporal dimensions.5. A consequent R package (“caRbs”) for use by scientists and managers.Using the Australian Institute of Marine Science’s (AIMS) Long-Term Monitoring Program (LTMP) data on coral cover and fish biomass between 2016–2020 to parameterise the caRbs package, we quantified spatial and temporal patterns in carbonate production and bioerosion and identified the thresholds in coral cover required to maintain a positive carbonate budget for the GBR. Results highlighted the importance of Acropora sp. as the primary contributor to the carbonate budget, although massive Porites sp. were identified as key framework builders to maintain carbonate production. Carbonate production varied latitudinally, with higher calcification rates at low latitudes, while there were no clear spatial or temporal trends in primary bioerosion on the GBR. Parrotfish (Chlororus microrhinus) was the primary driver of bioerosion on fore-reef habitats. The study concluded that overall estimates of the carbonate budget on the GBR ranged between -4.9 and 28.4 kg m-2 yr-1 (~73% of sites had net positive budgets), and the threshold of coral cover to maintain a positive carbonate budget on the GBR were estimated at ~17%.Key take home messages from a management perspective are:1. For mid and outer shelf reef slope communities, overall coral cover needs to exceed 17% for reefs to have a positive carbonate budget.2. We do not yet have a corresponding threshold for inshore reefs where a number of processes, including bioerosion and biogeochemistry, can be quite different.3. The fact that an individual reef may sit below the 17% threshold does not necessarily indicate that there is a problem; it may just be in an early stage of recovery.4. The caRbs package presents an important tool to evaluate trends of reef state over time, asking how the proportion of reefs sitting below and above the 17% threshold is changing and to ask whether one region has a higher proportion of ‘positive reefs’ than another.5. A positive carbonate budget is a pre-requisite for many reef services but does not necessarily imply that all services, including accretion with sea-level rise and provision of high-quality habitat for fish and coastal protection, will occur at historical rates. Further work is needed to link carbonate budget trends explicitly to ecosystem functions. This work has started by linking carbonate budgets to reef accretion (Perry et al. 2018) but there is more to do

Near Threshold Photoproduction of η Mesons off the Proton

We have measured precise total and differential cross sections for the reaction γp→pη from threshold to 790 MeV at the MAMI accelerator in Mainz with the neutral meson spectrometer TAPS. Resonance parameters of the S11(1535) resonance and the electromagnetic coupling γp→S11 have been extracted from the data. Contributions from the D13(1520) resonance to η photoproduction in the threshold region have been identified for the first time via interference terms in the angular distributions

Neutral pion photoproduction from the proton near threshold

Differential and total cross sections for the photoproduction of neutral pions from the proton have been measured for incident photon energies from 140-270 MeV, using the photon spectrometer TAPS at the tagged photon beam of the 855 MeV Mainz Microtron. The energy dependence of the s- and p-wave multipoles close to threshold was deduced from a multipole fit and a multipole analysis. The extracted s-wave amplitude E(0+) at threshold is found to be significantly smaller than the prediction of the classical low energy theorems, but is in reasonable agreement with the chiral perturbation theory