Polymer chain stiffness versus excluded volume: A Monte Carlo study of the crossover towards the wormlike chain model

When the local intrinsic stiffness of a polymer chain varies over a wide range, one can observe both a crossover from rigid-rod-like behavior to (almost) Gaussian random coils and a further crossover towards self-avoiding walks in good solvents. Using the pruned-enriched Rosenbluth method (PERM) to study self-avoiding walks of up to $N_b=50000$ steps and variable flexibility, the applicability of the Kratky-Porod model is tested. Evidence for non-exponential decay of the bond-orientational correlations $<\cos \theta (s) >$ for large distances $s$ along the chain contour is presented, irrespective of chain stiffness. For bottle-brush polymers on the other hand, where experimentally stiffness is varied via the length of side-chains, it is shown that these cylindrical brushes (with flexible backbones) are not described by the Kratky-Porod wormlike chain model, since their persistence length is (roughly) proportional to their cross-sectional radius, for all conditions of practical interest.Comment: 6 pages, 5 figures, to be published in Europhys. Lett. (2010

Time-dependent density-matrix functional theory for biexcitonic phenomena

We formulate a time-dependent density-matrix functional theory (TDDMFT) approach for higher-order correlation effects like biexcitons in optical processes in solids based on the reduced two-particle density-matrix formalism within the normal orbital representation. A TDDMFT version of the Schr\"odinger equation for biexcitons in terms of one- and two-body reduced density matrices is derived, which leads to finite biexcitonic binding energies already with an adiabatic approximation. Biexcitonic binding energies for several bulk semiconductors are calculated using a contact biexciton model

Tailoring laser pulses with spectral and fluence constraints using optimal control theory

Within the framework of optimal control theory we develop a simple iterative scheme to determine optimal laser pulses with spectral and fluence constraints. The algorithm is applied to a one-dimensional asymmetric double well where the control target is to transfer a particle from the ground state, located in the left well, to the first excited state, located in the right well. Extremely high occupations of the first excited state are obtained for a variety of spectral and/or energetic constraints. Even for the extreme case where no resonance frequency is allowed in the pulse the algorithm achieves an occupation of almost 100%

Transverse Double-Spin Asymmetries for Muon Pair Production in pp-Collisions

We calculate the rapidity dependence of the transverse double-spin asymmetry for the Drell-Yan process to next-to-leading order in the strong coupling. Input transversity distributions are obtained by saturating the Soffer inequality at a low hadronic mass scale. Results for the polarized BNL-RHIC proton-proton collider and the proposed HERA-N fixed-target experiment are presented, and the influence of the limited muon acceptance of the detectors on measurements of the asymmetry is studied in detail.Comment: 7 pages including 5 figures; significantly shortened, to be published in Phys. Rev.

Efficiency of Bti-based floodwater mosquito control in Sweden -four examples

Abstract: Mass-occurrence of floodwater mosquitoes, mainly Aedes sticticus, in the River Dalälven floodplains in central Sweden has caused public health issues and economic losses for many decades. In the summer of 2000, the problem escalated and the Biological Mosquito Control project was initiated with the aim of reducing mosquito nuisance. Larviciding, based on Bacillus thuringiensis israelensis (Bti), was chosen as the optimal method. However, high abundance of blood-seeking floodwater mosquitoes after Bti-treatments on some occasions raised questions about the effectiveness of the treatments. This study evaluated the effect of Bti-larviciding on abundance of larval and adult floodwater mosquitoes in four selected study areas, each represented by a CDC-trap site and a 5 km radius. The four areas differed with respect to their mosquito control history and the coverage of larval habitats with Bti-larviciding. The Bti-treatments provided a significant reduction of mosquito larval abundance, and normally 100% reduction was achieved. Thus, high abundance of blood-seeking mosquitoes could not be explained by insufficient larval control by Bti. However, a significant negative correlation was found between high numbers of blood-seeking floodwater mosquitoes and the coverage of larval habitats with Bti-larviciding within 5 km around the trap site. Consistently low numbers of mosquitoes (less than 1000 per trap/night) were only found in the two areas with high treatment coverage of larval habitats (97-100%). Evaluating the mosquito control efficiency showed that larval habitat coverage of at least about 95% is required in order to accomplish consistent low floodwater mosquito numbers. The conclusion from this analysis is that the coverage of larval habitats with Bti-larviciding in parts of the River Dalälven floodplains has to increase in order to guarantee an improvement of the public health problems caused by Aedes sticticus and other floodwater mosquitoes to both humans and animals in the region

Chaotic Scattering in the Regime of Weakly Overlapping Resonances

We measure the transmission and reflection amplitudes of microwaves in a resonator coupled to two antennas at room temperature in the regime of weakly overlapping resonances and in a frequency range of 3 to 16 GHz. Below 10.1 GHz the resonator simulates a chaotic quantum system. The distribution of the elements of the scattering matrix S is not Gaussian. The Fourier coefficients of S are used for a best fit of the autocorrelation function if S to a theoretical expression based on random--matrix theory. We find very good agreement below but not above 10.1 GHz

Shear-free radiating collapse and conformal flatness

Here we study some general properties of spherical shear-free collapse. Its general solution when imposing conformal flatness is reobtained [1,2] and matched to the outgoing Vaidya spacetime. We propose a simple model satisfying these conditions and study its physical consequences. Special attention deserve, the role played by relaxational processes and the conspicuous link betweeen dissipation and density inhomogeneity.Comment: 13 pages Latex. Some misprints in eqs.(17), (30) and (35) have been correcte

Particle production from nonlocal gravitational effective action

In this paper we show how the nonlocal effective action for gravity, obtained after integrating out the matter fields, can be used to compute particle production and spectra for different space-time metrics. Applying this technique to several examples, we find that the perturbative calculation of the effective action up to second order in curvatures yields exactly the same results for the total number of particles as the Bogolyubov transformations method, in the case of masless scalar fields propagating in a Robertson-Walker space-time. Using an adiabatic approximation we also obtain the corresponding spectra and compare the results with the traditional WKB approximation.Comment: 22 pages, LaTeX, no figures. Corrected version with new comments and results. To appear in Phys. Rev.