C-axis oriented growth of magnetron sputtered YBaCuO thin films on MgO substrates

Highly textured YBaCuO thin films were sputtered on MgO (100)-oriented single crystal substrates at ambient temperature followed by an anneal in oxygen for 1 h at temperatures up to 920 °C. X-ray diffractograms of the highly textured films indicate an orientation of the c-axis of the YBaCuO lattice perpendicular to the substrate surface. There are strong indications that the oriented c-axis growth is due to a CuO self-flux effect. Auger measurements reveal a copper diffusion profile into the substrate down to a depth of more than 400 nm.     

Validation of CASCADE code using the monitor reactions

Production cross-sections of monitor reaction ²⁷Al(x, y)²⁴Na for proton, neutron and deuteron projectiles have been estimated from the CASCADE code from tens of MeV/n to tens of GeV/n energy and compared them with the available experimental data. It has also been shown that using the CASCADE code production cross-section of ²⁷Al(d, y₂)²⁴Na reaction can be obtained from the proton and neutron projectiles. Implicitly, this provides an alternative way of knowing production cross-section of ²⁷Al(n, y ₁)²⁴Na reaction which may be used to monitor the neutron flux in a wide range of energy. However, in the paper need of experimental determination of cross-sections for neutron projectile has been stressed. Similarly, cross-section of other monitor reactions like ²⁷Al(p, y ^′)²²Na and ²⁷Al(n, y ^′ ₁)²²Na are also calibrated as function of energy up to several GeV.     

A simplified Glauber model for hadron–nucleus cross sections

A model for hadron–nucleus cross sections based on a simplified Glauber approach is proposed. The predictions of the model are compared with experimental data for the inelastic and the total hadron–nucleus cross sections from available databases. The model has been implemented in the framework of the Gent4 toolkit.     

The temperature dependence of the resistive response of superconducting Pb films upon picosecond optical excitation

We report a study of the temperature dependence of the transient resistivity of superconducting Pb thin films excited by short duration (70 ps) laser pulses of varying irradiances. The results are analyzed in terms of a model proposed by Elesin which features the existence of a nonstationary, inhomogeneous, intermediate state.     

Anisotropic elastic properties of microtubules

The present study aims at providing insight into the acceleration mechanism of a bubble chain rising in shear-thinning viscoelastic fluids. The experimental investigation by the Particle Image Velocimetry (PIV), birefringence visualisation and rheological simulation shows that two aspects are central to bubble interactions in such media: the stress creation by the passage of bubbles, and their relaxation due to the fluids memory forming an evanescent corridor of reduced viscosity. Interactions between bubbles were taken into account mainly through a linear superposition of the stress evolution behind each bubble. An analytical approach together with the rheological consideration was developed to compute the rise velocity of a bubble chain in function of the injection period and bubble volume. The model predictions compare satisfactorily with the experimental investigation.     

Direct observation of counterion organization in F-actin polyelectrolyte bundles

Attractions between like-charged polyelectrolytes have been observed in a variety of systems (W.M. Gelbart, R.F. Bruinsma, P.A. Pincus, V.A. Parsegian, Phys. Today 53, September issue, 38 (2000)). Recent biological examples include DNA, filamentous viruses, and F-actin. Theoretical investigations on idealized systems indicate that counterion correlations play a central role, but no experiments that specifically probe such correlations have been performed. Using synchrotron X-ray diffraction, we have directly observed the organization of multivalent ions on cytoskeletal filamentous actin (a well-defined biological polyelectrolyte) and found an unanticipated symmetry-breaking collective counterion mechanism for generating attractions. Surprisingly, the counterions do not form a lattice that simply follows actins helical symmetry; rather, the counterions organize into frozen ripples parallel to the actin filaments and form structures reminiscent of charge density waves. Moreover, these 1D counterion charge density waves form a coupled mode with twist deformations of the oppositely charged actin filaments. This counterion organization is not sensitive to thermal fluctuations in temperature range accessible to protein-based polyelectrolyte systems. Moreover, the counterion density waves are pinned to the spatial periodicity of charges on the actin filament even if the global filament charge density is varied, indicating the importance of charge periodicity on the polyelectrolyte substrate.     

The interplay between surface micro-topography and -mechanics of type I collagen fibrils in air and aqueous media: An atomic force microscopy study

Calf skin type I collagen fibrils were regenerated from acidic solution and imaged with contact mode atomic force microscopy in air, water, and buffer solution. When imaged in air at a contact force of 20-150 nN, collagen fibrils exhibited a distinct transverse banding pattern with a period of 65 nm, consisting of high ridges and shallow grooves. The force dependence of the images suggests that such banding pattern is attributed to the transverse contraction of the fibril upon dehydration during sample preparation, which reflects the tangential mass density across the fibril. Imaging in water and phosphate buffer solution at a contact force of 15-80 nN revealed hydrated collagen fibrils with smooth surfaces. The rigidity of the collagen fibrils decreased considerably upon hydration. Scanning the cantilever tip in an aqueous medium at a contact force of 90-280 nN enabled us to probe subunit arrangement in the bulk region of the collagen fibril. The results indicate that the molecular assembly in the hydrated fibril is akin to that in the intact form. The image resolution was improved by stabilizing the collagen molecules through crosslinking with glutaraldehyde, which served to resolve microfibril-like structure on the fibril surface. Received 28 March 2000 and Received in final form 15 June 2001     

Universality classes of self-avoiding fixed-connectivity membranes

We present an analysis of extensive large-scale Monte Carlo simulations of self-avoiding fixed-connectivity membranes for sizes (number of faces) ranging from 512 to 17672 (triangular) plaquettes. Self-avoidance is implemented via impenetrable plaquettes. We simulate the impenetrable plaquette model in both three and four bulk dimensions. In both cases we find the membrane to be flat for all temperatures: the size exponent in three dimensions is ν = 0.95(5) (Hausdorff dimension d _H = 2.1(1)). The single flat phase appears, furthermore, to be equivalent to the large bending rigidity phase of non-self-avoiding fixed-connectivity membranes --the roughness exponent in three dimensions is ξ = 0.63(4). This suggests that there is a unique universality class for flat fixed-connectivity membranes without attractive interactions. Finally, we address some theoretical and experimental implications of our work. Received 23 June 2000 and Received in final form 25 October 2000     

Feasibility of high power refractory metal-foil targets for EURISOL

EURISOL foil-targets have to withstand a primary proton beam of 1 GeV kinetic energy and up to 100 μA beam current. These foil targets will be based on previous high-power target concepts, i.e. the RIST target [J.R.J. Bennett et al., Nucl. Instrum. Meth. Phys. Res. B 126, 117 (1997)] or high power targets used at TRIUMF [P. Bricault et al., Nucl. Instrum. Meth. Phys. Res. B 204, 319 (2003), M. Dombsky et al., Nucl. Instrum. Meth. Phys. Res. B 204, 191 (2003)]. A single target unit is capable of dissipating up to 25 kW, hence, several target units can be merged together by individual transfer lines to one common ion source. The single target units will be irradiated by a proton beam in a time sharing mode to distribute the primary proton beam current to the individual target units. In this feasibility study the necessary properties of high-power foil targets are discussed and the requirements to design a foil target according to the proton beam parameters [CITE] for the future EURISOL facility are given.