Catálogo de publicaciones - libros

Scientific computing on super-computers has become one of the standard methods of research in physics. This is clearly demonstrated by the articles in this section, which present a selection of projects related to physical re-search currently running at the HLRS. The presented work does not only cover a wide range of physics, it is also dominated by long-term projects which continuously have progressed over several years applying well estab-lished numerical methods.

We investigate the chain conformations and phase separation in binary polymer blends. Using large scale semi-grandcanonical Monte Carlo simulations and finite size scaling, we investigate the molecular extension and the intermolecular paircorrelation function in thin films with hard, non-preferentially adsorbing surfaces. The interplay between chain conformations, demixing and the validity of mean field theory is investigated for a large variation of chain lengths 16 ≤ ≤ 512. Three regimes of film thickness can be distinguished: (i) For film thicknesses much larger than the unperturbed chain extension , bulk behavior is observed, i.e., the critical temperature of demixing increases linearly with chain length, and the mean field theory becomes asymptotically correct for large . (ii) For ∼ , the critical temperature scales linearly, ∼ , but the mean field theory overestimates the prefactor even in the limit → ∞ (iii) For ultrathin films, the chain conformations are quasi-two-dimensional, ∼ √ and mean field theory completely fails.

Lattice Monte-Carlo simulations of quantum chromodyanmics (QCD) have shown that strongly interacting matter undergoes a phase transition at some temperature []. Above the transition temperatures hadrons (the experimentally observed strongly interacting particles) cease to exist and a new state of matter, the so-called quark-gluon plasma (QGP) was predicted to exist []. One of the most prominent features of QGP is the screening of static chromoelectric fields. At large distances (i.e. distances much larger than the inverse temperature) the screening is exponential and can be parametrized by a temperature dependent chromoelectric screening mass. Chromoelectric screening masses have been extensively studied by us in the last few years [–] in the framework of our project (Nr. 11725). More recently chromoelectric screening has been studied in terms of the free energy of static quark anti-quark pair [,,,]. Moreover we developed a technique to separate energy and entropy contributions to the free energy [, ].

Simulating the flow of suspensions is an extremely difficult and demanding problem. Suspensions are mixtures of fluid and granular materials, and each component alone is a challenge to numerical modelers. When they are combined in a suspension, neither component can be neglected, so all the difficulties of both fluids and grains must be solved, in addition to the new problem of describing the interaction between the two.

In the current project, we simulate thermal convection and generation of magnetic fields inside a fully convective star, like an M-dwarf or a T-tauri star. The numerical resolution required to reasonably well describe convection properties inside the star is 256 to 512.

Symbiotic systems consist of a red giant undergoing strong mass loss and a white dwarf. More than hundred symbiotic stars are known, but only about ten systems show jet emission. The most famous systems are R Aquarii, CH Cyg and MWC 560. While the first two objects are seen at high inclinations — a fact which makes it possible to study the morphology and structure of jets of symbiotic stars — the jet axis in MWC 560 is practically parallel to the line of sight. This special orientation provides the opportunity to observe the outflowing gas as line absorption in the source spectrum (Schmid et al. 2001). Therefore MWC 560 can be used to probe the short term evolution and the propagation of the gas outflow in jets from white dwarfs.

We discuss image potential states on an insulator surface, LiF(001)-(1×1), within ab-initio many-body perturbation theory. The image potential states originate from the interaction of electrons outside the surface with polarization charges inside the substrate. They are responsible for characteristic features in the electron energy loss spectrum of the material. The onset of excitation energies is at 9.2 eV, which is several eV lower than the bulk excitations.

We have used the vectorised and parallelised magnetohydrodynamics code NIRVANA on the NEC SX-5 and the new SX-6 installation in parallel mode to simulate the interaction of jets with a galactic wind that might be typical for the star-bursting radio-galaxies of the early universe.

In the following we give a summary of the computational physics articles presented in the frame of solid-state physics. In the project of Prof. P. Nielaba from the physics department in Konstanz new insights into the electronic transport in nano-wires, elastic constants of model colloids, pore condensates and phase transitions in nano-systems in ex-ternal potentials and reduced geometry have been obtained. Despite the fact that many new experimental techniques have studied these properties in sys-tems of the size of the a few nanometers, the theoretical investigations are still in an initial stage. In this field computer simulations have become more and more important since the nano-systems in reduced geometry contain typically between 10 and 10.000 particles. This size is nearly ideal for the application of computer simulation methods. The summary given by P. Nielaba then de-scribes in detail the computational efficiency implemented at the HLRS in Stuttgart and the results obtained in the projects in 2003.

Elastic and structural properties of model colloids have been studied with particular emphasis on the effect of quenched impurities and of external fields. The structural and electronic properties of atomic wires has been analysed. In the following sections an overview is given on the results of our recent computations on quantum effects, structures and phase transitions in such systems.