B. Willems

On the apsidal motion in close binaries due to the tidal deformations of the components

The paper is devoted to a confrontation of the apsidal-motion rates in close binaries due to the tidal perturbations of the stellar components that are predicted by Sternes formula (1939) with the corresponding apsidal-motion rates that are determined in the framework of the theory of the dynamic tides. Sternes formula is derived in the supposition that the orbital period and the stars rotational period are suciently long so that, in accordance with an earlier suggestion of Cowling (1938), the star is almost adjusted to the gravitational eld of the companion. From the point of view of the theory of the dynamic tides, the second-degree tide is then approximated at each instant by an appropriate linear combination of three second-degree tides which are considered to be static. In this limiting case, the rate of secular apsidal motion predicted by Sternes formula agrees, up to large orbital eccentricities, with the rate of secular apsidal motion determined in the framework of the theory of the dynamic tides and depends on the stars central mass condensation. For close binaries with shorter orbital periods, the use of Sternes formula leads to deviations because of the increasing influence of the compressibility of the stellar fluid and resonances of dynamic tides with lower-order g + -modes. The relative deviations may to amount to a few tens of percents for models of zero-age main sequence stars of 5M ,1 0M ,a nd 20M.

Granular superconductivity in metallic and insulating nanocrystalline boron-doped diamond thin films

The low-temperature electrical transport properties of nanocrystalline boron-doped diamond (b-NCD) thin films have been found to be strongly affected by the systems granularity. The important differences between the high and low-temperature behaviour are caused by the inhomogeneous nucleation of superconductivity in the samples. In this paper we will discuss the experimental data obtained on several b-NCD thin films, which were studied by either varying their thickness or boron concentration. It will be shown that the low-temperature properties are influenced by the b-NCD grain boundaries as well as by the appearance of an intrinsic granularity inside these granules. Moreover, superconducting effects have been found to be present even in insulating b-NCD films and are responsible for the negative magnetoresistance regime observed at low temperatures. On the other hand, the low-temperature electrical transport properties of b-NCD films show important similarities with those observed for granular superconductors.

Negative magnetoresistance in boron-doped nanocrystalline diamond films

We report on the observation of a negative magnetoresistance (NMR) regime in boron-doped nanocrystalline diamond films at low temperatures. A comparative analysis of our experimental results and those reported for systems composed of superconducting granules embedded in an insulating matrix (also referred as granular films) suggest the presence of superconducting regions inside the insulating films as causing the NMR. By considering the latter scenario, the experimental observations are explained by modeling the systems as consisting of a distribution of superconducting granules whose global properties are tuned by the intergrain distance.

Dislocation Structures in Diamond: Density-Functional Based Modelling and High-Resolution Electron Microscopy

The core structures of perfect 60 and edge dislocations in diamond are investigated atomistically in a density-functional based tight-binding approach, and their dissociation is discussed both in terms of structure and energy. Furthermore, high resolution electron microscopy is performed on dislocation cores in high-temperature, high-pressure annealed natural brown diamond, and HRTEM image simulation allows a comparison of theoretically predicted and experimentally observed structures.